Engineers and technicians stack the left aft assembly for the Artemis II SLS (Space Launch System) solid rocket booster inside the Vehicle Assembly Building at NASA’s Kennedy Space Center in late November. Credit: NASA/Glenn Benson
Investigation into Artemis I heatshield reveals link between material permeability and skip reentry maneuver.
On December 11, 2022, the Orion spacecraft from NASA’s Artemis I mission streaked into Earth’s atmosphere at more than 24,500 miles per hour at the end of a 25-day test flight that took it as close as 62 miles to the surface of the Moon. Unlike NASA’s Apollo missions, when Artemis I hit Earth’s upper atmosphere, it employed a maneuver known as skip entry, entering the atmosphere, skipping out of it, then reentering.
Skip entry decreases a spacecraft’s velocity while increasing the distance it can travel through Earth’s atmosphere from its entry point. Where the Apollo spacecraft could travel 1,752 miles after entry, Orion can fly as much as 5,524 miles. This gives Artemis missions much greater latitude and precision in reaching the splashdown point where rescue ships are waiting.
When the recovery team aboard the USS Portland pulled the Orion capsule out of the Pacific Ocean, the charred heat shield, which bore the brunt of nearly 5,000° F reentry temperatures, showed that an ablative material known as Avcoat wore away differently than engineers expected, with small pieces of the char layer cracking and breaking off the spacecraft, rather than slowly eroding away.
“The root cause investigation began almost immediately after our observations and our team has been pretty relentless in focusing on every possible root cause,” said NASA Deputy Administrator Pam Melroy, speaking at a recent NASA press conference.
After NASA’s Orion spacecraft was recovered at the conclusion of the Artemis I test flight and transported to NASA’s Kennedy Space Center in Florida, its heat shield was removed from the crew module inside the Operations and Checkout Building and rotated for inspection. Credit: NASA
The investigation was sweeping, with NASA bringing in an array of experts from the U.S. Department of Defense, the U.S. Department of Energy, the aerospace industry, and more. The teams used facilities across the U.S. to analyze the heat shield, replicate the issue, and determine the cause.
“They started with a fault tree assessment that led directly to … advances, not just in the understanding of the interactions of the vehicle’s material loss properties and its interaction with the aerothermal environment, but [they] also invented and innovated new tests and non-destructive evaluation techniques for us to be able to actually observe this phenomenon in real time in the ARC jets, and the upgrades to the ARC jets that we created as part of this investigation,” said Amit Kshatriya, deputy associate administrator of NASA’s Moon to Mars Program Office, speaking at the press conference.
Artemis I was an uncrewed test flight. NASA collected data about the heat shield through a suite of pressure sensors, strain gauges, and thermocouples embedded in the ablative material during spaceflight. Combined with about 200 Avcoat samples cut from the heat shield for analysis, this data enabled the team to develop a comprehensive picture of what happened during the reentry of Artemis I.
One of the keys to the investigation came when the team was able to link permeability of the Avcoat with cracking and char loss.
“The heat shield was not uniform in terms of its permeability. There were places where it was actually more permeable than the rest, the small percentage,” Kshatriya said. “And in those places, we did not witness in-plane cracking. That was the key clue for us.”
“So, we were able to take that and then reproduce it in multiple different environments and multiple different facilities—the generation of those cracks and the environment under which those cracks would exist,” Kshatriya added.
Technicians at Textron in Wimington, MA, apply Avcoat ablative material to the composite honeycomb structure attached to the Exploration Flight Test-1 (EFT-1) Orion heat shield carrier structure on May 22, 2013.
Credit: NASA/Radislav Sinyak
The team determined that during the skip entry, between the first entry and the second, because of the specific characteristics of that moment, hot gases built up at a rate faster than the Avcoat’s permeability could release it in many areas of the heat shield. As the pressure built up, it created in-plane cracking, and some char material was released.
“So, our path forward is pretty clear,” Kshatriya said. “We know, because of the test program that we executed, we know how to produce Avcoat with the desired permeability standards. We have sized up all of our production capability in order to quickly produce heat shields. But it’s still going to take us a little bit of time to finish the qualification of that material. That should be ready to go sometime this year.”
Because the issue was traced to an interaction between the heat shield and the skip entry trajectory, Artemis II, which is fitted with a similar heat shield, will use a different trajectory for reentry, avoiding the buildup of gases. Beginning with Artemis III, NASA will begin using heat shields with the permeability standards identified during the investigation for use with skip entry.
On December 5, NASA’s executive council met, accepting unanimous recommendations to move forward with the current Artemis II Orion capsule and heat shield, using a modified entry trajectory that will prevent gases from building up in the ablative layer. This clears the way for the team of Artemis II to move toward an April 2026 target launch date. The launch target for Artemis III, which will return humans to the surface of the Moon, is mid-2027.
“We are now, as we speak, stacking the solid rocket boosters and then comes the Artemis II core stage, which is in the [Vertical Assembly Building at Kennedy Space Center]. We’ll stack that and the launch vehicle stage adapter. All of this is going on for this next step. The crewed flight test. The safety of our astronauts is always first in our decisions. It is our North Star. We do not fly until we are ready. We do not fly until we are confident that we have made the flight as safe as possible for the humans on board,” said NASA Administrator Bill Nelson, speaking at the press conference.
To learn more about the Artemis Program, click here.
