hydrogen tanks for currently planned operations. However, if the expansion of scientific, commercial, and military operations occurs, the capability to store vast quantities of cryogenics will become very important. An additional application of the cryogenic storage capabilities of the ET would be to partially disassemble the manned mission to the plantes. The tradeoff here is a slightly heavier than required tank already in space versus a specially designed lightweight fuel tank brought to orbit (82). As is currently flown, the ET is too heavy to use as a fuel tank in this application (51). However, if the tank is partially disassembled and only the hydrogen tank used, this may be a possible application. The program cost savings will once again be in launch and developmental costs for the specialized equipment. A cryogenic storage facility also has military applications (25). These would include a refueling base for a small earth to orbit vehicle, a refueling base for military OTVs designed to fly classified payloads, and an ET based battle station for SDI purposes. The battle station could use liquid hydrogen stored in the ET with NERVA engine based generator flown in an ACC to power any desired high energy weapon. B. Gas and Water Storage The ET can also be used to store gasses and water on orbit. The ground testing of both tanks provide a more than sufficient safety margin for 14.7 psi (one atmosphere) pressurization (48). Gasses can be stored in the ET indefinitely on orbit (95). Due to its structural design, a hydrogen tank can also be used as a structural member if pressurized. Water is also an important item in LEO and can be stored indefinitely in the tank. The temperature can be easily controlled by both external and internal techniques. The available storage volumes of 147,000 and 405,000 gallons in the oxygen and hydrogen tanks are far larger than required for planned future operations. For example, a single oxygen tank can store the water equivalent of 50 - 150 shuttle visits of scavenged residual cryogenics (95). Water also becomes a energy storage medium. Fuel cells in the station can convert the scavenged cryogenics into water. This excess electrical energy can be used to electrically raise the orbit of the station. This would save fuel and shuttle visits by allowing orbital maintenance without the use of
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