NASA isn't fixing the heat shield on Artemis II

Last week, NASA announced that they will delay the next flight of the Artemis program, Artemis II, from September 2025 to April 2026. They also announced the results of the comprehensive investigation into problems with the Orion spacecraft’s heat shield, and that the agency would fly Artemis II as-is, but with a modified re-entry trajectory to mitigate the issue.

Since then, NASA has come under fire from pretty much every angle. Some say NASA is being too conservative, because the charring issue would not have put astronauts in danger. Others think NASA is being way too cavalier in flying Artemis II as is; the heat shield has already been bolted on the spacecraft. To change the heat shield would mean a years-long delay, and some think NASA is too deadline oriented and is compromising on astronaut safety. Still others understand the decision but don’t trust NASA because of their reluctance to release information around the issue.

Let’s break it down.

Orion’s heat shield on Artemis I

Let’s start with a brief summary of the problems with Orion’s heat shield.

But the short story is that during the Artemis I flight, Orion’s heat shield behaved in unexpected ways. The heat shield covers the bottom of the capsule, 16 feet in diameter, and is made of a material called AVCOAT.

AVCOAT is the same ablative material that was used in the heat shield for the Apollo missions. We have a lot of experience with it and we know how it’s supposed to behave.

What happens is during re-entry into Earth’s atmosphere, Orion can experience temperatures of up to 5,000 degrees F or 2,760 degrees C. At those temperatures, Orion’s titanium skeleton and carbon fiber skin would basically melt. The AVCOAT ablates away, charring in order to protect the capsule (and the crew inside). Or, at least, that’s what it’s supposed to do.

After Orion touched down in December 2022, and examination of the capsule began, NASA noticed that the AVCOAT heat shield layer had chunks missing. The material was charred, and that’s supposed to happen.

What is not supposed to happen is that the charred material cracked and broke off in chunks. This happened in about 100 spots on the Orion capsule. NASA needed to know why.

How was Artemis I different than Orion’s previous flight?

The thing is, we’ve flown Orion before, on an Exploration Flight Test, or EFT-1 in December 2014. The heat shield has been tested and there were no problems on EFT-1. But there were some big differences between EFT-1 and Artemis I.

Let’s talk first about skip entry.

The velocity of coming back from the moon is much greater than orbital or sub-orbital re-entry velocity. So NASA takes steps to slow the Orion capsule down using a technique called skip entry. The capsule basically dips in and out of the atmosphere, which reduces the spacecraft’s velocity. When it pulls back up into the atmosphere, this is called a “skip dwell,” which will become important later.

This was a technique first attempted on Artemis I.

There were other relevant differences between Artemis I and EFT-1, though.

First of all, the basics: the re-entry velocity and re-entry temperature of the flight test were both lower than they were for Artemis I. EFT-1 was a four and a half-hour orbital flight test. It didn’t leave Earth orbit and certainly didn’t go to the moon. While NASA did raise Orion’s orbit to test the heat shield, re-entry velocity maxed out at around 20,000 mph / 32,186 kph and a temperature of 4,000 degrees F / 2,200 degrees C.

In contrast the maximum re-entry velocity for Artemis I was around 25,000 mph / 40,000 kph. That’s Mach 32 (32 times the speed of sound), and temperatures were up to 5,000 degrees F or 2,760 degrees C.

The application of AVCOAT was also different between Artemis I and the Orion EFT. For the Orion flight test, the AVCOAT was applied individually. Workers filled 300,000 honeycomb cells by hand, one by one, with the ablative material, after which they heat cured it. That’s also how it was done for Apollo. As you can imagine, this is a painstaking and labor intensive process.

For Artemis I, Lockheed Martin and NASA changed the application process (Lockheed Martin builds Orion, but it’s under a cost plus contract, which means NASA owns and operates the vehicle, and is intimately involved with every step and every decision. It’s very different from, say, NASA contracting with SpaceX for astronaut flights to the ISS.)

