Final Report Summary - CHATT (Cryogenic Hypersonic Advanced Tank Technologies)
Executive Summary:
In future aviation and particularly in hypersonic systems new propellants will be used, such as liquid hydrogen, liquid methane and possibly liquid oxygen. Previous FP7 funded studies in Europe such as FAST20XX, ATLLAS or LAPCAT investigated advanced vehicles with these fuels for passenger transport like the SpaceLiner or Lapcat A2 and some of their constituent materials and associated propulsion challenges. The question of cryogenic propellant storage inside an airliner – although of critical importance but by far not yet mastered – had not been addressed in Europe in comparable detail.
The need for more detailed investigations on liquid hydrogen or methane tanks in future airliners is not only urgent in hypersonic aviation, but is also essential for environmental reasons in subsonic aviation. New materials and design concepts are required, such as fiber based composite materials, in order to reduce the tank weight and to increase the structural performance. This is particularly important if the tank has load carrying functions. Different to current rocket launch systems, the durability through hundreds or even thousands of flight cycles must be assured. Tank liners are another essential element of a tank design in order to assure the material compatibility over long durations.
The Cryogenic Hypersonic Advanced Tank Technologies (CHATT) project addressed these challenges within a comprehensive and multifold approach. The focus has been on sophisticated experimental material research campaigns, culminating in the development, manufacturing and testing of four different demonstrator tank structures, each involving individual technologies or design philosophies. On the material research level various CFRP material and layup combinations as well as liner material and CFRP/liner combinations have been evaluated in order to assess their applicability for future advanced hydrogen tank structures. Special attention has been paid on material behavior in cryogenic temperature environments, including micro-crack initiation, thermal cycling, and fatigue. Based on these results, suitable material systems have been selected and utilized for manufacturing of the four tank structures. The largest tank built is a cylindrical vessel with approximately 3 m in length and 1 m in diameter, utilizing a CFRP shell design wound on a polymer liner. Another, smaller cylindrical vessel has been manufactured with innovative dry-wind technology. A third vessel employs a liner-less design, while the fourth tank has been designed and manufactured as a multi-bubble tank suited for non-circular fuselage cross-sections. Testing of the tank structures involved pressurization and mechanical loading tests, as well as cryogenic fluid fill and drain experiments using liquid nitrogen.
The material research and tank manufacturing campaigns in CHATT have been complemented by comprehensive system investigations for different types of hypersonic vehicles. These not only provided the reference environmental and load conditions for material and tank research, but also included related topics such as propellant management, propellant cross-feed designs for parallel staged vehicles, cabin environment control and power generation by using cryogenic fluids, and flight control under presence of fluid movement. In particular the propellant sloshing problem has been addressed on a theoretical as well as experimental level. High-fidelity CFD codes have been improved enabling detail investigations of sloshing phenomena. Simultaneously, approximate analytic models were assessed on how far the impact of sloshing on flight vehicle control could be simulated in a fast way. Via coupled analysis, also the impact of sloshing on vehicle aero-elastics and structural sizing could be assessed. Furthermore, analytical and/or numerical analysis tools for cylinder, multi-lobe, and multi-bubble tanks including insulations were developed. These codes have been coupled with high-order optimization suites for overall optimization of tanks.
Further associated topics to cryogenic tanks have been investigated as well, theoretically and experimentally, such as ceramic heat exchanger technologies, roughness induced boiling, aerogels for insulation applications, or tank structural health monitoring.
One of the central findings of the CHATT-project supported by manufacturing and material tests is the very promising potential of thin-ply CFRP technologies for cryogenic tank applications. Extensive material research and demonstrator tube testing campaigns proved the superior performance of thin-ply approaches for delaying or even preventing cryogenic temperature induced micro-cracking. The results indicate that liner-less and ultra-lightweight cryogenic tank systems could be realized using thin ply approaches.
Project Context and Objectives:
In future aviation and particularly in hypersonic systems new propellants will be used, such as liquid hydrogen, liquid methane and possibly liquid oxygen. Previous FP7 funded studies in Europe such as FAST20XX, ATLLAS or LAPCAT investigated advanced vehicles with these fuels for passenger transport like the SpaceLiner or Lapcat A2 and some of their constituent materials and associated propulsion challenges. The question of cryo¬genic propellant storage inside an airliner – although of critical importance but by far not yet mastered – had not been addressed in comparable detail. The need for more detailed investigations on liquid hydrogen or methane tanks in future airliners is not only urgent in hypersonic aviation, but is also essential for environmental reasons in subsonic aviation. Air traffic has experienced strong growth over a long time, and it is predicted that such growth will continue at rates of 4 – 5 % annually over the next decades. On the other hand, it is generally accepted that the emission of greenhouse gases, most notably of carbon dioxide (CO2), resulting from human’s activities, cannot be allowed to continue increasing if adverse global climate change is to be avoided. Liquid hydrogen, produced on the basis of renewable energy, is the only known new fuel meeting the requirements of the political guidelines of former EU president Barroso.
The previous CRYOPLANE study’s analysis had shown that hydrogen could be a suitable alternative fuel for the future of aviation. Nevertheless, due to the missing materials, parts, components and engines, further R&D work has to be performed until hydrogen can be used as an aircraft fuel. Cryogenic fuel propulsion is already operational in advanced launcher systems and Europe has some expertise with the Ariane rocket. However, the airliner systems will require more complex technology (compared to those existing in today’s launchers like Ariane 5), such as ultra-light-weight and reusable propellant tank systems. The propellant tank technologies are critical for the vehicle operations, cost and safety. The extent to which the technological issues can be solved also has an important impact on the trade-off and choice of the future propellants, in particular between liquid hydrogen and liquid methane.
New materials and design concepts are required, such as fiber composites, in order to reduce the tank weight and to increase the structural performance. This is particularly important if the tank has load carrying functions. Different to current rocket launch systems, the durability through hundreds or even thousands of flight cycles must be assured. Tank liners can be another essential element of a tank design in order to assure the material compatibility over long durations. Embedded structural health monitoring systems are required to assure acceptable turn-around time between flights, to optimize the service and maintenance effort and to allow low design margins without lowering overall safety.
Propellant management is imperative for achieving reliable and efficient vehicle operation. The sloshing of cryogenic fluids close to their boiling conditions in tanks of horizontal take-off vehicles is not yet mastered.
One of the core objectives of CHATT is to investigate Carbon Fiber Reinforced Plastic (CFRP) cryogenic pressure tanks. Composite pressure vessels have an excellent potential for light weight design, because the fiber architecture can be manufactured in a way that all dominating loads are transferred along the reinforcement fibers. The most efficient structures carry their loads via in-plane stresses. The structural efficiency of a pressure vessel is defined as the ratio of the pressurized volume and the mass of the pressure vessel. To minimize the weight, iso-tensoid structures provide the lightest solutions. “Iso-tensoid” simply means that all locations within the structure are under the same level of tensile stress. Fiber reinforced materials are structurally the most efficient material for pressure vessels because there is the possibility to direct the right amount of fibers according to the orientation and the magnitude of the principal stresses, which makes it an iso-tensoid structure. Carbon fibers are currently known to have the highest specific strength and stiffness which makes it the material of choice when it comes to aerospace structures. On top of that, CFRP have a very small thermal expansion coefficient (CTE) and the mechanical properties of carbon fibers are consistent from temperatures between 0 K and 1000 K. This makes it worthwhile to investigate CFRP cryogenic pressure tanks for hypersonic vehicles. To manufacture composite pressure vessels it is recommendable to choose a filament winding process because this process provides the required boundary conditions concerning thread-tension and lay-up control. Even though small remaining porosities are not critical for the structural integrity, they can still be a potential leakage especially for gaseous contents. For this reason a gas-tight liner on the inside of the pressure vessels is mandatory for most technical applications. For low temperature applications the increasing brittleness of the liner has to be considered to ensure gas-tightness. For typical carbon epoxy composites the vessel structure itself is not critical under cryogenic boundary conditions. Another crucial point that has to be taken into account is the incompatible thermal expansion of some material combinations especially when local temperature gradients cannot be avoided. The resulting forces can lead to critical stresses especially in the liner structure. New composite material combinations are continuously being developed for potential use as lightweight tank structures. However, the realisation of a lightweight, well-insulated and safe storage system for cryogenic propellant is a delicate engineering challenge. In aerospace applications, the lightweight property is a fundamental performance parameter. Depending on the type of application, different designs may be favourable and tanks may thus be built as single-, double-wall or sandwich structures. Apart from low heat-transfer and the ability to withstand thermo-mechanical loading, other key issues are easy inspection and health monitoring. For advanced tank wall materials the high specific strength at cryogenic temperatures is the most essential parameter. Fracture toughness and stiffness are also important properties. Potential wall candidate materials with high specific strength are monolithic metals, polymer matrix composites and discontinuously reinforced metal matrix composites. Ceramic materials also have high specific strength but low fracture toughness and are thus not suitable tank wall candidate materials.
In the US, the DC-XA composite LH2 tank demonstrator resulted in a 34% weight saving compared to an aluminium tank. Vessels for extremely low molecular weight gases such as helium and hydrogen must be practically leak-free. The DC-XA is the first successful demonstration of a leak-free liquid hydrogen composite tank, but the shell thickness was in this case oversized to avoid leakage. The X-33 demonstrator tank failed during ground testing due to polymer matrix micro-cracking and leakage into the sandwich core material causing delamination between the core and the inner composite skin. The tank showed leakage with subsequent damage, so-called “cryopumping”. The failure contributed to the complete termination of the vehicle development program. Northrop Grumman and NASA later completed a nine-month test series to demonstrate a composite cryogenic tank filled with LH2 and pressurised. The problems that brought the X-33 to a halt were proven to be solved in 40 load cycles performed without failure. The program highlighted the need to improve the permeation resistance of liner material and limit the composite micro-cracking in wall construction details. In Japan Higuchi et al. demonstrated in a flight test a successful development of a metal-lined CFRP cryogenic tank for a reusable rocket. The design strategy was based on the fact that micro-cracks are inevitable. A metallic liner was then used as a gas barrier. In this way the CFRP does not need to be gas tight. The tank is designed as a high pressure vessel, thus avoiding a mechanical fuel pump. The tank was pressurised above the elastic limit of the metallic liner, in order to avoid liner separation at cryogenic temperatures. Reported lessons learned during the project where related to the type of welding flux, O-ring seals and elaboration with different adhesives to avoid interlayer separation, face sheet delamination and finally drain and tank ventilation. The tank was also equipped with strain-gauges, thermocouples and an advanced optical sensor health monitoring system plus an onboard hydrogen gas detector. Thermal and stress cycling as well as flight test were demonstrated successfully.
Experiences in the past sometimes showed difficulties in achieving true lightweight designs when necessary safety factor penalties associated with advanced composite materials are regarded. The handling at cryogenic temperatures potentially give rise to large temperature gradients across the tank wall which need to be limited by using a lightweight, low-conductivity insulation system in order to limit fuel loss and tank pressure build-up. The main challenge when using composite tank structures is the permeation resistance. Polymeric films and coatings, using various polymers and elastomers films, have been considered as potential gas barriers. However, literature and manufacturer material data on hydrogen permeation is very limited. Apart from permeation, the resistance to hydrogen embrittlement is important. In these aspects, the linerless approach may be desirable from a weight point of view. The challenge in developing a cryogenic CFRP tank is finding a solution for the problems caused by differences in thermal expansion coefficients. Although carbon fiber composites have an extremely low CTE on a macroscopic scale, this is not the case on a microscopic scale and ordinary matrices cannot cope with cryogenic temperatures. This implies that the dry wound carbon fiber pressure vessel of ALE is a good alternative but dry wound structures do not have the ability to carry multiple load conditions. On top of this, if a liner is required, there is also the challenge to overcome the differences in CTE of the liner with respect to the structural shell.
Previously reported research activities on cryo-CFRP-tanks within Europe were limited to some studies on future launcher applications. The research in CHATT intended to increase the knowledge within Europe, with advancement from a pure material science level to a practical cryogenic tank demonstrator level. This project performed the first steps towards a common European development of future aerospace reusable lightweight composite cryogenic CFRP tanks. The fundamental building blocks in the tank design have been experimentally demon¬strated within this project by investigating the advantages and disadvantages of using liner- vs. linerless- tank designs and issues related to the realisation of more complex geometrical tank shapes.
Four different subscale CFRP-tanks were planned to be designed, manufactured, and tested under mechanical and thermal loads within the scope of the CHATT project. All advanced cryogenic tank technologies investigated within CHATT have been driven by system demands of future hypersonic passenger configurations. All cryo-tank technologies should eventually be assessed by the system requirements. These vehicles had already been proposed a few years before and were under study in the EU-funded cooperative projects LAPCAT and FAST20XX: LAPCAT A2, LAPCAT M8, and the SpaceLiner. Thus, the vehicles had already reached a certain level of maturity in their respective propulsion demands and overall size. However, the cryogenic tank systems had not been previously studied in any detail and major challenges concerning tank weight, sloshing, and insulation had not been addressed prior to CHATT.
