Next Generation Ground Testing for Spacecraft Re-entry

通过我提出的研究，我打算使未来的太空探索任务能够进入我们的太阳系，这在以前是不可能的。重返大气层的航天器会受到极端的热负荷，这种负荷可通过烧蚀防热罩来减轻。然而，飞行器周围的极高速流动的物理过程，以及烧蚀热屏蔽对流动的影响仍然没有很好地理解，导致热屏蔽质量的过高的安全裕度。隔热罩变得太重，并阻止遭受高热负荷的任务，如行星探索或样品返回场景。我将使用我们新的高速风洞T6来实验研究这些高焓流，并将T6升级为一种新型的混合设施，可以在飞行温度下对由真实的隔热材料制成的模型进行超高速测试。T6是新建的，于2018年投入使用，是欧洲唯一实现高达18 km/s的相关高速流条件的设施。等离子体发生器将被集成到T6的架构中，以便在模型暴露于高速气流之前对其进行预热。这保留了烧蚀-流动耦合的特征，并首次允许在空气动力学相似的流动中的真实的烧蚀比例模型，并使以前无法访问的影响的调查成为可能，并将使T6成为世界上第一个同类产品。我计划进行三种不同类型的实验，以超高速地球再入为目标：激波管中的激波层辐射研究，高超音速流场中再入舱的亚尺度模型测试，以及将T6升级为全新的混合等离子体脉冲设施。将通过激波管中的等效激波来模拟进入舱前形成的正激波。冲击波通过管中的窗口，在那里它被发射和吸收光谱询问。这允许空间分辨的温度，粒子密度和辐射热通量的测量。发射测量将与已经到位的实验装置进行，我将扩展到也包括吸收光谱。T6的铝激波管具有目前同类设备中最大的管径，这导致测量信号显著增加，从而获得新的高精度数据。我的目标是模拟高速地球重返大气层的气流条件，比如日本太空舱隼鸟号（Hayabusa）重返大气层时遇到的情况。此外，我将探讨下一代火星样品返回案例的使命方案。在激波管测试的基础实验之后，下一步是移动到模型周围的全流场。该模型将配备表面传热和压力传感器，以及光纤耦合到光谱仪的端口。这个实验将允许研究真实的几何形状周围的化学反应流，因此代表了激波管实验复杂性的额外增加。这将允许直接比较大量的数值模拟和直接测量的真实的飞行，在一个观察使命中捕获。在这个建议的方法的最后一步是把高焓地面测试到一个新的水平。产生等离子体，并通过喷嘴膨胀到模型所在的测试部分。在充分的等离子体加热后，模型已经达到飞行温度并开始分解。此时，超高速流开始，等离子体发生器同时关闭，剩余的等离子体被隔膜爆发的冲击冲出来。随后的流动现在面临着一个模型在飞行温度下，再现重要的以前无法访问的影响，如吹热屏蔽产品，表面氧化和表面复合。

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.