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Long-Term Advanced Propulsion Concepts and Technologies

Final Report Summary - LAPCAT (Long-Term Advanced Propulsion Concepts and Technologies)

In April 2005, the EC kicked-off a three years long project called 'Long-term advanced propulsion concepts and technologies' (LAPCAT) to initiate research on propulsion concepts for sustained hypersonic flight. The project was coordinated by ESA-ESTEC and was composed of a consortium of 12 partners from industry, research institutions and universities representing 6 countries.

The project objectives were to identify and assess critical propulsion technologies required to reduce long distance flights, e.g. from Brussels to Sydney, to less than two to four hours. Achieving this goal intrinsically requires a new flight regime for commercial transport with Mach numbers ranging from four to eight. At these high speeds, classical turbo-jet engines are not feasible and need to be replaced by advanced airbreathing engines. The identification of required breakthrough technologies goes hand in hand with the assessment of innovative propulsion concepts, and with steps towards their realisation.

The LAPCAT project allowed addressing some of the critical points crucially needed to enable high-speed transportation. Compared to classical aircraft designs, the difficulty for high-speed designs boils down in missing established rules and know-how to start off an iteration process for the different vehicle systems such as propulsion, structure, cooling, etc. Moreover, the high-speed aircraft require also a thorough integration of the propulsion unit with the airframe which is nearly non-existent for classical aircraft.

For the latter, the aerodynamic and engine layouts have little interference and they can be optimised nearly independently from each other. Though the importance of this interaction was anticipated by the team and reflected in the project layout, it demonstrated to be the main design driver.

Hence, the clear definition of the interfaces and the interactions of aerodynamics and propulsion need to be well addressed and assessed from the start. It also implicates that simplified but still representative models and tools need to be at hand for estimating the performance of the airframe and propulsion (intake, combustor and nozzle) during the first system design process.

Each of these modelling blocks needs to be well verified upfront by experimental and numerical means. LAPCAT has formed the basis for this methodology with the main emphasis on propulsion and combustion. Though each of these models have their strengths, they have also limitations and restrictions due to inherent assumptions. Hence it is also of importance to verify the overall vehicle performance including an operational propulsion unit on a more detailed scale.

These Nose-to-Tail (NtT) verifications should be performed both numerically and experimentally. Experimental set-ups of this kind are not evident and are hardly done. Having an engine at work during wind tunnel tests poses a lot of safety constraints and operational difficulties for the wind tunnel, not mentioning the complexity of the measurement techniques in hot environments and performance analysis such as net thrust and lift.

Numerical NtT verifications require three-dimensional (3D) computational fluid dynamics (CFD) techniques for the external aerodynamics with different levels of complexity for the propulsion part going from 0D to 3D. In particular, the latter approach is challenging in computer resources and robustness of the codes. These very same tools will then also allow to extrapolate to flight as there are no existing large high-speed ground facilities in Europe. These same NtT tools could then be used as the driver for a MDO-process to adapt the geometry or other parameters to improve the overall vehicle performance. A MDO approach has not yet been applied and adapted for high-speed cruise vehicles.

As new fuels, new flight altitudes and new speeds are considered, their impact on the environment needs to be explored as know-how is scarce for these applications. The use of hydrogen results in larger water vapour contents and Nox generation released at altitudes above 20 km. Similarly, the sonic boom impact needs to be assessed in terms of atmospheric propagation, sound level, carpet width and possible mitigation measures. Hydrogen as fuel has indicated its prominent role to achieve long-haul flight. Its cryogenic storage and large heat capacity are certainly sufficient to reject the integrated heat load. From this perspective, kerosene would already be disregarded as fuel. Only liquid methane could provide as hydrocarbon fuel this capability. Due to its natural abundance, methane would be worthwhile to explore its potential on vehicle system level.

The LAPCAT project was a very unique opportunity to group for the first time such a wide supersonic/hypersonic community at European level and work on a common EC project related to civil air transportation. Where each of the partner had a particular background and expertise needed for this program, everybody has also clearly realised that their research is only a part of a complete system and therefore their output depends a lot on the conditions dictated by the vehicle system. The outcome of LAPCAT will certainly form a guideline for the individual groups in identifying the conditions and topics of future research programs. It certainly has driven the project layouts of ATLLAS end LAPCAT II on European level.

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