Cryogenic propellants are not only of interest for hypersonic airliners but are also an interesting option for future subsonic passenger planes. The EU-funded study CRYOPLANE investigated several aspects of these propellants, but without looking deeply into the challenges of tank design and propellant feed. For aircraft with non-cylindrical fuselages, such as blended wing body configurations, more complex tank shapes, like multilobe, would be much more volume efficient, which is of high importance in achieving an economically viable configuration. The design of CFRP tanks with complicated geometries have been studied in CHATT.
Propellant management is imperative to achieve reliable and efficient vehicle operation. It is therefore the third pillar of the study and covers tank pressurization, fuel location/retention, and sloshing in horizontal tanks.
Apart from thermal aspects, sloshing of cryogenic fluids within the tanks can have a significant impact on flight operation as the liquid excited through vehicle movements may travel distances of considera¬ble lengths compared to the overall size of the aircraft. The vehicle may consequently experience a noticeable shift in its center of gravity and an impulse exchange between the propellant and the tank wall that will influence the vehicle behavior to a certain degree. Counter-measures such as anti-sloshing devices and tank design are susceptible to reduce these effects but will come at the cost of increased mass and production effort. In order to evaluate the need for such counter-measures, it is necessary to evaluate if the impact of sloshing on the flight mechanics can be tackled by an appropriate flight control design and to which extent these counter-measures are required. As a consequence a trade-off between hardware design and flight control development in order to minimize the impact of propellant sloshing is an important step in the design phase of a hypersonic vehicle. Little research has thus far been done on this essential aspect for a sound and robust vehicle design. Therefore, the CHATT study intended to focus on establishing engineering models for sloshing verified by numerical calculations and experiments. These models should then be applied to flight control simulations of the reference vehicle concepts allowing an evaluation of their overall feasibility.
The propellant feed system of aircraft using cryogenic fluids is considerably different to those in today’s airliners using JP. The lower densities of these liquefied gases require tanks of large sizes and hence long feed lines. The different thermodynamic characteristics of these fluids are to be considered in order to avoid cavitation in the lines or feed pumps. The propellant cross-feed between the two stages of the SpaceLiner enables a significant performance improvement. However, cross-feed between operational stages is highly innovative and has never been demonstrated in flight. A simulation of the steady and transient behavior in the propellant feed-system has been performed along the powered flight, performance and critical points are evaluated and recommendations are derived.
For the tank pressurization, novel concepts and systems might be needed for highly reusable propellant tanks. Depending on the vehicle type, in particular its propulsion system and mission profile, different tank pressurization system might be needed. For this reason alternative pressurization concepts and systems have been looked at in CHATT. For vehicles with an unpowered flight phase the tank pressurization might need to be sustained also during engine-off phases such as for structural reasons or to assure a re-ignition of the engines. In this case small and independent gas generators could be the preferred choice. Such systems could furthermore provide redundancy. Heat-exchangers are one of the most essential elements of such a system. In the past, however, metal based heat-exchangers have shown safety deficiencies in particular due to leaks. In winged systems with potentially more complex propellant movements, which can lead to rapid tank pressure variations, powerful pressurization systems might be required. Different types of ceramic gas generators have therefore been looked at in CHATT. Helium is commonly used as a pressurization gas for oxygen tanks. However the fact of He being a non-renewable natural resource, this is a major concern in particular for systems, which shall be operated on a daily basis. Concepts for helium recycling will therefore be developed and their feasibility will be assessed under the background of vehicle operation and cost objectives. The use of gaseous oxygen is another very attractive solution for oxygen tank pressurization. Potential performance penalties need to be traded-off against a number of benefits. One of which is, that gaseous oxygen is also required for the passenger cabin where it might be used for breathing as well as cooling and might also be used for attitude control.
The air-conditioning system for the hypersonic vehicles investigated in CHATT has been based on a similar system already briefly addressed in the FP6 study ATLLAS I, in which bleed air from the intake exhaust was cooled using cryogenic fuel, and then compressed to achieve the conditions required for the cabin air supply. Power for the compressor and other cabin sources was provided by a Rankine cycle on the cryogenic fuel. A similar open cycle approach has been intended for CHATT. The absorbed heat will be used to generate electrical and mechanical power. A detailed design is to be developed and a small scale (15 kW) prototype turbine to be constructed and tested.
Project Results:
The project has been broken down into three main technical activities (Workpackages WP2 to WP4) and one project management and coordination task (WP1). A central, steering role was applied to WP2 focusing on system requirements of advanced passenger airplanes, the development, test and implementation of engineering methods and tools. The two remaining workpackages WP3 and WP4 were dedicated to fundamental research with special focus on manufacturing and testing of fully integrated subscale hardware samples. Both WPs were serving as modules supporting the vehicle design and the verification of fast engineering methods. Each main work package represents a comprehensive work effort and is divided into major tasks.
WP2 System Analysis, Methods and Tools
Vehicle Reference Design Loads and Conditions (WP2.1)
All advanced cryogenic tank technologies investigated within CHATT are driven by system demands of future hypersonic passenger configurations. Such vehicles have been under study in other EU-funded cooperative projects LAPCAT and FAST20XX: LAPCAT A2, LAPCAT M8, and the SpaceLiner. Thus, the fundamental basis for all research activities in the CHATT project was the definition of reference vehicles for hypersonic passenger transport applications. These vehicles provided the reference environmental and loading conditions for all further investigations on material-, tank-, and propellant-management-level.
One supersonic transport aircraft being researched as part of CHATT is the LAPCAT A2 Mach 5 Civil Transport of Reaction Engines Limited. This aircraft design should be capable of flying from Europe to Australia in 5 hours carrying 300 passengers. The vehicle is intended to have about 20000 kilometers range calling for the use of liquid hydrogen as a fuel which also can be used to cool the vehicle and the air entering the engines via a precooler.
An interesting alternative to air-breathing hypersonic passenger airliners in the field of future high-speed intercontinental passenger transport vehicles might be a rocket-propelled, suborbital craft. Such a new kind of ‘space tourism’ based on a two stage RLV has been proposed by DLR under the name SpaceLiner. Ultra long-haul distances like Europe – Australia could be flown in 90 minutes. Another interesting intercontinental destination between Europe and North-West America could be reduced to flight times of about one hour.
Propellant Dynamics in Flight Control (WP2.2)
Propellant sloshing and propellant movement induced center of gravity (CoG) changes are a critical issue for hypersonic vehicles with large horizontal tanks for cryogenic fluids. Unlike conventional passenger aircraft that store kerosene fuels in segmented wing tanks, cryogenic propellants such as LOX and LH2 need to be stored in large cylindrical pressures vessels. This promotes significant propellant mass movements in case of flight maneuvers, which in turn not only causes the CoG to move, but also induces strong static and dynamic mechanical loads on the tank walls. This issue was investigated within CHATT for the LAPCAT A2 hypersonic cruiser. Propellant sloshing models developed in WP4 allowed for estimation of the propellant mass movements and resulting forces and moments. This was fed into a flight control analysis and enabled the derivation of the required control surface deflections in order to maintained trimmed flight. This flight control analysis in turn was finally coupled with an aeroelastics and structural analysis and optimization environment, which allowed the evaluation of the mutual interaction between fluid movement, flight control, and structural sizing.
Cryogenic Hydrogen Tank Design for Hypersonic Vehicles (WP2.3)
A major task of WP2 was research on optimum design of large LH2 tanks under realistic hypersonic flight vehicle conditions. The study tank has been taken from the LAPCAT Mach 5 cruiser. For this task, a sophisticated analytical optimization tool has been developed which is in particular suited for rapid trade-off studies, and enables the estimation of tank insulation and structural masses as a function of the applied loading and environmental conditions. This analytical analysis code was then coupled with a high-fidelity mixed variable optimization suite. The optimization results in particular allowed for the identification of the quantitative impact of individual design parameters on the overall tank mass. Furthermore, different insulation concepts were compared, and optimum solutions selected.
Tank System Engineering Methods (WP2.4)
Next to cylindrical LH2 tanks, also multi-lobe and multi-bubble tanks designs were addressed. This type of tank architecture is required for airframes with non-circular cross-sections. Typical examples are air-breathing cruisers with a high level of engine/airframe integration, or advanced sub-sonic blended wing-body configurations. Comprehensive analytical models were developed and implemented which allow for the design of CFRP tanks with an arbitrary number of lobes or bubbles. The models developed for the latter also served as the input for the multi-bubble tank finally manufactured in WP 3.
The models were exploited for extensive parametric analyses of different lobe/bubble numbers and material compositions. The analytical models for multi-bubble tanks were subsequently translated into FE-models, and the latter in turn coupled with the same mixed variable optimization environment that was already applied for the cylindrical LH2 tanks in WP2.3. This approach successfully found the optimum bubble configuration for the tank with the maximum structural efficiency within the prescribed constraints. The chosen constraint is the inverse of the “reserve factor” which is a function of maximum Von Mises stress and tensile strength and the composite failure criterion defined by the Tsai-Wu coefficient.
Although CHATT is mainly focused on CFRP tank structures, a comparison with metal and hybrid design is interesting as well in order to define benchmarks for evaluating performance improvements. A corresponding literature study complemented by structural analysis simulations for different concepts was done. The investigated concepts include vacuum isolated tanks, vacuum isolation with supports, foam suspended core concepts, and floating core with hoop wrapping designs.
One of the reference vehicles, the SpaceLiner, is a parallely staged rocket propelled hypersonic transport, where the ascent of the passenger carrying stage is supported by a reusable booster stage. Preliminary analyses indicated that significant performance increases are possible via the application of a so-called cross-feed system, where the booster tanks fed the passenger stage rocket engines during joint ascent. Consequently, the passenger stage tanks are still completely filled when booster separation occurs. Within CHATT, different options for cross-feed systems have been investigated, compared, and where appropritate also analyzed in detail. Based on these results, a cross-feed system for the SpaceLiner has been designed.
CFRP tanks for cryogenic applications in future hypersonic transport systems will require suitable health-monitoring (HM) systems in order to early detect damages and by this to allow for safe vehicle operations even after hundreds or thousands of load cycles. Thus, also health-monitoring systems were addressed in CHATT, whereas an extensive literature research was done to identify suitable HM candidates and to elaborate their particular advantages and disadvantages. A special focus was placed on electrical resistance based HM systems. Based on these theoretical studies, a functional system was designed WP 3.
Tank Pressurization and Cabin Environment Control (WP2.5)
Tank pressurization and cabin environment control studies were another part of WP2 in CHATT. The first pillar of this task investigated the utilization of tank-pressurization with gasified cryogenic propellants. This is of critical importance since the usually used helium pressurization gas in launcher applications is rare and extremely expensive. Already 8 % of today’s Helium consumption is allocated to space launch vehicles. Routine operations of hypersonic transport aircraft on a daily basis would exceed this amount potentially by orders of magnitude. Within a second pillar cabin environment control and passenger oxygen supply was analyzed. Due to the extreme temperature environments associated with hypersonic flight, fresh air cannot simply be extracted from the incoming air flow as usually done in case of subsonic passenger aircraft. However, the onboard stored cryogenic propellants offer attractive opportunities for cooling of cabin air and may also be used for driving electrical power generators. These studies performed the basis for an associated turbine design in WP4.
WP3 Advanced Cryogenic CFRP Tank Structures
Literature Survey and Concept Down-Selection (WP3.1)
The basis of the material experiments and tank designs were extensive literature studies. The objective was on the one hand to define relevant environmental conditions and material requirements. On the other hand, the state of the art concerning CFRP and liner material systems, tank designs, and manufacturing methods was to be assessed and the results to be compiled. Based on these results, suitable material systems were selected for material experiments as well as for the design and manufacturing of the tanks.
Liner Design (WP3.2)
Investigation of liner materials and liner/CFRP combinations for cryogenic applications was one of the most important tasks within CHATT. While many metallic materials such as aluminum alloys are well suited for cryogenic tanks, CFRP and liner materials suffer from various drawbacks when subjected to cryogenic environments and fluids such as hydrogen. Critical issues include for instance the initiation of micro-cracks, hydrogen permeation, chemical incompatibility, or thermal expansion mismatch between CFRP and liner material. Thus, a comprehensive experimental material research campaign was launched, with the results having been exploited for tank material selections.
Material samples for the systems as selected in the literature research were subjected to mechanical loads in room temperature (RT) as well as at cryogenic temperature levels. A special focus was placed on the development of micro-cracks and their propagation from the CFRP shell through the liner material or vice-versa. At lower temperature conditions almost all liner candidates were found to be prone to cracking due to increased brittleness. Also thermal cycling experiments for liner materials were conducted within this task.
Finally, analytical and numerical models were set up for simulation and prediction of stress and damage initiation in liner and liner-laminate combinations. The experimental results were exploited to validate and improve the simulation models.
