Final Report Summary - RECREATE (REsearch on a CRuiser Enabled Air Transport Environment)
Executive Summary:
“I’m in the simulated cockpit of an airliner, watching the displays as we close in on the tanker above us. By pushing a series of buttons, I have started the final, automated approach toward the refueling boom, through which 30,000 pounds of fuel will be pumped into the fuel tanks.” - These words in an Aerospace America article by Philip Butterworth-Hayes impressively summarize the highlights of 3.5 years of focussed research into the new cruiser-feeder concept of air transport, which has the verified potential to significantly reduce fuel consumption. The results laid out in this summary have been widely presented in papers to the scientific aerospace community and via the media to the general public.
Starting out with the earlier assessments of the benefits of cruiser-feeder operations which had pointed out the aerial refuelling concept as a promising concept, the REsearch on a CRuiser Enabled Air Transport Environment (RECREATE) project has been able to show in a consolidated, congruent manner that civil air-to-air refuelling (AAR) can be implemented in an airworthy and economically and ecologically beneficial way. A number of research methodologies coming from different fields and institutes were employed to cover the basic questions about the feasibility of what could be a complete renewal of the existing air transport system. The research project conducted by nine European research institutes, universities and small business partners successfully concluded in January 2015.
Due to the complexity of analysing a whole transport system and based on the choices and assumptions made for a comparison, only a range for the fuel reduction can be given. The conservative, aircraft-design driven and bottom-up derivation amounts to 11% fuel reduction, including also the fuel used by the tanker aircraft. The statistical, top-down specification shows an upper range of 23% fuel reduction. Implementing AAR on a large scale will enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports. This in turn also leads to local economic consequences.
Besides fuel reductions, there are also similar advantages of weight reductions. For a word-wide system, we can think in terms of a series of “optimum” cruisers with different passenger capacities (e.g. 200 to 300) and range capabilities (e.g. 2000 – 3500 nm). With AAR, the benefits increase as the total (refuelled) range increases. This will allow a greater flexibility to cope with “thin” and “thick” routes around the World.
From a design point of view, the conventional military refuelling configuration would be replaced by a forward-swept boom configuration which has been shown to be the most promising option, for safety, economical and passenger comfort reasons. Further the civil tankers need to be fairly small, capable of refuelling 2 or 3 cruisers. The tanker radius is between 500 - 1000 nm (2 - 2.5 hours of flight). So a civil tanker is very different from a military tanker which is usually designed for long endurance and multiple roles.
Future research required for making civil AAR a reality should include the conception of realistic business cases for air transport organisations, aircraft fuel providers and other stakeholders. Regarding airworthiness, a roadmap for civil certification has been developed. Technical research should focus on the design of automated flight control systems and on the airworthiness of the forward swept boom configuration, including ground and flight test demonstrations.
Project Context and Objectives:
Introduction
Current forecasts estimate a growth above 4% in worldwide air traffic per year for years to come. It is widely agreed that an equivalent rise of fuel consumption and CO2 emissions in aviation will not be acceptable. Current aircraft and propulsion technology developments as well as major step-change contributions to fuel burn reduction are required for the second half of this century. A major step may come from breakthrough technology development or a radical change in operations. One radical change in operations to alleviate the rise in fuel consumption and CO2 emissions are cruiser-feeder operations.
The definition of the cruiser-feeder concept states that the payload - passengers and/or cargo - is transported for the largest part of the way by one transport aircraft, called the cruiser. During flight, a mid-air contact with another aircraft is initiated. This second aircraft, the feeder, either joins the cruiser physically for a large part of the journey, or couples for a limited time to exchange passenger, cargo and/or fuel. Such a contact can take place one or several times during one journey. An obvious special case of cruiser-feeder operations, air-to-air refuelling (AAR), has been studied in the past. Estimations of the attainable reduction in fuel burn was shown in research preceding this project. These estimations suggested further in-depth research to underpin the data with refined analysis.
The European Union 7th Framework Programme funded project REsearch on a CRuiser Enabled Air Transport Environment (RECREATE) has been conducted by the following nine European research institutes, universities and small business partners:
• Nationaal Lucht- en Ruimtevaartlaborato¬rium NLR (The Netherlands, RECREATE coordinator),
• Deutsches Zentrum für Luft - und Raumfahrt DLR (Germany),
• Totalförsvarets Forskningsinstitut FOI (Swe¬den),
• Technische Universität München TUM (Ger¬many),
• Delft University of Technology DUT (The Netherlands),
• The Queen's University of Belfast QUB (United Kingdom),
• Zurich University of Applied Sciences ZHAW (Switzerland),
• Dr Rajendar Kumar Nangia (United King¬dom),
• Nuclear Research and Consultancy Group NRG (The Netherlands).
From 01 August 2011 until 31 January 2015, the feasibility of cruiser-feeder operation concepts has been studied, aiming at assessing the implications of a recreation of air transport operations.
Top level objective
The top level objective of the RECREATE research is to demonstrate on a preliminary design level that the cruiser-feeder concept can comply with airworthiness requirements for civil aircraft. For convenience of the reader, a generic and non-process orientated definition of airworthiness is: the ability of an aircraft or other airborne equipment or system to operate without significant hazard to aircrew, ground crew, passengers (where relevant) or to the general public over which such airborne systems are flown. A clear perspective of compliance with airworthiness requirements is a conditio sine qua non for further research and development of this out-of-the-box concept. The subsequent Scientific and Technological (S&T) objectives are:
1. to substantiate on a conceptual and preliminary design level that viable and acceptable concepts exist for cruiser-feeder operations;
2. to identify and qualify the necessary procedures and steps and required facilities to assure airworthiness of cruiser-feeder operations;
3. to confirm that the reported benefits of cruiser-feeder operations are consistent with the refined analysis and high fidelity simulation.
To achieve the S&T objectives, a collaborative research effort has been conducted with respect to:
• preliminary aircraft design dedicated to cruiser-feeder aircraft,
• the means of compliance to satisfy airworthiness requirements enabling cruiser-feeder aircraft,
• automatic flight control and flight simulation dedicated to cruiser-feeder aircraft.
Scientific and technological methodology
The scientific and technological methodology chosen for the RECREATE research consists of three integrating and three disciplinary research activities. The three integrated research activities and the three disciplinary work packages supporting the integrated work packages are listed below. An evolutionary delivery approach has been adopted, with two iterates (initial and final) for the disciplinary activities and three iterates (initial, updated and final) for the integrating activities.
Objectives of WP1: Concept for cruiser-feeder operations
First, a baseline concept of operations is established, the baseline being a conventional air transport without cruiser-feeder operations. Starting from this baseline, a cruiser-feeder concept of operations is defined, analysed and used to generate system requirements and other requirements, for instance the Top Level Aircraft Requirements.
Objectives of WP2: Airworthiness of cruiser-feeder operations
Civil aircraft adapted for use in military air-to-air refuelling operations are airworthy, providing a starting point for research on airworthiness of future cruiser-feeder operations. Applicable regulations and user requirements are studied and derived, and all potentially necessary means of compliance are being identified. A high level Functional Hazard and System Safety Assessment for cruiser-feeder operations is being made, finally leading to safety and airworthiness validation means of compliance with airworthiness requirements that are applicable to cruiser-feeder operations.
Objectives of WP3: Benefits of cruiser-feeder operations
Starting with the analysis of the chosen baseline and based on data generated in the other RECREATE work packages, environmental, dispatch reliability and economic delta analyses are conducted. The investigation of these potential benefits is essential for any future consideration of this operations concept.
Objectives of WP4: Conceptual and preliminary design
The requirements needed for a viable and acceptable airworthy concept will likely lead to new aircraft designs. For this reason, conceptual aircraft design is required for both the feeder, cruiser and aerial transfer boom system. This conceptual design is refined in a subsequent preliminary aircraft design phase. For the preliminary aircraft designs, an aerodynamic database on sizing, performance and stability & control will be created on the preliminary design level.
Objectives of WP5: Automatic flight control
Cruiser-feeder operations need to be highly automated if they are to function globally. The design of an automatic flight control system is important as well as understanding requirements and limitations of the system. In cruiser-feeder operations three bodies are involved, feeder aircraft, transfer boom and cruiser aircraft, which are to be controlled as one connected system. This is a challenge especially in all weather conditions. To understand the requirements of the distributed control system, models accounting for turbulence and elasticity are needed. A conceptual and preliminary design is made of a multi body automatic flight control system and a human-machine interface for the pilots of the cruiser and the feeder aircraft.
Objectives of WP6: Flight simulation
In order to develop a multi body cooperative automatic flight control system, flight simulation models are implemented to identify critical interactions, such as for instance due to atmospheric gust or wake-vortex interaction of the leading aircraft. Finally, flight simulation models are used for the identification of human factors. This is important in order to verify that the proposed manoeuvres are acceptable regarding safety and pilot workload.
Project Results:
“I’m in the simulated cockpit of an airliner, watching the displays as we close in on the tanker above us. By pushing a series of buttons, I have started the final, automated approach toward the refueling boom, through which 30,000 pounds of fuel will be pumped into the fuel tanks.” - These words in an Aerospace America article by Philip Butterworth-Hayes impressively summarize the highlights of 3.5 years of focussed research into this new concept of air transport, which has the verified potential to significantly reduce fuel consumption. The results laid out in this summary have been widely presented in papers to the scientific aerospace community and via the media to the general public (please also refer to the dissemination activities summarized below).
Starting out with the earlier assessments of the benefits of cruiser-feeder operations which had pointed out the aerial refuelling concept as a promising concept, the RECREATE project has been able to show in a consolidated, congruent manner that civil air-to-air refuelling can be implemented in an airworthy and economically and ecologically beneficial way. A number of research methodologies coming from different fields and institutes were employed to cover the basic questions about the feasibility of what could be a complete renewal of the existing air transport system.
Due to the complexity of analysing a whole transport system and based on the choices and assumptions made for a comparison, only a range for the fuel reduction can be given. The conservative, aircraft-design driven and bottom-up derivation amounts to 11% fuel reduction, including also the fuel used by the tanker aircraft. The statistical, top-down specification shows an upper range of 23% fuel reduction. Implementing AAR on a large scale will enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports. This in turn also leads to local economic consequences.
Besides fuel reductions, there are also similar advantages of weight reductions. For a word-wide system, we can think in terms of a series of “optimum” cruisers with different passenger capacities (e.g. 200 to 300) and range capabilities (e.g. 2000 – 3500 nm). With AAR, the benefits increase as the total (refuelled) range increases. This will allow a greater flexibility to cope with “thin” and “thick” routes around the World.
From a design point of view, the conventional military refuelling configuration would be replaced by a forward-swept boom configuration which has been shown to be the most promising option, for safety, economical and passenger comfort reasons. Further the civil tankers need to be fairly small, capable of refuelling 2 or 3 cruisers. The tanker radius is between 500 - 1000 nm (2 - 2.5 hours of flight). So a civil tanker is very different from a military tanker which is usually designed for long endurance and multiple roles.
Future research required for making civil AAR a reality should include the conception of realistic business cases for air transport organisations, aircraft fuel providers and other stakeholders. Regarding airworthiness, a roadmap for civil certification has been developed. Technical research should focus on the design of automated flight control systems and on the airworthiness of the forward swept boom configuration, including ground and flight test demonstrations.