For Artemis I and Artemis II, they manufactured AVCOAT blocks that are carefully designed to fit a particular position and they’re bonded onto Orion’s carbon fiber skin. They worked with 186 blocks total, and each block is anywhere from one to three inches thick. It’s a huge time and efficiency savings, and it was supposed to work the exact same way. But this was the first thing that occurred to me when we learned about the heat shield problems.

Now, NASA hasn’t actually released the independent review team’s findings — I’ll talk about that decision a little later — so we have very little insight into how this all came together. But what we know now is that some of these differences did matter. Let’s break down what NASA disclosed last week.

The root cause of the liberated char on Orion

If you saw my coverage of Boeing Starliner, then you know NASA cares a lot about “root cause.” If astronauts had been aboard Artemis I, nothing would have happened to them. They would have returned safely, and wouldn’t have even experienced an increase in temperatures inside the cabin.

But NASA has had experience with crew returning safely when something is acting in a way it shouldn’t, and they’ve learned the hard way with both Challenger and Columbia that if they don’t take the time to understand the root cause of aberrant behavior, it can lead to tragedy. So they have basically spent the two years since Artemis I ended looking for the root cause to this issue.

NASA and the independent review team found that one of the issues that contributed to the char loss from Orion’s heat shield was the skip entry technique. What they learned is basically, during the skip dwell (which remember is when the capsule lifts back up into the atmosphere), the heating rates decreased and gases started to form inside the AVCOAT.

The other issue here is permeability of the AVCOAT. Basically, the ablated AVCOAT produces this gas when it burns without oxygen. But as a result, the AVCOAT must be permeable, because if it’s not, that’s what causes the gases to be trapped, versus being able to be released.

The lowered heat during the skip dwell produced less char but increased production of gases. These gases were unable to permeate the outer AVCOAT layer so the internal pressure within the heat shield built up. That led to cracking, and then to the liberation of the charred material.

Why didn’t NASA know this before Artemis I?

Now, there are a few reasons NASA was unaware of this issue before Artemis I. It’s important to remember that in a lot of ways, this was still a test flight. That’s why it was uncrewed, to test the entire vehicle, Orion and SLS, to iron out any kinks before putting crew on it. But still, why didn’t NASA catch this if they have a lot of experience using AVCOAT?

Well, first, the AVCOAT used during the Apollo mission was robust, and we had a lot of experience with it — but it was different in some ways than what’s being used on Orion. Specifically, there were materials used that we can’t use today because of environmental regulations.

Plus, a lot of knowledge on heat shield construction was lost when the Constellation program was cancelled under President Obama. (This is also a concern with the new administration, if they slash budgets and personnel, a lot of institutional engineering knowledge will be lost. We’ve seen it before). So, basically, there was a gap in knowledge on how to formulate the AVCOAT. That meant there were very small changes in the material between Apollo and now that affected performance.

On top of that, there is the issue of the AVCOAT blocks, which I mentioned previously. Because this was a big change, NASA and Lockheed Martin tested this extensively to make sure it worked before they put it on the Artemis I capsule. And they got the construction of it right.

The thing they missed is specifically in regard to the skip entry. They assumed that basically, if it worked higher in the atmosphere and lower in the atmosphere, it would work the entire way down. But they didn’t take the specific skip dwell time, and changing temperatures, into account. And when they were developing this technique and testing it, NASA did not have the capability to test the varied heating rates during skip entry (they have since developed the capability, which is how they were able to recreate the issue.)

Now, let’s talk about that block AVCOAT application. It turns out that the hand honeycomb technique of Apollo provided a separation between the AVCOAT.

It would absolutely crack, but because it was basically in a titanium honeycomb design, those cracks wouldn’t spread across the vehicle. When it’s in blocks, and the blocks are designed in such a way that they fit together seamlessly, if one area of AVCOAT cracks, it’s not limited to that area.

So, why don’t they just go back to this hand-applied method? Well, Orion is a lot bigger than Apollo. The Apollo command module was 12.8 feet in diameter versus Orion’s 16 feet. The material that the capsule itself is made of is also different. What NASA and Lockheed Martin discovered when they applied the AVCOAT by hand to the EFT capsule was that it was much harder to do than during the Apollo area. Because of the size and material of the capsule, it was flexing on them and it wasn’t a great process. That’s why they shifted to the blocks method.

