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Innovative propellants in hybrid propulsion technology and its applications in space transportation

Final Report Summary - ORPHEE (Innovative propellants in hybrid propulsion technology and its applications in space transportation)

Executive summary:

Hybrid technology appears as an innovative, high performance and promising propulsion technique in number of space missions. By combining characteristics taken from both solid and liquid propulsions, this technology is expected to provide mainly high performance with throttleability and stop - restart capabilities. However, the current state of the art outlines that the standard fuels (mainly based on a hydrocarbonic polymer) suffer a low regression rate which induces complex grain shapes and low loading ratio. In order to achieve advanced fuels and to acquire a better knowledge, the Operative Research Project on Hybrid Engine in Europe (ORPHEE) was conducted from 2009 to end 2011, on the cooperation of nine partners (SME, Astrium SAS and Astrium GmbH, Avio, Onera, DLR, Politecnico di Milano, University of Naples, University Polytechnic of Bucharest, Thyia), and supported by funding from the European Commission?s Seventh Framework Programme (FP7/2007-2013). The overall logic of this project was devoted to increase the readiness of advanced fuels from a TRL1 to TRL2-3 and propose a coherent approach dealing with promising applications identification, mathematical models building, demonstrators and test bench designs, and technological roadmap definition.

A first study aimed to select potential interesting fuels by characterising the fuel composition and its combustion at small scale. A specific task was devoted to the scaling up of these compositions in order to reach a formulated grain mass of 7kg. One focused on the process of metallic powders addition and the verification of the grains integrity and homogeneity. The obtained fuel grains were fired by using gaseous oxygen. Accurate study of test results allowed describing the regression rate evolution in function of global mass flux and geometric shape of the grain and gave experimental data for numerical tools validation. More specifically, these tests provided information on the aptitude of metallic powders to increase the regression rate at large scale. It is observed that their effect is much less important than it was demonstrated at small scale.

Based on these results, two preliminary road-maps have been built by the ORPHEE partners. They address suitable hybrid propulsion demonstrators that are dedicated to space applications (selected through a market survey and system analysis). Technological needs in terms of development and associated time periods are presented a TRL of 6 for the two demonstrators have been identified.

Project context and objectives:

ORPHEE - Operational Research Project on Hybrid Engine in Europe - is a project supported by funding from FP7 programme by the European Commission (EC); it started in January 2009 and ended in December 2011.

Nowadays, chemical propulsion is based on solid technologies (launch applications like first stage booster) or liquid technologies (primary stages, upper stage engines). Complementary, hybrid propulsion technology, as defined in ORPHEE, appears as a new capability for advanced space transportation system. Engines based on this innovative propulsion concept can provide advantages like thrust performance, throttling (thrust modulation), versatility (easy adaptation to various configurations), simplicity, safety.

This hybrid technology will help to consolidate the long term sustainability, needed by the European propulsion space community to remain independent.

Classical hybrid propulsion principle is based on the injection of a liquid or gaseous oxidiser into the engine combustion chamber where it reacts with a solid fuel to generate hot gases providing the thrust. Enlarging the burning surface is the current proposed solution to reach the needed performance level. With current solid fuels, it dramatically increases the solid grain volume and the engine weight, limiting the applications. The regression rate is a key parameter controlling the solid fuel grain design. Its increase is a very attractive solution to reduce the grain volume.

The main objectives of ORPHEE are to increase versatility of space propulsion systems, to ensure a significant increase in the performance of a hybrid engine, to improve the solid fuel technological maturity from TRL one to three, to gather the European skills on hybrid propulsion and to contribute to European access to space.

In near future, the availability of new hybrid engines will allow the access to new space transportation missions. By consolidating the knowledge on this innovative technology and by implementing solutions in upcoming space agencies roadmaps, the European space propulsion community will strengthen its global competitiveness.

To achieve the overall objectives described above, numerous measurable objectives have been identified in term of knowledge development and physics understanding, but also in term of technological development:

1. performance requirements for two space transportation applications, i.e. upper stage engine and booster engine
2. solid fuel formulation characterized by an increased regression rate to reach 3 to 4 mm/s
3. Test-firing at lab-scale, which consisted of test definition, material supplying, grain manufacturing
4. Feasibility of models for mastering, namely liquid oxidiser injection, solid fuel regression, combustion efficiency and design parameter influent on operating effect (chamber geometry, operating pressure)
5. Demonstrator design for both applications and test bench design for demonstrating advantages of hybrid engines.

The project is organised in five main work packages (WPs).

Project results:

Management

About dissemination, in a first step a website has been created (at http://www.orphee-fp7-space.eu).

In second step, a specific area with controlled access has been validated to share the documentation and favour the exchanges. This private area managed by the coordinator has been an important tool for the communication and the exchange inside the consortium ORPHEE.

An exploitation plan has been defined and describes how to use the in the widest way the ORPHEE results coming from:

1. tests on material characteristics
2. numerical simulations
3. design justification
4. market survey and
5. technological road-map.

Then, the exploitation logic has been presented together with the description of the exploitation methodology in terms of targets, tools and selection criteria model.

Finally, it has been indicated how the exploitation output has to support the potential future activities, in relation with the activities outlined by the technological road-map of the most ambitious target of the hybrid technology development: the formulation and the manufacturing of an advanced fuel grain able to withstand the requirements of the first stage booster of a launch vehicle.

System level definition including missions analysis and requirements L
% The mission's definition and propulsive performance requirements are defined to guide the works on material (MWP300) and the modelling (MWP400) to induce preliminary designs of demonstrators (MWP500).

The activity of the first year of this WP was dedicated to the selection of potential interesting applications or platforms with regard to market analysis and hybrids propulsion advantages.

Market survey and missions selection

This first activity deals with a market survey concerning the hybrid propulsion for potential space applications and/or platforms. After having presented the current space market, some elements are given for the future with regard to the main following sectors (not given in any priority):

1. commercial launches,
2. institutional launches
3. space exploration
4. space tourism
5. miscellaneous.

An evaluation on each previous sector is done in order to bring out the main potential application of such a propulsion system.