In the following part, the research results are presented per work package in more detail. The individual results have also been presented during the final meeting in Amsterdam on 29 January 2015. These presentations have been added in Appendices A through G. All research results have been reported in the corresponding WP reports (24 deliverables).
WP1: Concept for cruiser-feeder operations
The objective of this integrating activity has been to de¬velop, iterate, select and describe two cruiser-feeder concepts, for further study. One chosen concept is realizable in the distant future (at least 50+ years from now), the other concept however is expected to be realizable in the medium-term (within 20+ years from now). This concept requires development and acceptance of new airworthiness regulations, but can be done with today’s technology.
First, a conventional baseline without cruiser-feeder operation has been defined to compare the benefits of future concepts with today’s technology, see Table 1:
Table 1: Baseline concept
Cruiser
Capacity 250 passengers
Range 5000 nm
Specific fuel con-sumption 0.525
Given the broad definition of cruiser-feeder opera¬tions, iterations conducted at the start of this activity involved a large diverging number of cruiser-feeder concepts. In a subsequent convergence sweep, two concepts were down selected for further investigation. The first final concept is civil air-to-air refuel¬ling, of which the overall characteristics are shown in Table 2:
Table 2: First final concept: Air-to-air refuelling as special case of cruiser-feeder operations
Cruiser Feeder
Capacity 250 pas¬sengers Fuel offload capacity 35000 lb / 3 con¬tacts
Range 2500-3000 nm Range / en¬durance 500 nm / ~4 hours
MTOW 100000 kg AAR enve¬lope < 24000 ft, Mach < 0.8
Specific fuel con-sumption 0.525 AAR proce¬dure 20 min
(5 min wet contact)
Concepts with transfer of payload and passengers based on engines burning chemical fuel have been shown not be economically feasible, as the overall weight of the system and thus the total amount of fuel burnt are too high. However, if the cruiser can be propelled by a nuclear power source, the efficiency is very high compared to the reference case, even if the total weight of the system is higher. Although the nuclear cruiser cannot be shown to meet airworthi¬ness requirements with today’s technology, this con¬cept has been retained for study because it cannot be excluded that new nuclear physics will be discovered and confirmed in the future. The second final concept (Table 3) concerns a nuclear pro¬pelled cruiser where the transfer of passengers and cargo is done via a life supporting container mecha¬nism.
Table 3: Second final concept: cruiser-feeder with nuclear propelled cruiser
Cruiser
Capacity 1000 passengers
MTOW 900000 kg
Range / endurance 60000 nm / 1 week
Cruise speed M = 0.8
Docking speed M = 0.7
Cruise altitude > 36000 ft
Payload transfer Single container station concept (100 passengers each)
The two studied concepts address long-distance air travel but with very different techniques. Table 4 shows the level of detail in which both concepts have been studied during the project.
Table 4: Level of aircraft design applied to two concepts
WP AAR Orbiting Nuclear Cruiser
WP 1 Concept X X
WP2 Airworthiness X (x) To some extent
WP3 Benefits X (x) To some extent
WP4 Design Concept Design and
Preliminary Design Concept Design only
WP5 Control X (conventional AAR) -
WP6 Simulation X (conventional AAR) -
WP7 Dissemination X X
The AAR concept, as outlined in the WP1 reports, is clearly within reach for aviation if the industry and concerned legal bodies decide to go into this direction. As for the orbiting nuclear concept it is not within reach anytime soon but the concept, as such, has great potential.
AAR can play an important role dealing with the sustainability challenge aviation faces. If the final operational cruiser-feeder concept presented in this report is introduced on today´s system as it is, it has the potential of bringing large benefits:
Reduce fuel burn and direct CO2 emission by 10-20%, depending on the assumptions made for the analysis.
Local environment – Noise and local air quality will be better or on the same levels as today.
The number of movements and aircraft will increase; however, the total mass of the system will be significantly lower. Operational constraints on the system with present traffic load, such as scheduling, workload on feeder bases and impact from weather hazards (mainly turbulence), seem manageable. Figure 1 gives a visual impression of the performed traffic simulations of the entire traffic taking place today within 48 hours.
It has previously been outlined that aviation faces the dilemma that short flights which are the most common are the cause of the congestions whereas long flights burn most of the fuel and hence have the largest environmental impact. With continued urbanisation and more megacities on earth, the use of air transport should be reserved for long distance travel where no other viable options exist. Following the Intergovernmental Panel on Climate Change (IPCC), high speed rail can substitute short-distance air travel up to 800 km and in some case even up to 1500 km (a good example today is Beijing – Shanghai). This is one clear way of mitigating greenhouse emissions from air travel and also alleviate noise and air pollution problems which many of the world’s megacities face already today.
Apart from major fuel and weight savings on long flights, AAR has the potential to reduce the number of short flights since the smaller, more efficient, AAR-cruisers inherently give an opportunity to serve more point to point connections. To set-up a new intercontinental “ point-to-point connection” from an airport in a “mid-sized city” that today has no (or few) intercontinental connections, will of course be an easier business case with smaller efficient RECREATE cruisers as compared to the larger baseline cruiser. Every new long distance point to point connection will reduce “hub travel” to some degree. The potential and sensitivities going from a hub – spoke system to more point to point connections has been studied in WP3.
A complete removal of the hub-spoke system in the near term is not realistic, but for the future the system has to be pushed away from the big hub solution. As for technical development of air-to-air refuelling concept, other novel fuel transfer configurations with the tanker behind the cruiser (or non-centreline set-ups) can improve aircraft efficiency, system performance and safety. In WP4 some of these ideas have been exploited. Civil AAR should also not be viewed in isolation. A possible introduction of alternative fuels would also benefit AAR as well as other novel operational concepts like formation flying for civil aircraft which also could be used together with civil AAR.
WP2: Airworthiness of cruiser-feeder operations
The objective of this integrating activity has been to develop a route towards airworthiness of cruiser-feeder operations. Bringing two aircraft in close vicinity of one another in mid-air, either for docking or for re¬fuelling, is potentially dangerous. Today, no civil certification regulations concerning cruiser-feeder operations exist. However, the airworthiness of such a concept must be established along the guidelines of the current regulatory system of the safety of civil aircraft operation and with an equivalent level of safety.
Based on the concepts of operations described, a Functional Hazard and System Safety Analysis of the selected cruiser-feeder concepts has been performed to map all hazards not covered by current airworthiness regulations, for both cruiser-feeder concepts. Using the operational scenarios relevant for collision risk, performance, availability and integrity requirements for the functions of the air-to-air refuelling system have been determined. A schematic for this system modelling is shown in Figure 2.
These were input for the system design which has been performed in other RECREATE Work Packages. Simulations based on physical models of the air-to-air refuelling system have been performed in WP5 for the most critical collision risk scenarios to verify that the air-to-air refuelling system design meets the safety requirements. Non-collision risk related hazards of the receiver-tanker have been identified and safety measures to mitigate these hazards have been proposed.
Collision risk related hazards for the nuclear cruiser-shuttle concept are similar to those for the conventional fuel receiver-tanker concept. Non-collision risk related hazards of the nuclear propelled cruiser have been identified and safety measures to mitigate these hazards have been proposed.
As a second important step, an approach for the certification of air¬worthiness of civil cruiser-feeder operations has been de¬veloped taking into account existing military regula¬tions for aerial refuelling. An evaluation of the exist¬ing EASA Certification Specifications and Ac¬ceptable Means of Compliance for Large Aeroplanes (CS-25 Amendment 11) has been made, resulting in an overview of applicable specifications and specifi¬cations to be amended for cruiser-feeder operations. Following the evaluation, an approach to certifica¬tion has been proposed. Because of their similarity of operations this approach has been based on the regulations covering automated landing, the EASA CS-AWO (All Weather Operations). The similarity lies in the ob¬jective to achieve an accurate and safe approach to a specific position. The amended regulations and Ac¬ceptable Means of Compliance (CS-AWO AMC) cover the required safety level, the considered conditions re¬garding aircraft, operations and environment, the performance requirements, performance demonstra¬tion and failure conditions.
Simulations have been used to get quantified data for these models, for normal operation and for the most critical scenarios:
For wind shear or turbulence encounter the most critical situation is the station keeping position with the boom almost connected or just discon¬nected (boom very close to cruiser) and a sudden upward movement of the cruiser.
For system failures the most critical situation is the station keeping position and a sudden engine failure, with or without the boom almost con¬nected or just disconnected.
It has to be noted that real ground and flight testing to demonstrate the safe operation of nuclear propulsion is not justifiable due to the inherent risk of the release of radioactivity. Here, the development of high fidel¬ity simulation with a very high level of confidence far beyond the current state-of-the-art will be required.
Finally, a tentative future roadmap for certification of air-air refuelling of commercial air transport aircraft has been provided. The roadmap describes the steps that must be taken to formally establish regulations and acceptable means of compliance.
WP3: Benefits of cruiser-feeder operations
The objective of this integrating activity has been to analyse economic benefits, dispatch reliability and environ¬mental effects of cruiser-feeder operations using the defined baseline as reference. The impact on fuel consumption, exhaust emissions, local noise produc¬tion and shareholder value have been studied.
Benefits analyses of the updated concept indicate a range of fuel and mass savings for the cruiser aircraft for cruiser-feeder concepts in which only fuel is transferred (Fig. 3). Remark that it is a challenge for an opti¬mized conventional configuration cruiser to be as efficient as an aircraft optimized for long range, be¬cause of the relatively larger fuselage and fixed weight of some of the equipment, furnishings etc. On the conservative side, fuel savings for the cruiser only - derived bottom-up by aircraft design - amount to around 21% for one refuelling. With two refuellings, this value could be increased, and further investiga¬tions on the effect of thrust-to-weight ratio and re¬serve fuel are ongoing. From this benefit the fuel consumed by the tanker(s) has to be subtracted. If the tanker is very efficient with a fuel ratio - i.e. ratio fuel given / fuel consumed - equal to 8, the tanker reduces the total efficiency of the cruiser-feeder combination by about 5-6%. In case of a more realistic tanker with a fuel ratio of 4, the impact of the tanker is a reduc¬tion by about 10-12%. A lower bond of fuel saving is in the order of 11%, which is still a huge fuel saving compared to nowadays standards in the industry. On the optimistic side and based on a top-down, statisti¬cal approach, an upper bond of 23% fuel saving for the cruiser-feeder combination has been calculated previously.
The environmental impact next to the reduction of fuel burn and thus CO2 emissions has been studied as well. A consequence of smaller and lighter cruiser aircraft compared to today’s long range passenger aircraft is a reduction of noise at the departure/arrival and hub airports, which are mostly located in densely populated areas. A noise reduction in turn leads to economic benefits through reduced noise fees and a rise in property value near existing airports. The noise impact through the additional heavy load near the required tanker bases has not been investigated. The tanker bases are assumed to be located in sparsely populated areas, where noise is not of a great influence.
To examine the insertion of the cruiser-feeder concept into a representative air traffic scenario based on the Star Alliance fleet, route optimization is conducted to account for interactions between cruiser and feeder weights and efficiencies, locations for feeder bases, feeder radius of operation and cruiser route modifications. Feeders with higher efficiency achieve the best results in the traffic simulations at a feeding capacity of 2 or 3 refuelling, with operations conducted close to the feeder base. However, there is significant variability in these findings, with less optimal findings when considering those routes serviced on European-Asia routes in contrast to transatlantic routes. Small disruptions to refuelling operations can largely be absorbed due to inefficient use of feeder aircraft at a number of the feeder bases, but large scale unplanned disruptions can have a significant impact, resulting in multiple failed operations and severe impact on the network stability.
