Once the temperatures get reasonable, it might be aluminum or or carbon fiber. The passenger/cargo/avionics section are probably the only areas where this would be true, so a bulk of the rocket will be steel.
When Canondale hit the big time, they had one model year where they made the dropouts out of solid aluminum and in the next model year the derailleur hanger was bolted on. It's the easiest part to bend and there's no repairing it. Bad enough for road bikes, spectacularly dumb for mountain bikes.
Nope, it's just steel. Outside shell is the tank.
I think the military was looking at a similar material to replace depleted uranium with something non-radioactive (depleted uranium still experiences alpha particle decay, which is fine if you don't aspirate or ingest it, but a shell can pulverize on impact with a target. Also it's still a heavy metal even if you don't take rads from it).
I thought the point of depleted uranium projectiles was simply the density of uranium, giving them large impact energy for a given volume. If that's the case, titanium (4.5 g/cm^3) will not even be as good as steel (8 g/cm^3), let alone uranium (19 g/cm^3).
I am so used to seeing space stuff done in clean rooms, and things taking years and years to come to fruition (e.g. hearing about and seeing NASA or ESA probes etc getting made) that to see what is essentially a bunch of guys in a field just welding something together blows my mind.
I mean, is it just the outside that looks like that and inside it is ultra-exotic materials and tanks? Of course the engines are sophisticated machinery, but what about the insides? Are we just seeing the outer shell and it's all unobtanium nano-tube composite on the inside?