Cylindrical Tanks with Liner (WP3.3)
A cylindrical tank was manufactured by filament-winding on a polymer liner at the DLR in Braunschweig. The tank design followed the CFRP-net geometry. The tank is about 3 m in length with a cylindrical length of 2.4 m and a diameter of 1 m. The total volume is 1.9 m³.
A combination of glass fibers and CFRP was utilized, whereas the CFRP was used for a second winding on top of the glass layers. The last layer is a second CFRP hoop layer. The tank was finally wrapped with a peel ply for removal of excess resin. Subsequently, the tank was cured in an autoclave. After manufacturing, the tank was sent to WP4 for mechanical testing, cryogenic experiments, and sloshing campaigns.
Beyond the wet-wound tanks, a dry filament wound cryogenic cylindrical demonstrator tank has been designed, produced by ALE and tested. This tank has a cylindrical mid-section with two isotensoid shaped ends. The tank has an approximate length of 0.57 m, a diameter of 0.29 m, and a volume of 33 l.The main risk of using a dry filament wound tank in cryogenic environment is that in unpressurized state the fibers could separate from the liner due to the difference in their CTE and start to relocate.
ALE has developed in-house software called PresVes which is able to simulate fiber circuits and fiber patterns according the netting theory and is written in Matlab. PresVes is able to export machine code for the tumble winder at ALE. Furthermore, the fiber network can be exported as a truss element mesh in order to perform FEA.
The liner has a cylindrical mid-section with isotensoid domes. The outer contour of the liner is determined using PresVes. The produced wall thickness is around 2 mm and has been determined iteratively using FEA. The length over diameter ratio is L/D = 2. Liners are produced using the blow moulding process. Therefore, the material selected for the liner is PE because the previously foreseen Vectra LCP grade A435 is not suitable for blow moulding. Unfortunately however, the CTE of PE is no longer favorable in combination with the T700 fibers due to its high value. The approximate liner mass is 0.38 kg.
FEM analyses of the dry wound tank have been performed to evaluate the following aspects:
• Deformations caused by cool down from room temperature to LH2 temperature
• Burst pressure validation by failure analysis of the fiber network, the liner and the closure parts at LH2 temperature
The maximum fiber strain that occurs at 12 bar internal pressure (burst pressure requirement) and at LH2 temperature is 0.13%. This strain magnitude is below the allowable strain of T700. With a calculated safety factor of 1.03% / 0.13% ≈ 7.9 it is concluded that the fiber network meets the load requirements.
The function of the dry carbon fiber layup is to carry the load due to the internal tank pressure. The liner and all overlying layers are assumed as non-load sharing. Toray T700 24k untwisted rovings were selected, whereas two different winding layers were placed in the following order:
• Helix layer; which is a fully wrapped layer that covers the liner completely. Two T700 24k rovings are placed in 63 circuits with 8 segments filled with 8 bundles. The mass of the fibers in this layer is estimated at 0.653 kg.
• Hoop layer; which covers the cylindrical part of the liner. The hoop layer covers the helix layer and has 108 circuits. The mass of these fibers is approximately 0.325 kg.
The dome closure consists of an aluminum insert and an aluminum counterpart. The insert is installed onto the liner by a snap connection.
Winding of the helix layer and winding of the hoop layer was done at the ALE workshop. Taped fiber ends and a transparent foil prevent fiber movement. A total of three tanks have been produced by ALE. The tank has been subject to pressurization tests at ALE and subsequently was delivered to the DLR cryolab in Bremen for cryo-testing in WP4. Fill and drain tests using liquid nitrogen have been performed including a sequence of several pressurization and venting cycles.
Design of Cylindrical Tanks without Liner (WP3.4)
Linerless tank designs are highly interesting for cryogenic applications since they avoid interface problems between CFRP shell and liner, in particular concerning CTE mismatch, and additionally may save weight. Actually, the linerless tank demonstrators built in Sweden are not closed volume tanks but rather tubes with a cylindrical section and open ends. The reason for choosing the tube configuration is the ability to validate the design by easy testing in the relevant loading conditions internal pressure, low temperatures and axial load. The manufacturing of the tube demonstrator tank was performed at Swerea SICOMP while the testing was executed at FOI. The linerless demonstrator tank concept is based on the utilization of thin-ply laminates and the superior mechanical properties these novel materials show. Both, all thin-ply tank concepts and hybrid concepts were evaluated. Several subscale demonstrator tubes were manufactured by filament winding on a steel mandrel with an external diameter of 165 mm to verify the results and test various concepts.
Numerous processing methods have been studied including one and two step winding and curing, variable winding tension, induction heating and shrink tape. However, most of the studied combinations of traditional material- and process parameters have a small effect on the operational stress state in the tank. The intended step-change in material performance was instead reached by utilizing a novel spread-tow material from the Swedish Oxeon company named TeXtreme®. As laminate material the Carbon Fiber Spread Tow TeXtreme® 50 UD TR50S WO /20:50, unidirectional, at 50 μm ply thickness has been used. Spread tow is a relatively new material and has a huge potential for performance improvements in many applications.
Finite Element analysis was used for various demonstrator cases, such as hybrid laminates (traditional and thin-ply composites) as well as pure thin-ply composites (only using TeXtreme®), to find the optimum lay-up of the demonstrator tubes considering both the real load case as well as the selected test conditions used at FOI for testing of the demonstrator tubes. Mechanical testing on specimen level performed at Swerea SICOMP also show that the critical transverse micro-crack initiation stress increases approximately from 60 to 120 MPa depending on the ply thickness
The TeXtreme® material had never been used for wet filament winding before the CHATT project started. Initial test tubes were hence manufactured by Swerea SICOMP to verify the quality of the laminates. The selected winding angle was 89.8º (tangential) with 2 mm laminate thickness. The void content in the manufactured samples were measured and found at 5 %, which is unacceptably high. The problem was traced to the poor processability regarding permeability in the thickness direction.
This manufacturing problem was solved by introducing an innovative solution called HOMS (HOles, Momentary or Stationary) in the spread tow weave to facilitate impregnation. The idea is to temporarily push the carbon fibers sideways in a gentle manner without damaging the fibers during manufacturing. Additional test tubes with the new method were manufactured by SICOMP using the same process parameters, lay-up and thickness as the previous test tubes including the HOMS procedure. The void content in the manufactured samples were measured and the void content was approximately 0 %, which is considered high quality manufacturing.
Several tubes with different fibre architecture, different laminate thickness and orientation were manufactured. Some of the tubes were produced with the primary objective of optimizing the manufacturing procedure and some of them for testing at FOI in terms of performance as demonstrator tanks.
For the final optimized hybrid demonstrator casing, four layers of T700 have been wound at ±45° and 20 layers of TeXtreme® at ±25°. The tube’s ends are wound with glass fiber at 90° to serve as tabs for testing. The optimization of the lay-up and material in the final demonstrator tube was performed for the specific load case used at FOI: -150°C, an inner pressure of 3 bars and increasing the axial load until failure. Helium gas was used for tracing the leak rate throughout the test together with strain gauges to measure the strain at critical locations in the demonstrator (WP3.6).
The results of the mechanical testing performed at FOI validated the desired performance of the design configuration used for the linerless tank concept. The axial load reached 998 kN which corresponds to an axial strain of 1.6 % before failure and subsequent leakage through the tank wall was detected.
Design of Cryogenic Tanks with Complex Shape (WP3.5)
Tanks with a more complex shape than cylinders and spheres offer the potential of an improved volumetric efficiency inside the fuselage or wing of hypersonic vehicles. A relatively simple structure has been selected for the CHATT scaled prototype of a multibubble tank built at TU Delft which, however, contains all the specific design and manufacturing issues of such a complicated spheres arrangement. It has been decided to design, evaluate and produce a planar arrangement of identical spheres with double symmetry. The radii of the four bubbles of the intended prototype are all at the same 150 mm. The distance between the centers of the incomplete spheres is equal to .
Mechanical and thermal loads were derived from the SpaceLiner passenger stage LH2-tank. These loads are representative for hypersonic applications although a multibubble tank is unlikely to be selected in the SpaceLiner for carrying cryogenic fluids. However, water of the active cooling system could be stored very efficiently inside the available volume of the wing root using this special tank shape.
Two external ports with a circular cross section were designed at the two front ends of the sub-scale tank. These ports will be used for filling and draining of the tank prior and after operation. The external ports are reinforced by metal bosses in order not to significantly thicken the shell. An internal structural web was designed at the sub-scale tank, thus dividing it into two chambers. The use of an internal structural web provides a structural support at the intersection. Holes were designed at the structural web, in order to create communication ports allowing liquid to move between sections and evenly distribute the pressure within the vessel. A reinforcing pad was created adjacent to the opening in order to reduce stress concentrations created near the hole.
The second subscale tank configuration analyzed was a composite overwrapped subscale tank with a hoop fiber reinforcing the intersections and thus providing structural support. An external UD carbon tow (roving) is applied over the tank wall from the outside to the inside under tension, thus forming a ∞-sign. Each hoop fiber-wrapping cycle starts from the top section of the tank at the junction intersection and continues to the central hollow tube covering all unreinforced junctions at longitudinal and circumferential directions. Additionally, the area where the four intersections meet and the circular tube starts should have high radius of curvature, since the entrapped hoop fibers should be stretched against the tank wall surface. This way it will keep the sub-scale tank compact and provide support without adding extra weight to the tank. As a result the concept of having reinforcement webs at the liner and adding extra weight at the tank can potentially be dropped, and thus allowing a maximization of the structural efficiency of the sub-scale tank by using hoop fibers. On the other hand there is a manufacturing challenge, since the fibers must be very carefully wrapped over the intersections and the tube for an effective load transfer between the laminate membrane and the tows.
The POM (polyoxymethylene) liner was made in a closed mould by rotation moulding and the selected tank wall material was 913C carbon/epoxy. A dynamic mechanical analysis (DMA) to evaluate the tensile modulus values of the selected liner POM material has been performed at tension mode over the temperature range -130°C up to 130°C and -130°C up to 207°C for the composite.
This complex multibubble tank integrating a hoop fiber was finally manufactured. The liner was overwrapped by cross ply woven fabrics [90/0]s that are stacked under an angle of 45° with respect to each other, thus forming a quasi-isotropic laminate. In a next step the liner, which was then covered by the stacked fabrics, was packed into a bleeder foil and a vacuum foil. The toughened epoxy resin was inserted by the vacuum infusion technique. Post processing in an oven or autoclave was necessary to remove any remaining voids and ensure complete curing of the resin.
After finalization of tank manufacturing, the tank was filled with water and pressurized for mechanical testing. Failure occurred at 5.1 bars.
Testing and Health Monitoring (WP3.6)
A dedicated task was allocated to testing and health monitoring. Mechanical testing equipment was set up and combined with a newly developed Helium leak detection system. Several liner-less demonstrator tubes with different layup-scheme as manufactured in WP3.4 (section 1.3.2.4) were subjected to mechanical testing, and the resulting Helium leak rates were measured.
Detailed failure analyses were conducted for the demonstrator tubes in order to evaluate the failure mechanisms, compare them with pure material sample behaviour, and increase confidence in failure prediction.
Aerogels (WP3.7)
Hypersonic vehicles with cryogenic temperature levels inside the tanks, and hot airflow over the external moldline, require high performance, reliable, and extremely light-weight insulation systems. Aerogels have been identified as interesting alternatives to classical foam or multi-layer based insulation systems. Aerogels or aerogel-based hybrid or composite materials can have much lower thermal conductivity compared to other insulation systems, while simultaneously offering extremely low weight and a wide temperature range. Within CHATT, a completely new highly porous aluminum oxide cryogel has been developed that can be applied for insulation applications well beyond 1000°C and exhibits excellent chemical resistance. According to literature data, the porous alumina structure of typical gels usually collapses above 800-900°C. The new material developed by ELTE instead is able to maintain its porous structure up to 1600°C. Compared to conventional aerogels, the new material therefore offers significantly wider temperature ranges and is also cheaper. In particular the production method is very cost effective. Investigations of the structure of samples over a 2 year time span furthermore proved the long-term applicability.
WP4 Thermo-Fluids Studies
Sloshing Modelling (WP4.1)
A major research effort was put into the analysis and prediction of propellant sloshing in large tanks. The resulting static and dynamic forces on the tanks walls, and the CoG movements associated with large propellant mass shifts, are a highly critical design issue for future hypersonic vehicles utilizing cryogenic propellants. The sloshing activities in CHATT included several sub-topics. On the high-order side, a high-fidelity numerical CFD code was further developed and adapted in order to enable the simulation of sloshing in cylindrical vessels with a high spatial resolution. The implemented model is based on high-order discontinuous Galerkin discretization of the incompressible Navier-Stokes equations. The code was applied to the large, cylindrical demonstrator tank manufactured by DLR. Simultaneously, a second CFD analysis with a different methodology was performed for the test tank as well. Subsequently, the calculation results of both codes were compared.