The increased ground traffic at both airport and refuelling bases raises challenges for dispatch reliability with increasing number of flights and the impact on local communities. As highlighted in the traffic simulations, dispatch reliability must now also account for the refuelling operation delay and the impact on ground handling capability of both minor disruptions requiring diversions of aircraft to alternative refuelling positions and accommodating additional aircraft landings in the event of major disruptions resulting in aborted operations.
AAR has been shown to be beneficial in terms of fuel reduction compared to non-AAR, even when considering a network based on more point-to-point connections, less on a hub-and-spoke system. Implementing AAR on a large scale will thus also enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports, with the related economic and en¬vironmental consequences.
Regarding economic benefits achievable in a cruiser-feeder configuration, evaluation of the configuration to include realistic route and demand analysis confirms that the economic viability of the concept is tied to the correct implementation of the underlying feeder network in order to maximise benefit. Transformation of the network to the cruiser-feeder enabled environment without consideration of available seat miles significantly reduces the profitability (and hence viability) of the concept, and in order to match previous revenue generation levels without passing additional costs onto passengers, additional aircraft are required which in turn reduces the environmental and cost benefits achievable over the baseline to approximately 14% and 12% respectively. However, improved scheduling of cruisers to match feeder availability may significantly enhance these cost benefits through enabling better utilisation of feeders capable of multiple refuelling exercises, and in particular, enable the adoption of new concepts such as the point to point network. It has been shown that with the use of AAR, the increased fuel burn associated with the additional routes serviced can be reduced significantly, and a significant increase in available seat miles within the network can be achieved without a corresponding proportional increase in operational cost.
WP4: Conceptual and preliminary design
The objective of this activity has been to conduct conceptual and preliminary design iterations of dedicated cruiser and feeder aircraft according to two the chosen concepts:
The aerial refuelling concept, where passenger aircraft (the cruisers) receive fuel from tanker (the feeders)
The passenger exchange concept, where one large dimensions aircraft with nuclear propulsion (the cruiser) exchanges passengers in flight, with various shuttle aircraft (feeders), by means of pre-loaded containers.
Families of cruiser and feeder aircraft are generated as well as refuelling boom designs. The generated design data is used in support of the benefits analyses, the air¬worthiness analyses and the auto¬matic flight control system development.
Cruiser and feeder aircraft specifically designed and optimised for their task have been shown to increase the benefits achievable by air-to-air refuelling. The gain results on the one hand from the fact that a long-range cruiser aircraft designed for a specific range including refuelling will have a reduced Operating Empty Weight (OEW) as it will be designed for a smaller Maximum Fuel Weight (MFW) and thus a smaller Maximum Take-Off Weight (MTOW). De¬sign studies of long-range passenger aircraft able of being refuelled have been performed with conceptual and preliminary design tools. The design work on aircraft and refuelling systems has demonstrated that aerial refuelling for civil aircraft is technically feasible. Indeed, it is standard in military operations. From an engineering point of view no showstopper has been identified. However the developments (refuelling system, tanker, engines) would require huge investments, drastically reducing the economic viability of the concept.
It has been shown that the tanker design has a huge impact on the overall efficiency. Current military tankers, however, are not optimized for the refuelling task but are converted cargo and passenger aircraft. Dedicated tanker design studies have shown the ad¬vantages of a joint-wing tanker concept.
The initial conceptual design phase lead to optimistic results compared to the following preliminary design work. This can be attributed mainly to the following factors:
In the conceptual sizing and design process constant empty mass fractions have been assumed, which are valid for long-range aircraft but not for the short/medium range refuelled aircraft. More realistic values deducted from the preliminary design work should be in the order of 0.5 to 0.6 for the refuelled designs.
Similarly, we assumed an L/D ratio of 19.9 equivalent to long-range aircraft. This assumption is invalid if the aircraft wing size is properly reduced for the more lightweight refuelled aircraft. This leads to a an increase of the relative importance of the fuselage drag. The preliminary design results indicate that, depending on the number of refuellings, the L/D ratio should be one to two points lower.
For flight control reasons the refuelling had to be scheduled at a reduced altitude. Each refuelling therefore requires a descent and climb segment which leads to an additional fuel consumption of about 0.5 to 1% per refuelling operation.
Therefore, the results of the more accurate preliminary design work are less optimistic and show lower reductions in fuel consumption for the cruiser aircraft than the initial estimations.
Based on all findings, we estimate the overall fuel savings for the cruiser alone to range between 15 and 20% of the block fuel. The alternate concept of staged flights would lead to fuel savings in the order of 10 to 15% but would need no tanker aircraft.
Adding the fuel consumption of the tanker aircraft reduces the fuel saving s for the aerial refuelling concept by about 5%, leading to overall savings between 10 and 15%.
A trade-off study showed that a non-conventional, inverted receiver-tanker configuration is beneficial and essentially the more viable configuration. Such a configuration increases the safety of the pas-senger aircraft from possible debris from the tanker or re¬fuelling boom after a collision, removes possible passenger discomfort due to flying in the tanker wake and limits the amount of extra pilot training required to the much smaller number of tanker pilots. Fur¬thermore, the passenger aircraft need a minimum of refurbishment for the new manoeuvres, and thus have a minimum loss of cruise efficiency, while the costly surplus thrust requirements and supporting equipment lay with the smaller number of tankers.
A number of different concepts for a forward ex¬tending boom have been studied. A big chal¬lenge lies in the controllability and aeroelastic stabil¬ity for this concept. Preliminary aeroelastic analysis results show that a design space free from static and dynamic aeroelastic instabilities exists.
Preliminary design with aerodynamic CFD computa¬tions for a refuelling boom has been done for the conventional con¬figuration. An aerodynamic database was created containing the results of about 800 computations for a number of relative boom position parameters and the control surfaces’ de¬flection angles. This preliminary aircraft design data has supported the development of an automated flight control system.
Several design studies have investigated the nuclear cruiser concept in cooperation with the Nuclear Research and Consul¬tancy Group NRG. A pressurized container exchange concept was adopted for the in-flight trans¬fer of passengers, crew, cargo and consumables, see Figure 5.
On a conceptual basis, weight estimations, aerody¬namic design and design of a nuclear propulsion system for the cruiser aircraft were performed; see Figure 6 for a conceptual design sketch. Op¬tions for a Brayton cycle and a Rankine cycle propul¬sion system were studied.
WP5: Automatic flight control
An early as¬sessment showed that successful cruiser-feeder operations will require automatic flight control systems for the cruiser air¬craft, the feeder aircraft and the refuelling boom to enable automatic conduction of the manoeuvre to achieve an adequate level of safety and availability. The objective of this activity was to develop this auto¬mated in-flight refuelling system, including sensor suites with redundant measurement technologies, actuators and controllers, and to model that system in a realistic simulation environment. The work had to consider, extend and comply with the airworthiness requirements defined in WP2 as well as the operational concepts elaborated in WP1.
It could be demonstrated that aerial refuelling of civil transport aircraft as one possible concept of cruiser-feeder operations is viable and safe. Since manual control is no option with respect to the high levels of safety and availability, an automatic flight control system was developed, comprising the following main components:
Cruiser-feeder operations and maneuver design: An approach maneuver was designed that accounts for operational and regulatory aspects. The envisaged approach trajectory starts at the minimum separation that is required for independently controlled flight and leads to a very close proximity in range of the fuel transfer boom. Tasks related to the automation system to be performed by the cruiser and feeder pilots were defined, which allowed a realistic evaluation of the automatic flight control system by piloted simulations conducted in the scope of work package 6. Requirements were put on the refuelling boom envelope and system design, which were used as basis for detailed boom design in the scope of WP4.
Sensor selection and sensor data fusion algorithms: Adequate sensors were selected for relative position estimation between the aircraft as well as between the boom tip and the receptacle. An exhaustive sensor suite was defined comprising inertial measurement units, code phase DGPS receiver, radio frequency ranging transponders and electro-optic sensors. The sensor suite along with the sophisticated sensor data fusion algorithms led to highly accurate measurements required for close formation flight with high availability and integrity, i.e. sensor faults are reliably detected and excluded, leading to high confidence in the estimated variables.
Guidance and control: Automatic flight control laws for the cruiser, feeder, and the boom were developed. A decoupled approach was chosen. The cruiser controller is based on the standard PID concept and tuned with emphasis on tracking performance. The developed refuelling boom controller is based on exact linearization of the constrained boom system where the boom hinge point motion is forced by the tanker motion. The combination of cruiser control laws for relative position control and boom control laws for engagement of the connection between the aircraft has proven functionality for the considered cruiser-feeder scenario.
Mode selection and phase transition: A finite state machine was developed and implemented that ensures a safe and predictable behavior of the coupled cruiser-feeder system during all phases relevant for a successful and safe conduction of automated aerial refuelling of civil transport aircraft.
Supervision: Algorithms were developed that monitor control and navigation performance as well as external disturbances. Based on geometric and stochastic parameters, these algorithms ensure a timely abort in case of system malfunctions or exceedance of admissible external disturbance levels.
All the afore mentioned components were implemented in an exhaustive simulation environment, also considering effects of the correlated wind turbulence field and aircraft elasticity. A screenshot of such a simulation is shown in Figure 7.
Using this simulation environment it could be demonstrated that aerial refuelling can be automatically conducted. For project RECREATE it had not only to be successfully demonstrated but also safety had to be proven according to the findings of WP2. It could be shown by simulation that a collision between the aircraft, which is the most safety-critical event, is less probable than specified in the safety requirements derived in WP2. For that, a special simulation method was applied (subset simulation) since conventional Monte Carlo simulation is not feasible for calculating the very low probabilities associated to the examined events. In general the sensitivity of the safety simulation results is the higher the lower the probability is. Especially for the case where a probability of 〖10〗^(-9) was proven only minor changes in the aircraft models might lead to considerable differences in the resulting thresholds. However, the objective of project RECREATE was to prove that close formation flight could be conducted safely according to civil certification specifications. Although the simulation models did not exactly represent a special type of aircraft, all contributing subsystems were simulated to a reasonable extent. This allows the conclusion that with similar configurations the considered safety requirements could be complied by using the control, navigation, and supervision algorithms developed, implemented, and tested in the scope this work package.
The knowledge gained in the scope of WP5 concerning feasibility, safety, and also the results of the piloted simulations conducted in the scope of WP6 based on the developed automatic flight control system provide a good baseline for possible next steps, especially the demonstration of automatic close formation flight by flight tests. Additionally, the introduced sensor suite, data fusion, and safety simulation algorithms can be directly applied to different aircraft configurations discussed in the scope of WP4.
WP6: Flight simulation
The objective of this activity has been to, firstly, verify the developed models and flight control systems in flight simulator experiments. Secondly, the impact of the human-machine interface and contributing human factors, such as pilot workload, on the safe and re¬liable execution of the refuelling manoeuvres are assessed by professional airline pilots.
