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Aimed at improving heat shields and enabling exploratory missions to planets and moons with extreme atmospheric conditions, the work was carried out by scientists at the DIII-D National Fusion Facility at General Atomics, in San Diego, California.
Future missions to the outer Solar System will need sophisticated heat shield materials that do not currently exist, the researchers said. The extreme heating conditions needed to study new shield materials are very difficult to achieve on Earth, however.
During high-speed atmospheric entries of up to 160,000km/h – such as those required in missions to the gas giants – the atmospheric gas surrounding the spacecraft turns into plasma and spacecraft temperatures increase to more than 5,500ºC. To protect scientific payloads, heat shield materials burn – also known as ablating – in a controlled manner, pulling the excess heat away from the core of the spacecraft.
Previous heat shield testing has used lasers, plasma jets, and hypervelocity projectiles. No single method could simulate the exact heating conditions of a high-speed atmospheric entry, however. This meant that past models of heat shield behaviour sometimes over- or under-predicted ablation, with potentially disastrous results.
The new experiments at DIII-D demonstrated that the hot plasma created by a fusion reactor during operation offers a novel and potentially improved way of modelling heat shield behaviour, especially for entries into Venus or the gas giants, the researchers said.
“Certain regions of the plasma in DIII-D closely approximate the conditions created when heat shields impact planetary atmospheres at extreme velocities,” said Dr Dmitri Orlov of the University of California San Diego, who led the multi-institutional team. “Our intent with these experiments was to leverage both these conditions and DIII-D’s rich suite of diagnostic instruments to develop a more accurate model of heat shield behaviour.”
Dr Orlov, Dr Eva Kostadinova of Auburn University, Alabama, and a team of researchers used the facility to study the ablation rates of carbon samples under extreme conditions, and to refine predictive models for carbon-based heat shield behaviour.
“DIII-D features relatively long plasma discharges with well-controlled stable conditions at the edge, where the heat flux and the flow speed are similar to those experienced during atmospheric entries,” said Dr Kostadinova. “This allowed us to simulate some of the most extreme conditions heat shields have experienced, such as the entry of the Galileo probe to Jupiter’s atmosphere, without the need to launch our test samples at high velocities.”
The team gathered a range of valuable data on the behaviour of the samples. By using scaling techniques, they extrapolated the results to larger projectiles and longer exposures, which allowed for comparison with experimental data from previous space flight missions and other on-ground testing facilities. The results could help develop the advanced heat shield materials necessary for planned missions to Venus and the moons of Jupiter.
The American Physical Society research was funded by the US Department of Energy.
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