To reduce long-distance flights, for example from Brussels to Sydney, to less than two to four hours, advanced propulsion concepts and technologies need to be identified and assessed. This requires a new flight regime with Mach numbers ranging from four to eight. At these high speeds, classical turbo-jet engines need to be replaced by advanced air-breathing engines.
Two major directions on a conceptual and technological level were considered: ram-compression and active compression. The latter has an upper Mach number limitation but can accelerate a vehicle up to its cruise speed. Ram-compression engines need an additional propulsion system to achieve their minimum working speed.
The key objectives were the definition and evaluation of:
- different propulsion cycles and concepts for high-speed flight at Mach 4 to 8 in terms of turbine-based and rocket-based combined cycles;
- critical technologies for integrated engine/aircraft performance, mass-efficient turbines and heat exchangers, high-pressure and supersonic combustion experiments and modelling.
The project duration of 36 months is expected to result in:
- a definition of requirements and operational conditions on a system level for high-speed flight;
- dedicated, experimental databases on supersonic and high-pressure combustion and flow phenomena specific to high-speed aerodynamics;
- setting-up and validating physical models integrated into numerical simulation tools on supersonic and high-pressure combustion, turbulence and transition;
- feasibility of weight performance of turbine and heat exchanger components.
A sound technological basis for the industrial introduction of innovative advanced propulsion concepts in the long term (20-25 years) was provided, defining the most critical RTD-building blocks by developing and applying dedicated analytical, numerical and experimental tools along the following road map:
- two air-breathing engines for a commonly agreed reference vehicle(s) and trajectory point(s);
- dedicated combustion experiments on supersonic and high-pressure combustion, including potential fuels and interaction with flow-field turbulence;
- modelling and validation of combustion physics on the basis of chemical kinetics and fuel spray vaporisation models and turbulence affecting the combustion;
- aerodynamic experiments for major engine components (intakes, nozzles, full engines), interaction of vehicle and propulsion aerodynamics resulting in a database;
- evaluation and validation of advanced turbulence models to evaluate unsteady, separated flow regimes and to develop transition models based on intermittency-related parameters;
- performance prediction of contra-rotating turbines and light cryogenic fuel heat exchangers.
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.
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.
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.