Two research flight simulators have been adapted to conduct these civil air-to-air refuelling manoeuvres. The developed simulation models and the automatic flight control systems were integrated in the real-time simulation environments of the research flight simulators at NLR and DLR, GRACE (Generic Research Aircraft Cockpit Environment ) re¬spectively GECO (Generic Experimental Cockpit).
A Human-Machine-Interface (HMI), which consists of the information displays and control mechanisms available to the pilots in the cockpit, has been devel¬oped (Figure 8).
During the experiments these two flight simulators were run in a coupled mode, the tanker aircraft being modelled in GECO and the cruiser aircraft in GRACE. The required connection of several host computers in two countries was made via internet using a so-called Distributed Interactive Simulation connection (DIS).
The experiments are split in two phases, the first of which has been conducted by four crews of profes¬sional pilots from two major European airlines in 2013. This first phase of experiments focussed on the evaluation of the nominal procedures of all ma¬noeuvre phases, and also included a forced abort manoeuvre due to a too high approach speed.
The evaluation is based on the feedback by the pilots on the cruiser-feeder concept as a whole, the opera¬tional procedure, the Human-Machine Interface and recommendations for improvement. All pilots who flew the experiments reported that they had the im¬pression that air-to-air refuelling of civil aircraft, can be performed within present day safety levels with such a highly automated flight control system. They also indicated that the proposed air-to-air refuelling manoeuvre does not require specific additional skills from the pilots. With the results of the successful first phase of simulator sessions, the operational concept and the developed flight control and monitoring systems has been improved. For example, additional and improved information on the refuelling process are offered to the pilots in the cockpit.
The improved concept has been evaluated in the phase 2 flight simulation experiments during eight experiment days between September 30th and October 16th 2014.
These phase 2 simulation experiments included more complex ma¬noeuvres, and non-nominal and emergency condition
In the second part of the experiment days, the experimental scenarios were flown. These scenarios included a couple of critical situations where pilots needed to closely observe the manoeuvre and decide whether the safety of the operation was still maintained. The following conditions were introduced during the different experiment scenarios:
Light turbulence
Increasing turbulence up to moderate
Engine failure on the cruiser aircraft
Engine failure on the feeder aircraft
Refuelling system malfunction, decreasing fuel flow until complete blockage
Failure of the Estimated Safety Margin indicator during the approach
Very low visibility, only anti-collision lights visible at close range
Uncommanded speed brake deflection on cruiser aircraft during refuelling
Multiple failures introduced one after the other, Relative Position Indicator on feeder aircraft, Estimated Safety Margin indicator, refuelling system and engine failure on feeder aircraft.
The most important question of the experiment has been whether the pilots feel that the RECREATE system that was presented in the simulation will provide sufficient safety and the required level of control when it will be implemented in real life. After all the experiment runs and during the debrief at the end of the experiment day the pilots rated the overall acceptance of the presented RECREATE system between 6 - “Some improvement needed” and 10 - “Very acceptable”. The average rating from all experiment runs is 7.7 for the cruiser pilots and 7.4 for the feeder pilots. With an average rating between 7 - “A few improvements needed” and 8 - “Acceptable” this is a very good result and this shows that all pilots feel that the presented RECREATE system can really work when it will be implemented in real life.
Besides the acceptance of the RECREATE system it is also important that the work load of the pilots remains within acceptable limits and preferably comparable to present day levels. From the selected concept where the cruiser approaches the feeder from behind it is clear that the workload of the cruiser pilots would be higher than the workload of the feeder pilots. The ratings from the pilots confirmed this. It can be concluded that the workload of performing the air refuelling operation with the RECREATE system and procedures is comparable or less than present day approach and landing operations. The pilots noted that it was very easy to learn how to operate the RECREATE system and monitor the execution of the automated approach and station keeping. Figure 9 gives an impression of the view of the tanker and the markers on the tanker aircraft, seen from the cruiser cockpit during refuelling
The safety of the operation was on average rated between “Good” and “Satisfactory”. In just a couple of experiment runs some pilots rated the safety “Unacceptable”. But again this was during scenarios where deliberately multiple systems were failed simultaneously. This is not a realistic condition but a test case to see the reaction of the pilots to extreme situations. There has not been a single experiment run where the safety of the operation actually got compromised. It can be concluded that the safety of the operation under realistic conditions was at least satisfactory.
In 97% of the experiment runs the cruiser pilots indicated that they had sufficient control over the automated RECREATE system and the aircraft when required. During the debrief even 100% indicated that in general they had sufficient control. Overall it can be concluded that if the systems are working properly, the pilots had sufficient control of the RECREATE systems and their aircraft.
The overall conclusion of the human-in-the-loop evaluations of the presented RECREATE concept is that all pilots that participated in the experiments believe that this concept can be implemented in real life and can be operated by the pilots from the cruiser and the feeder aircraft while maintaining the required safety levels and with an acceptable workload for the pilots. They also indicated that little training will be required to get used to the operation of the automated execution of the air refuelling by the RECREATE system. Of course some aspects still need to be improved before actual implementation on an aircraft but no major issues were identified that would be hard to solve. It would be very interesting to investigate if this RECREATE concept can be developed further towards actual implementation.
Potential Impact:
When considering a potential implementation of such a complete restructuring of the intercontinental long-range air transport system, as the one proposed in this feasibility study, the huge task of informing relevant groups becomes evident. WP7 had the objective to create impact of the RECREATE project on a European (and global) scale through focussed dissemination of the RECREATE project outcome to the aeronautical science community, advisory groups, policy makers and not in the least the general public, as the potential users of this service.
In order to communicate the results to the aeronautical sciences community and to policy makers, a total of 24 scientific papers have been published at authoritative international conferences (Royal Aeronautical Society - RAeS, AIAA, ICAS, DLRK and others) throughout the project run time. More than 20 additional presentations were given at various opportunities and to various audiences (e.g. RAeS, NATO RTO, AirTN Forum, university students) by members of the project consortium.
Scientific publications of the RECREATE project:
• K. de Cock, R.K. Nangia: Research on a CRuiser Enabled Air Transport Environment (RECREATE), AVT-209 Workshop 2012.
• F. Morscheck: Tanking Strategies for Transatlantic air-to-air refueling operations, DLRK 2012.
• R. McRoberts, J.M. Early, F. Morscheck, M. Price & B. Korn: Implication of Tanker Mission Concept on the Benefits Evaluation of a Civil Air-to-Air Refuelling Transport System, AIAA Aviation 2013.
• G. La Rocca, M. Li, M. Chiozzi: Feasibility study of a nuclear powered blended wing body aircraft for the Cruiser/Feeder concept, CEAS 2013.
• D. Löbl, F. Holzapfel: High Fidelity Simulation Model of an Aerial Refueling Boom and Receptacle, DLRK 2013.
• F. Morscheck: Analyses on a Civil Air to Air refueling Network in a Traffic Simulation, ICAS 2014.
• G. La Rocca, P. van der Linden, M. Li, Elmendorp: Conceptual Design of a Passenger Aircraft for Aerial Refueling Operations, ICAS 2014.
• D. Löbl, F. Holzapfel: Simulation Analysis of a Sensor Data Fusion for Close Formation Flight, AIAA GNC 2014.
• H.S. Timmermans, G. La Rocca: Conceptual Design of a Flying Boom for Air-to-Air Refueling of Passenger Aircraft, ICCMSE 2014.
• R. McRoberts , J.M. Early , M. Price: Improving Feasibility of Point to Point Operations Through Civil Aerial Refuelling, AIAA Aviation 2014.
• S. Zajac, K. de Cock: Overview of the Research on a Cruiser Enabled Air Transport Environment (RECREATE) project, RAES AAC 2014.
• T. Mårtensson, R. Nangia: Towards Design of Efficient Cruiser-Feeder Concepts for Civil AAR, RAES AAC 2014.
• T. van Birgelen: Air-to-Air Refuelling of Civil Air Transport Aircraft - a Certification Approach, RAES AAC 2014.
• F. Morscheck: Interference Liability of a civil Air to Air refueling Traffic Network, RAES AAC 2014.
• M. Li, G. La Rocca: Conceptual Design of Joint-Wing Tanker for Civil Operations, RAES AAC 2014.
• L. Manfriani, M. Righi: Aerodynamic Design of a Boom for Air-to-Air Refuelling, RAES AAC 2014.
• W.W.M. Heesbeen, D. Löbl, J. Groeneweg: Human Aspects of Air-to-Air Refuelling of Civil Aircraft - A Human-in-the-Loop Study, RAES AAC 2014.
• J. Wang, D. Löbl, T. Raffler, F. Holzapfel: Kinematic Modeling and Control Design for an Aerial Refueling Task, RAES AAC 2014.
• H.S. Timmermans, G. La Rocca: Feasibility Study of a Forward Extending Flying Boom for Passenger Aircraft Aerial Refuelling, RAES AAC 2014.
• M. van Lith, H.G. Visser, H. Hosseini: Modeling an Operations Concept for Commercial Air-to-Air Refueling Based on a Vehicle Routing Problem Formulation, ICAS 2014.
• L. Manfriani, M. Righi: Aerodynamic Modelling of a Refuelling Boom, ICAS 2014.
• D. Löbl, F. Holzapfel: Closed-Loop Simulation Analysis of Automated Control of Aircraft in Formation Flight, DLRK 2014.
• R. McRoberts, J.M. Early, F. Morscheck, M. Price, B. Korn: Tanker Mission Implication on a Civil Aerial Refuelling Transport System’s Benefit Evaluation, Journal of Aircraft, Vol. 52/Issue 1, 01/01/2015, pp. 320-328.
• D. Löbl, F. Holzapfel: Subset Simulation for Estimating Small Failure Probabilities of an Aerial System Subject to Atmospheric Turbulences, AIAA AFM 2015.
Furthermore, a dedicated session of the cruiser-feeder concept has been organized within the framework of the Applied Aerodynamics Conference 2014 of the Royal Aeronautical Society. A total of nine presentations, containing the intermediate results of all work packages, were given during this session. The conference was held in Bristol on 22-24 July 2014. The resulting papers have been published in the proceedings of the conference. Although difficult to measure, attention to the Cruiser-Feeder concept and the outcome of the RECREATE project has certainly been drawn by this successful session which was attended by aerospace scientists and managers from universities and companies worldwide. Useful discussions took place following the presentations. It has also received some attention by international journalists.
The final phase 2 of the flight simulator experiments in the research simulators of NLR and DLR, GRACE and GECO has been the climax of the research conducted in the RECREATE project, with the work of all work packages (but especially that of WP5 and 6) contributing to these experiments. The experiments were completed in October 2014. This was taken as opportunity to draw attention of the general public to the final results of RECREATE by publishing a press release on 27 October 2014. This directly resulted in four major and several small articles in newspapers and technical magazines, and in two radio interviews with RECREATE researchers.
During the project run time, a total of 18 newspaper and magazine articles in English, Dutch and German have been published. Three radio interviews were given on Dutch radio stations. A website with complete coverage of the dissemination efforts and two videos produced by NLR and presented via YouTube help to generate awareness of the concept for a broad public. It can be concluded that the dissemination activities of the project results have been very successful, especially in the Dutch press and for the (English speaking) aeronautical science community.
List of Websites:
All publishable outcome of the RECREATE projects is also being disseminated via the public RECREATE homepage: www.cruiser-feeder.eu. Contact information is also available via the webpage.