The analysis of the market survey (performed before the financial crisis that occurred just after this analysis) shown that the number of launches will be in constant growth during the next years, in relation to the increasing demand of satellites for communication, earth observation, space exploration and of needs of manned spaceflights (moon, mars), whatever the demand comes from governments or private companies.

Moreover the emerging countries aspire to develop systems allowing to reach space (at minimum LEO) in an autonomous way, which can change the deal of the future markets and the way to meet them. The coming of new entrants could change the way of thinking of new launch systems because these new players could propose offers more striking in term of cost, robustness and preferment at long term, if not earlier.

In these conditions, some new propulsive systems have to be developed to obtain systems always more efficient and attractive. Hybrid propulsion can provide advantages such as simplicity, safety aspects, thrust performance, thrust control and cost reduction.

Based on these advantages, an evaluation of potential applications for hybrid propulsion is performed by applying a specific rating to each application with regard to market access and retained parameters.

This study shows that the two or three main platforms where hybrid propulsion can be applied are:

1. the exploration (automatic or manned spacecraft) thanks to the safety inherent of hybrid propulsion, the throttlability, its reignition capacity and its compacity
2. the upper stages for its potential high thrust and its ability to be extinguished and reignited
3. the low cost launchers mainly for the cost objective.

In comparison with the space market, it can be noticed that there is a quite good consistency between the future needs (space exploration, low cost launchers to reach low orbits) and the capacities of hybrid propulsion in term of performance, robustness, safety and costs.

Of course, the results obtained in this study will have to be updated with regard to technology maturity progress or new possible applications not yet identified, especially during the very next years to find the fuel formulation (coupled with the best oxidiser) able to consolidate the hybrid propulsion advantages emphasized in open literature, while minimizing the current drawbacks (combustion instabilities, unburned masses).

The favourable possibilities of the hybrid propulsion (versatility with the thrust modulation, re-ignition, safety, a specific impulse comparable to semi-cryo technology, potential cost reduction) allow to envisage using it on various space transportation applications.

The selected and studied platforms are:

1. launchers with a re-ignitable upper stage to maximize the payload on an orbit requiring re-ignition(s)
2. exploration Landers towards the Moon, to perform the whole descent and a soft-landing
3. launchers with a booster (or first stage) to manage the dynamic pressure.

Mission analysis and requirements

The analysis has preliminary designed the hybrid propulsive systems and related stages which could favourably replace the already existing propulsive systems, namely hybrid upper stage (HUS), Moon or Mars lander and low cost first stage.

This design of hybrid propulsion system leads to a motor which meets high level requirements defined by mission analysis with simplicity and efficiency. A second loop has been done with the demonstrator works with respect to the results of the studies on advanced fuel.

Trade-off and solid fuel optimisation: make a significant progress in the fuel composition.

Trade-off

In relation with the system level definition (including missions analysis and requirements), a trade-off has been completed to define the orientations for the solid fuel.

The results of the AHP (Analytical Hierarchy Process) trade off reviewed the 2 off-the-shelf reference standard fuels in hybrid propulsion. The first one is HTPB and the second one pure Wax. The results obtained for the two standard fuels are given in order to establish a reference with all considered oxidisers (LOx, Gaseous O2, N2O4, H202 (99%) and N2O). These reference values have to be compared with the AHP results of the future advanced fuel formulations. The work performed is related to the definition of the criteria used in the AHP. The selective criteria are:

1. Theoretical Isv max,
2. Theoretical rho*Is max,
3. Regression rate in function of GOx,
4. Storability and ageing (fuel/Oxidiser) before first combustion,
5. Mechanical properties (solid fuel),
6. Toxicity (fuel/Oxidiser) of combustion products and oxidiser,
7. Safety and security of fuel formulation products and oxidiser before combustion,
8. Oxidiser cooling capability,
9. Stability in time and in space (fuel and Oxidiser) after first ignition (upper stage application only).

A specific rating is applied for each criterion according to the selected application (Launcher, Upper stage or Lander).

As main results, priority factors were defined.

The ranking of the potential Fuel/Oxidiser related to the application are:

1. Lander applications: Interest of paraffin with H2O2 (99%) linked with a much higher regression rate (Martian: 593 vs. 575 Lunar: 594 vs. 575); N2O4 appears as an alternative solution for Martian Lander (533 for HTPB & 528 for paraffin); LOx is also interesting for Lunar Lander (540 for HTPB).

Upper stage application:

1. Interest of HTPB with LOx
2. H2O2 (99%) appears as an alternative solution with HTPB

Booster Stage

1. Interest of HTPB with LOx
2. H2O2 (99%) appears as an alternative solution with HTPB.

All these results are obtained by considering a high concentration of hydrogen peroxide. An update could be performed in order to investigate the effect of a lower concentration on the rating.

Depending on last up-to-come tests on advanced fuels, a review of the obtained results has been performed. A preliminary result is presented dealing the use of H2O2 with 15% of H2O instead of pure water. HTPB/MagnesiumH2 and HTPB/nanoaluminium have also been addressed with an assumption of a 20% increase of the regression rate.

The interest of the use of nano aluminium powder, especially when a more dense fuel is required, has been identified. Concerning the oxidiser, the interest of LOx appears clearly for booster and upper stage. For Lander applications, for which a long storage capability is mandatory, N2O4 become a good candidate. Hydrogen peroxide can be considered as a potential interesting oxidiser only for high concentration (higher than 85 %).

Solid Fuel characterisation and optimisation

Both ballistic and mechanical aspects have been addressed and are presented in the following sections. Two experimental facilities have been used for ballistic:

1. the SPLab radial burner (typical grain shape is 30mm length, 20mm in diameter and 10 to 16mm diameter for the central bore)
2. the SPLab 2D Slab burner (samples have a rectangular shape (15 x 22 x 4 mm).

For both facilities, reference is defined on pure HTPB grain tess and then a ballistic characterisation of solid fuels based on Hydroxyl-Terminated Polybutadiene (HTPB) loaded with Magnesium Hydride (MagnesiumH2) or other additives is presented.

HTPB-based fuels loaded with different mass fractions of MagnesiumH2 were tested under 100 % oxygen. Operating conditions were chamber pressure of 10 bar and sample geometry was characterized by initial diameter of 6 mm.

