manner. This is perceived as the primary problem in the use of the tank on-orbit. As a result of the altitude of delivery and its size, the ET has a finite orbital lifetime due to aerodynamic and solar drag. Any program which will rely on the tank has to plan to expend reaction mass of some sort in order to keep the tank in orbit. Orbital lifetimes can be as long as years or as short as days depending on the altitude the ET is released, the attitude at which it is stored, and the activity of the sun (56). There are several tradeoffs which can be utilized to solve this problem. First, the residual cryogenics can be used to power low thrust gas burning hydrogen/oxygen thrusters which can boost the ET up to a very long lived orbit over the period of a few days (46). If this is done, little to no recoverable cryogenic residuals will remain in the ET, but the ET will be in a very long lived orbit. Second, the orbiter can attach a tether, set up a libration with the tank, release at the proper point, and send the ET into a much higher orbit and deorbit the shuttle. There are two problems with this technique. First, it rotates a billion dollar spacecraft and a massive ET around one another on the end of a rope. Second, it has never been done on this scale before. Contamination is the second major problem with the ET. The entire ET is coated with Spray-On Foam Insulation (SOFI) which will outgas on orbit in space. This pollution may not be tolerable in the evironment inhabited by the space station. A potential fix is to strip the foam off the ET after release in orbit. Most of the foam can be stripped by the equivalent of a hot-wire cheese slicer. The remainder can be polished off by leaving the ET 'hanging in the breeze’ over the period of a month or two exposed to the molecular oxygen present in the proposed orbits (14). There are other problems with the ET on orbit. Having more mass and volume available than is required for currently planned operations in orbit are factors. The partial disassembly of a tank has never been attempted in space and needs to be demonstrated. These problems are all solvable and in some cases may not be problems at all but advantages. VII. Conclusions It is clearly in the national interest to do whatever is reasonable to better utilize equipment already purchased. With the External Tank, we have the potential to provide for an enormous expansion in space capabilities for minimal expenditures - Extremely Low Cost and Extremely High Return. With this in mind, the following suggestions are made:
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