A third method involves trading the cryogenics as they boil off for orbital altitude. There have been several proposals for low thrust, gas burning hydrogen - oxygen rocket engines for station keeping and orbit raising. The engines proposed by Martin Marietta are 1,500 Ibf thrust with a specific impulse (Isp) of 375 seconds (46). A proposed arrangement of four of these engines mounted on the intertank as thrusters using the residuals left. The tradeoff here is between orbital lifetime desired and the amount of scavenged cryogenics desired. As was demonstrated by the reentry of Skylab, the orbital lifetime of large space objects is often far less than predicted. One of the primary considerations with taking the tank into orbit is keeping it there. The orbit of the tank will decay over time due to aerodynamic drag and effects of the solar activity. The desire is to put the tank into the highest possible orbit. Cross section ’into the wind* is also a factor. The gravity gradient stabilized tank with the 'nose down’ attitude is in the worst possible attitude for long orbital lifetimes because it presents the largest area to the wind. The best attitude is either end into the direction of motion. The following graphs detail calculated orbital lifetimes for a single external tank in three different storage modes (56). It is particularly important to control the orbital altitude of the tank if brought into orbit. The reentry point and impact footprint are extremely difficult to predict for an uncontrolled reentry (56). The leads to a requirement to reenter the tank after launch, reenter the tank after it has served its pupose on orbit, or to keep it in orbit using active measures. Thus, any plan which proposes external tank applications on-orbit must address this issue. Some form of active propulsion like small thrusters or an alternate form of orbital maintenance such as momentum transfers involving tethers or the low pressure cryogenic boiloff thruster needs to be used. This is not a particularly difficult problem to solve. It is however, a fact of life for every structure in earth orbit. There are current studies of appropriate propulsion for keeping the NASA space station in orbit. The last problem with the External Tank in orbit concerns outgassing from the SOFI on orbit. This amounts to an unwanted fouling of the environment and the equipment in the vicinity of the tank. Outgassing has been called the fatal problem with the use of the tank on-orbit (14). Like most problems in
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