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NASA High-Speed Wind Tunnel Testing to Improve Heat Shield Design

Mach 6 heat shield model test
A view of the Mach 6 wind tunnel from above. The round, white object in the center is the heat shield being tested.
Credits: NASA/David C. Bowman

Heat shields, meant to protect people and payloads as they traverse through the harsh conditions of entering an atmosphere, have been used by NASA since the Apollo era more than 50 years ago. But what happens as parts of the heat shield burn up during entry, and how does that impact their performance? NASA researchers are working to find out.

More than 100 ceramic models of various heat shield designs have been tested in NASA Langley Research Center’s Mach 6 wind tunnel since 2016. The facility allows testing at six times the speed of sound, which represents about 4,600 miles per hour at sea-level on Earth – fast enough to cross the Atlantic in just one hour. To recreate Mach 6 wind speeds, dry air is fed into a chamber, building pressure to a target, and is then accelerated through narrowing nozzles to hypersonic speed. The air rushes through the test section of the tunnel and over whatever model is there. As the air exits the test section through widening nozzles, it decelerates and is then drawn out via a vacuum pump.

This testing is part of the Entry Systems Modeling project within NASA’s Game Changing Development Program, which is developing new approaches to improve predictions of the spacecraft pressure and temperature entry environments and to simulate the responses of the heat shield systems to those environments.

Orion heat shield
The Orion heat shield uses blocks of ablative material to protect its crew during re-entry.
Credits: NASA/Isaac Watson

Many of the heat shield models tested represent different types of thermal protection systems similar to those on current and future NASA missions. An Orion-like heat shield made up of blocks of ablative material, or material meant to burn up; a woven thermal protection system like Low-Earth Orbit Flight Test of an Inflatable Decelerator; heat shields with panels like Mars 2020 and Mars Science Laboratory; and an ablating heat shield like the one being designed for the Mars Sample Return mission have all been tested.

LOFTID
The Hypersonic Inflatable Aerodynamic Decelerator heat shield uses a woven, flexible thermal protection system.
Credits: NASA/David C. Bowman

The ceramic models, built in Langley’s model shop, are covered in a thermographic phosphor compound – similar to what was used in old cathode ray tube televisions – that fluoresces, or glows, under ultraviolet light. Using a unique technique developed at Langley in the 1990s, researchers measure light coming off the model and correlate it to temperature changes, from which engineers can determine how hot different parts of the heat shield got during the simulated atmospheric entry.

Data is collected on how roughness of the heat shield models – caused by ablation, or burn-off of material, gaps in between panels, or material used to fill gaps between panels – affects overall heat shield performance under different environments and loads.

“The goal is to better define margins so that we don’t have to over-design,” said Brian Hollis, Mach 6 test lead for ESM at Langley. “These data, along with flight data from projects like the Mars Entry, Descent, and Landing Instrumentation 2 measurement on the Mars 2020 lander during atmospheric entry, will help us reduce the design margins, potentially saving weight and cost.”

The Entry Systems Modeling project is managed at NASA’s Ames Research Center in California’s Silicon Valley and funded by NASA’s Game Changing Development Program, part of NASA’s Space Technology Mission Directorate, in partnership with the Planetary Science Division of NASA’s Science Mission Directorate. Technical support of the project is spread across Ames, Langley, NASA’s Johnson Space Center in Houston, and NASA’s Glenn Research Center in Cleveland. ESM also has partnerships with other government agencies and many universities. The Game Changing Development program aims to advance exploratory concepts and deliver technology solutions that enable new capabilities or radically alter current approaches.