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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