Preliminary tests were performed on sample geometry with initial port diameter of 4 mm. This configuration allows achieving an initial oxidiser mass flow rate of around 130 kg/m2/s.

Paraffin based-fuels

Several wax-based solid fuels formulations added with nano-sized Aluminum powders (n-Al) and with a Magnesium hydride (MagnesiumH2) powder were tested in the 2D slab experimental setup available at SPLab. Firing tests were performed in double slab configuration, with 50 x 10 x 4 mm sample geometry. Tests were carried out at an oxygen mass flux (GOx) spanning in the range 100 to 350 kg/m2s, comparing aluminized and Magnesium hydride (MagnesiumH2) filled fuels. %:%: Aluminium powders used have average particle diameter of 50 and 100 nm (Alex50 and Alex100, respectively). The magnesium hydride powder has average particle diameter of 50-150 microns (bimodal distribution). Two types of paraffins are used as binders: a Gel Wax (GW), with chemical formula C12H26, and a Solid Wax (SW), with chemical formula C24H50. In order to overcome the limit of the poor mechanical properties typical of paraffin waxes, a structural strengthening based on Poly-Urethane Foam (PUF) is used.

The fuel mass flow rate can be expressed as the ratio between the fuel mass consumed during combustion and the burning time, where the initial fuel sample mass before combustion and the corresponding final mass after the firing test are measured.

In all the tested fuels, magnesium hydride or aluminum addition both result in enhanced regression rate.

For GW-based fuels, the best regression rate results are obtained with aluminum addition, while in SW-based fuels the best results are obtained with Magnesium hydride addition. This can be explained taking into account the specific characteristics of the different waxes used. GW has elasto-plastic behaviour and higher surface tension than SW, which displays plastic behaviour, and these features induce a higher tendency to droplets entrainment for SW-based fuels. Addition of highly reactive but coarse particle sized filer such as magnesium hydride, or addition of less reactive butfiner particle sized filler such as the aluminum powders considered in this work, lead to different effects depending on the type of wax used. In SW-based fuels, the high tendency for droplet entrainment ensures that particles combustion can fully exploit the additive potential for energy release; in such a fuel, additive average particle size is less effective than additive type in enhancing average regression rate, and magnesium hydride high reactivity results in the best performance. On the other hand, finer particle size is more effective in low-entrainment fuel type (GW), because in this case the coarse particles combustion onset is hampered, while inner particles burning is easier.

No difference in performance is perceived with different nano-sized aluminum powders containing SW-based fuels. This observation concerns that additive average particle size is less effective than additive type in enhancing average regression rate for this kind of fuels.

Reinforcement of mechanical properties with PUF

PUF, inserted in the formulations as reinforcing material, has the effect to increase both the storage modulus and the melting temperature. In the temperature range approximately below 35 °C, the storage modulus of the reinforced fuel is close to that of PUF. When temperature increases, the modulus decreases for the paraffin softening effect. However, before the melting occurrence, the storage modulus is still rather high. Pure GW melts at 48 °C; if reinforced by means of PUF, the melting temperature increases up to 77-78 °C. The presence of PUF inhibits the entrainment phenomenon development, but PUF allows the paraffin containment up to a temperature of 77 °C.

Investigations focused on the correlation between the foam cell size and regression rate should point out the important role of cell size. PUF has the advantages of a thermosetting polymer, characterized by a low interaction with the flame. PUF very low density allows a very good compromise: it has good properties without inhibition of the entrainment effect typical of paraffin-based fuels. A similar investigation was performed for SW; results confirm the strengthening effect of the PUF. However, the fragile behaviour of SW is not fully solved by this fuel reinforcing approach. The very promising interest of this composition, from a ballistic point of view, does not have a similar interesting behaviour from a mechanical point of view. Tensile tests point out the fragile behaviour of the material.

New material

To overcome this difficulty, new fuels have been investigated. They include a 15% mass fraction of SEBS and kerosene. A strong increase of G and decrease of melting temperature is observed when a fraction of paraffin is replaced by solid wax. Observed trends show that kerosene is a suitable material to decrease viscosity. All the investigated homogeneous compositions allow overcoming the problem of waxes brittleness.

Lab scale motor testing

Fuel formulation

Based on the previous work, formulations have been scaled up. These grains underlined the importance of mechanical properties and some adaptations in the manufacturing process leading to a significant improvement in the fuel grains quality. Several firings of pure HTPB and HTPB/MagnesiumH2 grains have been performed, leading to a reference regression rate law. It was also demonstrated the interest of the addition of a small amount of MagnesiumH2 to increase the regression rate.

For the last year, grains involving new formulations have been manufactured:

1. HTPB/NanoFe : a small amount of nano iron was added to the HTPB fuel to test the potential increase of the regression rate due to oxidation of the iron. It is believed that the important energy released at the surface could enhance the pyrolysis of the HTPB.
2. HTPB/NanoAluminium: The particles were uncoated and their average size is of the order of 100 nm.
3. HTPB/Magnesium: this additive replaced a second lot of magnesium hydride powder as none was found available with noticeable difference with the first one.
4. HTPB/NanoAluminium: The second quality of nano aluminium used a coated powder. The passivating agent is Viton which could react very energetically and then increase the heat released at the surface of the grain.
5. HTPB/Magnesium/NanoFe: to test the potential effect of nano iron particles on a HTPB fuel with additives.

Firing tests

The firing provided interesting data on the behaviour of the pressure evolution depending on the nature of the fuel.

As expected, the pressure time history of pure HTPB grains did not show any oscillation. This result confirmed previous ones. The most surprising result was obtained with HTPB containing 15% of nano aluminium. The powder was uncoated, so the protecting layer surrounding the particles is only composed by alumina. During the firing, an oscillatory behaviour appeared at a relatively low frequency (around 5 Hz which is rather different from the frequency of the 1st longitudinal acoustic mode).

Regression rates of the fired test are plotted in the next figure. The mean values of the regression rate provide the same tendency as the literature law found in the literature (Sutton). However, the HTPB law seems to be lower than previous results obtained with the same experimental device (but manufacturing process was different). An explanation is proposed, based on the possible formation of alumina slag on the nozzle throat which is periodically removed.

