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Uses for the Shuttle's External Fuel Tank

(This file represents a portion of the messages that I originally posteed on the SIG in mid-1983 dealing with the proceedings of the Princeton Conference on Space Manufacturing. A considerable portion of the Conference sessions dealt with the question of possible use of the Space Shuttle's external tank. Since this seems to generate interest on the SIG from time to time, I decided to consolidate the messages dealing with that subject into a file and upload it to XA.)

E.T., it seems, is not only a movie and the name of it's principal character but also refers to the Space Shuttle's external tank and the space program's most hotly debated topic at the moment seems to be what to do with the latter. Everyone appears to agree that dropping the tank in the Indian Ocean as is presently done is not a good idea, especially since it reduces the Shuttle's payload. After that, however, the consenus breaks down. The 1983 Princeton Conference on Space Manufacturing produced it's share of discussion on this point.

The problem is that the tank cannot simply be taken into orbit and left there. Under "worst case" conditions, that is at solar maximum and with the tank encountering the atmosphere broadside, it's orbit would decay in about eighteen months. Moreover, the tank is big enough to survive re-entry and cause damage if it hits anything. (One NASA representative particpat -ing in the conference remarked to me that this had given NASA serious concern when it was first proposed that the Shuttle be used to rescue the Solar Maximum Mission satellite. At that time the only launch profile which would enable the Shuttle to rendezvous with the satellite would have resulted in the tank landing in the vicinity of Hawaii. This problem has since been solved, I gather because it proved possible to get more thrust from the Shuttle's engines.)

The simplest proposal offered at the conference was to attach the tank to a 1-km. tether with a 500 kg. wieght on the other end. This would cause the tank to fly nose-on to the atmosphere, increasing it's orbital liftime to 35 years. (I'm not clear as to whether this method could be used to tether tanks together like beads on a string.) The trouble is that this would only postpone the problem not solve it, and after the Skylab experience it is doubtful that public opinion would be willing to accept the arguement that a permanant solution would be found in the meantime.

On the other hand, the most ambitious proposal regarding the external tank was to use it as a space station or habitat. A variant of this, proposed by Martin-Marietta, is to modify the tank by the addition of a "caboose" at the lower end. While the caboose would be only a fraction of the size of the tank, it would itself be volumetrically larger than Skylab. On the inside, the caboose would be either a cargo pod containing equipment which the crew of the Shuttle would install in the remainder of the tank or a "construction shack" where the crew would live while making the rest of the tank into a space station, the necessary materials being brought up in the Shuttle payload bay either on the same or later flights.

There are however serious objections to this proposal. To begin with, the caboose would have to be located right between the motors of the solid rocket boosters. While Martin-Marietta expresses confidence that it can solve the problems this would create, others are not so sure. The remark making the rounds in the aerospace industry is that if the caboose can be made to work Martin-Marietta's next project should be to construct something in the "theological place of eternal punishment," since the physical environment there can't be much worse than that to which the caboose would be subject.

An even more serious problem is the economics of orbital construction work. A NASA conference participant, an expert on human factors remarked to me that "what their talking about is manhandling a box through a hatch, bolting it down somewhere, connecting it to another box and hoping that everything works." This, of course would have to be done over and over again, a time consuming task even in a shirtsleeve environment and an even more time consuming if the job has to be done in a space suit.

The man-day costs, that is the costs of keeping one man in orbit for one day, using the Space Shuttle are well in excess of $100,000. If the construction crew can work from a space station (e.g. the caboose) the man-day cost drops substantially but still remains in the medium five figure range. Moreover, the implication I get is that the work necessary to turn the Shuttle's external tank into a space habitat would have to be performed largely by humans, since neither teleoperated nor robot machines with the necessary levels of intelligence or dexterity are likely to be availible any time soon. Combine this with the fact the NASA official I previously mentioned estimated that the proposal that was put forward would entail 75% of the work involved in getting a completed space station being done in orbit, and you get some idea of the nature of the economic problems involved.

My own thought is that some economies of scale may be possible. That is, the more people working in orbit the lower the man-day costs become. If this is so some "bootstrapping" will be possible. In other words, the first phase of the job will be to create additional living quarters to enable the crew to be enlarged. This necessarily assumes that a major factor in man-day costs is not the supplying of oxygen, water and food. If that is not the case, bootstrapping will have to await the development of closed cycle life support systems.

