Research Training Group SCRamjet
GRK 1095/2: Aero-Thermodynamic Design of a Scramjet Propulsion System for Future Space Transportation Systems
For future, reusable space transportation systems, as well as for hypersonic flight vehicles, the main design problem is to reliably sustain operation in supersonic combustion mode. This requirement also affects overall vehicle layout. A scramjet propulsion system is very likely to offer an economic alternative to classical, expendable and hence expensive rocket driven systems and is one of the key technologies for hypersonic flight. As a result, the main scientific objective of all the projects networked within the Research Training Group GRK 1095 is the design and the development of a scramjet demonstrator. This highly interdisciplinary field requires extensive use of experimental, analytical and numerical methods and tools. The demonstrator engine will be completely integrated and will include all required engine components such as forebody, inlet, isolator, combustion chamber and nozzle.
Several partly coupled problems in different scientific areas emerge in the design process. For example, in the field of aero- and gasdynamics, precompression of the airflow is of main interest. In particular, unsteady shock-boundary layer interactions in the ramp flow as well as the so called shock train, as the main part of the internal compression, have to be investigated. Other examples include the nozzle flow. In the field of aero-thermodynamics the main research activities are focused on supersonic combustion itself, which is, of course, one of the key-problems of the whole project. In addition, materials research is included. We look at possibilities to make fibre composites applicable for the thermally highly stressed combustion chamber. Finally, several subprojects are dealing with the analysis of the overall system, in preparation for the highly complex integration of the single components into the demonstrator. Besides the technical part of the project also a very wide educational program is included. So, on one side the scholars have the possibility to deepen their knowledge on different topics. On the other side the quality of the scientific work can be enhanced. A particular feature of the Research Training Group GRK 1095 is the involvement of three German universities as well as the German Space Agency (DLR). Thus, scientists of the Universität Stuttgart, the RWTH Aachen, the Technische Universität München and the DLR Cologne are working together in the scramjet field.
Over the last years, large international efforts were undertaken to develop an air breathing propulsion system, based on scramjet technology, for the flight in the hypersonic velocity regime. To prove that this technology is viable, experiments with demonstrators have to be carried out. At the moment, different kinds of demonstrators are in use or are planned. On the one hand, complete scramjet powered hypersonic flight vehicles can be used with a two-stage propulsion system. The first stage, a conventional rocket, is required to reach scramjet operation conditions. After stage-separation and ignition of the scramjet, the vehicle performs a self-powered aerodynamic flight. This concept has successfully been tested within the NASA Hyper-X program. Furthermore such a concept will be the basic idea of the European LEA testing program and the Japanese research activities at JAXA. On the other hand, single scramjet demonstrators have been used. Examples include the Russian KHOLOD hypersonic flight lab, a combination of a rotationally symmetric scramjet and a rocket. Here, the rocket permanently produces thrust, even during the operation of the scramjet. A clear analysis of the functional principle of the scramjet is hence very difficult. Another type of flying test bed consists of a scramjet demonstrator placed as a payload on the top of a rocket. Here, the rocket boosts the demonstrator to the apogee of its trajectory. Under the influence of gravity, the demonstrator accelerates and falls back towards the Earth's surface. When operational conditions are reached, the engine is ignited. This type of setup worked successfully within the Australian HyShot program and presents a guiding concept for the Research Training Group GRK 1095. To complete the list of activities in scramjet demonstrator testing, we also mention the French national military program PROMETHEE and the European LAPCAT program. This overview of research activities, all of them realised with enormous scientific and financial effort, clearly demonstrates the international top-ranking position of scramjet technology as a propulsion system for hypersonic vehicles or future reusable space transportation systems.
In Germany, in the year 2005, the Research Training Group GRK 1095/1 was established on the base of already existing scientific know-how in the fields of scramjet technology and hypersonic vehicles design. For more than 14 years, internationally well established basic results have been generated in the three former Special Research Centres (SFB 253, 255 and 259). These results and the already existing national and international scientific network, especially between the participating universities and institutes, enabled the working group in the last years to make a contribution to the actual research activities on scramjet systems. In the meantime, the GRK 1095 has been operated for about five years and has been very successful. The involved scientists have contributed a lot of new scientific results to the known literature on the physics of scramjet. It is this database, that currently represents the main advantage over all the other research groups and provides an excellent starting point for all planned future research activities. The available results, as well as all the developed methods and tools, are being enhanced and represent the foundation of the educational program of the scholarship holders working in the 23 subprojects. From the technical point of view, a scramjet is a highly integrated system with very strong interactions between all engine elements. Therefore, it is not possible to develop the components separately. As soon as the first isolated numerical simulations and wind tunnel test are done, it is indispensable to merge the components, i.e. forebody with inlet, isolator, combustion chamber and nozzle, into a demonstrator engine for further development. This interaction between the different components can be interpreted as an effigy of the cooperation and the networking of the whole Research Training Group.
