Next Generation Ground Testing for Spacecraft Re-entry

With my proposed research, I intend to enable future space exploration missions into our Solar System that have not been possible before. Re-entering spacecraft are exposed to extreme heat loads, which are mitigated by ablative heat shields. However, the physical processes of the extreme high speed flow around the vehicle, and the influence of the ablating heat shield on the flow are still not well understood and result in exorbitant safety margins for the heat shield mass. Heat shields become too heavy and prevent missions that suffer from high heat loads like planet exploration or sample return scenarios.
I will use our new high-speed wind tunnel T6 to investigate these high-enthalpy flows experimentally, and upgrade T6 to a novel hybrid facility that enables hyper-velocity testing of models at flight temperatures that are made of real heat shield materials. T6 is newly built, commissioned in 2018, and is Europe's only facility to achieve the relevant high-speed flow conditions of up to 18 km/s. A plasma-generator will be integrated into the architecture of T6 to pre-heat models before they are exposed to the high-speed flow. This retains the characteristics of an ablation-flow coupling and allows for the first time a real ablating scaled model in an aerodynamically similar flow and enables the investigation of effects that were previously inaccessible and would make T6 the first of its kind world-wide.

I plan to conduct three different types of experiments that target hypervelocity Earth re-entry: Shock layer radiation studies in a shock tube, sub-scale model testing of a re-entry capsule in a hypersonic flow field, and the upgrade of T6 to an entirely novel hybrid plasma-impulse facility. The normal shock formed in front of an entry capsule will be experimentally simulated through an equivalent shock travelling through a shock tube. The shock passes a window in the tube where it is interrogated by emission and absorption spectroscopy. This allows the spatially resolved measurement of temperatures, particle densities, and radiative heat flux. Emission measurements will be conducted with an experimental setup that is already in place, which I will extend to also include absorption spectroscopy. The Aluminium shock tube of T6 has the largest tube-diameter of current comparable facilities, which leads to a significant increase of measurement signal enabling new high accuracy data. I will target flow conditions that replicate high-speed Earth re-entry, such as encountered during the re-entry of the Japanese capsule Hayabusa. In addition, I will explore next generation mission scenarios for a Mars sample return case.

The next step after the fundamental experiments of shock tube testing is moving to a full flow field around a model. The model will be equipped with surface heat transfer and pressure sensors, as well as ports for optical fibres coupled into a spectrograph. This experiment will allow the investigation of the chemically reacting flow around a real geometry and therefore represents an additional increase in complexity from the shock tube experiments. This will allow the direct comparison to a wealth of numerical simulations and direct measurements of the real flight that were captured during an observation mission.

The final step in the methodology of this proposal is to bring high enthalpy ground testing to a new level. A plasma is generated and is expanded through a nozzle into the test section where the model is located. After sufficient plasma heating the model has reached flight temperature and starts to decompose. At this moment, the hyper-velocity flow is started, the plasma generator is switched off simultaneously, and the remaining plasma is flushed out by the incoming shock of the diaphragm burst. The subsequent flow now faces a model at flight temperature that reproduces important previously inaccessible effects like blowing of heat shield products, surface oxidation and surface recombination.

The excitement of space exploration will encourage young students to pursue careers in science and engineering. To fuel this process, regular news releases will be provided through social media (Twitter, Facebook) and through the hypersonics group website and NWTF website. Exciting videos and images with simplified explanations will be uploaded. These will be passed to the University of Oxford outreach team to support of their advocacy of STEM. The research will further be brought to public attention through means I already am familiar with: An exhibition at the Oxford science festival in year 2 and 3 and through a collaborative art and science project in year 4. I will work together with our outreach officer to explore new avenues to reach the broader public. Regarding the long-term impact of my proposed project, the research produced will ultimately expand our current capability to explore the solar system and help us understand the nature of the universe. I will focus on conveying this message in the public engagement campaigns.

The upgrade of T6 to a novel hybrid plasma/hypervelocity facility will benefit other academics in the UK through the National Wind Tunnel Facility scheme which allows external acces to T6. The direct collaboration with the German DLR will enable a broader impact to academic colleagues in the field. The combination of two different testing principles in the upgraded version of T6 will bring the two communities of plasma and hypervelocity testing together and spark new collaboration possibilities. In addition, other related fields of plasma and nuclear physics, gas dynamics, chemical kinetics, and internal combustion engineering will have an interest in T6 as a facility to investigate high-temperature chemical kinetics.

The understanding produced and experimental capability will be the groundwork of novel thermochemistry modelling into the future. This will give further European and UK re-entry capability (ESA, UKSA) and improve defence capabilities of the UK (DSTL). The engagement of DSTL with the group's word class facilities and research will boost the effectiveness of how colleagues can perform research and engage with academia. I have already started this process with DSTL's materials group through the EPSRC funded transpiration cooling grant, however, the proposed research could expand its impact to different departments. I will invite members of DSTL and EOARD to regular seminar meetings of the hypersonics group where I will present the state of my current research, giving external parties the opportunity to influence and steer the testing matrix of my proposed research.

Airbus Space and Defence and Fluid Gravity Engineering will have UK capability for future civil and defence applications which rely upon non-equilibrium thermochemistry modelling which will be improved in this research programme. Implementation of new non-equilibrium thermochemistry models into the FGE codes will give it an initial advantage in aerothermal prediction software. Unique experimental testing capability will secure participation in future international space programmes.

Development of non-equilibrium thermochemistry and ablation coupling has direct impact in future capabilities for the UK defence through the improvement of knowledge on missile signatures and interactions with vehicle aerodynamics. The access to testing of ablation-flowfield interaction will be unique and give the UK defence sector an advantage internationally. Through the involvement of DSTL in the hypersonics seminar this capability can be directly exploited.