[sobellian]

I'd say material science since the only non-ablative material we can use is too brittle compared to a normal fuselage. I really hope they succeed but it's a pretty fundamental problem to have unanswered this deep into the program development (and gating Artemis no less). Also hard to judge their progress without the data their heat shield team is getting, see https://x.com/mcrs987/status/1978183753114505496 for example. It's great that they can tolerate loss of vehicle & have better margins due to the steel fuselage but for Artemis and Mars they need to solve it or they'll be burning up hardware fast, literally.