The addition of nano-aluminium powder seems to not increase the regression rate whereas a little positive effect is observable for the magnesium hydride.

The measured regression rates are post-treated in order to demonstrate an increase of the regression rate with regard to the reference HTPB fuel (considered as the reference). In that purpose, the overall data are interpolated by a regression rate law.

As it can be seen, nanometric additive provide poor increase limited to a factor of below 10%. Same tendency is obtained for nanometric iron particles. However higher values are provided by magnesium based additives (MagnesiumH2 or metallic Magnesium). For pure Magnesium, the increase is larger than 10%. The best result is obtained with the unique test with nanometric iron and Magnesium additive. The obtained value is close to 50%, but other test should be performed to confirm this trend.

A test has been carried out to assess the feasibility of motor throttling in a large range. Pure HTPB has been fired in this test and a feeding pressure ratio of 20/6 has been selected.

Oxygen flow rate varied in the ratio 90.5/26.5; the first step lasted from 4 to 11 s and the second from 11 to around 24 s. Over these two ranges, chamber pressure increased from 4 atm up to around 12 atm in a ratio slightly lower than that of oxygen mass flow rate, not showing any pressure oscillations. This may demonstrate the capability of hybrid rockets to be throttled in a wide range.

Hybrid engine modelling: provide numerical models to simulate the operating of a hybrid engine

This main WP is dedicated to the numerical simulation with the objective of a better understanding of the physical phenomena and a possible control of parameters that are directly involved in a hybrid engine operating.

Fuel combustion modelling

The aim was to enhance our understanding of the hybrid combustion processes in order to point out the most sensitive parameters. We targeted reliable assessments of classical oxidiser / fuel couples, based on information supplied by appropriate combustion models.

Classic hybrid propulsion is based on a relatively simple basic mechanism: an oxidizing gas ignites with propellant gases generated by the pyrolysis of the solid fuel, and this pyrolysis is fed by the heat steaming from the combustion flame. This pattern occurs mainly with classic fuels such as HTPB or polyethylene. Study of the behaviour of new fuels such as paraffin based ones, should take into account more complex phenomena such as the development of a liquid film over the fuel surface or the existence of a reactive solid phase within the gaseous phase.

In order to analyse hybrid combustion, it is therefore crucial to know the fuel regression rate. It is tightly linked to the fuel surface temperature, in such an intricate way that basic (or 'linear') models include this temperature through an exponential term, such as e-E/RT. Therefore it is also paramount to know this wall temperature at every point of the fuel surface. By adapting the classic theory of the turbulent boundary layer to the flow inside the fuel internal channel, it can be shown that the wall temperature in a given section is estimated from the general characteristics of the flow when it reaches this section. Some founding works discovered the dominating role of diffusion phenomena through the turbulent boundary layer in hybrid combustion. With a few closure hypotheses, it is possible to create a parabolic equations system that describes the internal flow inside.

The combustion reaction takes place in a flame area away from the fuel surface. We adopt the hypothesis of an infinitesimally thin flame area, characterised by its distance f(x) at the fuel surface, this distance obviously depending on the abscissa x. In the stoichiometric ratio, this area is distinguished by mass fractions of both macro species [ox] and [fu].

After a successful validation phase, the calculation easily reproduced the general characteristics of test that uses a pure HTPB grain. The gaseous phase reaction model considers that the HTPB pyrolyses in a unique monomer like butadiene C4H6. The calculation that best fits the experiment was obtained for a particular low value (0.1) of the global emissivity (radiative exchanges between the flame and the fuel surface). However it can be noticed that some details of experimental trends are not detected by the calculation, for instance, the time history of the burnt layer which exhibits a linear evolution during the first half of the firing is modelled by a relatively constant value in the calculation.

The monodimensional calculations performed in the frame of ORPHEE provided interesting trends even if they do not close the discussion on the phenomena that contribute to the operating of a hybrid engine. These calculations also substantiate the non-negligible part played by the radiative exchanges with the HTPB.

Oxidiser injection modelling

AST/G contributed to the ORPHEE project by numerically simulating a hybrid combustor with the spray combustion tool Rocflam-II. The first task was dedicated to the chemistry modelling. To achieve this activity, two basic approaches are possible, namely a global chemistry and an equilibrium-based PPDF-approach. Both have been tested and used for simulating hybrid engine operating:

1. The global modelling has the ability to take into account finite rate chemistry effects, but due to the increase in computational effort with increasing number of species and reactions must simplify the occurring chemistry greatly.
2. On the other hand, the PPDF approach already in use for the simulation of LOx-hydrocarbon thrust chambers at Astrium has drawbacks in the description of the pyrolysis gases; the chosen equilibrium modelling gives a wrong gas composition. However, the effect on global engine data such as mixing, performance etc has to be evaluated as simulations are available and cannot be predicted in advance.

For validation of this numerical approach, a hybrid test case, identified by DLR Lampoldshausen during the first year of the ORPHEE activities, has been considered for simulation. This test case is originally based on DLR in-house solid fuel ramjet firing dating back to the 1980s where an HTPB solid fuel grain was burnt with air as oxidiser.

For chemistry modelling, the global chemistry considers finite rate chemistry effects, but due to the increase in computational effort with increasing number of species and reactions, simplifications are necessary. Simulations with the PPDF-approach show a reasonable good agreement with the available test data both in regard to the measured chamber pressure (for given propellant mass flows) and the static temperature measurement within the combustor.

A three-step global reaction scheme with 5 species has also been implemented in Rocflam-II and tested on the DLR solid fuel ramjet test case. It turned out that there is no further advantage with respect to the usual equilibrium-based chemistry (PPDF-) approach with 12 species.

Within the logic to come from the solid fuel ramjet simulations to a more hybrid motor like LOx/HTPB combustor, a pre-chamber was added to the original Air/HTPB combustor. It could be observed that a recirculation system resulting from the step inlet in the original test case but vanishing in the case of a pre-chamber, had an important influence: the missing recirculation resulted in decreased mixing between oxidiser and fuel and thus lower temperatures and combustion efficiency.

