As a rough guess, the wall would need about 6mm of aluminum. That'd add about 22 metric tons of aluminum, plus bracing between the two walls. It could easily knock up to 30 tons off the payload once you account for some kind of sealing ring around your proposed removable ends and the structure to convey loads and act as a replacement for the ET.
Awesome, and thanks for the information. So for a significant portion of the mission payload, we get a large, reusable Space Station Module, plus a payload to put inside it, all in one go?
Once in orbit, it'd be days or weeks of intensely manual EVA to unbolt the ends of this notional container, pull the ET out, and bolt the ends back on.
I have trouble with these figures, could you clarify why it would take so long to separate them? I don't know the reasons that would make this assessment likely, so if you explained them, then I might be able to propose some time saving alternatives/improvements. I would expect to have an EVA component of most missions, and definitely all that involved using such a "Sheathing" to comprise large, cheap, reusable Space Station Modules.
In the end, all you gain is a giant empty can not much different than an ET with altered insulation for much more work. It's not really worth bothering with. If you need a module that large for a one-off mission and already have room for an inline payload fairing on the stack, it's much better to just fit out the module on the ground and launch it in the fairing. If you want to try and make a module out of something you fly every flight, using the basic ET as your base is far better.
If your so inclined, please provide a detailed explanation of this, as my idea for an ATL US Space Program would involve an ongoing mission to steadily expand the US presence in space, and that as an integral portion of this would include labor-saving, power tools designed to be used in zero/micro G environments, as such would be needed nearly every mission excluding resupply missions.
For instance, having watched the repair video for the Hubble, I was like, "How could they possibly have done it that way, when there could have been a much easier effort had they lofted a reusable repair bay to conduct the work inside of?"
I wouldn't see much in the way of One-Off uses, as once we have paid the cost to get the thing into orbit, leaving it there under-utilized would simply be seen as something for a future mission to correct.
Even there, as others mentioned, it's been such a pain when previously studied that it's never actually been done in practice. Shear volume on orbit has never been enough of a problem to warrant it when crew time--the major resource such a wet workshop consumes--is actually the most limited factor.
This would change, though, would it not, if such work was not handled as a one-off, do it a couple times over the course of the entire program kinda thing, but rather as an every mission kinda thing. In other words, they would take the time to think things through, and provide the labor saving tools and equipment to make these tasks far less time consuming and tedious, since they would be doing such things on most missions.
Like providing the equivalent of an auto mechanic with a "dolly on wheels" to service the underside of a car, or the equivalent of the lumberjacks belt to climb trees, so too would I see the astronauts being equipped with the tools of the trade.