It needs to act as both a tank wall for cryogenic liquids that won't crack and as a backing for a heat shield that won't melt. Tough trade-off. Melting point o

It needs to act as both a tank wall for cryogenic liquids that won't crack and as a backing for a heat shield that won't melt. Tough trade-off. Melting point of aluminum is too low. Burning point of epoxy/carbon fiber is also probably quite low, but it's also famously hard to work with and get reliably good results that you can actually inspect. Titanium might work, but is also had to work with and tends to crack.