Good agreement was reached between simulation and test concerning the radial static temperature profile and the combustion efficiency. This confirmed the chosen simulation approach, including the chemistry modelling, where the equilibrium table based PPDF-approach proved to be superior to a global reaction scheme. As a further step towards the simulation of LOx/HTPB, the air in the DLR solid fuel ramjet test case was replaced by gaseous oxygen, proofing the expected behaviour.

A cryogenic study with LOx/HTPB was then performed on the same test chamber configuration. The first and most important result is that a small amount of fuel must be added in the pre-chamber to ensure convergence of the simulation. Without such fuel, the temperatures in the pre-chamber are extremely low preventing the oxidiser from evaporating. It is worth to be noted that a similar behaviour was reported by AMROC during the development of hybrid thrusters. Non-vaporized droplets lead to combustion instabilities in real tests, which were significantly reduced when adding a hypergolic fuel in the pre-chamber.

A parametric study was finally performed including a variation of different droplet sizes and distributions, droplet temperatures and injection slot geometries. In most cases the simulation results were in accordance with experience. The variation in droplet size classes did not disclose substantial differences in the combustion temperature profiles, which was considered to be mainly due to an accumulation of the evaporated oxidiser along the center section of the port.

Besides these first validation simulations with a prescribed, fixed solid fuel evaporation rate along the grain surface, a fully coupled solution linking the solid fuel pyrolysis to the hot gas temperature profile developing inside the combustor was concluded to be essential for treating hybrid injection and combustion in a more comprehensive manner, an issue to be dealt within the forthcoming modelling period.

The last case, a GOx/HTPB subscale combustor fired by the University of Naples in 2009 was simulated using a two-way coupled approach, where the regression rate and thus fuel mass flow and temperature is not prescribed as in the preceding simulations, but computed from the surrounding flowfield. These simulations globally showed good agreement to test data in terms of the regression rates, pressure and mixture ratio values. In the simulation, the combustion chamber is divided in a discrete number of axial patches, each with an individual regression rate. The extreme of only one patch considers an axially constant regression rate over the entire solid block.

As a summary of the simulation performed on the Napoli test case, the next figure depicts the predicted local regression rate profiles along the assumed solid fuel. Examples are given for both start and end of burn. The diagram shows a very good agreement to the measured averaged regression rate, determined by weighing the solid fuel grain before and after the hot run. For both the one- and the ten-section-approach, Rocflam-II predicts a higher regression rate at start of the burn compared to end of burn. For the ten-section-approach, the regression rate is axially almost constant at start of burn, whereas a strong decay in axial direction is observed for the end of burn.

The first and possibly most important result was that for the chosen hypothetical test case, the combustion efficiency only reached 84%, which is a low value. This clearly indicates a need to further work with the optimisation of the performance, which could be realized in a combined experimental / analytical effort. It must be emphasized that rather low combustion efficiency values between 82 and 91 % were also reached in case of gaseous oxidiser. Hence, the low performance appears to be a general problem of hybrid combustion and not specific to liquid oxidisers.

In the numerical simulations, the mean mass diameter (MMD) of the injected spray was varied between 30 and 80 µm. In a real engine, this could characterize different injection systems causing varying atomisation levels. The smaller the MMD was chosen, the higher the combustion efficiency was, which is in line with the expectations. However, the influence was rather small with only 1% of performance increase for the quite high variation of MMD between 80 and 30 µm.

As second parameter, the injection area was investigated. In one simulation, the injection was realized over the whole faceplate whereas in an alternative approach, the injection covered a smaller fraction of the faceplate ('partial injection'). The partial injection approach turned out to result in higher turbulence levels and thus better mixing of fuel and oxidiser, leading to a combustion efficiency increased by 1 %.

As third parameter, the injection temperature influence was investigated. To remain within reasonable limits, this value was varied between 100 and 110K. This investigation showed almost no influence on the combustion efficiency; hence the oxidiser injection temperature is not a controlling parameter for hybrid engine performance.

As a concluding remark, the main identified controlling parameters for hybrid thrust chambers concerning liquid injection are the droplet size of the injected spray and the area over which the injection is realized. Nevertheless, it has to be kept in mind that the rather low performance values are hardly influenced by the liquid injection, but is an inherent problem for hybrid combustion also present in case of gaseous oxidisers. Attempting to increase the performance could lead to concepts such as multi-port grains where the interaction surface between oxidiser and fuel is largely increased.

The Rocflam-II code has been validated on test data in this case. As far as liquid oxygen is concerned, no reliable test measurements are available, which could be used to validate and verify the Rocflam-II code. It can nevertheless be stated that the simulation results for liquid oxidiser look reasonable.

As a possible next step and outlook, it would be desirable to enlarge the validation base by a test case using liquid oxidisers to close this gap.

Turbulent effect and scaling up

Technical solutions

A first part presents technical solutions and technologies regarding hybrid propulsion systems. One studied the latest articles and books related to this issue and selected the most interesting technical solutions and technologies given by the authors, in order to accomplish the goal of the project. The presentation is structured for future data base development; so, for each important task of the report are presented all the solutions that were taken into account in the above publications.

An extended literature survey covering mainly the late 20 years has been performed on the subject of hybrid rocket engines oxidiser-fuel pairs, their performances and types of materials used. Based on the experimental and theoretical investigations into main operating processes in hybrid engines reveal the feasibility of constructing high power and top operating characteristics HRE capable of deploying large payloads.

Injectors position and maximum oxidiser mass flow rate

Important results regarding the influence of injector shape and configuration are shown. Several types of injectors are studied and graphs with the influence on the regression speed are presented. A serie of firing tests was carried out to investigate the influence of the oxidiser injection on the solid fuel regression rate behavior in a hybrid rocket engine. For this purpose, a conical subsonic nozzle as the injector of the gaseous oxidiser was selected to generate non-uniform conditions at the entrance of the fuel port. Gaseous oxygen and polyethylene fuel cylindrical grains were used. When the oxygen was fed by this kind of injector, the fuel regression in the region of the oxygen impingement on the grain?s surface was increased several times, which led to irregular fuel consumption with the characteristic after-burn port shape typical of solid fuel ramjets having a rearward-facing step at the air inlet.

Comparisons between axial and radial injectors are shown with performances outlined for each type of injector.

