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During the launch of STS vehicle, the two stage 0 Solid Rocket Boosters (SRB) were jettisoned and recovered after they gently parachuted into the ocean:
But what happened with the STS External Tanks after separation as shown in figure 3 on the image above? Were they recoverable and reusable, or were they a single use only? And did they stay in orbit, or splash down into the ocean some miles downrange after stage separation?
No, the previously used External Tanks (ETs) disintegrated in the atmosphere before they fell into the sea.
Notably, Buzz Aldrin and others proposed different ideas for reuse of the tank in orbit, and allegedly NASA said that they would be willing to take external tanks to orbit if a private company would use them. No private effort ever stepped up to the plate. Now that the Shuttle has been retired no more external tanks will be taken up.
Source:
NASA did not at any point actually reuse an external tank in any way. However...
NASA did have tentative plans for utilization of the tanks in orbit. These plans were scrapped, however. The primary factors being (1) decreased payload capacity to stable orbit †, (2) risk of insulating foam falling off and becoming space debris, and (3) lack of need for large tanks in orbit during the majority of the Space Shuttle Program era.
The tanks could have been worked to allow for attaching an airlock, and using the tank volume for storage or habitation, or melted for metals in orbit. The primary issue remains the insulation, and preventing it from becoming a hazard of its own.
Note that for reasons of weight, there is no external skin, and even the paint (which may have helped bind the insulation) was dropped for weight savings in later flights; a considerable mass (several hundred kilograms) of fabric or foil as an outer layer would suffice, as would a complete redesign of the tank to put the insulation between the inner tanks and the outer structural shell.
This also would not have been the first time a fuel tank was converted to a space habitation - Skylab was a modification of a Saturn S-IVB's tankage into a space habitat, albeit the work was done on the ground.
Also, keep in mind that the shuttle's normal operating regime isn't a stable height for stations - most potential customers looking to take advantage of the "free tanks" would have to have shipped up other components to make use of them, and had NASA deliver the tank to a more stable, higher orbit.
† while the OMS burn to ensure the tank falls would no longer be needed, the mass being carried to orbit means the OMS burn is including the tank mass, and thus proportionately longer for the same change in craft vector. This means, for the same orbit, more OMS fuel being used overall.
A big issue with boosting the tank to orbit, would be the foam insulation. It was believed it would come off in chunks, like popcorn, causing immense amounts of orbital debris, potentially in the orbit you wished to store it at. Which could be really bad news.
