savings of $3.5 Billion over the period. The savings comes primarily from the launch cost savings. The scavenging operation can be performed in a variety of ways. These inlcude scavenging into the orbiter after MECO, scavenging into an ACC, scavenging into the space station after rendezvous, and scavenging into a free flyer. Each tank will provide an average of $30 million worth of residual cryogenics available for scavenging. Another use of the ET as propellant is to powder the aluminium and use it as reaction mass in a Aluminium - Oxygen - Hydrogen rocket for OTV applications. Analysis of OTV traffic models show that current technology engines (RL-10) are sufficient if the OTV fuel requirements do not exceed the availability of scavenged cryogenics. However an aluminium/oxygen/hydrogen engine rather than an advanced cryogenic oxygen/hydrogen engine appears to be the most cost effective choice. This is once again due to the savings in launch costs because 40-50% of the reaction mass is already in orbit as tanks. The analysis of the aluminium rocket engine includes the $1-2 billion R&D costs, the high production costs, and the cost of flying a processing plant to grind the tanks into powder. Even with the inclusion of all these additional costs, the aluminium engine is potentially far cheaper than an advanced cryogenic engine because the mass requirements to orbit are far lower. Each tank used in this application is worth about $107 million in powdered aluminium (computed at $2000 per pound to LEO). The problems associated with these type engines are known. It is not clear at this time why the choice not to develop these engines in the 1960s was made. There were fairly serious problems with propellent transport that may be solved by zero 'G* conditions in orbit. There was also a problem with the time constraints of the Apollo program. The scientists investigating the Aluminium engine are well aware of the past history of the engine. They feel that the problems are solvable and that the aluminium engine has great potential for OTV applications. IV. Structures The ET can be used in any number of structural applications. These range from partial disassembly to complete melting and refining opreations in orbit. The ET can be partially disassembled and reassembled into a variety of rigid structures. The tank domes can be removed and the hydrogen tank barrel
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