“I’m in the simulated cockpit of an airliner, watching the displays as we close in on the tanker above us. By pushing a series of buttons, I have started the final, automated approach toward the refueling boom, through which 30,000 pounds of fuel will be pumped into the fuel tanks.” - These words in an Aerospace America article by Philip Butterworth-Hayes impressively summarize the highlights of 3.5 years of focussed research into the new cruiser-feeder concept of air transport, which has the verified potential to significantly reduce fuel consumption. The results laid out in this summary have been widely presented in papers to the scientific aerospace community and via the media to the general public.
Starting out with the earlier assessments of the benefits of cruiser-feeder operations which had pointed out the aerial refuelling concept as a promising concept, the REsearch on a CRuiser Enabled Air Transport Environment (RECREATE) project has been able to show in a consolidated, congruent manner that civil air-to-air refuelling (AAR) can be implemented in an airworthy and economically and ecologically beneficial way. A number of research methodologies coming from different fields and institutes were employed to cover the basic questions about the feasibility of what could be a complete renewal of the existing air transport system. The research project conducted by nine European research institutes, universities and small business partners successfully concluded in January 2015.
Due to the complexity of analysing a whole transport system and based on the choices and assumptions made for a comparison, only a range for the fuel reduction can be given. The conservative, aircraft-design driven and bottom-up derivation amounts to 11% fuel reduction, including also the fuel used by the tanker aircraft. The statistical, top-down specification shows an upper range of 23% fuel reduction. Implementing AAR on a large scale will enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports. This in turn also leads to local economic consequences.
Besides fuel reductions, there are also similar advantages of weight reductions. For a word-wide system, we can think in terms of a series of “optimum” cruisers with different passenger capacities (e.g. 200 to 300) and range capabilities (e.g. 2000 – 3500 nm). With AAR, the benefits increase as the total (refuelled) range increases. This will allow a greater flexibility to cope with “thin” and “thick” routes around the World.
From a design point of view, the conventional military refuelling configuration would be replaced by a forward-swept boom configuration which has been shown to be the most promising option, for safety, economical and passenger comfort reasons. Further the civil tankers need to be fairly small, capable of refuelling 2 or 3 cruisers. The tanker radius is between 500 - 1000 nm (2 - 2.5 hours of flight). So a civil tanker is very different from a military tanker which is usually designed for long endurance and multiple roles.
Future research required for making civil AAR a reality should include the conception of realistic business cases for air transport organisations, aircraft fuel providers and other stakeholders. Regarding airworthiness, a roadmap for civil certification has been developed. Technical research should focus on the design of automated flight control systems and on the airworthiness of the forward swept boom configuration, including ground and flight test demonstrations.
Project Context and Objectives:
Introduction
Current forecasts estimate a growth above 4% in worldwide air traffic per year for years to come. It is widely agreed that an equivalent rise of fuel consumption and CO2 emissions in aviation will not be acceptable. Current aircraft and propulsion technology developments as well as major step-change contributions to fuel burn reduction are required for the second half of this century. A major step may come from breakthrough technology development or a radical change in operations. One radical change in operations to alleviate the rise in fuel consumption and CO2 emissions are cruiser-feeder operations.
The definition of the cruiser-feeder concept states that the payload - passengers and/or cargo - is transported for the largest part of the way by one transport aircraft, called the cruiser. During flight, a mid-air contact with another aircraft is initiated. This second aircraft, the feeder, either joins the cruiser physically for a large part of the journey, or couples for a limited time to exchange passenger, cargo and/or fuel. Such a contact can take place one or several times during one journey. An obvious special case of cruiser-feeder operations, air-to-air refuelling (AAR), has been studied in the past. Estimations of the attainable reduction in fuel burn was shown in research preceding this project. These estimations suggested further in-depth research to underpin the data with refined analysis.
The European Union 7th Framework Programme funded project REsearch on a CRuiser Enabled Air Transport Environment (RECREATE) has been conducted by the following nine European research institutes, universities and small business partners:
• Nationaal Lucht- en Ruimtevaartlaborato¬rium NLR (The Netherlands, RECREATE coordinator),
• Deutsches Zentrum für Luft - und Raumfahrt DLR (Germany),
• Totalförsvarets Forskningsinstitut FOI (Swe¬den),
• Technische Universität München TUM (Ger¬many),
• Delft University of Technology DUT (The Netherlands),
• The Queen's University of Belfast QUB (United Kingdom),
• Zurich University of Applied Sciences ZHAW (Switzerland),
• Dr Rajendar Kumar Nangia (United King¬dom),
• Nuclear Research and Consultancy Group NRG (The Netherlands).
From 01 August 2011 until 31 January 2015, the feasibility of cruiser-feeder operation concepts has been studied, aiming at assessing the implications of a recreation of air transport operations.
Top level objective
The top level objective of the RECREATE research is to demonstrate on a preliminary design level that the cruiser-feeder concept can comply with airworthiness requirements for civil aircraft. For convenience of the reader, a generic and non-process orientated definition of airworthiness is: the ability of an aircraft or other airborne equipment or system to operate without significant hazard to aircrew, ground crew, passengers (where relevant) or to the general public over which such airborne systems are flown. A clear perspective of compliance with airworthiness requirements is a conditio sine qua non for further research and development of this out-of-the-box concept. The subsequent Scientific and Technological (S&T) objectives are:
1. to substantiate on a conceptual and preliminary design level that viable and acceptable concepts exist for cruiser-feeder operations;
2. to identify and qualify the necessary procedures and steps and required facilities to assure airworthiness of cruiser-feeder operations;
3. to confirm that the reported benefits of cruiser-feeder operations are consistent with the refined analysis and high fidelity simulation.
To achieve the S&T objectives, a collaborative research effort has been conducted with respect to:
• preliminary aircraft design dedicated to cruiser-feeder aircraft,
• the means of compliance to satisfy airworthiness requirements enabling cruiser-feeder aircraft,
• automatic flight control and flight simulation dedicated to cruiser-feeder aircraft.
Scientific and technological methodology
The scientific and technological methodology chosen for the RECREATE research consists of three integrating and three disciplinary research activities. The three integrated research activities and the three disciplinary work packages supporting the integrated work packages are listed below. An evolutionary delivery approach has been adopted, with two iterates (initial and final) for the disciplinary activities and three iterates (initial, updated and final) for the integrating activities.
Objectives of WP1: Concept for cruiser-feeder operations
First, a baseline concept of operations is established, the baseline being a conventional air transport without cruiser-feeder operations. Starting from this baseline, a cruiser-feeder concept of operations is defined, analysed and used to generate system requirements and other requirements, for instance the Top Level Aircraft Requirements.
Objectives of WP2: Airworthiness of cruiser-feeder operations
Civil aircraft adapted for use in military air-to-air refuelling operations are airworthy, providing a starting point for research on airworthiness of future cruiser-feeder operations. Applicable regulations and user requirements are studied and derived, and all potentially necessary means of compliance are being identified. A high level Functional Hazard and System Safety Assessment for cruiser-feeder operations is being made, finally leading to safety and airworthiness validation means of compliance with airworthiness requirements that are applicable to cruiser-feeder operations.
Objectives of WP3: Benefits of cruiser-feeder operations
Starting with the analysis of the chosen baseline and based on data generated in the other RECREATE work packages, environmental, dispatch reliability and economic delta analyses are conducted. The investigation of these potential benefits is essential for any future consideration of this operations concept.
Objectives of WP4: Conceptual and preliminary design
The requirements needed for a viable and acceptable airworthy concept will likely lead to new aircraft designs. For this reason, conceptual aircraft design is required for both the feeder, cruiser and aerial transfer boom system. This conceptual design is refined in a subsequent preliminary aircraft design phase. For the preliminary aircraft designs, an aerodynamic database on sizing, performance and stability & control will be created on the preliminary design level.
Objectives of WP5: Automatic flight control
Cruiser-feeder operations need to be highly automated if they are to function globally. The design of an automatic flight control system is important as well as understanding requirements and limitations of the system. In cruiser-feeder operations three bodies are involved, feeder aircraft, transfer boom and cruiser aircraft, which are to be controlled as one connected system. This is a challenge especially in all weather conditions. To understand the requirements of the distributed control system, models accounting for turbulence and elasticity are needed. A conceptual and preliminary design is made of a multi body automatic flight control system and a human-machine interface for the pilots of the cruiser and the feeder aircraft.
Objectives of WP6: Flight simulation
In order to develop a multi body cooperative automatic flight control system, flight simulation models are implemented to identify critical interactions, such as for instance due to atmospheric gust or wake-vortex interaction of the leading aircraft. Finally, flight simulation models are used for the identification of human factors. This is important in order to verify that the proposed manoeuvres are acceptable regarding safety and pilot workload.
Project Results:
“I’m in the simulated cockpit of an airliner, watching the displays as we close in on the tanker above us. By pushing a series of buttons, I have started the final, automated approach toward the refueling boom, through which 30,000 pounds of fuel will be pumped into the fuel tanks.” - These words in an Aerospace America article by Philip Butterworth-Hayes impressively summarize the highlights of 3.5 years of focussed research into this new concept of air transport, which has the verified potential to significantly reduce fuel consumption. The results laid out in this summary have been widely presented in papers to the scientific aerospace community and via the media to the general public (please also refer to the dissemination activities summarized below).
Starting out with the earlier assessments of the benefits of cruiser-feeder operations which had pointed out the aerial refuelling concept as a promising concept, the RECREATE project has been able to show in a consolidated, congruent manner that civil air-to-air refuelling can be implemented in an airworthy and economically and ecologically beneficial way. A number of research methodologies coming from different fields and institutes were employed to cover the basic questions about the feasibility of what could be a complete renewal of the existing air transport system.
Due to the complexity of analysing a whole transport system and based on the choices and assumptions made for a comparison, only a range for the fuel reduction can be given. The conservative, aircraft-design driven and bottom-up derivation amounts to 11% fuel reduction, including also the fuel used by the tanker aircraft. The statistical, top-down specification shows an upper range of 23% fuel reduction. Implementing AAR on a large scale will enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports. This in turn also leads to local economic consequences.
Besides fuel reductions, there are also similar advantages of weight reductions. For a word-wide system, we can think in terms of a series of “optimum” cruisers with different passenger capacities (e.g. 200 to 300) and range capabilities (e.g. 2000 – 3500 nm). With AAR, the benefits increase as the total (refuelled) range increases. This will allow a greater flexibility to cope with “thin” and “thick” routes around the World.
From a design point of view, the conventional military refuelling configuration would be replaced by a forward-swept boom configuration which has been shown to be the most promising option, for safety, economical and passenger comfort reasons. Further the civil tankers need to be fairly small, capable of refuelling 2 or 3 cruisers. The tanker radius is between 500 - 1000 nm (2 - 2.5 hours of flight). So a civil tanker is very different from a military tanker which is usually designed for long endurance and multiple roles.
Future research required for making civil AAR a reality should include the conception of realistic business cases for air transport organisations, aircraft fuel providers and other stakeholders. Regarding airworthiness, a roadmap for civil certification has been developed. Technical research should focus on the design of automated flight control systems and on the airworthiness of the forward swept boom configuration, including ground and flight test demonstrations.