What does this mean for Artemis II?

Artemis II is designed to be about 10 days long and it’s going to be the first mission with crew. Astronauts won’t land on the moon — the mission will go around the moon and come back to Earth on a free-return trajectory, but because of this they will experience those higher temperatures and the higher re-entry velocity, versus just hanging out in Earth orbit.

Now, Artemis II has been delayed from September 2025 to no earlier than April 2026. (The first moon landing, Artemis III, is now delayed from September 2026, a date that there’s really no way they could have made, to mid-2027.)

NASA didn’t make this direct connection, but the extensive Orion heat shield testing is likely the cause of that delay. There are other problems — ground systems issues, abort system fixes. But basically I think that they’ve been delaying big decisions because they’ve been waiting on the final word about the heat shield, and that makes it too strenuous to continue towards that September 2025 date.

The other big change is that NASA is altering the re-entry profile of Artemis II and moving forward with this heat shield, rather than pulling it off the spacecraft and making a new one. If they’d chosen to replace the Artemis II heat shield, that would have pushed Artemis II from September 2025 to late 2026/early 2027, and then Artemis III would have been the end of 2028.

It’s important to note that these changes will constrain available launch windows for Artemis II — so while Artemis I had a window every 12 days, Artemis II will have that cut by about 50 percent, which is considerable. This is going to be a much more difficult launch, and Artemis I was not easy if you recall (though extensive tanking tests should cut down on the propellant loading problems we had during that launch cycle.)

For the re-entry profile, one thing they’re doing is reducing the amount of skip dwell time. NASA believes the generation of gas that will occur during the modified Artemis II skip dwell time is low enough that they won’t build up enough pressure to crack the heat shield. They’ve run these tests and they are confident with their decision.

For Artemis III on, the heat shield will incorporate changes to make the AVCOAT more permeable and avoid the gas pressure buildup. But one concern for Artemis II is that the heat shield for that capsule is actually less permeable than the heat shield for Orion on Artemis I.

According to an article on Ars Technica, which included an interview with Paul Hill, the leader of the independent review team, one big concern was that in order to examine the strength of the attachment between the AVCOAT blocks and Orion’s carbon fiber skin, NASA uses ultrasound testing. They found this difficult for Artemis I because of the permeability of the AVCOAT, so they made the Artemis II heat shield even more impermeable — which means gases will build up even faster than they did for Artemis I.

Did NASA make the right decision?

As a result of this revelation, and generally the decision to fly a capsule with a known flaw in its heat shield, NASA’s come under some fire. It’s no secret that the agency is in a time crunch here. The rhetoric about beating China to the moon during this press conference was very interesting, considering I’d never really heard much about that from NASA previously. In my mind that makes it very clear they’re concerned about Trump slashing budgets because he’s not happy with the progress of Artemis and are trying to emphasize forward progress. That makes sense.

But NASA is not a deadline driven organization. Even at their darkest moments, during Challenger and Columbia, they weren’t caving to schedule pressures and knowingly compromising safety. There’s an amazing book on this called The Challenger Launch Decision by Diane Vaughan that I highly recommend that makes clear that the conventional wisdom about Challenger is wrong. It’s so influential that Vaughan was consulted during the Columbia accident investigation.

Instead, it was a combination of problems, including NASA’s organizational culture, that led to these tragedies. I had a lot of questions about whether their culture had changed after Columbia, and I dove into this question and NASA’s risk culture when discussing Boeing Starliner, which was the biggest safety issue the organization had faced since Columbia.

But I think the thoroughness of the process during that period showed a lot of us that NASA’s safety culture in intact. Their determination to find the root cause of the thruster issues, even as everyone was criticizing them, gave me some confidence.

And that extends to here. Paul Hill, the leader of the independent review board, was a flight director at NASA. He knows NASA inside and out. He also understands the organization at its worst moments — Hill led the accident investigation team for Columbia that was in charge of recovering debris, analyzing all the sensor information from various government agencies, and radar testing. He also was the flight director for the return to flight mission after Columbia, and he understood intimately the thoroughness necessary for an investigation like this.