For the final validation of the codes, sloshing experiments with water were conducted on a 6DOF hexapod system with the large cylindrical tank manufactured in WP 3. Fill levels of 500 and 800 liters were considered. Axial and lateral sloshing experiments with different frequencies and amplitudes have been conducted, and the reaction forces and accelerations were recorded. The results led to modelling improvements, which in turn enabled to upscale the sloshing models from the demonstrator tanks size to large scale hypersonic vehicle LH2 tanks.
This capability was then exploited in order to simulate sloshing in the LAPCAT A2 LH2 tanks at 3 selected flight points along the reference trajectory. Based on the numerical results, simpler analytical damped spring mass models were developed for flight control purposes in preliminary system design. This was fed into the flight control analyses in WP 2. With the help of the analytical model, the impact of sloshing and fluid movement in a cylindrical tank on the vehicle CoG and control surface deflection requirements can approximately be determined. This was done for the LAPCAT A2 reference vehicle for different flight conditions. Comparisons of the analytical model and the CFD data are in sufficiently good agreement for preliminary system design purposes.
Another sub-topic was the effect of sloshing on damage initiation in CFRP tanks. Thereby, cryogenic sloshing was considered as a cyclic mechanical and thermal loading to the structure. Resulting tensile and bending stresses can be regarded as relevant loading cases to evaluate the risk of failure initiation. Consequently, different CFRP laminate samples were subjected to cyclic mechanical loading at different temperatures. The resulting fatigue behavior in terms of crack density was investigated at RT and at -50°C. It was found that the advantageous performance of thin plies for cryogenic tank applications is also present for cyclic sloshing loads, with significant lower crack densities compared to thicker layers. Based on the experimental results, an initial analytical model for cyclic sloshing related failure was set up and validated.
Heat Transfer and Boiling Studies (WP4.2)
Heated gasified oxygen can replace helium as pressurization gas in LOX-tanks. For this purpose, safe and reliable heat exchangers are required. Corresponding ceramic heat exchanger systems and technologies were investigated in the CHATT project. Different potential design solutions were modelled in numerical analysis tools, and thermal and mechanical loads were simulated. Based on the results, a final design for the heat exchanger was selected. The theoretical studies were complemented by a series of manufacturing trials utilizing the C/SiC based ceramic Cesic material. Thereby, problems were initially faced during milling and grinding due to the filigree architecture of the heat exchanger design and the brittleness of the material. These issues could however be solved. Finally, a complete heat exchanger was manufactured and tested. The ceramic core is encapsulated in a metallic shell, whereas the latter has to carry the mechanical loads due to the low tensile strength of Cesic. The final heat exchanger was eventually tested with liquid nitrogen.
A separate pillar of this task was addressing fluid boiling behavior as a function of tank wall surface roughness. An experimental setup was created that allowed for utilizing rectangular tank bottom walls with different surface roughness, and subsequently observing in detail the fluid heat-input induced boiling behavior. A special feature of these investigations was the identification and trajectory tracing of individual bubbles. A total of 6 separate experiments with 3 different roughness levels were conducted. The experimental results clearly reveal the role of the surface properties in promoting nucleate boiling and the final analysis of the data shows a distinct relationship between critical radius and surface roughness.
Compatibility of Materials (WP4.3)
Experiments on sample levels, as well as for demonstrator tanks were conducted in order to evaluate mechanical and thermal loads in the tank wall and the associated materials. The main purpose was to assess the transferability of loads from demonstrator size level to large scale tanks. The tank level investigations also included a static LN2 fill test for the large cylindrical tank manufactured by DLR, with around 50 % fill level, which was successfully passed by the demonstrator tank. Even after several hours of exposure, no damage could be observed.
Furthermore, the long-term behavior of CFRP materials when subjected to cryogenic fuels was experimentally investigated. Liquid oxygen compatibility was tested via ignition tests of CFRP specimen. The LH2 compatibility was investigated by submerging samples into LH2 and performing permeability tests. This was complemented by gas permeability experiments at different temperature levels using Helium as test gas. Thermal cycling tests were conducted as well with 50 cycles of immersing samples into LN2, and subsequent re-warming to ambient temperature. Also tensile strength, CTE and heat conductivity measurements were accomplished on sample level.
Cryogenic Rankine Cycle Studies (WP4.4)
The cooling cycles defined for the M5T and A2 concept aircraft both use air as the working fluid in what is known as an “Air Cycle”. The objective is to provide cold low pressure air at the turbine outlet when provided with warm high pressure air at the compressor inlet. Air cycle machines are used in all conventional aircraft cabin cooling systems, the distinguishing feature of the machine developed in this study is that the turbine will not drive a compressor but instead it will drive an alternator to provide electrical power in the absence of any other rotating machinery. Another unique feature of this open cycle is that a recuperator is used in place of the conventional external-air heat exchanger to pre-cool the air entering the turbine. The expander which converts the energy of the working fluid into mechanical power is to be a radial turbine. Within CHATT, a corresponding turbine was designed, manufactured and tested. Mounting of the turbine on a test rig was realized via an innovative foil bearing. The rotor was manufactured by 3D printing using maraging steel. The stator was made from plastic resin. During testing, the turbine was accelerated up to 140000 rpm, whereas high peak efficiency levels (> 90 %) were found.
Potential Impact:
Evaluation of achieved TRL
The Technology Readiness Level (TRL) is a well-known and well established indicator of the readiness of certain application to reach operational status. The definition of the different TRLs according to NASA is reprinted in the Final Report. Note that the section relevant to research projects like CHATT is TRL 1 to 5 reaching from Basic Technology Research up to Technology Development.
In the Annex of the CHATT Part B a table on the intended progress of the investigated technologies in terms of TRL-grading is provided This table lists individual items describing in short the objectives in technology development along with their present (e.g. 2010) TRL and the intended increase towards the TRL-scale shown in the right column.
WP Nr. Technology description Present (2010) TRL Envisaged TRL for CHATT in 2015
2.2 Flight control development sloshing case 1 2/3
2.2 Aeroelastic modelisation of tanks 2 3
2.3 Tank preliminary design model 2 5
2.4 Tank System Engineering Methods 3 5
3.2 Liner Design 3 4
3.3 Tank Design with Liner 3 4
3.3 Cryogenic dry wounded carbon fiber pressure vessel 2 4
3.4 Tank Design without Liner 3 4
3.5 Tank Design with Complex Shape 3 4
3.6 Testing and Health Monitoring 4 5
3.7 Tank insulation (Aerogels) 2 3
4.1 High Order Methods for Sloshing Simulation 2 5
4.1 Sloshing Tests and Modeling 2 4
4.2 CMC Heat exchanger technologies 1 3
4.4 Cryogenic Rankine Cycle 2 4
At the end of the CHATT project it is of interest to evaluate retrospectively the actually achieved TRL in the technologies as well as in the simulation models. A realistic assessment is also crucial for the preparation of any follow-on work. At the final progress meeting of CHATT (PM6) the participants discussed on June 23rd 2015 the actually achieved progress and the self-assessment is documented in the table below. It is interesting to note that the pre-CHATT TRL is in some cases now assessed as considerably below what was believed state-of-the-art in 2010. This could be explained by a better understanding of the complexity of certain issues than was available at the time of the CHATT project proposal.
It is to be acknowledged that the evaluation in the table is a self-assessment of the project participants. This should be followed by a judgement of independent experts. Nevertheless, significant progress has definitely been achieved in key areas of cryogenic composite tanks and related technologies.
WP Nr. Technology description Previous TRL Achieved TRL
2.2 Flight control development sloshing case 1 2
2.2 Aeroelastic modelisation of tanks for hypersonic airplane 1-2 2-3
2.3 Tank preliminary design model for hypersonic airplane 1 3
2.4 Tank System Engineering Methods 3 3
3.2 Liner Design 2-3 4
3.3 Tank Design with Liner 2-3 3
3.3 Cryogenic dry wound carbon fiber pressure vessel 2 3
3.4 Tank Design without Liner 2 3
3.5 Tank Design with Complex Shape 3 4
3.6 Testing and Health Monitoring 1 2-3
3.7 Tank insulation (Aerogels) 2 3
4.1 High Order Methods for Sloshing Simulation 2 3
4.1 Sloshing Tests and Modeling for hypersonic airplane 2 3
4.2 CMC Heat exchanger technologies 1 2
4.4 Air Cycle 1 2
4.4 Turbine / Foil bearings 2 3
Discussion of Important Impacts of the Project Results
One of the most important results of the CHATT project is the implementation of thin-ply technologies for liner-less cryogenic CFRP tank applications. In the past, the utilization of composite materials for cryogenic tanks in aerospace vehicles was severely handicapped by the necessity of complementing the CFRP shell by an internal liner. The liner is required in particular for preventing micro-crack-initiation in the CFRP, which not only adversely affects the structural integrity of the tank, but also provide leak paths for the fluids inside the tank. Liners however increase the weight of the tank and may lead to fatal interface issues between liner and CFRP shell, in particular due to differential thermal expansion. During CHATT, extensive experimental research campaigns on specimen as well as demonstrator tube level proved the superior performance of thin ply approaches for cryogenic tanks. Thin ply based laminates have been found to largely reduce or even eliminate micro-cracking in cryogenic environments. This opens very promising perspectives for cryogenic liner-less CFRP tanks. For future hypersonic transport vehicles, this may even be considered as a break-through technology. Of similar or even higher importance is the potential impact on the space launcher industry. If thin ply approaches ultimately prove to be suitable for ultra-lightweight and reusable LH2 and/or LOX tanks, a completely new category of launch vehicles may be developed, which in turn would greatly foster the exploitation of space for mankind. Nevertheless, the current TRL for thin plies in cryogenic tanks is still at 3 because the thin-ply technology is not yet implemented in a functional tank.
Thin ply approaches are not only suitable for cryogenic applications, but may offer superior mechanical properties compared to conventional CFRP’s in general due to significantly higher plasticity stress. Although this thin ply effect is well known in material science, thin ply approaches were hardly used in history due to high costs and manufacturing issues. Therefore it is highly noteworthy, that also the production process of thin ply laminates was improved during the CHATT project, which now allows for the production of high quality and virtually void-free thin-ply laminates.
Design, manufacturing, and testing of different CFRP tank structures revealed the difficulties when using composite structures for cryogenic environments. However, this enabled also the development of solutions strategies and led to improved design approaches for future tanks. In particular notable is the progress on dry-wound tanks and multi-bubble tanks. The dry-wound tank research proved that this potentially ultra-lightweight design concept is in principle suitable for cryogenic applications, but requires liner materials that provide low thermal expansion mismatch between fibers and liner. The manufacturing and testing of a multi-bubble tank on the other hand largely improved the understanding of the complicated design issues and greatly increased the confidence in designing cryogenic CFRP tanks of this innovative technology. This opens perspectives for cryogenic propellant tanks not only in complex shaped hypersonic vehicles, but also in sub-sonic airliners with non-circular fuselage cross-sections such as advanced blended wing-body configurations. This in turn would allow for a drastic reduction of environmental pollution of sub-sonic air-traffic when switching from kerosene to hydrogen fuels.
Cryogenic tanks for future airliners not only require lightweight tanks structures, but also lightweight, durable, and reliable insulation systems. In this area, a major technological advancement could be achieved within the project by the development of a new type of cryogels. The new material extents the temperature ranges of conventional aerogels by several hundred degrees, while simultaneously being much cheaper. The applications of this new material are multifold in all areas where lightweight and cheap insulations for wide temperature ranges are required.
Significant progress was also made on the modelling and analysis of sloshing. The research results of CHATT perhaps the first time looked into an identification of the impact of longitudinal sloshing in large horizontal tanks on the flight control of future aerospace vehicles, as well as on resulting aero-elastic effects and structural sizing. Since this crucial issue is typically ignored in hypersonic vehicle pre-design due to either lack of problem awareness, or lack of simulation capabilities, CHATT essentially improved the quality of hypersonic vehicle design and allows for more reliable feasibility assessments of hypersonic vehicle configurations. Moreover, the CFD code developments for sloshing considerations during CHATT may be applied to other fluid dynamics problems in many sectors. Furthermore, the detailed investigation of propellant cross-feed systems greatly improved the understanding of this technology, and may support the development of multi-stage transport vehicles, such as the DLR SpaceLiner.
Progress was also achieved on topics associated with cryogenic propellants, such as heat transfer and boiling problems. Manufacturing studies and finally the construction and testing of a ceramic heat-exchanger greatly increased the fundamental knowledge on ceramic materials for cryogenic heat-transfer applications, but also revealed the critical issues associated with this technology, that need to be overcome in future studies. The experimental boiling studies in turn largely improved the understanding of roughness induced boiling. This is not only relevant for cryogenic tanks in hypersonic vehicles, but also for problems of heat transfer into fluid storage vehicles in general.
In summary, CHATT resulted in valuable progress for the realization of future hypersonic airliners, as well as environmentally friendly and economically highly efficient sub-sonic blended wing-body aircraft. The potential benefits for other technology areas are multifold, in particular when considering the universal applicability and importance of lightweight composites, insulation materials, and lightweight pressure vessels.