Fundamental design concept
For the very successfully finished first program phase in the beginning a basically two-dimensional design concept was selected for the scramjet demonstrator which consists of a long double-wedge shaped forebody, an inlet with two outer compression ramps and moderate deflection angles, a subsequent diffusor (isolator), a supersonic combustion chamber and a single expansion ramp nozzle (SERN). The geometrical shape and basic dimensions result from the long-term goal to mount the demonstrator on a sounding rocket that boosts the payload to a specified height. At the apogee, the demonstrator separates from the rocket. As the system falls down to earth it gathers speed until the necessary velocity and dynamic pressure are reached to ignite the scramjet. Depending on the trajectory, a couple of seconds of supersonic combustion under real flight conditions are achievable.
After the very successful review in January 2009 and with respect to the main goal of the project in the actual running second phase the whole program moved more into the direction of the investigation of a possible flight experiment after the end of research training group. Consequently, some components of the whole concept had to be changed. The most important modification was done in the design of the inlet. The up to now used two-dimensional inlet design required relatively long outer compression surfaces with moderate ramp angles to reach the needed compression ratio without separation. Looking to the aerodynamic and stability requirements of a real flight configuration, the overall length of the complete inlet shouldn't be too large. So it was necessary to reduce the length while using a three dimensional inlet shape. Putting into account the complexity of a three-dimensional inlet flow, at the moment this seems to be a useful way for the whole program.
The now even stronger flight-experiment oriented design of the demonstrator is again used in order to have a technically relevant and applied test case, although the Research Training Group itself will not realize such a flight experiment. Flight dynamic problems, like stability and trajectory control, are also not addressed in the second term of the project.
Anyway the test case, a stationary flight at an altitude of 32 km and at a flight Mach number of 8, is still considered as a main "boundary condition for all involved single projects.
The main goal of the Research Training Group is the aero-thermodynamic design of a scramjet propulsion system which integrates all parts of such an engine, i.e. forebody, inlet, isolator, combustor and nozzle, and where each part is optimized for the chosen engine design point. Furthermore, thermomechanical analyses with regard to high-temperature materials for the combustion chamber as well as numerical system analyses of the complete engine are carried out. Even though the actual sounding rocket flight experiment is not part of the Research Training Group, the described scramjet demonstrator represents the overall objective of all projects involved. By aiming for that objective, each project develops individual goals which, however, strongly interact with other projects and therefore have been highly tuned to one another.
Due to the change in the inlet design it is of great importance to investigate the very sensitive three-dimensional inlet flow quite carefully. Especially the combination of ramp and sidewall compression has a great influence on the design setup of the complete engine. Consequently, experimental and numerical analyses have to be conducted to yield the influence of this specific forebody geometry on the inflow condition of the inlet and, thus, on the overall engine mass flux. Depending on the complex three-dimensional geometry and also on the flight conditions (Mach number, Reynolds number), the boundary layer conditions are analyzed to determine defined flow conditions for the subsequent outer compression which is of greatest importance for the experimental as well as numerical investigations of all downstream processes.
Furthermore, the impact of the three-dimensional boundary layer of the forebody combined with the cowl lip-shock on the flow physics inside the inlet and the subsequent isolator is of great interest. Especially, the state of the incoming boundary layer as well as separation effects is investigated. Moreover the stability of the shock system (shock oscillation) as well as the shock boundary layer interaction is in the focus of investigation.
The inlet, which consists of a three-dimensional configuration combined with the engine cowl, and the subsequent isolator are aiming for a maximum pressure gain for the combustor inflow and are designed with respect to the specific needs and restrictions of the demonstrator. The investigation of engine unstart and whether or not bleeding is necessary as well as the effect of small changes in the inflow (e.g., angle of attack) on the engine mass flux (spillage) is of major interest.
