These volumes can be accessed through three foot diameter inspection hatches in the respective tank domes. The domes can also be completely removed by pulling the attachment bolts if the desire is to open the entire end of the tank. The mass of the tank is also an advantage. At an average mass of 69,000 pounds apiece, the tank can provide the inertia and strength required for large scale operations in space. Once released, the tank will assume a gravity gradient stabilized attitude with the long axis pointing at the center of the earth (56). A telescope mounted on this platform could rely partialy on the mass and inertia of the tank itself rather than an active three axis stabilization system. The tank can be used as a workbench or a strongback in orbit. This concept utilizes the actual structural strength of the tank itself as a construction platform. The construction of large space structures need not be based on specially designed lightweight members that could not support their own weight on the surface. Due to reserve, pressurization, and ullage requirements, the tank should contain an average of 15,000 pounds (5,000 to 40,000 pounds) of residual cryogenics when it arrives in orbit (16). This liquid hydrogen and liquid oxygen can be scavenged from the tank shortly after launch for a variety of uses on-orbit (69). These residuals also pose a safety problem which must be addressed. The current use of the liquid oxygen tank pressurization gas is to tumble the tank after it is jettisoned for reentry. The on-orbit problem is that there is concern about the boiloff rate and possible overpressurization and rupture of the tank. There are several ways to deal with this problem. The first is to scavenge the residuals relatively early with a set of catch tanks located on the aft end of the tank. This can also be done with the required equipment located in the payload bay of the orbiter but is less desirable because it uses payload bay space that can be better sold to customers. A second method would be to install a heat reflector to keep the sunlight off the tank. The first Skylab crew did this to the Skylab to make if habitable. The ET could be made a better storage container for cryogenics by wrapping it with mylar blankets to retard boiloff (46). The next figures detail boiloff rates for residual cryogenic propellants.
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