In future aviation and particularly in hypersonic systems new propellants will be used, such as liquid hydrogen, liquid methane and possibly liquid oxygen. Previous FP7 funded studies in Europe such as FAST20XX, ATLLAS or LAPCAT investigated advanced vehicles with these fuels for passenger transport like the SpaceLiner or Lapcat A2 and some of their constituent materials and associated propulsion challenges. The question of cryogenic propellant storage inside an airliner – although of critical importance but by far not yet mastered – had not been addressed in Europe in comparable detail.
The need for more detailed investigations on liquid hydrogen or methane tanks in future airliners is not only urgent in hypersonic aviation, but is also essential for environmental reasons in subsonic aviation. New materials and design concepts are required, such as fiber based composite materials, in order to reduce the tank weight and to increase the structural performance. This is particularly important if the tank has load carrying functions. Different to current rocket launch systems, the durability through hundreds or even thousands of flight cycles must be assured. Tank liners are another essential element of a tank design in order to assure the material compatibility over long durations.
The Cryogenic Hypersonic Advanced Tank Technologies (CHATT) project addressed these challenges within a comprehensive and multifold approach. The focus has been on sophisticated experimental material research campaigns, culminating in the development, manufacturing and testing of four different demonstrator tank structures, each involving individual technologies or design philosophies. On the material research level various CFRP material and layup combinations as well as liner material and CFRP/liner combinations have been evaluated in order to assess their applicability for future advanced hydrogen tank structures. Special attention has been paid on material behavior in cryogenic temperature environments, including micro-crack initiation, thermal cycling, and fatigue. Based on these results, suitable material systems have been selected and utilized for manufacturing of the four tank structures. The largest tank built is a cylindrical vessel with approximately 3 m in length and 1 m in diameter, utilizing a CFRP shell design wound on a polymer liner. Another, smaller cylindrical vessel has been manufactured with innovative dry-wind technology. A third vessel employs a liner-less design, while the fourth tank has been designed and manufactured as a multi-bubble tank suited for non-circular fuselage cross-sections. Testing of the tank structures involved pressurization and mechanical loading tests, as well as cryogenic fluid fill and drain experiments using liquid nitrogen.
The material research and tank manufacturing campaigns in CHATT have been complemented by comprehensive system investigations for different types of hypersonic vehicles. These not only provided the reference environmental and load conditions for material and tank research, but also included related topics such as propellant management, propellant cross-feed designs for parallel staged vehicles, cabin environment control and power generation by using cryogenic fluids, and flight control under presence of fluid movement. In particular the propellant sloshing problem has been addressed on a theoretical as well as experimental level. High-fidelity CFD codes have been improved enabling detail investigations of sloshing phenomena. Simultaneously, approximate analytic models were assessed on how far the impact of sloshing on flight vehicle control could be simulated in a fast way. Via coupled analysis, also the impact of sloshing on vehicle aero-elastics and structural sizing could be assessed. Furthermore, analytical and/or numerical analysis tools for cylinder, multi-lobe, and multi-bubble tanks including insulations were developed. These codes have been coupled with high-order optimization suites for overall optimization of tanks.
Further associated topics to cryogenic tanks have been investigated as well, theoretically and experimentally, such as ceramic heat exchanger technologies, roughness induced boiling, aerogels for insulation applications, or tank structural health monitoring.
One of the central findings of the CHATT-project supported by manufacturing and material tests is the very promising potential of thin-ply CFRP technologies for cryogenic tank applications. Extensive material research and demonstrator tube testing campaigns proved the superior performance of thin-ply approaches for delaying or even preventing cryogenic temperature induced micro-cracking. The results indicate that liner-less and ultra-lightweight cryogenic tank systems could be realized using thin ply approaches.
Project Context and Objectives:
In future aviation and particularly in hypersonic systems new propellants will be used, such as liquid hydrogen, liquid methane and possibly liquid oxygen. Previous FP7 funded studies in Europe such as FAST20XX, ATLLAS or LAPCAT investigated advanced vehicles with these fuels for passenger transport like the SpaceLiner or Lapcat A2 and some of their constituent materials and associated propulsion challenges. The question of cryo¬genic propellant storage inside an airliner – although of critical importance but by far not yet mastered – had not been addressed in comparable detail. The need for more detailed investigations on liquid hydrogen or methane tanks in future airliners is not only urgent in hypersonic aviation, but is also essential for environmental reasons in subsonic aviation. Air traffic has experienced strong growth over a long time, and it is predicted that such growth will continue at rates of 4 – 5 % annually over the next decades. On the other hand, it is generally accepted that the emission of greenhouse gases, most notably of carbon dioxide (CO2), resulting from human’s activities, cannot be allowed to continue increasing if adverse global climate change is to be avoided. Liquid hydrogen, produced on the basis of renewable energy, is the only known new fuel meeting the requirements of the political guidelines of former EU president Barroso.
The previous CRYOPLANE study’s analysis had shown that hydrogen could be a suitable alternative fuel for the future of aviation. Nevertheless, due to the missing materials, parts, components and engines, further R&D work has to be performed until hydrogen can be used as an aircraft fuel. Cryogenic fuel propulsion is already operational in advanced launcher systems and Europe has some expertise with the Ariane rocket. However, the airliner systems will require more complex technology (compared to those existing in today’s launchers like Ariane 5), such as ultra-light-weight and reusable propellant tank systems. The propellant tank technologies are critical for the vehicle operations, cost and safety. The extent to which the technological issues can be solved also has an important impact on the trade-off and choice of the future propellants, in particular between liquid hydrogen and liquid methane.
New materials and design concepts are required, such as fiber composites, in order to reduce the tank weight and to increase the structural performance. This is particularly important if the tank has load carrying functions. Different to current rocket launch systems, the durability through hundreds or even thousands of flight cycles must be assured. Tank liners can be another essential element of a tank design in order to assure the material compatibility over long durations. Embedded structural health monitoring systems are required to assure acceptable turn-around time between flights, to optimize the service and maintenance effort and to allow low design margins without lowering overall safety.
Propellant management is imperative for achieving reliable and efficient vehicle operation. The sloshing of cryogenic fluids close to their boiling conditions in tanks of horizontal take-off vehicles is not yet mastered.
One of the core objectives of CHATT is to investigate Carbon Fiber Reinforced Plastic (CFRP) cryogenic pressure tanks. Composite pressure vessels have an excellent potential for light weight design, because the fiber architecture can be manufactured in a way that all dominating loads are transferred along the reinforcement fibers. The most efficient structures carry their loads via in-plane stresses. The structural efficiency of a pressure vessel is defined as the ratio of the pressurized volume and the mass of the pressure vessel. To minimize the weight, iso-tensoid structures provide the lightest solutions. “Iso-tensoid” simply means that all locations within the structure are under the same level of tensile stress. Fiber reinforced materials are structurally the most efficient material for pressure vessels because there is the possibility to direct the right amount of fibers according to the orientation and the magnitude of the principal stresses, which makes it an iso-tensoid structure. Carbon fibers are currently known to have the highest specific strength and stiffness which makes it the material of choice when it comes to aerospace structures. On top of that, CFRP have a very small thermal expansion coefficient (CTE) and the mechanical properties of carbon fibers are consistent from temperatures between 0 K and 1000 K. This makes it worthwhile to investigate CFRP cryogenic pressure tanks for hypersonic vehicles. To manufacture composite pressure vessels it is recommendable to choose a filament winding process because this process provides the required boundary conditions concerning thread-tension and lay-up control. Even though small remaining porosities are not critical for the structural integrity, they can still be a potential leakage especially for gaseous contents. For this reason a gas-tight liner on the inside of the pressure vessels is mandatory for most technical applications. For low temperature applications the increasing brittleness of the liner has to be considered to ensure gas-tightness. For typical carbon epoxy composites the vessel structure itself is not critical under cryogenic boundary conditions. Another crucial point that has to be taken into account is the incompatible thermal expansion of some material combinations especially when local temperature gradients cannot be avoided. The resulting forces can lead to critical stresses especially in the liner structure. New composite material combinations are continuously being developed for potential use as lightweight tank structures. However, the realisation of a lightweight, well-insulated and safe storage system for cryogenic propellant is a delicate engineering challenge. In aerospace applications, the lightweight property is a fundamental performance parameter. Depending on the type of application, different designs may be favourable and tanks may thus be built as single-, double-wall or sandwich structures. Apart from low heat-transfer and the ability to withstand thermo-mechanical loading, other key issues are easy inspection and health monitoring. For advanced tank wall materials the high specific strength at cryogenic temperatures is the most essential parameter. Fracture toughness and stiffness are also important properties. Potential wall candidate materials with high specific strength are monolithic metals, polymer matrix composites and discontinuously reinforced metal matrix composites. Ceramic materials also have high specific strength but low fracture toughness and are thus not suitable tank wall candidate materials.
In the US, the DC-XA composite LH2 tank demonstrator resulted in a 34% weight saving compared to an aluminium tank. Vessels for extremely low molecular weight gases such as helium and hydrogen must be practically leak-free. The DC-XA is the first successful demonstration of a leak-free liquid hydrogen composite tank, but the shell thickness was in this case oversized to avoid leakage. The X-33 demonstrator tank failed during ground testing due to polymer matrix micro-cracking and leakage into the sandwich core material causing delamination between the core and the inner composite skin. The tank showed leakage with subsequent damage, so-called “cryopumping”. The failure contributed to the complete termination of the vehicle development program. Northrop Grumman and NASA later completed a nine-month test series to demonstrate a composite cryogenic tank filled with LH2 and pressurised. The problems that brought the X-33 to a halt were proven to be solved in 40 load cycles performed without failure. The program highlighted the need to improve the permeation resistance of liner material and limit the composite micro-cracking in wall construction details. In Japan Higuchi et al. demonstrated in a flight test a successful development of a metal-lined CFRP cryogenic tank for a reusable rocket. The design strategy was based on the fact that micro-cracks are inevitable. A metallic liner was then used as a gas barrier. In this way the CFRP does not need to be gas tight. The tank is designed as a high pressure vessel, thus avoiding a mechanical fuel pump. The tank was pressurised above the elastic limit of the metallic liner, in order to avoid liner separation at cryogenic temperatures. Reported lessons learned during the project where related to the type of welding flux, O-ring seals and elaboration with different adhesives to avoid interlayer separation, face sheet delamination and finally drain and tank ventilation. The tank was also equipped with strain-gauges, thermocouples and an advanced optical sensor health monitoring system plus an onboard hydrogen gas detector. Thermal and stress cycling as well as flight test were demonstrated successfully.
Experiences in the past sometimes showed difficulties in achieving true lightweight designs when necessary safety factor penalties associated with advanced composite materials are regarded. The handling at cryogenic temperatures potentially give rise to large temperature gradients across the tank wall which need to be limited by using a lightweight, low-conductivity insulation system in order to limit fuel loss and tank pressure build-up. The main challenge when using composite tank structures is the permeation resistance. Polymeric films and coatings, using various polymers and elastomers films, have been considered as potential gas barriers. However, literature and manufacturer material data on hydrogen permeation is very limited. Apart from permeation, the resistance to hydrogen embrittlement is important. In these aspects, the linerless approach may be desirable from a weight point of view. The challenge in developing a cryogenic CFRP tank is finding a solution for the problems caused by differences in thermal expansion coefficients. Although carbon fiber composites have an extremely low CTE on a macroscopic scale, this is not the case on a microscopic scale and ordinary matrices cannot cope with cryogenic temperatures. This implies that the dry wound carbon fiber pressure vessel of ALE is a good alternative but dry wound structures do not have the ability to carry multiple load conditions. On top of this, if a liner is required, there is also the challenge to overcome the differences in CTE of the liner with respect to the structural shell.
Previously reported research activities on cryo-CFRP-tanks within Europe were limited to some studies on future launcher applications. The research in CHATT intended to increase the knowledge within Europe, with advancement from a pure material science level to a practical cryogenic tank demonstrator level. This project performed the first steps towards a common European development of future aerospace reusable lightweight composite cryogenic CFRP tanks. The fundamental building blocks in the tank design have been experimentally demon¬strated within this project by investigating the advantages and disadvantages of using liner- vs. linerless- tank designs and issues related to the realisation of more complex geometrical tank shapes.
Four different subscale CFRP-tanks were planned to be designed, manufactured, and tested under mechanical and thermal loads within the scope of the CHATT project. All advanced cryogenic tank technologies investigated within CHATT have been driven by system demands of future hypersonic passenger configurations. All cryo-tank technologies should eventually be assessed by the system requirements. These vehicles had already been proposed a few years before and were under study in the EU-funded cooperative projects LAPCAT and FAST20XX: LAPCAT A2, LAPCAT M8, and the SpaceLiner. Thus, the vehicles had already reached a certain level of maturity in their respective propulsion demands and overall size. However, the cryogenic tank systems had not been previously studied in any detail and major challenges concerning tank weight, sloshing, and insulation had not been addressed prior to CHATT.