Regression rate and scalability

Several results regarding regression speed in hybrid rocket motors are presented. Two fits of regression speed as a function of G are presented and a maximum saturation value for G is suggested to exist based on the study of those fits. Regression speed results for paraffin/GOx are presented (Stanford research work). Tables with detailed experimental data are presented showing relations between various experimental parameters of the test motors used.

Database

HYBRID database is a data structure oriented for hybrid rocket engine:

1. Browsing and systematisation of a large number of papers in the field of hybrid rocket engine and structuring this information in the preliminary Technical Notes
2. Designing and building a relational database and its elements
3. Decomposition of the simple elements (pictures, tables, relationships, papers, comments), removing duplicates
4. Definition of relevant keywords, and cross-checking structures
5. Defining query structures
5. Definition of reports of 'unique item' and global reports.

As a result, a portable, medium-sized database (about 160 MB) was built. It is structured with five tables that cover more than 500 entries gathering 150 Papers, 133 Pictures, 176 Comments, 40 Tables.

Turbulent Effects

To enhance knowledge in scaling up by database building and determination of key parameters involved in scale increase, a specific focus has been done on the influence of turbulence as a controlling parameter. To achieve this goal, a numerical model is used to extrapolate the experimental and theoretical results in the frame of upper scales. This model was calibrated by using experimental data. In this purpose, a numerical code considering Large Eddy Simulation (LES) of stabilized premixed and partially premixed combustion is built. A LES numerical algorithm allows handling the entire range of combustion regimes and equivalence ratios approach is developed for this purpose. The literature review of turbulence modelling is detailed in the last report of UPoB.

To summarise, the work performed is shared as follows:

1. a database, initiated since the beginning of ORPHEE, was built. It contains major information regarding hybrid engine, main results and experiments
2. an improvement of hybrid calculus model with the use of turbulence to understand its effects
3. study of the fundamental aspects related to the RANS modelling. This development is relatively extensive because in ORPHEE project and, generally in industrial applications, RANS modelling is the preferred alternative due to simplicity and reasonable computational effort
4. development of two original RANS models for Low Reynolds Number flows, which are validated using experimental and numerical results available in literature
5. theoretical and numerical foundations of LES modeling for combustion problems
6. a detailed analysis of these models for hybrid combustion problems
7. the effect of the scalability is then studied thanks to an original simplified calculus model - internal hybrid ballistic model - applied to similar hybrid engines at different scales.

Concerning the proposed RANS models (original for some of them), a specific focus is done on the velocity and thermal fields. The interest of the Low Reynolds number turbulence models is that they can simulate simultaneously the complete developed turbulence flows and the near wall flows. By introducing the adapted time turbulent scales to each region present in a turbulent flow, it is possible to obtain an amended expression of the apparent turbulent viscosity and of the turbulent dissipation equation.

To deduce the damping functions and the closure constants of the model, one uses the asymptotic behavior of variables resulted on the analysis of the experimental and of the Direct Navier-Stokes simulations (DNS) results. The validation of the model was performed through a comparison of the results with DNS and experimental results available in literature. The prediction results, by the present turbulence model, are found very accurate with reference data.

The detailed analysis of turbulence was then carried out through six test cases:

1. Case one: Simulation of turbulent combustion problems. Since all RANS models assume turbulence in equilibrium, a PDF combustion model was chosen which is compatible with flow thermodynamic equilibrium
2. Case two: Simulation of a 2D hybrid combustion of HTPB with oxygen; a finite-rate/EDC chemistry-turbulence interaction model versus the PDF-approach
3. Case three: two-dimensional (2D) hybrid combustion of polypropylene with nitrous oxide; kinetics is modelled by a four chemical reactions mechanism representative of propylene combustion
4. Case four: This test is related to the classical type of hybrid rocket engine; Single-step propylene kinetic mechanism
5. Case five: Similar to case 4 but the oxidant is fed through a narrow pipe (injector) in the pre-chamber; Single-step propylene kinetic mechanism is assumed
6. Case six: LES simulations of 3D hybrid combustion of HTPB with oxygen in geometry similar to that considered in first three cases.

Without considering details of the simulation of each test case, the main conclusions drawn from this study are:

1. Despite its undisputable practical usefulness for industrial computations, RANS modelling is not the state-of-the-art technique for quantitative predictions of turbulent reacting flows
2. Stationary methods cannot address a number of crucial issues in turbulent combustion, such as ignition, extinction, and combustion instabilities
3. Turbulence greatly enhances the mixing process by increasing the surface area of the thin mixing layers where most of the molecular diffusion occurs.
4. The interaction between turbulent mixing and combustion chemistry is extremely complex and remains an active research area. When the two SST simulations, one with PDF and the other with finite reactions, are compared we notice that the general trend is similar but the fine details advocate in the favour of finite reaction simulation. The most noticeable difference is that PDF model under-predicts temperature profiles values and over-predicts the height of the flame front above the fuel slab (the locus of the maximum temperature.
5. Turbulent non-premixed flames contain a wide range of length scales. For a given flame geometry, the largest scales of turbulence are determined by the overall width of the hardware that contain the flow. Therefore, the largest scales of turbulent motion are typically independent of Reynolds number. As the Reynolds number increases, the turbulent fluctuations in the velocity and mixture fraction cascade go down to the progressively smaller eddies, increasing the dynamic range of the length scales.
6. LES solvers, once devoted to academic configurations, can now handle the complex geometries and moving parts found in industrial applications. Some details are presented bellow.

Computations have been performed on two grids with different resolutions (4,276,275 and 7,166,470 nodes) and constant step in the y-direction and variable step in the other two directions (x and z). A null velocity and 900 K temperature are the initial conditions applied in every cell of the computational domain.

This simulation showed the formation of a variety of large turbulent structures right above the fuel grain that transport energy, mass, and momentum to the main flow. Even from the beginning of the cross flow, the flame front is anchored at the fuel grain leading edge. From this configuration, it is also observed large variations of velocity (1-30 m/s), of temperature (900-3600 K, of O2 and CO2 mass fractions (0.05-0.95). The main cause is the development of a high vorticity magnitude patch above the fuel slab that extends towards middle channel.

