Wet workshop
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Wet workshop is the idea of using a spent rocket stage as a makeshift space station. A liquid-fuel rocket primarily consists of two large, airtight fuel tanks; it was realized that the fuel tanks could be retrofitted into the living quarters of a space station. A large rocket stage would reach a low Earth orbit and undergo later modification. This would make for a cost-effective reuse of hardware that would otherwise have no further purpose, but the in-orbit modification of the rocket stage could prove difficult and expensive.
Werner von Braun proposed a wet workshop concept for launch on the Saturn V. His design replaced the S-IVB stage with a shell containing a cylindrical equipment section. On reaching orbit the S-II stage would be "vented" to remove any remaining fuel, and then a large access hatch in the top of the hydrogen tank would be opened. The cylindrical cargo module would then be inserted hydraulically into the tank through this opening, sealed, and then the tank would be repressurized to form a large living space. Power would be provided by solar cells lining the outside of the S-II. However, this proposal would require an entire Saturn V to launch the station, and at the time the design was being considered by Apollo X, all the Saturn V's were already earmarked for lunar missions.
In the meantime a number of the proposed Earth-orbit tests for Apollo had been canceled, leaving a small number of Saturn IB launchers available for use. A station could be built in an S-IVB instead, which both the Saturn V and Saturn I used. However, the lower throw-weight of the Saturn I meant that the power of the S-IVB would be needed to place the station in orbit. Again, the need to fire the stage that would eventually form the station suggested the use of a wet workshop concept. To allow this, the resulting design included metal mesh floors that were designed to allow the fuel to pass through them during launch.
Considerable work along these lines was carried out. Ironically, when the later Apollo missions were canceled (18 through 20), a supply of Saturn V's became available. However, instead of switching back to von Braun's original S-II-derived design, so much work had been done on the S-IV-derived system that they decided to continue along those lines instead. The Saturn V had enough power to place the station in orbit without firing the S-IVB, and the design was flown as a "dry workshop", even though it retained many of its original wet workshop design features.
Several similar conversions of the Space Shuttle's external tank (ET) have also been studied. During launch the ET accelerates to about 98% of orbital speed before being dropped and deliberately spun in order to increase its drag. A number of people have proposed keeping the ET attached into orbit, bleeding off any remaining fuel through the SSME's, which would be "left open". One such test had been scheduled, but was canceled after the Space Shuttle Challenger disaster dramatically changed safety rules.
The ET would provide a huge working space, and one major problem with various wet workshop designs is what to do with all the space. The oxygen tank, the smaller of the two tanks inside the ET, is itself much larger than the entire Space Station Freedom in its fully expanded form. Additionally, getting access to the interior is possible though "manholes" used for inspection during construction, but it is not clear if realistic amounts of building materials could be inserted into the tank after reaching orbit. Nevertheless the problem has been studied repeatedly, a number of such studies are collected in the links section below.
A similar concept, the "Aft Cargo Carrier", was studied by Martin Marietta in 1984. This consisted of a large cylindrical cargo container bolted onto the aft end of the ET, which offered the same volume as the Space Shuttle orbiter's cargo bay, but able to carry bulkier loads. The same basic layout was also used for a short-duration station design. Although not a wet workshop in the conventional sense, the station piggybacks on the fuel tank and is therefore related to some degree.
