Hypervelocity impact in stellar media: Spacecraft Heat Shield study in DIII-D* *

A study of carbon ablation at high heat flux relevant to hypervelocity spacecraft entries was performed in the DIII-D tokamak as part of the Frontiers in Science campaign. Exploration missions to the Solar System’s gaseous giants and hyperbolic re-entries into the Earth’s atmosphere require spacecraft heat shields that can withstand high velocity (>10 km/s) and extreme heat flux (>10 MW/m2). Testing and modeling [1,2] material performance in this regime is challenging due to lack of adequate ground testing facilities. Conditions in DIII-D L-mode edge plasma reproduce the flow velocity and high heat flux experienced during the Galileo probe’s entry into the atmosphere of Jupiter. Three types of samples were used for the experiments: stationary graphite rods [3] protruding from the floor of the vessel, 1-mm-diameter porous carbon spheres, and 700-micron-diameter glassy carbon spheres injected from the floor into the edge plasma. In the graphite rod experiments, the mass loss rates as a function of heat fluxes determined from an extensive array of spectroscopic measurements are found to agree with semi-empirical ablation models obtained from spacecraft flight data. Experimental results for the pellet trajectories and mass loss rates of the porous and glassy carbon pellets are confirmed using the UEDGE-DUSTT simulations. These pellet experiments are also compared against simulations of carbon-based meteorite atmospheric entries [4]. It is found that due to thermo-mechanical stress the glassy pellets shatter fragments with sharp edges. Similar spallation was observed during Galileo probe’s entry into the Jovian atmosphere and is also expected to affect survivability of meteors in Earth’s atmosphere. We demonstrate that scaling between DIII-D experimental results, available flight data, and numerical models can be used to address questions ranging from optimization of heat shields for future planetary missions to understanding extraterrestrial delivery of organic material to planet surfaces.

[1] Matsuyama S., J of Thermophysics and Heat Transfer 19, no. 1 (2005): 28–35

[2] Park C., J of Thermophysics and Heat transfer 23.3 (2009): 417-424.

[3] D.M. Orlov, ASME IMECE2021-73326 (2021).

[4] C. Mehta, Life 2018, 8(2), 13