RECREATE – REsearch on a CRuiser Enabled Air Transport Environment

By Dries Visser (TU Delft – Aerospace Engineering)
Project website: www.cruiser-feeder.eu

The collaborative project REsearch on a CRuiser Enabled Air Transport Environment (RECREATE) is about the introduction and airworthiness of cruiser-feeder operations for civil aircraft. The RECREATE project, funded through the 7th Framework Programme of the European Commission, is currently being conducted by a consortium comprising ten European partner organizations, including Delft University of Technology. In this project, which started in 2011, so-called cruiser-feeder operations are investigated as a promising pioneering idea for energy efficient long-haul air transport in the second half of this century. In cruiser-feeder operations, feeder aircraft dock to long-haul passenger (cruiser) aircraft and transfer payload and/or fuel to the cruiser, to then return to the nearby feeder base.

The top level objective of the project is to demonstrate at a preliminary design level that cruiser-feeder operations (as a concept to reduce fuel burn and CO2 emission levels) can be shown to comply with the airworthiness requirements for civil aircraft. The underlying Scientific and Technological (S&T) objectives are to determine and study airworthy operational concepts for cruiser-feeder operations, and to derive and quantify benefits achievable, not only in terms of CO2 emission reduction, but also those attributable to economic, operational and passenger convenience.

The soundness of the concept of cruiser-feeder operations for civil aircraft fuel burn reduction was clearly demonstrated already during the initial stage of the project. Taking air-to-air refuelling operations as an example, a comprehensive analysis revealed a fuel burn reduction potential and a CO2 emission reduction of 20% for a typical 6000 nautical miles flight and a payload of 250 passengers (taking the additional fuel consumption of the tanker into account). This reduction potential can be considered large by any measure. It is obvious that the fuel benefits of multi-staging can also be enjoyed by conducting intermediate stops (rather than through in-flight refueling). However, it is readily clear that this would lead to a significantly increased travel time for passengers.

Benefit of multi-staging:  “40% more fuel is required to carry the same  payload over 15,000 [km] in one leg than in three legs.” (Green, 2003)

Benefit of multi-staging: “40% more fuel is required to carry the same
payload over 15,000 [km] in one leg than in three legs.” (Green, 2003)

Good progress has been made in the RECREATE project with regard to the top level objective, by showing the way forward on how to create identified missing regulations and new technology in order to achieve airworthiness in the future. A first iteration of identifying cruiser-feeder concepts of operation was made, taking the requirement of reaching airworthiness in the future into account. The benefits in terms of CO2 emission reduction have been derived and quantified for this first iterate. A conceptual and preliminary design study of the cruiser and feeder aircraft required has been made, and a first version of automatic flight control concepts necessary to achieve airworthiness and to reduce the workload of the pilots has been developed and tested. Concepts to transfer payload and fuel have been studied as well. The environmental impact of the concept was explored, both with reference to fuel burn, emissions and noise, highlighting the ways in which the concept can improve the current state of the air transportation system and contrasted with the impact on operations, economic viability and concept flexibility. Flight simulations to investigate the initial cruiser-feeder concept of operations have been recently started.

The RECREATE research project started out in August 2011 by exploring the design space of different cruiser-feeder concepts. Due to the pioneering character of the cruiser-feeder concept, it proved to be impossible to down-select all concepts to just one concept. Consequently, two RECREATE concepts have been retained for further investigation in the project. One RECREATE cruiser-feeder concept is based on transfer of fuel only. Realisation of this civil air-to-air refuelling concept can be projected in the not too distant future. Cruiser top level aircraft requirements have been specified as a 250 seat passenger aircraft with a nominal range of 2500 to 3000 nm, to be extended to long ranges with aerial refuelling. The corresponding tanker aircraft has to be capable to deliver 35.000 lb fuel and to loiter for a period of 4 hours along with a number of requirements concerning the refuelling process.

Air-to-air refueling of civil aircraft:  advanced configuration tankers equipped with a forward extending boom,  refuel (non-manoeuvring) cruisers from below, while maintaining typical cruise speed.

Air-to-air refueling of civil aircraft: advanced configuration tankers equipped with a forward extending boom,
refuel (non-manoeuvring) cruisers from below, while maintaining typical cruise speed.

The other RECREATE cruiser-feeder concept is an ultra-long-range cruiser with a propulsion system independent of consumables (nuclear), featuring a transfer of payload (passengers) by means of interchangeable, preloaded containers including a life-support system. Cruiser top level aircraft requirements have been specified for a 1,000 seat passenger aircraft. The nuclear-powered cruiser is envisioned to continuously conduct transoceanic flight cycles, remaining aloft for weeks at a time.

Nuclear-powered RECREATE concept: (left) feeder, (centre) cruiser, and (right) docking.

Nuclear-powered RECREATE concept: (left) feeder, (centre) cruiser, and (right) docking.

Cruiser-feeder airworthiness requirements have been studied for the first iteration concepts. Critical airworthiness, continued airworthiness, flight crew licencing and operations regulations for cruiser–feeder civil air transport operations have been identified and analysed, taking into account the identified systems requirements and design characteristics, and leading to identification of topics which could require specific new regulations and means of compliance for cruiser-feeder operations. A full evaluation of the civil European regulations CS-25 and in addition the military guidelines MIL-HDBK 516B and JSSG-2009 has also been performed. The main results are that for the non-nuclear propulsion related hazards, risk mitigating measures are judged to be feasible. For the nuclear propulsion related hazards on the other hand, an exception has been made. It has been decided to consider for the nuclear propulsion related hazards only long term feasibility, anticipating future technology to ensure equivalent levels of safety in the second half of this century.

The conceptual and preliminary design study of the aircraft required for both RECREATE initial concepts has delivered first conceptual and preliminary designs for both feeder, cruiser and boom. Three cruiser aircraft configurations have been developed at preliminary design level and are being evaluated. A conceptual study of a nuclear-propelled blended wing body aircraft has also been conducted.

Comparison of optimal 250 seat conventional cruiser designs: (left) a multi-stage design, featuring a 2500 Nm range, (right) a long-haul design, featuring a 7,500 Nm design range.

Comparison of optimal 250 seat conventional cruiser designs: (left) a multi-stage design, featuring a 2500 Nm range,
(right) a long-haul design, featuring a 7,500 Nm design range.

In the current system of long-haul air traffic operations, routes and flight timings are not organized such that full advantage can be taken of the cruiser-feeder configuration. In order to extract the maximum benefit from the cruiser-feeder configuration, careful re-organization and re-planning of cruiser routes, schedules and feeder operations is required. In order to make best use of the potential benefits, network optimizations have been conducted to account for interactions between cruiser and feeder efficiencies, locations of feeder bases, feeder flight ranges, and cruiser route extensions. Feeders with higher efficiency achieve the best results in traffic simulations at a feeding capacity of 2 or 3 refuelling operations close to the feeder base. Based on the outcome of the optimization study, apractical lower bound of fuel saving seems to be in the order of 10%, which, although substantially less than the theoretical savings potential, still implies a huge fuel saving compared to current industry standards.

In-flight refueling location: (left) refueling locations midway, (right) refueling points close to tanker base.

In-flight refueling location: (left) refueling locations midway, (right) refueling points close to tanker base.

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