Cryogenic propellants are not only of interest for hypersonic airliners but are also an interesting option for future subsonic passenger planes. The EU-funded study CRYOPLANE investigated several aspects of these propellants, but without looking deeply into the challenges of tank design and propellant feed. For aircraft with non-cylindrical fuselages, such as blended wing body configurations, more complex tank shapes, like multilobe, would be much more volume efficient, which is of high importance in achieving an economically viable configuration. The design of CFRP tanks with complicated geometries have been studied in CHATT.
Propellant management is imperative to achieve reliable and efficient vehicle operation. It is therefore the third pillar of the study and covers tank pressurization, fuel location/retention, and sloshing in horizontal tanks.
Apart from thermal aspects, sloshing of cryogenic fluids within the tanks can have a significant impact on flight operation as the liquid excited through vehicle movements may travel distances of considera¬ble lengths compared to the overall size of the aircraft. The vehicle may consequently experience a noticeable shift in its center of gravity and an impulse exchange between the propellant and the tank wall that will influence the vehicle behavior to a certain degree. Counter-measures such as anti-sloshing devices and tank design are susceptible to reduce these effects but will come at the cost of increased mass and production effort. In order to evaluate the need for such counter-measures, it is necessary to evaluate if the impact of sloshing on the flight mechanics can be tackled by an appropriate flight control design and to which extent these counter-measures are required. As a consequence a trade-off between hardware design and flight control development in order to minimize the impact of propellant sloshing is an important step in the design phase of a hypersonic vehicle. Little research has thus far been done on this essential aspect for a sound and robust vehicle design. Therefore, the CHATT study intended to focus on establishing engineering models for sloshing verified by numerical calculations and experiments. These models should then be applied to flight control simulations of the reference vehicle concepts allowing an evaluation of their overall feasibility.
The propellant feed system of aircraft using cryogenic fluids is considerably different to those in today’s airliners using JP. The lower densities of these liquefied gases require tanks of large sizes and hence long feed lines. The different thermodynamic characteristics of these fluids are to be considered in order to avoid cavitation in the lines or feed pumps. The propellant cross-feed between the two stages of the SpaceLiner enables a significant performance improvement. However, cross-feed between operational stages is highly innovative and has never been demonstrated in flight. A simulation of the steady and transient behavior in the propellant feed-system has been performed along the powered flight, performance and critical points are evaluated and recommendations are derived.
For the tank pressurization, novel concepts and systems might be needed for highly reusable propellant tanks. Depending on the vehicle type, in particular its propulsion system and mission profile, different tank pressurization system might be needed. For this reason alternative pressurization concepts and systems have been looked at in CHATT. For vehicles with an unpowered flight phase the tank pressurization might need to be sustained also during engine-off phases such as for structural reasons or to assure a re-ignition of the engines. In this case small and independent gas generators could be the preferred choice. Such systems could furthermore provide redundancy. Heat-exchangers are one of the most essential elements of such a system. In the past, however, metal based heat-exchangers have shown safety deficiencies in particular due to leaks. In winged systems with potentially more complex propellant movements, which can lead to rapid tank pressure variations, powerful pressurization systems might be required. Different types of ceramic gas generators have therefore been looked at in CHATT. Helium is commonly used as a pressurization gas for oxygen tanks. However the fact of He being a non-renewable natural resource, this is a major concern in particular for systems, which shall be operated on a daily basis. Concepts for helium recycling will therefore be developed and their feasibility will be assessed under the background of vehicle operation and cost objectives. The use of gaseous oxygen is another very attractive solution for oxygen tank pressurization. Potential performance penalties need to be traded-off against a number of benefits. One of which is, that gaseous oxygen is also required for the passenger cabin where it might be used for breathing as well as cooling and might also be used for attitude control.
The air-conditioning system for the hypersonic vehicles investigated in CHATT has been based on a similar system already briefly addressed in the FP6 study ATLLAS I, in which bleed air from the intake exhaust was cooled using cryogenic fuel, and then compressed to achieve the conditions required for the cabin air supply. Power for the compressor and other cabin sources was provided by a Rankine cycle on the cryogenic fuel. A similar open cycle approach has been intended for CHATT. The absorbed heat will be used to generate electrical and mechanical power. A detailed design is to be developed and a small scale (15 kW) prototype turbine to be constructed and tested.
Project Results:
The project has been broken down into three main technical activities (Workpackages WP2 to WP4) and one project management and coordination task (WP1). A central, steering role was applied to WP2 focusing on system requirements of advanced passenger airplanes, the development, test and implementation of engineering methods and tools. The two remaining workpackages WP3 and WP4 were dedicated to fundamental research with special focus on manufacturing and testing of fully integrated subscale hardware samples. Both WPs were serving as modules supporting the vehicle design and the verification of fast engineering methods. Each main work package represents a comprehensive work effort and is divided into major tasks.
WP2 System Analysis, Methods and Tools
Vehicle Reference Design Loads and Conditions (WP2.1)
All advanced cryogenic tank technologies investigated within CHATT are driven by system demands of future hypersonic passenger configurations. Such vehicles have been under study in other EU-funded cooperative projects LAPCAT and FAST20XX: LAPCAT A2, LAPCAT M8, and the SpaceLiner. Thus, the fundamental basis for all research activities in the CHATT project was the definition of reference vehicles for hypersonic passenger transport applications. These vehicles provided the reference environmental and loading conditions for all further investigations on material-, tank-, and propellant-management-level.
One supersonic transport aircraft being researched as part of CHATT is the LAPCAT A2 Mach 5 Civil Transport of Reaction Engines Limited. This aircraft design should be capable of flying from Europe to Australia in 5 hours carrying 300 passengers. The vehicle is intended to have about 20000 kilometers range calling for the use of liquid hydrogen as a fuel which also can be used to cool the vehicle and the air entering the engines via a precooler.
An interesting alternative to air-breathing hypersonic passenger airliners in the field of future high-speed intercontinental passenger transport vehicles might be a rocket-propelled, suborbital craft. Such a new kind of ‘space tourism’ based on a two stage RLV has been proposed by DLR under the name SpaceLiner. Ultra long-haul distances like Europe – Australia could be flown in 90 minutes. Another interesting intercontinental destination between Europe and North-West America could be reduced to flight times of about one hour.
Propellant Dynamics in Flight Control (WP2.2)
Propellant sloshing and propellant movement induced center of gravity (CoG) changes are a critical issue for hypersonic vehicles with large horizontal tanks for cryogenic fluids. Unlike conventional passenger aircraft that store kerosene fuels in segmented wing tanks, cryogenic propellants such as LOX and LH2 need to be stored in large cylindrical pressures vessels. This promotes significant propellant mass movements in case of flight maneuvers, which in turn not only causes the CoG to move, but also induces strong static and dynamic mechanical loads on the tank walls. This issue was investigated within CHATT for the LAPCAT A2 hypersonic cruiser. Propellant sloshing models developed in WP4 allowed for estimation of the propellant mass movements and resulting forces and moments. This was fed into a flight control analysis and enabled the derivation of the required control surface deflections in order to maintained trimmed flight. This flight control analysis in turn was finally coupled with an aeroelastics and structural analysis and optimization environment, which allowed the evaluation of the mutual interaction between fluid movement, flight control, and structural sizing.
Cryogenic Hydrogen Tank Design for Hypersonic Vehicles (WP2.3)
A major task of WP2 was research on optimum design of large LH2 tanks under realistic hypersonic flight vehicle conditions. The study tank has been taken from the LAPCAT Mach 5 cruiser. For this task, a sophisticated analytical optimization tool has been developed which is in particular suited for rapid trade-off studies, and enables the estimation of tank insulation and structural masses as a function of the applied loading and environmental conditions. This analytical analysis code was then coupled with a high-fidelity mixed variable optimization suite. The optimization results in particular allowed for the identification of the quantitative impact of individual design parameters on the overall tank mass. Furthermore, different insulation concepts were compared, and optimum solutions selected.
Tank System Engineering Methods (WP2.4)
Next to cylindrical LH2 tanks, also multi-lobe and multi-bubble tanks designs were addressed. This type of tank architecture is required for airframes with non-circular cross-sections. Typical examples are air-breathing cruisers with a high level of engine/airframe integration, or advanced sub-sonic blended wing-body configurations. Comprehensive analytical models were developed and implemented which allow for the design of CFRP tanks with an arbitrary number of lobes or bubbles. The models developed for the latter also served as the input for the multi-bubble tank finally manufactured in WP 3.
The models were exploited for extensive parametric analyses of different lobe/bubble numbers and material compositions. The analytical models for multi-bubble tanks were subsequently translated into FE-models, and the latter in turn coupled with the same mixed variable optimization environment that was already applied for the cylindrical LH2 tanks in WP2.3. This approach successfully found the optimum bubble configuration for the tank with the maximum structural efficiency within the prescribed constraints. The chosen constraint is the inverse of the “reserve factor” which is a function of maximum Von Mises stress and tensile strength and the composite failure criterion defined by the Tsai-Wu coefficient.
Although CHATT is mainly focused on CFRP tank structures, a comparison with metal and hybrid design is interesting as well in order to define benchmarks for evaluating performance improvements. A corresponding literature study complemented by structural analysis simulations for different concepts was done. The investigated concepts include vacuum isolated tanks, vacuum isolation with supports, foam suspended core concepts, and floating core with hoop wrapping designs.
One of the reference vehicles, the SpaceLiner, is a parallely staged rocket propelled hypersonic transport, where the ascent of the passenger carrying stage is supported by a reusable booster stage. Preliminary analyses indicated that significant performance increases are possible via the application of a so-called cross-feed system, where the booster tanks fed the passenger stage rocket engines during joint ascent. Consequently, the passenger stage tanks are still completely filled when booster separation occurs. Within CHATT, different options for cross-feed systems have been investigated, compared, and where appropritate also analyzed in detail. Based on these results, a cross-feed system for the SpaceLiner has been designed.
CFRP tanks for cryogenic applications in future hypersonic transport systems will require suitable health-monitoring (HM) systems in order to early detect damages and by this to allow for safe vehicle operations even after hundreds or thousands of load cycles. Thus, also health-monitoring systems were addressed in CHATT, whereas an extensive literature research was done to identify suitable HM candidates and to elaborate their particular advantages and disadvantages. A special focus was placed on electrical resistance based HM systems. Based on these theoretical studies, a functional system was designed WP 3.
Tank Pressurization and Cabin Environment Control (WP2.5)
Tank pressurization and cabin environment control studies were another part of WP2 in CHATT. The first pillar of this task investigated the utilization of tank-pressurization with gasified cryogenic propellants. This is of critical importance since the usually used helium pressurization gas in launcher applications is rare and extremely expensive. Already 8 % of today’s Helium consumption is allocated to space launch vehicles. Routine operations of hypersonic transport aircraft on a daily basis would exceed this amount potentially by orders of magnitude. Within a second pillar cabin environment control and passenger oxygen supply was analyzed. Due to the extreme temperature environments associated with hypersonic flight, fresh air cannot simply be extracted from the incoming air flow as usually done in case of subsonic passenger aircraft. However, the onboard stored cryogenic propellants offer attractive opportunities for cooling of cabin air and may also be used for driving electrical power generators. These studies performed the basis for an associated turbine design in WP4.
WP3 Advanced Cryogenic CFRP Tank Structures
Literature Survey and Concept Down-Selection (WP3.1)
The basis of the material experiments and tank designs were extensive literature studies. The objective was on the one hand to define relevant environmental conditions and material requirements. On the other hand, the state of the art concerning CFRP and liner material systems, tank designs, and manufacturing methods was to be assessed and the results to be compiled. Based on these results, suitable material systems were selected for material experiments as well as for the design and manufacturing of the tanks.
Liner Design (WP3.2)
Investigation of liner materials and liner/CFRP combinations for cryogenic applications was one of the most important tasks within CHATT. While many metallic materials such as aluminum alloys are well suited for cryogenic tanks, CFRP and liner materials suffer from various drawbacks when subjected to cryogenic environments and fluids such as hydrogen. Critical issues include for instance the initiation of micro-cracks, hydrogen permeation, chemical incompatibility, or thermal expansion mismatch between CFRP and liner material. Thus, a comprehensive experimental material research campaign was launched, with the results having been exploited for tank material selections.
Material samples for the systems as selected in the literature research were subjected to mechanical loads in room temperature (RT) as well as at cryogenic temperature levels. A special focus was placed on the development of micro-cracks and their propagation from the CFRP shell through the liner material or vice-versa. At lower temperature conditions almost all liner candidates were found to be prone to cracking due to increased brittleness. Also thermal cycling experiments for liner materials were conducted within this task.
Finally, analytical and numerical models were set up for simulation and prediction of stress and damage initiation in liner and liner-laminate combinations. The experimental results were exploited to validate and improve the simulation models.
