External Tanks – OTA Report on Space Stations excerpts
CIVILIAN SPACE STATIONS AND THE U.S. FUTURE IN SPACE
Office of Technology Assessment, Congressional Board of the 98th Congress
OTA-ST1-241, Library of Congress Catalog Card Number: 84-601136
Superintendent of Documents, US Government Printing Office, Washington DC 20402
Excerpts taken from the OTA Report, Chap 3, Space Infrastructure, pp. 77-82
Shuttle External Tank (ET) –
Application of the ET as an infrastructure element is intriguing because of its large size, because it achieves a near-orbital velocity during normal Shuttle launch operations, and because it “comes free of extra cost” to orbit. As a result, several aerospace companies have studied the ET for possible use on orbit.
The ET has an interior pressurized volume of some 2,000 cubic meters in the form of two separate tanks – one for hydrogen, the other for oxygen.
In present Shuttle launch operations, the ET separates from the Shuttle and reenters the atmosphere after main engine cutoff. On average, at separation from the Shuttle, the ET still contains about 4,500 kg of liquid O2 and H2. The challenge is to identify practical methods of salvaging the tank and scavenging these residual propellants.
The ET in orbit, initially viewed as a construction shed and distribution center, might serve as a mounting structure for telescopes, large antennas, large solar power collectors, and experiment pallets, or it could be used as a component of inhabited infrastructure, in which case it would need windows and entry hatches. The most obvious use for the ET is for on-orbit modification, but assumes that the techniques and equipment needed to scavenge leftover fuel from the Shuttle and to store it for long periods in space are developed. Use as an uninhabited warehouse or unpressurized, sheltered workshop in space only requires that the tank be purged of residual fuel, since several access openings (larger than 1 meter diameter) already exist.
A concept to use ETs as components of habitable infrastructure has been developed by the Hughes Aircraft Co. In this concept, four ETs would be taken separately into orbit and then joined to form the spokes of a large wheel-like structure. Solar panels would be mounted on a rim connected to the outer ends of the ET spokes, providing 150 kW of power. The wheel would rotate, and a “despun” module at the hub of the wheel would provide zero gravity workspace. The basic feasibility of this “dual spin” system has been demonstrated on a much smaller scale in over 100 successful communications satellites built by Hughes. Modules attached to the outer ends of the ETs, carried into space as aft cargo carriers, would be available for habitation and pressurized workspace. Rotation of the wheel would provide artificial gravity in the spinning part of the facility and gyroscopic action for attitude control.
This innovative concept has several obvious advantages. There is no doubt that many human activities, such as eating, drinking, food preparation, showering, and dealing with human waste, would be much easier to carry on in the artificial gravity environment provided by this system. Any possible health problems associated with long-term living in microgravity, such as decalcification of bones and atrophy of muscle and connective tissue, could be avoided. In general, the presence of spin and a choice of gravity regimes, ranging from microgravity to artificial gravity simulating what we are used to on Earth, should prove to be useful in solving a number of human, scientific, and engineering problems.