Why don't they put the foam on the inside of the Shuttle external tank instead of the outside?

Question

Inside the tank are two pressure vessels, one for liquid oxygen, and another of liquid hydrogen. There is a gap between the outer shell and the pressure vessels. Why not put the foam on the pressure vessels or the inside of the outer shell. That way it couldn't fall off.

Answer 1

The foam maintains the internal propellants at an acceptable temperature, protects the skin surface from aerodynamic heat and minimizes ice formation. The foam needs to be on the outside of the tank mainly for the second two reasons.

So, if the foam was on the inside of tank, the exterior would burn up.

Answer 2

There is no gap. The tanks are each a single layer of aluminium, and they are joined by an intertank cylinder. The insulation is on the outside for several reasons.

1: Aluminium is actually stronger when it gets colder, and about the only metal that does not get brittle when at extremely low temperatures, so having it in contact with the cryogenic fuel is actually beneficial structurally.

2: If the insulation were inside it would allow the outside of the tank to get cold and for ice to form outside, which would fall off in great sheets during liftoff. Look at the Saturn V. The fuel tanks on that used internal insulation, and the cascades of ice that fell off on liftoff would be a disaster for the shuttle.

Answer 3

Why not mount the shuttle on top where foam couldn't hit it and put engines on the bottom of the fuel tank turning it into a booster rocket (basically the Ares V)? NASA misses the obvious.

Answer 4

If the insulation was put inside, then the skin of the shuttle would burn off before the insulation had a chance to perform its function!

Answer 5

That would take up space needed for the fuel. I wonder why don't they fix this problem once and for all?