In the following part, the research results are presented per work package in more detail. The individual results have also been presented during the final meeting in Amsterdam on 29 January 2015. These presentations have been added in Appendices A through G. All research results have been reported in the corresponding WP reports (24 deliverables).
WP1: Concept for cruiser-feeder operations
The objective of this integrating activity has been to de¬velop, iterate, select and describe two cruiser-feeder concepts, for further study. One chosen concept is realizable in the distant future (at least 50+ years from now), the other concept however is expected to be realizable in the medium-term (within 20+ years from now). This concept requires development and acceptance of new airworthiness regulations, but can be done with today’s technology.
First, a conventional baseline without cruiser-feeder operation has been defined to compare the benefits of future concepts with today’s technology, see Table 1:
Table 1: Baseline concept
Cruiser
Capacity 250 passengers
Range 5000 nm
Specific fuel con-sumption 0.525
Given the broad definition of cruiser-feeder opera¬tions, iterations conducted at the start of this activity involved a large diverging number of cruiser-feeder concepts. In a subsequent convergence sweep, two concepts were down selected for further investigation. The first final concept is civil air-to-air refuel¬ling, of which the overall characteristics are shown in Table 2:
Table 2: First final concept: Air-to-air refuelling as special case of cruiser-feeder operations
Cruiser Feeder
Capacity 250 pas¬sengers Fuel offload capacity 35000 lb / 3 con¬tacts
Range 2500-3000 nm Range / en¬durance 500 nm / ~4 hours
MTOW 100000 kg AAR enve¬lope < 24000 ft, Mach < 0.8
Specific fuel con-sumption 0.525 AAR proce¬dure 20 min
(5 min wet contact)
Concepts with transfer of payload and passengers based on engines burning chemical fuel have been shown not be economically feasible, as the overall weight of the system and thus the total amount of fuel burnt are too high. However, if the cruiser can be propelled by a nuclear power source, the efficiency is very high compared to the reference case, even if the total weight of the system is higher. Although the nuclear cruiser cannot be shown to meet airworthi¬ness requirements with today’s technology, this con¬cept has been retained for study because it cannot be excluded that new nuclear physics will be discovered and confirmed in the future. The second final concept (Table 3) concerns a nuclear pro¬pelled cruiser where the transfer of passengers and cargo is done via a life supporting container mecha¬nism.
Table 3: Second final concept: cruiser-feeder with nuclear propelled cruiser
Cruiser
Capacity 1000 passengers
MTOW 900000 kg
Range / endurance 60000 nm / 1 week
Cruise speed M = 0.8
Docking speed M = 0.7
Cruise altitude > 36000 ft
Payload transfer Single container station concept (100 passengers each)
The two studied concepts address long-distance air travel but with very different techniques. Table 4 shows the level of detail in which both concepts have been studied during the project.
Table 4: Level of aircraft design applied to two concepts
WP AAR Orbiting Nuclear Cruiser
WP 1 Concept X X
WP2 Airworthiness X (x) To some extent
WP3 Benefits X (x) To some extent
WP4 Design Concept Design and
Preliminary Design Concept Design only
WP5 Control X (conventional AAR) -
WP6 Simulation X (conventional AAR) -
WP7 Dissemination X X
The AAR concept, as outlined in the WP1 reports, is clearly within reach for aviation if the industry and concerned legal bodies decide to go into this direction. As for the orbiting nuclear concept it is not within reach anytime soon but the concept, as such, has great potential.
AAR can play an important role dealing with the sustainability challenge aviation faces. If the final operational cruiser-feeder concept presented in this report is introduced on today´s system as it is, it has the potential of bringing large benefits:
Reduce fuel burn and direct CO2 emission by 10-20%, depending on the assumptions made for the analysis.
Local environment – Noise and local air quality will be better or on the same levels as today.
The number of movements and aircraft will increase; however, the total mass of the system will be significantly lower. Operational constraints on the system with present traffic load, such as scheduling, workload on feeder bases and impact from weather hazards (mainly turbulence), seem manageable. Figure 1 gives a visual impression of the performed traffic simulations of the entire traffic taking place today within 48 hours.
It has previously been outlined that aviation faces the dilemma that short flights which are the most common are the cause of the congestions whereas long flights burn most of the fuel and hence have the largest environmental impact. With continued urbanisation and more megacities on earth, the use of air transport should be reserved for long distance travel where no other viable options exist. Following the Intergovernmental Panel on Climate Change (IPCC), high speed rail can substitute short-distance air travel up to 800 km and in some case even up to 1500 km (a good example today is Beijing – Shanghai). This is one clear way of mitigating greenhouse emissions from air travel and also alleviate noise and air pollution problems which many of the world’s megacities face already today.
Apart from major fuel and weight savings on long flights, AAR has the potential to reduce the number of short flights since the smaller, more efficient, AAR-cruisers inherently give an opportunity to serve more point to point connections. To set-up a new intercontinental “ point-to-point connection” from an airport in a “mid-sized city” that today has no (or few) intercontinental connections, will of course be an easier business case with smaller efficient RECREATE cruisers as compared to the larger baseline cruiser. Every new long distance point to point connection will reduce “hub travel” to some degree. The potential and sensitivities going from a hub – spoke system to more point to point connections has been studied in WP3.
A complete removal of the hub-spoke system in the near term is not realistic, but for the future the system has to be pushed away from the big hub solution. As for technical development of air-to-air refuelling concept, other novel fuel transfer configurations with the tanker behind the cruiser (or non-centreline set-ups) can improve aircraft efficiency, system performance and safety. In WP4 some of these ideas have been exploited. Civil AAR should also not be viewed in isolation. A possible introduction of alternative fuels would also benefit AAR as well as other novel operational concepts like formation flying for civil aircraft which also could be used together with civil AAR.
WP2: Airworthiness of cruiser-feeder operations
The objective of this integrating activity has been to develop a route towards airworthiness of cruiser-feeder operations. Bringing two aircraft in close vicinity of one another in mid-air, either for docking or for re¬fuelling, is potentially dangerous. Today, no civil certification regulations concerning cruiser-feeder operations exist. However, the airworthiness of such a concept must be established along the guidelines of the current regulatory system of the safety of civil aircraft operation and with an equivalent level of safety.
Based on the concepts of operations described, a Functional Hazard and System Safety Analysis of the selected cruiser-feeder concepts has been performed to map all hazards not covered by current airworthiness regulations, for both cruiser-feeder concepts. Using the operational scenarios relevant for collision risk, performance, availability and integrity requirements for the functions of the air-to-air refuelling system have been determined. A schematic for this system modelling is shown in Figure 2.
These were input for the system design which has been performed in other RECREATE Work Packages. Simulations based on physical models of the air-to-air refuelling system have been performed in WP5 for the most critical collision risk scenarios to verify that the air-to-air refuelling system design meets the safety requirements. Non-collision risk related hazards of the receiver-tanker have been identified and safety measures to mitigate these hazards have been proposed.
Collision risk related hazards for the nuclear cruiser-shuttle concept are similar to those for the conventional fuel receiver-tanker concept. Non-collision risk related hazards of the nuclear propelled cruiser have been identified and safety measures to mitigate these hazards have been proposed.
As a second important step, an approach for the certification of air¬worthiness of civil cruiser-feeder operations has been de¬veloped taking into account existing military regula¬tions for aerial refuelling. An evaluation of the exist¬ing EASA Certification Specifications and Ac¬ceptable Means of Compliance for Large Aeroplanes (CS-25 Amendment 11) has been made, resulting in an overview of applicable specifications and specifi¬cations to be amended for cruiser-feeder operations. Following the evaluation, an approach to certifica¬tion has been proposed. Because of their similarity of operations this approach has been based on the regulations covering automated landing, the EASA CS-AWO (All Weather Operations). The similarity lies in the ob¬jective to achieve an accurate and safe approach to a specific position. The amended regulations and Ac¬ceptable Means of Compliance (CS-AWO AMC) cover the required safety level, the considered conditions re¬garding aircraft, operations and environment, the performance requirements, performance demonstra¬tion and failure conditions.
Simulations have been used to get quantified data for these models, for normal operation and for the most critical scenarios:
For wind shear or turbulence encounter the most critical situation is the station keeping position with the boom almost connected or just discon¬nected (boom very close to cruiser) and a sudden upward movement of the cruiser.
For system failures the most critical situation is the station keeping position and a sudden engine failure, with or without the boom almost con¬nected or just disconnected.
It has to be noted that real ground and flight testing to demonstrate the safe operation of nuclear propulsion is not justifiable due to the inherent risk of the release of radioactivity. Here, the development of high fidel¬ity simulation with a very high level of confidence far beyond the current state-of-the-art will be required.
Finally, a tentative future roadmap for certification of air-air refuelling of commercial air transport aircraft has been provided. The roadmap describes the steps that must be taken to formally establish regulations and acceptable means of compliance.
WP3: Benefits of cruiser-feeder operations
The objective of this integrating activity has been to analyse economic benefits, dispatch reliability and environ¬mental effects of cruiser-feeder operations using the defined baseline as reference. The impact on fuel consumption, exhaust emissions, local noise produc¬tion and shareholder value have been studied.
Benefits analyses of the updated concept indicate a range of fuel and mass savings for the cruiser aircraft for cruiser-feeder concepts in which only fuel is transferred (Fig. 3). Remark that it is a challenge for an opti¬mized conventional configuration cruiser to be as efficient as an aircraft optimized for long range, be¬cause of the relatively larger fuselage and fixed weight of some of the equipment, furnishings etc. On the conservative side, fuel savings for the cruiser only - derived bottom-up by aircraft design - amount to around 21% for one refuelling. With two refuellings, this value could be increased, and further investiga¬tions on the effect of thrust-to-weight ratio and re¬serve fuel are ongoing. From this benefit the fuel consumed by the tanker(s) has to be subtracted. If the tanker is very efficient with a fuel ratio - i.e. ratio fuel given / fuel consumed - equal to 8, the tanker reduces the total efficiency of the cruiser-feeder combination by about 5-6%. In case of a more realistic tanker with a fuel ratio of 4, the impact of the tanker is a reduc¬tion by about 10-12%. A lower bond of fuel saving is in the order of 11%, which is still a huge fuel saving compared to nowadays standards in the industry. On the optimistic side and based on a top-down, statisti¬cal approach, an upper bond of 23% fuel saving for the cruiser-feeder combination has been calculated previously.
The environmental impact next to the reduction of fuel burn and thus CO2 emissions has been studied as well. A consequence of smaller and lighter cruiser aircraft compared to today’s long range passenger aircraft is a reduction of noise at the departure/arrival and hub airports, which are mostly located in densely populated areas. A noise reduction in turn leads to economic benefits through reduced noise fees and a rise in property value near existing airports. The noise impact through the additional heavy load near the required tanker bases has not been investigated. The tanker bases are assumed to be located in sparsely populated areas, where noise is not of a great influence.
To examine the insertion of the cruiser-feeder concept into a representative air traffic scenario based on the Star Alliance fleet, route optimization is conducted to account for interactions between cruiser and feeder weights and efficiencies, locations for feeder bases, feeder radius of operation and cruiser route modifications. Feeders with higher efficiency achieve the best results in the traffic simulations at a feeding capacity of 2 or 3 refuelling, with operations conducted close to the feeder base. However, there is significant variability in these findings, with less optimal findings when considering those routes serviced on European-Asia routes in contrast to transatlantic routes. Small disruptions to refuelling operations can largely be absorbed due to inefficient use of feeder aircraft at a number of the feeder bases, but large scale unplanned disruptions can have a significant impact, resulting in multiple failed operations and severe impact on the network stability.