Cylindrical Tanks with Liner (WP3.3)
A cylindrical tank was manufactured by filament-winding on a polymer liner at the DLR in Braunschweig. The tank design followed the CFRP-net geometry. The tank is about 3 m in length with a cylindrical length of 2.4 m and a diameter of 1 m. The total volume is 1.9 m³.
A combination of glass fibers and CFRP was utilized, whereas the CFRP was used for a second winding on top of the glass layers. The last layer is a second CFRP hoop layer. The tank was finally wrapped with a peel ply for removal of excess resin. Subsequently, the tank was cured in an autoclave. After manufacturing, the tank was sent to WP4 for mechanical testing, cryogenic experiments, and sloshing campaigns.
Beyond the wet-wound tanks, a dry filament wound cryogenic cylindrical demonstrator tank has been designed, produced by ALE and tested. This tank has a cylindrical mid-section with two isotensoid shaped ends. The tank has an approximate length of 0.57 m, a diameter of 0.29 m, and a volume of 33 l.The main risk of using a dry filament wound tank in cryogenic environment is that in unpressurized state the fibers could separate from the liner due to the difference in their CTE and start to relocate.
ALE has developed in-house software called PresVes which is able to simulate fiber circuits and fiber patterns according the netting theory and is written in Matlab. PresVes is able to export machine code for the tumble winder at ALE. Furthermore, the fiber network can be exported as a truss element mesh in order to perform FEA.
The liner has a cylindrical mid-section with isotensoid domes. The outer contour of the liner is determined using PresVes. The produced wall thickness is around 2 mm and has been determined iteratively using FEA. The length over diameter ratio is L/D = 2. Liners are produced using the blow moulding process. Therefore, the material selected for the liner is PE because the previously foreseen Vectra LCP grade A435 is not suitable for blow moulding. Unfortunately however, the CTE of PE is no longer favorable in combination with the T700 fibers due to its high value. The approximate liner mass is 0.38 kg.
FEM analyses of the dry wound tank have been performed to evaluate the following aspects:
• Deformations caused by cool down from room temperature to LH2 temperature
• Burst pressure validation by failure analysis of the fiber network, the liner and the closure parts at LH2 temperature
The maximum fiber strain that occurs at 12 bar internal pressure (burst pressure requirement) and at LH2 temperature is 0.13%. This strain magnitude is below the allowable strain of T700. With a calculated safety factor of 1.03% / 0.13% ≈ 7.9 it is concluded that the fiber network meets the load requirements.
The function of the dry carbon fiber layup is to carry the load due to the internal tank pressure. The liner and all overlying layers are assumed as non-load sharing. Toray T700 24k untwisted rovings were selected, whereas two different winding layers were placed in the following order:
• Helix layer; which is a fully wrapped layer that covers the liner completely. Two T700 24k rovings are placed in 63 circuits with 8 segments filled with 8 bundles. The mass of the fibers in this layer is estimated at 0.653 kg.
• Hoop layer; which covers the cylindrical part of the liner. The hoop layer covers the helix layer and has 108 circuits. The mass of these fibers is approximately 0.325 kg.
The dome closure consists of an aluminum insert and an aluminum counterpart. The insert is installed onto the liner by a snap connection.
Winding of the helix layer and winding of the hoop layer was done at the ALE workshop. Taped fiber ends and a transparent foil prevent fiber movement. A total of three tanks have been produced by ALE. The tank has been subject to pressurization tests at ALE and subsequently was delivered to the DLR cryolab in Bremen for cryo-testing in WP4. Fill and drain tests using liquid nitrogen have been performed including a sequence of several pressurization and venting cycles.
Design of Cylindrical Tanks without Liner (WP3.4)
Linerless tank designs are highly interesting for cryogenic applications since they avoid interface problems between CFRP shell and liner, in particular concerning CTE mismatch, and additionally may save weight. Actually, the linerless tank demonstrators built in Sweden are not closed volume tanks but rather tubes with a cylindrical section and open ends. The reason for choosing the tube configuration is the ability to validate the design by easy testing in the relevant loading conditions internal pressure, low temperatures and axial load. The manufacturing of the tube demonstrator tank was performed at Swerea SICOMP while the testing was executed at FOI. The linerless demonstrator tank concept is based on the utilization of thin-ply laminates and the superior mechanical properties these novel materials show. Both, all thin-ply tank concepts and hybrid concepts were evaluated. Several subscale demonstrator tubes were manufactured by filament winding on a steel mandrel with an external diameter of 165 mm to verify the results and test various concepts.
Numerous processing methods have been studied including one and two step winding and curing, variable winding tension, induction heating and shrink tape. However, most of the studied combinations of traditional material- and process parameters have a small effect on the operational stress state in the tank. The intended step-change in material performance was instead reached by utilizing a novel spread-tow material from the Swedish Oxeon company named TeXtreme®. As laminate material the Carbon Fiber Spread Tow TeXtreme® 50 UD TR50S WO /20:50, unidirectional, at 50 μm ply thickness has been used. Spread tow is a relatively new material and has a huge potential for performance improvements in many applications.
Finite Element analysis was used for various demonstrator cases, such as hybrid laminates (traditional and thin-ply composites) as well as pure thin-ply composites (only using TeXtreme®), to find the optimum lay-up of the demonstrator tubes considering both the real load case as well as the selected test conditions used at FOI for testing of the demonstrator tubes. Mechanical testing on specimen level performed at Swerea SICOMP also show that the critical transverse micro-crack initiation stress increases approximately from 60 to 120 MPa depending on the ply thickness
The TeXtreme® material had never been used for wet filament winding before the CHATT project started. Initial test tubes were hence manufactured by Swerea SICOMP to verify the quality of the laminates. The selected winding angle was 89.8º (tangential) with 2 mm laminate thickness. The void content in the manufactured samples were measured and found at 5 %, which is unacceptably high. The problem was traced to the poor processability regarding permeability in the thickness direction.
This manufacturing problem was solved by introducing an innovative solution called HOMS (HOles, Momentary or Stationary) in the spread tow weave to facilitate impregnation. The idea is to temporarily push the carbon fibers sideways in a gentle manner without damaging the fibers during manufacturing. Additional test tubes with the new method were manufactured by SICOMP using the same process parameters, lay-up and thickness as the previous test tubes including the HOMS procedure. The void content in the manufactured samples were measured and the void content was approximately 0 %, which is considered high quality manufacturing.
Several tubes with different fibre architecture, different laminate thickness and orientation were manufactured. Some of the tubes were produced with the primary objective of optimizing the manufacturing procedure and some of them for testing at FOI in terms of performance as demonstrator tanks.
For the final optimized hybrid demonstrator casing, four layers of T700 have been wound at ±45° and 20 layers of TeXtreme® at ±25°. The tube’s ends are wound with glass fiber at 90° to serve as tabs for testing. The optimization of the lay-up and material in the final demonstrator tube was performed for the specific load case used at FOI: -150°C, an inner pressure of 3 bars and increasing the axial load until failure. Helium gas was used for tracing the leak rate throughout the test together with strain gauges to measure the strain at critical locations in the demonstrator (WP3.6).
The results of the mechanical testing performed at FOI validated the desired performance of the design configuration used for the linerless tank concept. The axial load reached 998 kN which corresponds to an axial strain of 1.6 % before failure and subsequent leakage through the tank wall was detected.
Design of Cryogenic Tanks with Complex Shape (WP3.5)
Tanks with a more complex shape than cylinders and spheres offer the potential of an improved volumetric efficiency inside the fuselage or wing of hypersonic vehicles. A relatively simple structure has been selected for the CHATT scaled prototype of a multibubble tank built at TU Delft which, however, contains all the specific design and manufacturing issues of such a complicated spheres arrangement. It has been decided to design, evaluate and produce a planar arrangement of identical spheres with double symmetry. The radii of the four bubbles of the intended prototype are all at the same 150 mm. The distance between the centers of the incomplete spheres is equal to .
Mechanical and thermal loads were derived from the SpaceLiner passenger stage LH2-tank. These loads are representative for hypersonic applications although a multibubble tank is unlikely to be selected in the SpaceLiner for carrying cryogenic fluids. However, water of the active cooling system could be stored very efficiently inside the available volume of the wing root using this special tank shape.
Two external ports with a circular cross section were designed at the two front ends of the sub-scale tank. These ports will be used for filling and draining of the tank prior and after operation. The external ports are reinforced by metal bosses in order not to significantly thicken the shell. An internal structural web was designed at the sub-scale tank, thus dividing it into two chambers. The use of an internal structural web provides a structural support at the intersection. Holes were designed at the structural web, in order to create communication ports allowing liquid to move between sections and evenly distribute the pressure within the vessel. A reinforcing pad was created adjacent to the opening in order to reduce stress concentrations created near the hole.
The second subscale tank configuration analyzed was a composite overwrapped subscale tank with a hoop fiber reinforcing the intersections and thus providing structural support. An external UD carbon tow (roving) is applied over the tank wall from the outside to the inside under tension, thus forming a ∞-sign. Each hoop fiber-wrapping cycle starts from the top section of the tank at the junction intersection and continues to the central hollow tube covering all unreinforced junctions at longitudinal and circumferential directions. Additionally, the area where the four intersections meet and the circular tube starts should have high radius of curvature, since the entrapped hoop fibers should be stretched against the tank wall surface. This way it will keep the sub-scale tank compact and provide support without adding extra weight to the tank. As a result the concept of having reinforcement webs at the liner and adding extra weight at the tank can potentially be dropped, and thus allowing a maximization of the structural efficiency of the sub-scale tank by using hoop fibers. On the other hand there is a manufacturing challenge, since the fibers must be very carefully wrapped over the intersections and the tube for an effective load transfer between the laminate membrane and the tows.
The POM (polyoxymethylene) liner was made in a closed mould by rotation moulding and the selected tank wall material was 913C carbon/epoxy. A dynamic mechanical analysis (DMA) to evaluate the tensile modulus values of the selected liner POM material has been performed at tension mode over the temperature range -130°C up to 130°C and -130°C up to 207°C for the composite.
This complex multibubble tank integrating a hoop fiber was finally manufactured. The liner was overwrapped by cross ply woven fabrics [90/0]s that are stacked under an angle of 45° with respect to each other, thus forming a quasi-isotropic laminate. In a next step the liner, which was then covered by the stacked fabrics, was packed into a bleeder foil and a vacuum foil. The toughened epoxy resin was inserted by the vacuum infusion technique. Post processing in an oven or autoclave was necessary to remove any remaining voids and ensure complete curing of the resin.
After finalization of tank manufacturing, the tank was filled with water and pressurized for mechanical testing. Failure occurred at 5.1 bars.
Testing and Health Monitoring (WP3.6)
A dedicated task was allocated to testing and health monitoring. Mechanical testing equipment was set up and combined with a newly developed Helium leak detection system. Several liner-less demonstrator tubes with different layup-scheme as manufactured in WP3.4 (section 1.3.2.4) were subjected to mechanical testing, and the resulting Helium leak rates were measured.
Detailed failure analyses were conducted for the demonstrator tubes in order to evaluate the failure mechanisms, compare them with pure material sample behaviour, and increase confidence in failure prediction.
Aerogels (WP3.7)
Hypersonic vehicles with cryogenic temperature levels inside the tanks, and hot airflow over the external moldline, require high performance, reliable, and extremely light-weight insulation systems. Aerogels have been identified as interesting alternatives to classical foam or multi-layer based insulation systems. Aerogels or aerogel-based hybrid or composite materials can have much lower thermal conductivity compared to other insulation systems, while simultaneously offering extremely low weight and a wide temperature range. Within CHATT, a completely new highly porous aluminum oxide cryogel has been developed that can be applied for insulation applications well beyond 1000°C and exhibits excellent chemical resistance. According to literature data, the porous alumina structure of typical gels usually collapses above 800-900°C. The new material developed by ELTE instead is able to maintain its porous structure up to 1600°C. Compared to conventional aerogels, the new material therefore offers significantly wider temperature ranges and is also cheaper. In particular the production method is very cost effective. Investigations of the structure of samples over a 2 year time span furthermore proved the long-term applicability.
WP4 Thermo-Fluids Studies
Sloshing Modelling (WP4.1)
A major research effort was put into the analysis and prediction of propellant sloshing in large tanks. The resulting static and dynamic forces on the tanks walls, and the CoG movements associated with large propellant mass shifts, are a highly critical design issue for future hypersonic vehicles utilizing cryogenic propellants. The sloshing activities in CHATT included several sub-topics. On the high-order side, a high-fidelity numerical CFD code was further developed and adapted in order to enable the simulation of sloshing in cylindrical vessels with a high spatial resolution. The implemented model is based on high-order discontinuous Galerkin discretization of the incompressible Navier-Stokes equations. The code was applied to the large, cylindrical demonstrator tank manufactured by DLR. Simultaneously, a second CFD analysis with a different methodology was performed for the test tank as well. Subsequently, the calculation results of both codes were compared.
