pressurized also keeps the interior clean. The tank is then checked out, attached to the appropriate stack and launched in normal sequence. A typical launch inserts the orbiter and ET into a very low earth orbit where the tank is jettisoned to reenter somewhere over the Indian or Pacific Oceans (56). The orbiter then conducts an OMS burn to loft itself into the desired orbit. The tank is jettisoned with over 98% of the energy required to keep it in orbit. It is important to control the reentry of the tank to insure that it comes down in an unpopulated area. There is an alternate trajectory for launch called a direct insertion. This trajectory has been flown successfully with the ET splashing down near the Hawiian Islands. If the tank is retained even longer and taken into orbit, the orbiter can deliver the tank, orbiter payload, and additional orbiter payload into all possible STS orbits. It attains this capability by flying a more efficient trajectory and using the more efficient SSMEs to put it all the way into orbit. The following figure is a comparison of Shuttle performance to orbit without the ET, with the ET, and with an ET enhancement called the AFT Cargo Carrier (ACC) which will be discussed later. It is important to note that if the ET is taken to orbit, this action alone improves the STS payload capability by as much as 2,000 pounds to orbit (56). At an average launch cost of $2,000 per pound (69), this is a substantial enhancement of the STS capability. III. The ET on Orbit - Problems and Solutions Taking the ET into orbit presents several opportunities and capabilities that we do not yet have. It also produces some additional problems. The opportunities available are related to the mass and size of the tank itself. The capabilities are related to things that can be done with the tank on-orbit. The primary problem is related to safety considerations. All will be discussed in the following section. The size and mass of the external tank on-orbit compare favorably with past and proposed future orbital facilities. The 53,500 cubic foot volume of the hydrogen tank alone is far larger than any facility flown or planned. It is more than twice as large as the Skylab at 18,300 cubic feet (70). It is more than five times the volume of the proposed Space Operations Center (SOC) module of 7,100 cubic feet (75). The volume available in the oxygen tank at 19,500 cubic feet is also larger than all the above mentioned facilities.
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