Space Shuttle External Tank insulation

[Peter B]

Can someone remind me (I'm sure it's been explained before) why the ET insulation is on the outside of the tank, rather than inside the skin?

IIRC, the Saturn 5's S-II stage had the insulation under the skin. Why not with the ET?

Cheers

[danscope]

Don't want to contaminate the liquid with anything lining the tank. Not to foul pumps etc. Also, ice build-up on the outside.
Of course, they "Could" seal the foam with epoxy FRP. Weight and expense.
Nothing's easy.
Dan

[cjl]

FRP is HEAVY. That would vastly increase the weight.

[NEOWatcher]

Can someone remind me (I'm sure it's been explained before) why the ET insulation is on the outside of the tank, rather than inside the skin?

I would also assume that the tank and the skin work off each other for strength. Any separation between the two would require much more of a structure.

IIRC, the Saturn 5's S-II stage had the insulation under the skin. Why not with the ET?

First the design requirements were overall very different. Second, ice forming on a traditional rocket is not an issue (provided it falls off).
Do you remember all those sheets of ice falling off a SatV launch? I would assume there was considerably less insulation. (Fill and go...)

[Jim]

FRP is HEAVY. That would vastly increase the weight.

Considering that NASA stopped painting the surface of the insulation white as a weight-saving measure, they would be reluctant to cover it with anything.