Next Generation Ground Testing for Spacecraft Re-entry

通过我提出的研究，我打算使未来进入我们太阳系的太空探索任务成为可能，这在以前是不可能的。重新进入航天器的航天器暴露在极端的热负荷下，烧蚀的隔热罩可以减轻这种负荷。然而，车辆周围极高速流动的物理过程，以及烧蚀隔热罩对流动的影响仍然不清楚，导致隔热罩质量的安全裕度过高。隔热层变得太重，阻碍了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对这些高焓流动进行实验研究，并将T6升级为一种新型混合设备，可以在飞行温度下对由真实隔热材料制成的模型进行高速测试。T6是新建的，于2018年投产，是欧洲唯一达到相关高速流动条件的设施，最高可达18公里/S。T6的架构中将集成等离子体发生器，以便在模型暴露在高速流动之前对其进行预热。这保留了烧蚀-流动耦合的特征，首次允许在空气动力学相似的流动中建立真正的烧蚀比例模型，并能够研究以前无法获得的影响，使T6成为世界上第一个这样的模型。我计划进行三种不同类型的以超高速地球再入为目标的实验：激波管中的激波层辐射研究，高超声速流场中再入太空舱的小比例模型测试，以及将T6升级为一种全新的混合等离子体-脉冲设备。在进入舱前形成的正常激波将通过通过激波管传输的等效激波进行实验模拟。冲击波通过管子中的一个窗口，在那里它被发射光谱和吸收光谱所询问。这使得对温度、粒子密度和辐射热流的空间分辨率测量成为可能。排放测量将在已经到位的实验设置下进行，我将扩展到也包括吸收光谱分析。T6的铝制激波管的管径是目前同类设备中最大的，这使得测量信号显著增加，从而实现了新的高精度数据。我将针对复制高速地球重返大气层的流动条件，例如在日本太空舱Hayabusa重返大气层期间遇到的流动状况。此外，我还将探索火星样本返回案例的下一代任务场景。在激波管试验的基础实验之后，下一步是进入模型周围的全流场。该模型将配备表面换热和压力传感器，以及连接到光谱仪的光纤端口。这项实验将允许研究真实几何形状周围的化学反应流动，因此比激波管实验增加了额外的复杂性。这将允许与在观测任务中捕获的大量数值模拟和实际飞行的直接测量进行直接比较。这项提议的方法学的最后一步是将高焓地面测试提高到一个新的水平。产生等离子体，并通过喷嘴将其膨胀到模型所在的测试部分。在足够的等离子体加热后，模型已经达到飞行温度并开始分解。此时，超高速气流启动，等离子体发生器同时关闭，剩余的等离子体被隔膜破裂的冲击冲走。随后的流动现在面对的是一个在飞行温度下的模型，该模型再现了以前无法达到的重要效果，如隔热罩产品的吹扫、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.