Re: tremendous success [Artemis 1]
I guess we're ignoring the heat shield issues?
The heat shield is mostly made of the same, flight-proven stuff used on the Apollo heat shields, AVCOAT, so NASA selected it with high confidence that it would work. NASA's endorsing it for Artemis II because, again, they know it'll work when reentry is kept to an Apollo-style direct entry.
However, NASA's learned something new about AVCOAT during the Artemis I flight: it doesn't like skip maneuvers.
An atmospheric "skip" allows a re-entering vehicle to effectively bounce up in a ballistic arc and land somewhere other than the original target. The skip distance can be significant - Apollo capsules could skip over 1,000 miles, while the Orion capsules can skip 5,500 miles.
So if your Orion capsule is about to land in the wrong ocean or continent, then you can skip to a more preferable landing site. Useful, right?
Apollo never tested a skip maneuver and, as far as I know, a skip maneuver had never been tested before the 2022 Artemis 1 flight. This left a gap between theory and reality. The gap was that it was thought that a skip maneuver would create a gentler thermal environment for the heat shield. The theory made sense: the Orion capsule wouldn't just blaze in at lunar return speeds to a full stop (25,000mph to 0) but would get a briefer, lower peak heating, then have a chance to cool off as it skipped above the atmosphere, and finish reentry some minutes later. Lockheed's Orion skip page still talks about those 'thermal environment' benefits.
Orion's 2022 flight was uncrewed, so NASA decided to finally try the skip maneuver, and that's where reality proved theory wrong.
The problem with ablative heat shields like AVCOAT, PICA, and others is that they depend on polymers. In a normal reentry, these polymers pyrolize in the heat of reentry (consuming some heat energy), form fumes from decomposition ("blowing") that pushes the superheated plasma away from the surface, and then char, forming an insulating layer. This would continue as reentry stripped away the ablator, exposing fresh ablator to fight the good fight. Basically, there are several processes preventing heat from the plasma from soaking through the heat shield and reaching the contents (e.g., astronauts) behind the shield.
But they're still polymers, which start having mechanical and chemical problems at a few hundred degrees (on any temperature scale).
The two-part skip maneuver interrupted some of those ablative heat shield processes as the maneuver carried the scorching shield back into a vacuum. Rather than immediately dropping into the thick, cooling air of the troposphere and splashing into the cold ocean minutes after peaking heating, the plastic shield was heat-soaked in a vacuum. This roasting produced gas pockets and spallation inside the shield material.
And as the damage developed inside the heat shield, the shield was thrown back into the furnace again. Large chunks were torn from the spalling-damaged material by aerodynamic forces. NASA and Lockheed-Martin developed bladder control problems upon seeing the heat shield. (Which still worked - the capsule was intact.)
After figuring out the problem in 2024, NASA approved AVCOAT for the Artemis II flight, but took atmospheric skip maneuvers off the menu. AVCOAT works just fine when used as physics, if not the manufacturer, indicates.