The combustion chamber is the main focus of the proposed engine concept, where the achievement of a stable and truly supersonic combustion, aside from optimized test conditions in a lab, is the objective of experimental as well as numerical investigations. In accordance with the requirements of the demonstrator, a combustor, equipped with adjustable walls made out of high-temperature ceramic fibers, will be designed in terms of thermomechanical as well as system analysis aspects and will be built in a later program phase. The investigations concerning the supersonic combustion process itself are manifold: the influence of turbulent/chemical interaction on ignition and the numerical analysis of flame stabilization in supersonic flow are investigated as well as the heat load on the central injector inside the combustor.
Finally, a cooling concept for the nozzle is designed using complex numerical tools in accordance to the outflow conditions of the combustion chamber and the requirements of a real flight system in order to optimize the achievable thrust.
All obtained results are used for an overall system analysis, which is constantly updated and extended and at least which ties all projects together. In the second phase a special focus is set on the aerodynamic design of a possible flight configuration and also on the required in-flight instrumentation to ensure testing data of a future flight experiment after the end of the Research Training School.
The different projects of the Research Training Group can be divided into three categories:
- Project group A: „Aero-thermodynamical analysis“
- Project group B: „Combustion“
- Project group C: „Nozzle flow and system analysis“
The three project groups are made up of the following projects:
|Project group A: Aero-thermodynamical analysis|
|A1||Experimental investigations of the boundary layer transition of a double ramp configuration|
|A2||Experimental investigation of the shock boundary layer interaction of a double ramp configuration at different inflow conditions|
|A3||Design and characterization of a 3D scramjet inlet|
|A4||Experimental investigation of the internal flow conditions of a scramjet engine|
|A5||Investigation of 3D flow structures caused by side wall effects of a scramjet engine|
|A6||Numerical simulations of the unsteady effects in a scramjet intake|
|A7||Computational analysis of the relaminarisation in hypersonic intake flows|
|A8||Numerical investigation of the sensitivity on the design of a hypersonic intake|
|Project group B: Combustion|
|B1||Experimental investigations of the fuel injection and mixing and stability of a supersonic combustor|
|B2||Numerical investigation of a supersonic combustion chamber for different flight conditions|
|B3||Thermal and mechanical investigations of a central injector in a scramjet combustor|
|B4||Numerical investigation of the turbulence-chemical reaction interaction and the emission production in a scramjet combustor|
|B5||Unsteady simulations of supersonic combustion chambers|
|B6||Development of a numerical efficient combustion model for the interaction between chemical reaction and turbulence for LES|
|B7||Shock-boundary layer interaction for reactive flows|
|B8||Prevention of thermal cocking in supersonic combustors by using staged injection|
|Project group C: Nozzle flow and system analysis|
|C1||Aerodynamic design of a flight configuration and the related instrumentation|
|C2||Numerical simulation of a nozzle flow with cooling|
|C3||Experimental investigation of the expansion flow of an air breathing propulsion system under consideration of the temperature gradient between jet and outer flow|
|C4||Multi-field formulation for functionally graded high-temperature materials|
|C5||Coupled simulations for the systematic design optimization of scramjets|
|C6||Investigation of the thermal loads of a hypersonic propulsion system|
|C7||Modeling of a scramjet propulsion system|
To reduce the duration of each individual doctoral project to three years instead of five, the study program is closely linked to the Research Training Group objectives. Interdisciplinary elements are strengthened to enhance the quality of the education. Here, special emphasis is placed on basic knowledge in the natural sciences and in engineering as well as in mathematics, always applied to the topic at hand. The experiences of former Research Training Groups are incorporated into the present study program. However, new elements are also added to attract excellent students as well as take into consideration the growing importance of experiences abroad. Special emphasis is placed on the following elements:
- Individual study program comprising graduate-level lectures
- Lecture series of the scientific members of the Research Training Group and guest researcher
- Summer schools
- Seminars of external scientists
- Annual colloquiums of the Research Training Group fellows
This list of proven elements has been augmented by four new elements:
- For each fellow an individually conceived stay abroad for six months is intended.
- Some elements of the study program are organized by the fellows themselves to encourage their scientific autonomy (e.g., seminars, internet-communication-elements)
- Appointment of a consultant industry panel
- Involvement of an external patent attorney to identify possible applications
The home institutions of the scientific members of the Research Training Group are the Universität Stuttgart, the RWTH Aachen, the Technische Universität München and the DLR. Due to the regional separation, modern web-based communication techniques are routinely used. The research data of all projects is stored and exchanged using a RAID server located at the Universität Stuttgart. Special emphasis is placed on the two Post Docs, who are responsible for a large part of the coordination of all projects involved in the Research Training Group.