The increased ground traffic at both airport and refuelling bases raises challenges for dispatch reliability with increasing number of flights and the impact on local communities. As highlighted in the traffic simulations, dispatch reliability must now also account for the refuelling operation delay and the impact on ground handling capability of both minor disruptions requiring diversions of aircraft to alternative refuelling positions and accommodating additional aircraft landings in the event of major disruptions resulting in aborted operations.
AAR has been shown to be beneficial in terms of fuel reduction compared to non-AAR, even when considering a network based on more point-to-point connections, less on a hub-and-spoke system. Implementing AAR on a large scale will thus also enable more long-range point-to-point services and thus lead traffic away from existing hub airports and towards regional airports, with the related economic and en¬vironmental consequences.
Regarding economic benefits achievable in a cruiser-feeder configuration, evaluation of the configuration to include realistic route and demand analysis confirms that the economic viability of the concept is tied to the correct implementation of the underlying feeder network in order to maximise benefit. Transformation of the network to the cruiser-feeder enabled environment without consideration of available seat miles significantly reduces the profitability (and hence viability) of the concept, and in order to match previous revenue generation levels without passing additional costs onto passengers, additional aircraft are required which in turn reduces the environmental and cost benefits achievable over the baseline to approximately 14% and 12% respectively. However, improved scheduling of cruisers to match feeder availability may significantly enhance these cost benefits through enabling better utilisation of feeders capable of multiple refuelling exercises, and in particular, enable the adoption of new concepts such as the point to point network. It has been shown that with the use of AAR, the increased fuel burn associated with the additional routes serviced can be reduced significantly, and a significant increase in available seat miles within the network can be achieved without a corresponding proportional increase in operational cost.
WP4: Conceptual and preliminary design
The objective of this activity has been to conduct conceptual and preliminary design iterations of dedicated cruiser and feeder aircraft according to two the chosen concepts:
The aerial refuelling concept, where passenger aircraft (the cruisers) receive fuel from tanker (the feeders)
The passenger exchange concept, where one large dimensions aircraft with nuclear propulsion (the cruiser) exchanges passengers in flight, with various shuttle aircraft (feeders), by means of pre-loaded containers.
Families of cruiser and feeder aircraft are generated as well as refuelling boom designs. The generated design data is used in support of the benefits analyses, the air¬worthiness analyses and the auto¬matic flight control system development.
Cruiser and feeder aircraft specifically designed and optimised for their task have been shown to increase the benefits achievable by air-to-air refuelling. The gain results on the one hand from the fact that a long-range cruiser aircraft designed for a specific range including refuelling will have a reduced Operating Empty Weight (OEW) as it will be designed for a smaller Maximum Fuel Weight (MFW) and thus a smaller Maximum Take-Off Weight (MTOW). De¬sign studies of long-range passenger aircraft able of being refuelled have been performed with conceptual and preliminary design tools. The design work on aircraft and refuelling systems has demonstrated that aerial refuelling for civil aircraft is technically feasible. Indeed, it is standard in military operations. From an engineering point of view no showstopper has been identified. However the developments (refuelling system, tanker, engines) would require huge investments, drastically reducing the economic viability of the concept.
It has been shown that the tanker design has a huge impact on the overall efficiency. Current military tankers, however, are not optimized for the refuelling task but are converted cargo and passenger aircraft. Dedicated tanker design studies have shown the ad¬vantages of a joint-wing tanker concept.
The initial conceptual design phase lead to optimistic results compared to the following preliminary design work. This can be attributed mainly to the following factors:
In the conceptual sizing and design process constant empty mass fractions have been assumed, which are valid for long-range aircraft but not for the short/medium range refuelled aircraft. More realistic values deducted from the preliminary design work should be in the order of 0.5 to 0.6 for the refuelled designs.
Similarly, we assumed an L/D ratio of 19.9 equivalent to long-range aircraft. This assumption is invalid if the aircraft wing size is properly reduced for the more lightweight refuelled aircraft. This leads to a an increase of the relative importance of the fuselage drag. The preliminary design results indicate that, depending on the number of refuellings, the L/D ratio should be one to two points lower.
For flight control reasons the refuelling had to be scheduled at a reduced altitude. Each refuelling therefore requires a descent and climb segment which leads to an additional fuel consumption of about 0.5 to 1% per refuelling operation.
Therefore, the results of the more accurate preliminary design work are less optimistic and show lower reductions in fuel consumption for the cruiser aircraft than the initial estimations.
Based on all findings, we estimate the overall fuel savings for the cruiser alone to range between 15 and 20% of the block fuel. The alternate concept of staged flights would lead to fuel savings in the order of 10 to 15% but would need no tanker aircraft.
Adding the fuel consumption of the tanker aircraft reduces the fuel saving s for the aerial refuelling concept by about 5%, leading to overall savings between 10 and 15%.
A trade-off study showed that a non-conventional, inverted receiver-tanker configuration is beneficial and essentially the more viable configuration. Such a configuration increases the safety of the pas-senger aircraft from possible debris from the tanker or re¬fuelling boom after a collision, removes possible passenger discomfort due to flying in the tanker wake and limits the amount of extra pilot training required to the much smaller number of tanker pilots. Fur¬thermore, the passenger aircraft need a minimum of refurbishment for the new manoeuvres, and thus have a minimum loss of cruise efficiency, while the costly surplus thrust requirements and supporting equipment lay with the smaller number of tankers.
A number of different concepts for a forward ex¬tending boom have been studied. A big chal¬lenge lies in the controllability and aeroelastic stabil¬ity for this concept. Preliminary aeroelastic analysis results show that a design space free from static and dynamic aeroelastic instabilities exists.
Preliminary design with aerodynamic CFD computa¬tions for a refuelling boom has been done for the conventional con¬figuration. An aerodynamic database was created containing the results of about 800 computations for a number of relative boom position parameters and the control surfaces’ de¬flection angles. This preliminary aircraft design data has supported the development of an automated flight control system.
Several design studies have investigated the nuclear cruiser concept in cooperation with the Nuclear Research and Consul¬tancy Group NRG. A pressurized container exchange concept was adopted for the in-flight trans¬fer of passengers, crew, cargo and consumables, see Figure 5.
On a conceptual basis, weight estimations, aerody¬namic design and design of a nuclear propulsion system for the cruiser aircraft were performed; see Figure 6 for a conceptual design sketch. Op¬tions for a Brayton cycle and a Rankine cycle propul¬sion system were studied.
WP5: Automatic flight control
An early as¬sessment showed that successful cruiser-feeder operations will require automatic flight control systems for the cruiser air¬craft, the feeder aircraft and the refuelling boom to enable automatic conduction of the manoeuvre to achieve an adequate level of safety and availability. The objective of this activity was to develop this auto¬mated in-flight refuelling system, including sensor suites with redundant measurement technologies, actuators and controllers, and to model that system in a realistic simulation environment. The work had to consider, extend and comply with the airworthiness requirements defined in WP2 as well as the operational concepts elaborated in WP1.
It could be demonstrated that aerial refuelling of civil transport aircraft as one possible concept of cruiser-feeder operations is viable and safe. Since manual control is no option with respect to the high levels of safety and availability, an automatic flight control system was developed, comprising the following main components:
Cruiser-feeder operations and maneuver design: An approach maneuver was designed that accounts for operational and regulatory aspects. The envisaged approach trajectory starts at the minimum separation that is required for independently controlled flight and leads to a very close proximity in range of the fuel transfer boom. Tasks related to the automation system to be performed by the cruiser and feeder pilots were defined, which allowed a realistic evaluation of the automatic flight control system by piloted simulations conducted in the scope of work package 6. Requirements were put on the refuelling boom envelope and system design, which were used as basis for detailed boom design in the scope of WP4.
Sensor selection and sensor data fusion algorithms: Adequate sensors were selected for relative position estimation between the aircraft as well as between the boom tip and the receptacle. An exhaustive sensor suite was defined comprising inertial measurement units, code phase DGPS receiver, radio frequency ranging transponders and electro-optic sensors. The sensor suite along with the sophisticated sensor data fusion algorithms led to highly accurate measurements required for close formation flight with high availability and integrity, i.e. sensor faults are reliably detected and excluded, leading to high confidence in the estimated variables.
Guidance and control: Automatic flight control laws for the cruiser, feeder, and the boom were developed. A decoupled approach was chosen. The cruiser controller is based on the standard PID concept and tuned with emphasis on tracking performance. The developed refuelling boom controller is based on exact linearization of the constrained boom system where the boom hinge point motion is forced by the tanker motion. The combination of cruiser control laws for relative position control and boom control laws for engagement of the connection between the aircraft has proven functionality for the considered cruiser-feeder scenario.
Mode selection and phase transition: A finite state machine was developed and implemented that ensures a safe and predictable behavior of the coupled cruiser-feeder system during all phases relevant for a successful and safe conduction of automated aerial refuelling of civil transport aircraft.
Supervision: Algorithms were developed that monitor control and navigation performance as well as external disturbances. Based on geometric and stochastic parameters, these algorithms ensure a timely abort in case of system malfunctions or exceedance of admissible external disturbance levels.
All the afore mentioned components were implemented in an exhaustive simulation environment, also considering effects of the correlated wind turbulence field and aircraft elasticity. A screenshot of such a simulation is shown in Figure 7.
Using this simulation environment it could be demonstrated that aerial refuelling can be automatically conducted. For project RECREATE it had not only to be successfully demonstrated but also safety had to be proven according to the findings of WP2. It could be shown by simulation that a collision between the aircraft, which is the most safety-critical event, is less probable than specified in the safety requirements derived in WP2. For that, a special simulation method was applied (subset simulation) since conventional Monte Carlo simulation is not feasible for calculating the very low probabilities associated to the examined events. In general the sensitivity of the safety simulation results is the higher the lower the probability is. Especially for the case where a probability of 〖10〗^(-9) was proven only minor changes in the aircraft models might lead to considerable differences in the resulting thresholds. However, the objective of project RECREATE was to prove that close formation flight could be conducted safely according to civil certification specifications. Although the simulation models did not exactly represent a special type of aircraft, all contributing subsystems were simulated to a reasonable extent. This allows the conclusion that with similar configurations the considered safety requirements could be complied by using the control, navigation, and supervision algorithms developed, implemented, and tested in the scope this work package.
The knowledge gained in the scope of WP5 concerning feasibility, safety, and also the results of the piloted simulations conducted in the scope of WP6 based on the developed automatic flight control system provide a good baseline for possible next steps, especially the demonstration of automatic close formation flight by flight tests. Additionally, the introduced sensor suite, data fusion, and safety simulation algorithms can be directly applied to different aircraft configurations discussed in the scope of WP4.
WP6: Flight simulation
The objective of this activity has been to, firstly, verify the developed models and flight control systems in flight simulator experiments. Secondly, the impact of the human-machine interface and contributing human factors, such as pilot workload, on the safe and re¬liable execution of the refuelling manoeuvres are assessed by professional airline pilots.