Even if economies of scale are possible, it appears that a "Catch 22" situation exists both with respect to using the tank and space exploitation generally: Large scale space projects will be too expensive until we construct self sufficient space colonies with permanant populations working for more-or-less Earthside wages, but such structures cannot be built until large scale space projects become less expensive.

A comment from another conference participant suggests to me a futher variation to minimize the cost objections. He noted that repair or check-out of a satellite is more easily done hands-on in a shirtsleeve environment than in a space suit or using a remote manipulater. The Shuttle tank of course is divided into several compartments, the main ones being the oxygen and hydrogen tanks. My scheme assumes that it is feasible to build a living quarters caboose, install large pressure tight doors on either the oxygen or hydrogen tanks (preferably the hydrogen tank, since it is larger) and connect that tank to the other tank via a pumping system. The tank would be depressurized by pumping it's atmosphere into the other tank. The doors would be opened and the spacecraft to be worked on admitted. The doors would then be closed and the tank repressurized to become an orbital vehicle assembly building. The living quarters for the crew performing the work would of course be in the caboose.

The advantage of this is that while some equipment would have to be installed in the tank it would involve nowhere near as much work as turning the whole tank into living or working quarters. The disadvantage is that the doors would have to be installed after the tank arrived in orbit. (I doubt it would be possible to make them strong or tight enough to withstand the pressures generated when the tank contains propellant.) Since the doors would be large and massive and would have to fit to very close tolerances, lifting them into orbit and installing them once there would present major problems to put it mildly. Possibly the entire caboose compartment could be hinged so that it would swing aside when a space craft was taken in or out of the assembly area. In effect the caboose becomes the door. (O.K., you engineers. Tell me why this can't be done.)

Given all the problems connected with using the Shuttle tank, it's hardly suprising that one paper presented at the 1983 Princeton Confernece on Space Manufacturing suggested simply grinding it up into aluminum powder, melting it down and making aluminum wire. The equipment would be simple, the process based upon technology long used here on Earth and (although the paper did not say so) the operation could be largely automated. The question in my mind, though, is what you do with the wire? There would be no purpose in making aluminum wire in orbit unless space-made wire had some property which does not exist in wire made on Earth (e.g. much greater strength or conductivity) or large amounts of wire were usable in space (e.g. wire tethers).

The proposed use for the external tank which has the greatest chance of becoming reality in the near term is as an orbital storage facility for propellants. It was noted in one conference paper that on most STS missions payload limits are determined by volume, not weight. This will enable the Shuttle to carry substantial amounts of propellants into orbit. (It's not clear to me wether this is because the extra wieght can be made up by loading more propellant in the tank or wether the tank has to be full at launch in any case and less propellant is consumed.)

The plan involves one tank being left in orbit. On subsequent missions the orbiter would rendezvous with the first tank before detaching it's own external tank. Micro gravity is then applied to the second tank to settle the remaining propellant to the bottom from which it would be pumped to the first tank. The propellant would be used either to refuel orbital transfer vehicles or (since it consists of hydrogen and oxygen) in fuel cells to power a space station).

The use of the Shuttle's external tank as an orbital propellant storage facility, however, presents probelms of its own. It is by no means certain that long term storage of cryogenic propellants in space is feasible. At a very minimum, the storage tank must be kept cold. While the tank as launched is insulated, and while a passive sunshade would help, an active refrigeration unit will also be required. (Perhaps the sunshade could be also be used as a solar collector to provide power for the refrigerator.) Also, a refrigerator being merely a heat pump, the heat taken out of the tank has to be rejected into space by means of a radiator.

Another problem, not addressed at the conference, is that pumping propellant out of one tank into another will result in an orbiting empty tank. In other words, back to square one: How do you safely de-orbit the empty tank, or if you don't de-orbit it, than what do you do with it? The storage facility idea is valid only if you have an answer to that question. (Frustrating isn't it? Even more frustrating because hundreds of these tanks will be used in the Shuttle program.)

(The forgoing messages cover only one of several topics on the agenda of the 1983 Princeton Conference on Space Manufacturing which will repay extended discussion, including recent and contemplated electophoresis experiments on the Space Shuttle, which may launch a major space industry, Lunar and asteriod mining, materials processing and the proper mix of men and machines in space. Some of these were discussed in other segemnts of my series of messages concerning the conference. I still have these messages stored on disks and I hope <fingers crossed> to be able to upload these sometime in the future. In the case of others, I wasn't even able to get that far. I still have my notes but the next Princeton Conference on Space Manufacturing, scheduled for 1985, is liable to occur before I get around to typing them up and uploading them.)

 
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