There have been questions about whether NASA is caving to time pressure by using the Orion heat shield for Artemis II. But Hill said to Ars Technica that the concern about the impermeability of the heat shield was negated by the modified re-entry profile for Artemis II. It shouldn’t be an issue.

And as with Boeing Starliner, I think the amount of time they took to make this decision makes it clear they did not take it lightly.

Why did all of this take so long?

It’s also about making sure they completely understand the issue — not that they think they understand the issue. NASA was aware of the foam shedding issue before the Columbia disaster. They even saw the foam strike the wing of the orbiter during launch.

While some engineers within NASA thought this could be a problem, most of management dismissed it. What damage could foam do to the reinforced carbon-carbon Space Shuttle?

(This by the way is called normalization of deviance — nothing is supposed to shed during launch, but because this foam issue had happened before, they figured it was safe and moved on without trying to figure out why the foam was shedding or possible consequences of it happening.)

Even after the destruction of the spacecraft, many at NASA still didn’t believe that the foam strike was the culprit. It wasn’t until they tested with a reinforced carbon carbon wing panel from the Space Shuttle Atlantis, and the foam blew a hole in the panel, that they realized the foam had been the culprit.

It’s easy to think you know why something happened, especially when it’s as complicated as spaceflight. It’s really easy to look back and say, “Of course the foam was the problem.” But sometimes you can get so focused on what you think the problem is that you try to prove that is, indeed, the problem and you miss the real cause. NASA didn’t want to do that with Orion, they didn’t want to let their theories and preconceived notions and unconscious biases direct the testing. So they tested everything. That’s why it took so long.

I will also say I personally think that NASA’s structure contributed to the amount of time this took. The Artemis program has been spread across centers around the U.S. It’s a game NASA plays to keep Congress happy, and therefore continue to get funding. This means that there are a lot of individual places with deep knowledge about any given part of Artemis, but the broadness is sometimes missing. That’s why NASA established the Moon to Mars office, to be able to oversee all parts of Artemis, which is good.

But the thing is, Congress only authorized funding of the Moon to Mars office 18 months ago. Artemis I touched down in December 2022. We’re two years out from that. But the independent review team wasn’t convened until April of this year to oversee NASA’s testing of the heat shield problem and to make sure they were being thorough. To be clear, NASA has been working on this problem since they identified it after direct examination of the Orion capsule in 2022. But has it been as directed or intense since the Moon to Mars program took over? I don’t know about that.

My guess is there was about a year between when Artemis I touched down and when the Moon to Mars office was in place and operating efficiently that things weren’t moving all that fast. That’s part of why the office was established in the first place. This is just my speculation, but the timing explains why it took almost a year and a half for NASA to put the independent review team into place.

So they did the testing, with no deadline in mind so the team could focus on every possible cause with no time pressure, they had to identify the root cause which is a challenge because replicating complex re-entry conditions on the ground is hard, they had to test every other possibility to make sure they weren’t missing anything, and then once they found the root cause, they had to model re-entry scenarios to see what the risks are and whether Artemis II could fly safely with its current heat shield. THEN they had to develop a flight rational for flying Artemis II as is. It takes time, for sure.

But it seems that the independent review board unanimously agrees with NASA’s decision to fly Artemis II as is, and the testing has clearly been thorough. At some point you either trust NASA knows what it’s doing or you don’t, based on their history. I’m choosing here to think they did make the right decision, given everything else I’ve learned and seen.

Where is the independent review team report?

I do wish, though, that NASA had made the independent review team report available to the public. I’m honestly not sure why they didn’t, and that makes me wary. Especially considering NASA has been so cagey about this issue generally.

To be clear, NASA was very frank and straightforward about the heat shield problem at last week’s press conference. But that’s in stark contrast to the way they’ve treated this issue up until now. We didn’t even learn about the seriousness of the problem until April 2024, when the NASA Office of the Inspector General released a report with the first photos we’d seen of the damage.

I do understand NASA doesn’t want to release information until there’s something definitive to say, but as we saw with Boeing Starliner, not releasing information makes it seem like they have something to hide and it just drives people to speculate about what’s going wrong.

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