For the final validation of the codes, sloshing experiments with water were conducted on a 6DOF hexapod system with the large cylindrical tank manufactured in WP 3. Fill levels of 500 and 800 liters were considered. Axial and lateral sloshing experiments with different frequencies and amplitudes have been conducted, and the reaction forces and accelerations were recorded. The results led to modelling improvements, which in turn enabled to upscale the sloshing models from the demonstrator tanks size to large scale hypersonic vehicle LH2 tanks.
This capability was then exploited in order to simulate sloshing in the LAPCAT A2 LH2 tanks at 3 selected flight points along the reference trajectory. Based on the numerical results, simpler analytical damped spring mass models were developed for flight control purposes in preliminary system design. This was fed into the flight control analyses in WP 2. With the help of the analytical model, the impact of sloshing and fluid movement in a cylindrical tank on the vehicle CoG and control surface deflection requirements can approximately be determined. This was done for the LAPCAT A2 reference vehicle for different flight conditions. Comparisons of the analytical model and the CFD data are in sufficiently good agreement for preliminary system design purposes.
Another sub-topic was the effect of sloshing on damage initiation in CFRP tanks. Thereby, cryogenic sloshing was considered as a cyclic mechanical and thermal loading to the structure. Resulting tensile and bending stresses can be regarded as relevant loading cases to evaluate the risk of failure initiation. Consequently, different CFRP laminate samples were subjected to cyclic mechanical loading at different temperatures. The resulting fatigue behavior in terms of crack density was investigated at RT and at -50°C. It was found that the advantageous performance of thin plies for cryogenic tank applications is also present for cyclic sloshing loads, with significant lower crack densities compared to thicker layers. Based on the experimental results, an initial analytical model for cyclic sloshing related failure was set up and validated.
Heat Transfer and Boiling Studies (WP4.2)
Heated gasified oxygen can replace helium as pressurization gas in LOX-tanks. For this purpose, safe and reliable heat exchangers are required. Corresponding ceramic heat exchanger systems and technologies were investigated in the CHATT project. Different potential design solutions were modelled in numerical analysis tools, and thermal and mechanical loads were simulated. Based on the results, a final design for the heat exchanger was selected. The theoretical studies were complemented by a series of manufacturing trials utilizing the C/SiC based ceramic Cesic material. Thereby, problems were initially faced during milling and grinding due to the filigree architecture of the heat exchanger design and the brittleness of the material. These issues could however be solved. Finally, a complete heat exchanger was manufactured and tested. The ceramic core is encapsulated in a metallic shell, whereas the latter has to carry the mechanical loads due to the low tensile strength of Cesic. The final heat exchanger was eventually tested with liquid nitrogen.
A separate pillar of this task was addressing fluid boiling behavior as a function of tank wall surface roughness. An experimental setup was created that allowed for utilizing rectangular tank bottom walls with different surface roughness, and subsequently observing in detail the fluid heat-input induced boiling behavior. A special feature of these investigations was the identification and trajectory tracing of individual bubbles. A total of 6 separate experiments with 3 different roughness levels were conducted. The experimental results clearly reveal the role of the surface properties in promoting nucleate boiling and the final analysis of the data shows a distinct relationship between critical radius and surface roughness.
Compatibility of Materials (WP4.3)
Experiments on sample levels, as well as for demonstrator tanks were conducted in order to evaluate mechanical and thermal loads in the tank wall and the associated materials. The main purpose was to assess the transferability of loads from demonstrator size level to large scale tanks. The tank level investigations also included a static LN2 fill test for the large cylindrical tank manufactured by DLR, with around 50 % fill level, which was successfully passed by the demonstrator tank. Even after several hours of exposure, no damage could be observed.
Furthermore, the long-term behavior of CFRP materials when subjected to cryogenic fuels was experimentally investigated. Liquid oxygen compatibility was tested via ignition tests of CFRP specimen. The LH2 compatibility was investigated by submerging samples into LH2 and performing permeability tests. This was complemented by gas permeability experiments at different temperature levels using Helium as test gas. Thermal cycling tests were conducted as well with 50 cycles of immersing samples into LN2, and subsequent re-warming to ambient temperature. Also tensile strength, CTE and heat conductivity measurements were accomplished on sample level.
Cryogenic Rankine Cycle Studies (WP4.4)
The cooling cycles defined for the M5T and A2 concept aircraft both use air as the working fluid in what is known as an “Air Cycle”. The objective is to provide cold low pressure air at the turbine outlet when provided with warm high pressure air at the compressor inlet. Air cycle machines are used in all conventional aircraft cabin cooling systems, the distinguishing feature of the machine developed in this study is that the turbine will not drive a compressor but instead it will drive an alternator to provide electrical power in the absence of any other rotating machinery. Another unique feature of this open cycle is that a recuperator is used in place of the conventional external-air heat exchanger to pre-cool the air entering the turbine. The expander which converts the energy of the working fluid into mechanical power is to be a radial turbine. Within CHATT, a corresponding turbine was designed, manufactured and tested. Mounting of the turbine on a test rig was realized via an innovative foil bearing. The rotor was manufactured by 3D printing using maraging steel. The stator was made from plastic resin. During testing, the turbine was accelerated up to 140000 rpm, whereas high peak efficiency levels (> 90 %) were found.
Potential Impact:
Evaluation of achieved TRL
The Technology Readiness Level (TRL) is a well-known and well established indicator of the readiness of certain application to reach operational status. The definition of the different TRLs according to NASA is reprinted in the Final Report. Note that the section relevant to research projects like CHATT is TRL 1 to 5 reaching from Basic Technology Research up to Technology Development.
In the Annex of the CHATT Part B a table on the intended progress of the investigated technologies in terms of TRL-grading is provided This table lists individual items describing in short the objectives in technology development along with their present (e.g. 2010) TRL and the intended increase towards the TRL-scale shown in the right column.
WP Nr. Technology description Present (2010) TRL Envisaged TRL for CHATT in 2015
2.2 Flight control development sloshing case 1 2/3
2.2 Aeroelastic modelisation of tanks 2 3
2.3 Tank preliminary design model 2 5
2.4 Tank System Engineering Methods 3 5
3.2 Liner Design 3 4
3.3 Tank Design with Liner 3 4
3.3 Cryogenic dry wounded carbon fiber pressure vessel 2 4
3.4 Tank Design without Liner 3 4
3.5 Tank Design with Complex Shape 3 4
3.6 Testing and Health Monitoring 4 5
3.7 Tank insulation (Aerogels) 2 3
4.1 High Order Methods for Sloshing Simulation 2 5
4.1 Sloshing Tests and Modeling 2 4
4.2 CMC Heat exchanger technologies 1 3
4.4 Cryogenic Rankine Cycle 2 4
At the end of the CHATT project it is of interest to evaluate retrospectively the actually achieved TRL in the technologies as well as in the simulation models. A realistic assessment is also crucial for the preparation of any follow-on work. At the final progress meeting of CHATT (PM6) the participants discussed on June 23rd 2015 the actually achieved progress and the self-assessment is documented in the table below. It is interesting to note that the pre-CHATT TRL is in some cases now assessed as considerably below what was believed state-of-the-art in 2010. This could be explained by a better understanding of the complexity of certain issues than was available at the time of the CHATT project proposal.
It is to be acknowledged that the evaluation in the table is a self-assessment of the project participants. This should be followed by a judgement of independent experts. Nevertheless, significant progress has definitely been achieved in key areas of cryogenic composite tanks and related technologies.
WP Nr. Technology description Previous TRL Achieved TRL
2.2 Flight control development sloshing case 1 2
2.2 Aeroelastic modelisation of tanks for hypersonic airplane 1-2 2-3
2.3 Tank preliminary design model for hypersonic airplane 1 3
2.4 Tank System Engineering Methods 3 3
3.2 Liner Design 2-3 4
3.3 Tank Design with Liner 2-3 3
3.3 Cryogenic dry wound carbon fiber pressure vessel 2 3
3.4 Tank Design without Liner 2 3
3.5 Tank Design with Complex Shape 3 4
3.6 Testing and Health Monitoring 1 2-3
3.7 Tank insulation (Aerogels) 2 3
4.1 High Order Methods for Sloshing Simulation 2 3
4.1 Sloshing Tests and Modeling for hypersonic airplane 2 3
4.2 CMC Heat exchanger technologies 1 2
4.4 Air Cycle 1 2
4.4 Turbine / Foil bearings 2 3
Discussion of Important Impacts of the Project Results
One of the most important results of the CHATT project is the implementation of thin-ply technologies for liner-less cryogenic CFRP tank applications. In the past, the utilization of composite materials for cryogenic tanks in aerospace vehicles was severely handicapped by the necessity of complementing the CFRP shell by an internal liner. The liner is required in particular for preventing micro-crack-initiation in the CFRP, which not only adversely affects the structural integrity of the tank, but also provide leak paths for the fluids inside the tank. Liners however increase the weight of the tank and may lead to fatal interface issues between liner and CFRP shell, in particular due to differential thermal expansion. During CHATT, extensive experimental research campaigns on specimen as well as demonstrator tube level proved the superior performance of thin ply approaches for cryogenic tanks. Thin ply based laminates have been found to largely reduce or even eliminate micro-cracking in cryogenic environments. This opens very promising perspectives for cryogenic liner-less CFRP tanks. For future hypersonic transport vehicles, this may even be considered as a break-through technology. Of similar or even higher importance is the potential impact on the space launcher industry. If thin ply approaches ultimately prove to be suitable for ultra-lightweight and reusable LH2 and/or LOX tanks, a completely new category of launch vehicles may be developed, which in turn would greatly foster the exploitation of space for mankind. Nevertheless, the current TRL for thin plies in cryogenic tanks is still at 3 because the thin-ply technology is not yet implemented in a functional tank.
Thin ply approaches are not only suitable for cryogenic applications, but may offer superior mechanical properties compared to conventional CFRP’s in general due to significantly higher plasticity stress. Although this thin ply effect is well known in material science, thin ply approaches were hardly used in history due to high costs and manufacturing issues. Therefore it is highly noteworthy, that also the production process of thin ply laminates was improved during the CHATT project, which now allows for the production of high quality and virtually void-free thin-ply laminates.
Design, manufacturing, and testing of different CFRP tank structures revealed the difficulties when using composite structures for cryogenic environments. However, this enabled also the development of solutions strategies and led to improved design approaches for future tanks. In particular notable is the progress on dry-wound tanks and multi-bubble tanks. The dry-wound tank research proved that this potentially ultra-lightweight design concept is in principle suitable for cryogenic applications, but requires liner materials that provide low thermal expansion mismatch between fibers and liner. The manufacturing and testing of a multi-bubble tank on the other hand largely improved the understanding of the complicated design issues and greatly increased the confidence in designing cryogenic CFRP tanks of this innovative technology. This opens perspectives for cryogenic propellant tanks not only in complex shaped hypersonic vehicles, but also in sub-sonic airliners with non-circular fuselage cross-sections such as advanced blended wing-body configurations. This in turn would allow for a drastic reduction of environmental pollution of sub-sonic air-traffic when switching from kerosene to hydrogen fuels.
Cryogenic tanks for future airliners not only require lightweight tanks structures, but also lightweight, durable, and reliable insulation systems. In this area, a major technological advancement could be achieved within the project by the development of a new type of cryogels. The new material extents the temperature ranges of conventional aerogels by several hundred degrees, while simultaneously being much cheaper. The applications of this new material are multifold in all areas where lightweight and cheap insulations for wide temperature ranges are required.
Significant progress was also made on the modelling and analysis of sloshing. The research results of CHATT perhaps the first time looked into an identification of the impact of longitudinal sloshing in large horizontal tanks on the flight control of future aerospace vehicles, as well as on resulting aero-elastic effects and structural sizing. Since this crucial issue is typically ignored in hypersonic vehicle pre-design due to either lack of problem awareness, or lack of simulation capabilities, CHATT essentially improved the quality of hypersonic vehicle design and allows for more reliable feasibility assessments of hypersonic vehicle configurations. Moreover, the CFD code developments for sloshing considerations during CHATT may be applied to other fluid dynamics problems in many sectors. Furthermore, the detailed investigation of propellant cross-feed systems greatly improved the understanding of this technology, and may support the development of multi-stage transport vehicles, such as the DLR SpaceLiner.
Progress was also achieved on topics associated with cryogenic propellants, such as heat transfer and boiling problems. Manufacturing studies and finally the construction and testing of a ceramic heat-exchanger greatly increased the fundamental knowledge on ceramic materials for cryogenic heat-transfer applications, but also revealed the critical issues associated with this technology, that need to be overcome in future studies. The experimental boiling studies in turn largely improved the understanding of roughness induced boiling. This is not only relevant for cryogenic tanks in hypersonic vehicles, but also for problems of heat transfer into fluid storage vehicles in general.
In summary, CHATT resulted in valuable progress for the realization of future hypersonic airliners, as well as environmentally friendly and economically highly efficient sub-sonic blended wing-body aircraft. The potential benefits for other technology areas are multifold, in particular when considering the universal applicability and importance of lightweight composites, insulation materials, and lightweight pressure vessels.