One option for a future air transport system is the use of supersonic vehicles which can reach the antipodes in a few hours. In Europe, very limited research has been carried out in the field of supersonic transport vehicles above Mach 3. Concorde and other studies on supersonic transport in America and Japan limit the flight speed to Mach 2 to 2.4, which still allows the use of classical aluminium alloys.
For high-speed aircraft, the lift to drag ratio of the vehicle and the material and cooling issues for both airframe and engine are some of the key elements which force the designer to limit the flight Mach number.
A wide range of heat-resistant and lightweight materials is available nowadays but their definition and implementation requires the availability of vehicle system conditions and constraints.
Indeed, the expected benefits of economical, high-performance and high-speed civil-aircraft designs that are being considered for the future will be realised only through the development of lightweight, high-temperature composite materials for structure and engine applications to reduce weight, fuel consumption and direct operating costs.
While the LAPCAT project investigates propulsion systems for flight Mach number ranging between 3 and 8, this ATTLAS project looks into the vehicle aerodynamics and the testing of potential materials that can withstand the high heat loads encountered at these very high velocities.
The objectives of ATTLAS were:
- to evaluate two innovative supersonic aircraft concepts that will be able to provide acceptable levels of lift to drag ratios for flight Mach numbers ranging between 3 and 6;
- to identify and assess lightweight advanced materials that can withstand ultra high temperatures and heat fluxes enabling flights above Mach 3. At these high speeds, the classical materials used for airframes and propulsion units are no longer feasible and need to be replaced by high-temperature, lightweight materials, with an active cooling of some parts.
First, the overall design for high-speed transports was revisited to increase the lift/drag ratio and volumetric efficiency through the 'compression lift' and 'waverider' principles, taking into account sonic boom reduction.
Second, materials and cooling techniques and their interaction with the aero-thermal loads were addressed for both the airframe and propulsion components. The former focused on sharp leading edges, intakes and skin materials coping with different aerothermal loads, the latter on combustion chamber liners.
After material characterisation and shape definition at specific aero-thermal loadings, dedicated on-ground experiments were conducted. Both Ceramic Matrix Composites (CMC) and heat resistant metals were tested to evaluate their thermal and oxidiser resistance.
In parallel novel cooling techniques based on transpiration and electro-aerodynamic s principles were investigated. Combined aero-thermal experiments tested various materials specimens with a realistic shape at extreme aero-thermal conditions for elevated flight Mach numbers.
Dedicated combustion experiments on CMC combustion chambers allowed the reduction of combustion liner cooling resulting into NOx-reduction and overall thermal efficiency increase.
Finally, particular aero-thermal-material interaction strongly influence the aerothermal loadings. Conjugate heat transfer, transpiration cooling and compressible transition phenomena were investigated and modelled.
The study has shown that a cruise efficiency above three, i.e. L/D ratio times the propulsion efficiency, is recommendable for a long-haul cruiser. This can be achieved with the newly designed kerosene based Mach 3.5 vehicle M3T which is also well above the Concorde's figure of merit. With a 300 tons gross take-off weight, the 200 passenger vehicle achieves a range beyond 10 000 km (5 500 NM) after a 150 ton fuel burn. The hydrogen powered Mach 6 vehicle is however rather disappointing even after a dedicated optimisation process.
With a GTOW of 278 tonnes including the 110 tonnes of hydrogen fuel, the 200 passenger vehicle's range could be brought up with 10 to 20 % to 7 400 km (4 000 NM) which is still below the envisaged 9 000 km. This doesn't mean a Mach 6 is intrinsically not conceivable, but indicates rather that a 'classical' design as proposed by Lockheed is not recommendable and should be avoided. A different architectural design or an improved engine design, including intake and nozzle, is needed to make it attractive. This perspective is not out of scope as a Mach 5 A2 vehicle conceived during the LAPCAT project can achieve this critical cruise efficiency. The better performance for the latter is mainly due to a well designed engine concept. During the project, significant steps were realised and promising results demonstrated. Work has nevertheless to be continued to increase the TRL of these technologies and take benefit of the existing test facilities and multi-physics engineering, industrial and dedicated simulation tools.