The LO2 tank and the LH2 tank are welded aluminum structures (2219 & 2095 Al-Li on Super LightWeight Tank) that are covered with sprayed on polyurethane based foam insulation. The insulation is there to protect propellant quality (keep it cold) but mainly to prevent freezing of condensation on the exterior. Remember: at sea level, liquid hydrogen boils at -432 deg. F. Air would actually liqeufy at those temps on the outside of the LH2 tank. Both tanks would be COVERED with ice at liftoff (like the Saturn cryo tanks were) and all that ice breaking off and flying around once things start rocking & rolling would be a far far worse Orbiter hazard. It was OK during Apollo because the Command Module was up top (in line vehicle, not on the side like Shuttle).

ET: the Orion was an engine that Scotty Boulduc was going to sell or develop or something. It was basically (I think, IIRC) a KJ-66 derivative, but very heavy. I think a guy in Australia was going to be making it for him. It never amounted to much. I think they had a few prototypes, but that was it. Been a long time.