Question: I get the basics of how heat shields protect reentering spacecraft, and I heard they had some issues with the Artemis heat shields. Made me curious to learn more about them, as the Artemis 2 crew needs to depend on it for a safe return from the Moon. Could you clue me in on the science of heat shields? I’m sure there’s some amazing engineering involved. Thanks! — J.G. Cambridge, OH
Answer: There is a lot of interesting science behind heat shields. To cover all of it I’d need multiple posts, so I’ll just focus on the most critical science. As the graphic shows, the capsule (usually called the command module or crew module) is exposed to extreme heating from friction during reentry. Keeping that heat from reaching the crew, or payload, is the top priority.
When you reenter from low earth orbit (LEO), like from the ISS, you hit the atmosphere with a speed of around 7 km/s (17,500 mph). Friction with the air will heat the shield to 1650 °C (3000 °F). In the Artemis program, these capsules will be falling from a much greater distance — 385,000 km instead of 200–300 km. They’ll hit the atmosphere with a speed of 11.3 km/s (25,200 mph), heating the shield to 2760 °C (5000 °F).
NASA’s been using the same heat shield material since the Apollo missions. It’s a substance called Avcoat made of silica fibers in an epoxy resin. It’s an ablative material, meaning it’s designed to “evaporate” from the heat and form a thin boundary layer of gas between the shield and the impinging hot ionized air. This boundary layer limits heat transfer to the capsule. Behind the shield is a layer of high-performance ceramic insulation to further impede heat flow.
FYI, the Space Shuttle used a different technique involving refractory materials for its heat shield. These materials are extremely poor conductors of heat. They are actually ceramics made from pure quartz sand.
Despite all this engineering, in a post-flight analysis of the Artemis 1 heat shield, NASA identified more than 100 locations where the shield had lost large pieces of the ablative material during reentry. This is what it looked like back in the lab:
NASA, contractors, and an independent review team, launched an investigation. The analysis involved more then 100 tests at facilities across the country. Their conclusion was that the heat shield wasn’t porous enough to let the evaporating resin escape. Pressure built up inside it, and pieces of the shield blew away. Artemis 2’s heat shield uses a modified Avcoat formula with greater porosity.
The manufacturing process for Avcoat has evolved over the years, according to fabricator Lockheed-Martin. They originally had workers filling 300,000 honeycomb cells one by one with ablative material, then heat-cure and machine it to the proper smooth shape.
Today they use CAD to machine the Avcoat tiles (around 200 of them) to exactly fit into their positions. They go together like an easy 3D jigsaw puzzle and greatly reduce fabrication time. Tiles are bonded in place on the heat shield’s underlying carbon fiber skin using epoxy resin. They’ve already got the shield for Artemis 4 completed.
Here’s a look inside Lockheed-Martin’s fabrication plant showing an Artemis heat shield being assembled. Note the smooth contours that help direct air flow:
Of course, you need to keep the heat shield pointed in the general direction the capsule is moving. Steering is accomplished by multiple small engines called reaction control system thrusters. For Artemis capsules there’s an array of 12 thrusters that burn the reliable fuel hydrazine. The steering is normally automated, but the crew can also change the capsule orientation manually. It’s interesting to note that by varying the orientation of the heat shield relative to direction of motion, they can generate aerodynamic lift and extend the landing point downrange.
For the upcoming Artemis 2 mission, as an extra measure of safety, NASA has modified the reentry trajectory to follow a slightly steeper angle, resulting in a shorter downrange landing location and a shorter heating duration. This will limit how long capsule spends in the temperature range at which Artemis 1’s heat shield was damaged.
If you’d like to see what it looks like from inside a capsule during reentry, here’s a video of Apollo 11’s fiery descent. Strangely, I was unable to find it in NASA’s archives, so you’ll have to watch it on YouTube:
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