“We have since determined that as the capsule dipped in and out of the atmosphere, as part of that planned skip entry, heat built up in the outer layer of the heat shield, causing gases to form and become trapped in the heat shield,” says Pam. Melroy, NASA Deputy Administrator. “This caused internal pressure to build up and led to cracking and uneven shedding of that outer layer.”
An independent team of experts agreed with NASA's determination of the cause, Melroy said.
NASA Administrator Bill Nelson, Deputy Administrator Pam Melroy, Associate Administrator Jim Free and Artemis II Commander Reid Wiseman speak to reporters Thursday in Washington, DC.
Credit: NASA/Bill Ingalls
Counterintuitively, this means that NASA engineers will feel comfortable with the safety of the heat shield if the Orion spacecraft enters the atmosphere at a slightly steeper angle than on Artemis I and spends more time at higher temperatures.
When the Orion spacecraft climbed back out of the atmosphere during the return of the Artemis I skip, a period known as the skip-down, NASA said the heating rate slowed and thermal energy built up in the Avcoat material of the heat shield. This caused gases to form in the heat shield through a known process as pyrolysis.
“Pyrolysis is just burning without oxygen,” said Amit Kshatriya, deputy associate administrator of NASA's Moon to Mars program. “We learned that as part of that response, the permeability of the Avcoat material is essential.”
During the skip stay, “the production of those gases was higher than the permeability could tolerate, causing a pressure difference. That pressure led to cracks in the plane of the outer mold line of the vehicle,” Kshatriya said.
NASA didn't know this could happen because engineers tested the heat shield on the ground at higher temperatures than the Orion spacecraft encountered during flight to prove that the thermal barrier could withstand the most extreme possible heating during reentry.
“What we missed was this critical region in the center, and we missed that region because we didn't have the test facilities to produce the low-level energies that occur during skipping and stalling,” Kshatriya said on Thursday.
During the investigation, NASA replicated the charring and cracking after engineers devised a test procedure to expose Avcoat's heat shield material to the actual conditions of Artemis I's reentry.
So for Artemis II, NASA plans to adjust the reentry trajectory to shorten the dwell time of the skipped reentry. Let's add some numbers to illustrate the difference.
The distance traveled by Artemis I during the return phase of the mission was according to Kshatriya. This downward distance will be limited to no more than 1,775 nautical miles (2,042 miles; 3,287 kilometers) on Artemis II, effectively reducing the residence time the Orion spacecraft spends in the lower heating regime that led to the cracking of Artemis I.
NASA's inspector general report in May included new images of Orion's heat shield that the agency initially did not release after the Artemis I mission.
Credit: NASA Inspector General
With this change, NASA engineers don't expect to see the heat shield erosion they saw on Artemis I.He The gas generation that occurs during that flashover residence time is sufficiently low that the environment for crack initiation will not overwhelm the structural integrity of the char layer.”
For future Orion spacecraft, NASA and its Orion prime contractor, Lockheed Martin, will implement changes to address the heat shield permeability problem.
Waiting for what?
NASA officials discussed the heat shield issue and broader plans for the Artemis program during a news conference in Washington on Thursday. But the timing of the event added a layer of disbelief to much of what they said. President-elect Donald Trump, with SpaceX founder Elon Musk in his ear, has promised to cut wasteful government spending.
