9. 4 at the base of the orbiter. Standard shuttle avionics are also carried to provide ascent guidance, and to allow for commonality in systems. In short, the modification which results in the DOL is the replacement of the orbiter airframe with a simple payload protective cover, when the added weight of the life support systems, reentry thermal protection, and wings for landing are not justified by the mission. However, space shuttle main engines are quite expensive; too much so to throw away. For this reason, the engines and avionics of the DOL are recovered in space and returned to the surface for refurbishment and reuse by the space shuttle, thus utilizing the return capability of the shuttle to its maximum advantage. The mass of the systems to be returned from the DOL is 11,000 kilograms (9.3). The return capabiiity of the orbiter is 11,400 kilograms (9.4). It is clear that one orbiter can handle the return mass of only one DOL while maintaining a safety margin. Therefore the launch system used for the colony setup period, and also used during the construction period, is a 50-50 mix of space shuttles and DOL's. A DOL is launched with a payload mass of 75 metric tons into circular earth orbit at an altitude of 370 km, followed by a space shuttle with a 27.5 ton payload, which rendezvous with the DOL. The DOL shroud is jettisoned and both of the payloads are released. The engines and avionics from the DOL are taken inboard of the orbiter, which then reenters and lands at Kennedy Space Center. The DOL parts are off-loaded at the orbiter safing facility, and refurbished and reused, as is the orbiter. It should be noted that this flight alternation is merely a convenience, and DOL engines may be allowed to stockpile in orbit before their return. However, this method of recovery would necessitate an additional number of engines in the inventory, thus increasing total transport system costs. In either case, the number of shuttle launches must at least equal the number of DOL launches for complete engine recovery. IX.1.2: Interorbital Transfer Vehicle: Given a payload mass in low earth orbit, it is necessary to transport it to the site of interest, which in the case of the prototype space colony is either
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