Based on this preliminary simulation, a 3D-LES calculation was performed to study the development of the flame front and, in particular, the appearance of vortex filaments that spread the flame downstream in the cross flow direction.

Performance model

The aim of this task is to gather and synthesise the different modelling aspects involved in the physical phenomena occurring in the hybrid engine operating. The proposed global performance model is based on a one-dimensional unsteady approach for the reactive flow description. A preliminary phase consisted in developing an approximate Riemann solver before introducing hybrid combustion specific modelling.

Oxidiser can be injected into the combustion chamber either on gaseous or liquid state. In the later configuration, a Lagrangian approach is integrated in the one-dimensional digital code. The adopted two phase flow modelling is based on a standard 'd2' law for the evolution of the oxidiser particles diameter. An application of this code was performed in the purpose of validation of the new module. The direct comparison with a previous calculation in a gaseous case proved the correct integration of the new functionality.

A second step was achieved by simulating an experimental firing from the University of Naples. As it can be seen on next figure, the pressure history allows fitting the experimental data at the end of the test. The raison of the difference at the beginning is mainly due to the modelling of the regression rate for which no dependence of bore diameter is considered. However, it is to be noted that the 1D approach shows a pressure gradient along the engine axis. This behaviour is also characteristic of the firing even if the pressures levels and time of presence of the phenomena are overestimated by the numerical results.

A minor drawback of the one-dimensional approach has been identified: some pressure jumps appear at the geometry brutal variations (for example in premixing and post-combustion chambers). It seems that an assumption of a continuous geometry does not dramatically change the results.

Preliminary calculations have also shown the importance of particles diameter and injection velocity to define the zone where they vaporize. Nevertheless, if the vaporisation is completed in the pre-combustion chamber, the variation of droplets size or velocity does not affect the results on the performance.

Depending on the geometry, an instability phenomenon based on the ratio of oxidiser mass to the fuel mass has been demonstrated. Its origin seems to be related with the possibility of fuel gases to enter into the premixing chamber and hence generates a local increase of pressure. All these aspects have to be studies in detail in a scaling up design process.

Finally, this digital tool can be considered as operational for preliminary design applications.

Road-map and hybrid engine demonstrator designs including the definition of demonstrators' requirements

Basic requirements for demonstrators

Based on market survey and ORPHEE results, the requirements for two demonstrator design (booster and lander) have been updated.

Booster demonstrators

A demonstrator for a high thrust hybrid rocket engine for booster application has been designed. Based on system level requirements and by taking into account P8 (DLR test bench) constraints, a scaling analysis has been performed and design requirements have been defined for the design of a scale booster demonstrator combustion chamber. The design work was shared among the project partners AVIO, and AST/G. The document comprises justifications for the mechanical strength of the individual parts of concern, the selection of the injector concept and the expected pressure drop of the LOx injector. A preliminary CFD study has been performed to assess the pressure oscillations in the post-combustion chamber.

Lander demonstrators

A demonstrator for a hybrid rocket engine with a throttleable pintle injector has been designed. Based on system level requirements, design requirements have been defined for the conceptual design of a full scale lander demonstrator combustion chamber. The design work was shared among the project partners AVIO and AST/G. The document comprises justifications for the mechanical strength of the individual parts of concern, the selection of the injector concept and the expected pressure drop of the LOx injector. A preliminary computational fluid dynamics (CFD) study has been performed to assess the pressure oscillations in the post-combustion chamber.

Demonstrators diagnostics and bench needs

For the development of demonstrators, we need bench tests system and methodologies of diagnostics.

Several features of the selected test bench needed for hybrid demonstrator engine test programs are needed, namely oxidiser supply system, igniter supply system, measurement Command and Control system (MCC), thrust measurement device, low pressure environment simulation system (as needed for lander engine tests), permission of the authorities for testing the selected propellants.

The first three points (supply and MCC systems) are already available at the selected test bench. However, the last three points are of a more or less hypothetic nature: Whereas for the low pressure environment simulation system at least detailed analyses, drawings and a procurement planning (but no hardware yet) exist (CDR level), no more work than shown in this deliverable has been done for the thrust measurement device and no hybrid fuel test permission application has been prepared for the authorities responsible for the selected test bench.

A hybrid combustor is the combination of a solid fuel which is stationary stored in the combustion chamber and of a gaseous or liquid oxidiser pumped through a cylindrical hole within the mentioned solid propellant. Several measurement techniques applied on earlier engine and motor development programs have been chosen. Four classes of probing can be distinguished:

1. investigations of sound and pressure waves in order to characterize stress and pressure induced phenomena
2. optical measurements for the visualisation and characterisation of the plume, the combustion process itself, and the heat distribution in- and outside the chamber
3. ultrasonic sensors for the time dependent determination of the reducing thickness of the grain of the hybrid combustor during the hot run, and
4. a sampling system for gaseous stable intermediate combustion products as well as for solid particles in the pre- and post combustion chamber for later analysis.

The above mentioned observations of sound, electromagnetic wave emissions, the grain thickness and the chemical composition of the hot gas can help to monitor the burn-off and/or to promote further developments on the basis of the understanding of physical and chemical processes that take place within the hybrid combustion chamber.

Road map

Hybrid technologies consideration

The panel of hybrid technologies is large and can be roughly divided in two main categories:

1. Standard technologies are the technologies that are already used for some applications (ex: Space Ship 1). These technologies have a high TRL (TRL around 6), but the related performance and structural index are limited with regard to main applications.
2. Advanced technologies are the technology that allows to extend the range of performance of hybrid applications, for example motor design considered in ORPHEE, and to have competitive performance with other propulsion technologies. These technologies do not have a sufficient TRL to engage a development (TRL lower than 6).

In relation with the R&T scope of FP7, the ORPHEE roadmap is devoted to advanced technologies.

This roadmap presents the rationale of TRL increase for each target application considered, with technological work and schedule to mitigate risks, up to a level necessary to engage a product development (TRL 6).

Input data for the roadmap

The technology roadmap is based on the following preliminary designs:

1. Booster motor for launcher [ AVIO (AST/G, SME) ]
2. Lander motor [ AST/G (AVIO, SME) ].

The upper stage motor is not treated as a specific design, because main design topics are considered to be covered by the two other cases.

