ET Project - ET as Propellant Resource This section details external tank applications that are related to the use of the tank and what it carries as a fuels resource. The possibilities include scavenging the residual cryogenics remaining after launch, powdering the ET and using it as fuel in a special rocket engine, and the use of ETs as reaction mass in electric space engines. NASA has a requirement for 2.5 million pounds for cryogenic fuels in low earth orbit (LEO) based on potential orbital transfer vehicle (OTV) traffic models. The scavenging of residual cryogenics from the ET can fill over 92% of this requirement at a very low cost (69). The potential cost savings of these type operations is great when compared to the basic launch cost of $2,000 per pound to LEO (29, 32, 69). I. Scavenging of Residual Cryogenic Fuels As mentioned previously, the direct insertion trajectory will also deliver an average of 15,000 pounds of residual cryogenics into orbit. These residuals are available for scavenging, storage, or use immediately after MECO or longer if steps are taken to retard boiloff. In a propellent scavenging study, a plan utilizing residual cryogenics in the ET will meet the requirements of 2.5 million pounds of OTV fuels over a ten year period. The scavenging of residual cryogenic is cheaper than launching the required fuels into orbit in an orbiter based tanker by almost an order of magnitude. The numbers on the figure below are costs per pound of fuel delivered to orbit. Note how the ET/ACC based scavenging is by far the most economical fuels supply choice (69). The scavenging operation can be conducted in a variety of ways. If there is a requirement to dispose of the tank, the scavenging operation needs to be done in the 20 minutes of time available after MECO. This can be accomplished using equipment carried in the payload bay of the shuttle or in an ACC. If the ET is carried into orbit, the scavenging operation can be done later. This could use scavenging equipment carried in an ACC or a separate facility located at the space station. It could also use a free flying scavenging flyer carried in the ACC, the payload bay, or already on orbit. It could also use the payload bay based scavenging equipment. The payload bay based method is the most expensive way to scavenge residuals because it takes up payload bay space, payload mass, and requires orbiter modification. The ACC based
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