Two research flight simulators have been adapted to conduct these civil air-to-air refuelling manoeuvres. The developed simulation models and the automatic flight control systems were integrated in the real-time simulation environments of the research flight simulators at NLR and DLR, GRACE (Generic Research Aircraft Cockpit Environment ) re¬spectively GECO (Generic Experimental Cockpit).
A Human-Machine-Interface (HMI), which consists of the information displays and control mechanisms available to the pilots in the cockpit, has been devel¬oped (Figure 8).
During the experiments these two flight simulators were run in a coupled mode, the tanker aircraft being modelled in GECO and the cruiser aircraft in GRACE. The required connection of several host computers in two countries was made via internet using a so-called Distributed Interactive Simulation connection (DIS).
The experiments are split in two phases, the first of which has been conducted by four crews of profes¬sional pilots from two major European airlines in 2013. This first phase of experiments focussed on the evaluation of the nominal procedures of all ma¬noeuvre phases, and also included a forced abort manoeuvre due to a too high approach speed.
The evaluation is based on the feedback by the pilots on the cruiser-feeder concept as a whole, the opera¬tional procedure, the Human-Machine Interface and recommendations for improvement. All pilots who flew the experiments reported that they had the im¬pression that air-to-air refuelling of civil aircraft, can be performed within present day safety levels with such a highly automated flight control system. They also indicated that the proposed air-to-air refuelling manoeuvre does not require specific additional skills from the pilots. With the results of the successful first phase of simulator sessions, the operational concept and the developed flight control and monitoring systems has been improved. For example, additional and improved information on the refuelling process are offered to the pilots in the cockpit.
The improved concept has been evaluated in the phase 2 flight simulation experiments during eight experiment days between September 30th and October 16th 2014.
These phase 2 simulation experiments included more complex ma¬noeuvres, and non-nominal and emergency condition
In the second part of the experiment days, the experimental scenarios were flown. These scenarios included a couple of critical situations where pilots needed to closely observe the manoeuvre and decide whether the safety of the operation was still maintained. The following conditions were introduced during the different experiment scenarios:
Light turbulence
Increasing turbulence up to moderate
Engine failure on the cruiser aircraft
Engine failure on the feeder aircraft
Refuelling system malfunction, decreasing fuel flow until complete blockage
Failure of the Estimated Safety Margin indicator during the approach
Very low visibility, only anti-collision lights visible at close range
Uncommanded speed brake deflection on cruiser aircraft during refuelling
Multiple failures introduced one after the other, Relative Position Indicator on feeder aircraft, Estimated Safety Margin indicator, refuelling system and engine failure on feeder aircraft.
The most important question of the experiment has been whether the pilots feel that the RECREATE system that was presented in the simulation will provide sufficient safety and the required level of control when it will be implemented in real life. After all the experiment runs and during the debrief at the end of the experiment day the pilots rated the overall acceptance of the presented RECREATE system between 6 - “Some improvement needed” and 10 - “Very acceptable”. The average rating from all experiment runs is 7.7 for the cruiser pilots and 7.4 for the feeder pilots. With an average rating between 7 - “A few improvements needed” and 8 - “Acceptable” this is a very good result and this shows that all pilots feel that the presented RECREATE system can really work when it will be implemented in real life.
Besides the acceptance of the RECREATE system it is also important that the work load of the pilots remains within acceptable limits and preferably comparable to present day levels. From the selected concept where the cruiser approaches the feeder from behind it is clear that the workload of the cruiser pilots would be higher than the workload of the feeder pilots. The ratings from the pilots confirmed this. It can be concluded that the workload of performing the air refuelling operation with the RECREATE system and procedures is comparable or less than present day approach and landing operations. The pilots noted that it was very easy to learn how to operate the RECREATE system and monitor the execution of the automated approach and station keeping. Figure 9 gives an impression of the view of the tanker and the markers on the tanker aircraft, seen from the cruiser cockpit during refuelling
The safety of the operation was on average rated between “Good” and “Satisfactory”. In just a couple of experiment runs some pilots rated the safety “Unacceptable”. But again this was during scenarios where deliberately multiple systems were failed simultaneously. This is not a realistic condition but a test case to see the reaction of the pilots to extreme situations. There has not been a single experiment run where the safety of the operation actually got compromised. It can be concluded that the safety of the operation under realistic conditions was at least satisfactory.
In 97% of the experiment runs the cruiser pilots indicated that they had sufficient control over the automated RECREATE system and the aircraft when required. During the debrief even 100% indicated that in general they had sufficient control. Overall it can be concluded that if the systems are working properly, the pilots had sufficient control of the RECREATE systems and their aircraft.
The overall conclusion of the human-in-the-loop evaluations of the presented RECREATE concept is that all pilots that participated in the experiments believe that this concept can be implemented in real life and can be operated by the pilots from the cruiser and the feeder aircraft while maintaining the required safety levels and with an acceptable workload for the pilots. They also indicated that little training will be required to get used to the operation of the automated execution of the air refuelling by the RECREATE system. Of course some aspects still need to be improved before actual implementation on an aircraft but no major issues were identified that would be hard to solve. It would be very interesting to investigate if this RECREATE concept can be developed further towards actual implementation.
Potential Impact:
When considering a potential implementation of such a complete restructuring of the intercontinental long-range air transport system, as the one proposed in this feasibility study, the huge task of informing relevant groups becomes evident. WP7 had the objective to create impact of the RECREATE project on a European (and global) scale through focussed dissemination of the RECREATE project outcome to the aeronautical science community, advisory groups, policy makers and not in the least the general public, as the potential users of this service.
In order to communicate the results to the aeronautical sciences community and to policy makers, a total of 24 scientific papers have been published at authoritative international conferences (Royal Aeronautical Society - RAeS, AIAA, ICAS, DLRK and others) throughout the project run time. More than 20 additional presentations were given at various opportunities and to various audiences (e.g. RAeS, NATO RTO, AirTN Forum, university students) by members of the project consortium.
Scientific publications of the RECREATE project:
• K. de Cock, R.K. Nangia: Research on a CRuiser Enabled Air Transport Environment (RECREATE), AVT-209 Workshop 2012.
• F. Morscheck: Tanking Strategies for Transatlantic air-to-air refueling operations, DLRK 2012.
• R. McRoberts, J.M. Early, F. Morscheck, M. Price & B. Korn: Implication of Tanker Mission Concept on the Benefits Evaluation of a Civil Air-to-Air Refuelling Transport System, AIAA Aviation 2013.
• G. La Rocca, M. Li, M. Chiozzi: Feasibility study of a nuclear powered blended wing body aircraft for the Cruiser/Feeder concept, CEAS 2013.
• D. Löbl, F. Holzapfel: High Fidelity Simulation Model of an Aerial Refueling Boom and Receptacle, DLRK 2013.
• F. Morscheck: Analyses on a Civil Air to Air refueling Network in a Traffic Simulation, ICAS 2014.
• G. La Rocca, P. van der Linden, M. Li, Elmendorp: Conceptual Design of a Passenger Aircraft for Aerial Refueling Operations, ICAS 2014.
• D. Löbl, F. Holzapfel: Simulation Analysis of a Sensor Data Fusion for Close Formation Flight, AIAA GNC 2014.
• H.S. Timmermans, G. La Rocca: Conceptual Design of a Flying Boom for Air-to-Air Refueling of Passenger Aircraft, ICCMSE 2014.
• R. McRoberts , J.M. Early , M. Price: Improving Feasibility of Point to Point Operations Through Civil Aerial Refuelling, AIAA Aviation 2014.
• S. Zajac, K. de Cock: Overview of the Research on a Cruiser Enabled Air Transport Environment (RECREATE) project, RAES AAC 2014.
• T. Mårtensson, R. Nangia: Towards Design of Efficient Cruiser-Feeder Concepts for Civil AAR, RAES AAC 2014.
• T. van Birgelen: Air-to-Air Refuelling of Civil Air Transport Aircraft - a Certification Approach, RAES AAC 2014.
• F. Morscheck: Interference Liability of a civil Air to Air refueling Traffic Network, RAES AAC 2014.
• M. Li, G. La Rocca: Conceptual Design of Joint-Wing Tanker for Civil Operations, RAES AAC 2014.
• L. Manfriani, M. Righi: Aerodynamic Design of a Boom for Air-to-Air Refuelling, RAES AAC 2014.
• W.W.M. Heesbeen, D. Löbl, J. Groeneweg: Human Aspects of Air-to-Air Refuelling of Civil Aircraft - A Human-in-the-Loop Study, RAES AAC 2014.
• J. Wang, D. Löbl, T. Raffler, F. Holzapfel: Kinematic Modeling and Control Design for an Aerial Refueling Task, RAES AAC 2014.
• H.S. Timmermans, G. La Rocca: Feasibility Study of a Forward Extending Flying Boom for Passenger Aircraft Aerial Refuelling, RAES AAC 2014.
• M. van Lith, H.G. Visser, H. Hosseini: Modeling an Operations Concept for Commercial Air-to-Air Refueling Based on a Vehicle Routing Problem Formulation, ICAS 2014.
• L. Manfriani, M. Righi: Aerodynamic Modelling of a Refuelling Boom, ICAS 2014.
• D. Löbl, F. Holzapfel: Closed-Loop Simulation Analysis of Automated Control of Aircraft in Formation Flight, DLRK 2014.
• R. McRoberts, J.M. Early, F. Morscheck, M. Price, B. Korn: Tanker Mission Implication on a Civil Aerial Refuelling Transport System’s Benefit Evaluation, Journal of Aircraft, Vol. 52/Issue 1, 01/01/2015, pp. 320-328.
• D. Löbl, F. Holzapfel: Subset Simulation for Estimating Small Failure Probabilities of an Aerial System Subject to Atmospheric Turbulences, AIAA AFM 2015.
Furthermore, a dedicated session of the cruiser-feeder concept has been organized within the framework of the Applied Aerodynamics Conference 2014 of the Royal Aeronautical Society. A total of nine presentations, containing the intermediate results of all work packages, were given during this session. The conference was held in Bristol on 22-24 July 2014. The resulting papers have been published in the proceedings of the conference. Although difficult to measure, attention to the Cruiser-Feeder concept and the outcome of the RECREATE project has certainly been drawn by this successful session which was attended by aerospace scientists and managers from universities and companies worldwide. Useful discussions took place following the presentations. It has also received some attention by international journalists.
The final phase 2 of the flight simulator experiments in the research simulators of NLR and DLR, GRACE and GECO has been the climax of the research conducted in the RECREATE project, with the work of all work packages (but especially that of WP5 and 6) contributing to these experiments. The experiments were completed in October 2014. This was taken as opportunity to draw attention of the general public to the final results of RECREATE by publishing a press release on 27 October 2014. This directly resulted in four major and several small articles in newspapers and technical magazines, and in two radio interviews with RECREATE researchers.
During the project run time, a total of 18 newspaper and magazine articles in English, Dutch and German have been published. Three radio interviews were given on Dutch radio stations. A website with complete coverage of the dissemination efforts and two videos produced by NLR and presented via YouTube help to generate awareness of the concept for a broad public. It can be concluded that the dissemination activities of the project results have been very successful, especially in the Dutch press and for the (English speaking) aeronautical science community.
List of Websites:
All publishable outcome of the RECREATE projects is also being disseminated via the public RECREATE homepage: www.cruiser-feeder.eu. Contact information is also available via the webpage.