Nevertheless in the roadmap area, the stop-restart capability is identified as a specific roadmap issue, in order to precise the specific tasks necessary to mature this capability.

Structure of the roadmap The technology roadmap has been structured in several topics, in order to define more precisely needs and rationale necessary for each technology, in a 'bottom-up' approach.

On the basis of detailed sheets provided by each contributor, roadmap synthesis has been established for booster and lander applications.

Rationale of TRL increase

For each target application, the main driver of TRL increase is to have a stepped approach with main milestones validated by specific demonstration(s):

1. Separated subassembly demonstrations for first levels (TRL around 3)
2. Breadboard demonstrator(s) for a first validation of integration of technologies, at reduced scale or at scale 1 depending on final application (TRL 4 to 5)
3. Product demonstrator(s) or prototype(s) for the validation of the complete performance, at representative scale or scale 1 if possible (TRL 6).

For booster and lander application, breadboard demonstrator and scale 1 designs have been defined within the ORPHEE WPs.

Potential impact:

Through the impacts included in the general policy context as defined in the global work programme:

1. Sustainable development
2. Common Foreign and Security Policy
3. Lisbon Strategy.

Regarding the specific area 'Space technology', the project ORPHEE fully agrees with:

1. a progress towards the sustainable provision of technologies needed by the European Space to become non dependant
2. the consolidation of long term sustainability
3. the improvement of the economical aspects of a domain known to be demanding in terms of reliability, experimenting novel techniques (hybrid propulsion system) and methodologies (new materials manufacturing).

Hybrid propulsion technology offers a wide range of beneficial properties that have a direct impact on improved competitiveness:

1. technology readiness: based on standard solid propulsion and liquid propulsion implying large experience in risk management
2. improved safety during manufacturing, testing, handling, storage and operation
3. higher propulsive performances than solids and similar to the liquid technologies (i.e. LOx / kerosene)
4. high versatility during operation by a large thrust modulation ratio
5. grain design is key feature to optimise the overall performance (better structural ratio and as consequence lower volume)
6. operability increase and reduced servicing requirements
7. costs reduction: benefits of solid and liquid techniques, using of solid fuel subassembly which implies lower cost than full liquid solution technology.

The European space industry constitutes a major research, development and manufacturing sector, worldwide known. The space industry R&T in Europe represents a challenge for the next decades, aiming to maintain the know-how at the highest level, and confirming Europe as a major actor.

From decades, European space industry & research entities have been regularly working in close cooperation for the successful study, development and industrialisation of both Ariane IV and Ariane V launchers. As a consequence, European propulsion community has gained credibility and competitiveness.

But, to maintain this status, European propulsion community must face tremendous challenges in a short and midterm:

1. to maintain high scientific; technologic and industrial skills
2. to maintain scientific and R&D heavy installations, required for the development of new engine technology with the needed safety approach
3. to develop new methods and techniques, keeping competitive advantages regarding new comer countries in the space community
4. to offer high performance versatility to space transportation vehicles.

In that context ORPHEE has given to the European propulsion community the opportunity to tackle most of those challenges thanks to hybrid propulsion introduction for space application.

Besides the economic benefits, ORPHEE may have an impact on the societal interaction of space transportation industry within Europe, and thus on its sustainable development.

Employment aspects

For a sustainable employment in Europe the most important factor is global competitiveness within the framework of Space Transportation domain which requests high levels of safety, high qualified workers and costly infrastructures.

Space transportation industry is a mature industry. Due to publication spreading, knowledge becomes more and more available in low-wage countries. Employment can only be generated and preserved in Europe when production is cost-effective by making methods of process intensification available and reducing secondary costs significantly (infrastructural costs for safety, transport, environmental protection, etc.) by applying new technologies or methodologies. Moreover, by implementing this new Hybrid Engine technology, developed in USA, Israël?, new skilled and well educated employees will be requested in the European space industry, as well as in those branches that develop and provide the required technical equipment.

Safety aspects

The hybrid technology increases the safety issues about the chemistry process on the fuel side by concerning:

1. the synthesis routes on hydrides (aluminium; magnesium base) as additives for regression rates increase and specific impulse performances improvement
2. the formulation and manufacturing process of the fuel including additives.

Environmental aspects

Developing environmental aspects about the propellants chemistry (synthesis & formulation) requires the use of partly environmental hazardous materials. Consequently, enormous efforts exist in the European chemical industry to avoid the release of hazardous materials into the environment. Waste streams are disposed, recycled, or processed within closed loops to ensure safe and environmental friendly operations. Running chemical processes more efficient in terms of yield and selectivity will automatically reduce the amount of process waste.

Main dissemination activities and exploitation of results

Two major dissemination activities have been chosen by the partners:

1. Conferences: the different partners have presented the progress of the work during conferences. Twenty one main presentations have been done during the project.
2. A dedicated web site is available since the first year (see http://www.orphee-fp7-space.eu)

Initially a workshop was considered. In accordance with the REA, it has been decided to participate to one of the major conference in the world for the space activities the Space Conference 2012. Five ORPHEE's papers have been selected by the organisers, namely

1. University of Naples Federico II - Lab-scale Motor Firing Results on Advanced Solid Fuels for Hybrid Propulsion
2. Politecnico di Milano - Labscale Characterisation of Solid Fuels for Hybrid Propulsion
3. Astrium Space Transportation - Technology Demonstrators for Hybrid Rocket Propulsion in Europe,
4. Onera - Fuel combustion modelling in hybrid engine
5. SME (Safran Group) - An updated roadmap for hybrid propulsion.

These presentations will be part of the four sessions dedicated to hybrid propulsion. Thus, the ORPHEE project will be presented to a large community, aiming to initiate future cooperation.

The ORPHEE team has also proposed one of the EC experts on the project and four team members to chair one of the four sessions dealing with hybrid propulsion at this conference.

List of websites:

http://www.orphee-fp7-space.eu/

Contact: Philippe GAUTIER - Herakles (since May 2012 SME SA became Herakles) - 9 rue Lavoisier 91710 Vert Le Petit - France +33-164-991137, email: ph.gautier@safran-sme.fr (ph.gautier@herakles.com after September 2012).