NASA Going With External Resources on Orion Heat Shield

Last week, I posted an introduction to the heat shield issues on the Orion capsule used during the Artemis I mission at the end of 2022 as I'm trying to gather as much info as I can. Today, Ars Technica's Stephen Clark posted a followup article on the topic, emphasizing that NASA has asked a panel of outside experts to review their investigation. 

As should be obvious, this is potentially a very serious issue. 

Since the Artemis I mission, engineers conducted sub-scale tests of the Orion heat shield in wind tunnels and high-temperature arcjet facilities. NASA has recreated the phenomenon observed on Artemis I in these ground tests, according to Rachel Kraft, an agency spokesperson.

One of the things that caught my eye was a statement by made by Victor Glover, pilot for the Artemis II mission, in a recent interview with Ars. “More than any picture or report, I've seen that heat shield, and that really set the bit for how interested I was in the details.” His summary of the analysis and testing so far was more pointed. “There's no guarantee that changing the trajectory is the answer.” That has been the standard answer so far. Change the trajectory to reduce the demands on the heat shield. 

"In late April, NASA chartered an independent review team which includes experts outside the agency to conduct an independent evaluation of the investigation results," Kraft said in a statement to Ars. "That review, scheduled to be complete this summer, ensures NASA properly understands this condition and has corrective actions in place for Artemis II and future missions."

For his part, Glover thinks they have a good team and they'll eventually find the answer. “We’ve got the right people. If there is a solution, we’ll figure it out.” 

But settling on a solution will only come after NASA determines what caused the Orion spacecraft's heat shield to perform the way it did. And there's no guarantee NASA will find a definitive root cause. While engineers have recreated the char loss on sections of Orion's heat shield in ground tests, they can't test the full 16.5-foot-diameter (5-meter) heat shield on the ground or replicate the exact material response or flight environment experienced on Artemis I.

"We have a lot of extrapolation for scale that we have to do," said Jeremy Vander Kam, deputy manager for Orion's heat shield at NASA's Ames Research Center, shortly after the Artemis I mission in 2022. "Similarly, our test facilities can't reach the combination of heat flux, pressure, shear stresses, etc., that an actual reentering spacecraft does. We're always having to wait for the flight test to get the final certification that our system is good to go."

I know some of you have worked in fluid dynamics, which is what we're talking about here. The Artemis capsule reenters at 25,000 mph, turning the atmosphere into superheated plasma that makes those plasma photos from Starship look like someone was just shining a pink light on it. The interaction of that plasma with the heat shield is what they're trying to model and understand. This isn't laminar flow like a barely flowing river, this is more like full-tilt, raging whitewater. 

"This is a very, very complicated thing," Glover said. "The heat flow alone is really complicated to understand—the physical dynamic forces of all that wind and plasma swirling around. Imagine looking at a raging river, a whitewater river, and trying to analyze just one spot. What's happening at that spot? Recreate it. Draw it. It's really complicated."

The other issue is that the capsule for Artemis II is already built and undergoing tests at the KSC. To modify the heat shield now would require disassembling the Orion crew module from its European service module, and returning it to a place where that work could be done. That would delay the Artemis II launch, probably by a year or more beyond its current target schedule in September 2025. 

A view through the window of the Orion spacecraft during the Artemis I reentry. All those streaks in the bottom of half of the frame, white or yellowish to orange or peach-like are small pieces of material coming off the heat shield. Image Credit to NASA. I edited the photo slightly to enhance the contrast.

Again, the approach to reducing this problem being talked about is to modify the reentry. The problem there is that there are only a handful of possibilities and the whole approach is based on the thought that a different reentry reduces the stresses on the shield. But according to Glover, they saw chunks flying off the heat shield (their euphemism is "liberation") very early in the reentry when those stresses were lower. "If the damaging pieces start very early, there's no guarantee that changing the trajectory is the answer. It will change something, but it won't necessarily fix it. So we need to understand the root cause, if it's knowable."