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The External Tank: The Key to Space Exploration and Expansion?

by David Brin, Ph.D.

While it is very well to talk about colonies in high orbits, we must pass through a wide desert before reaching those promised lands. The prospect of investing for decades before rewards are reaped may prove too forbidding for the politician, who answers to the taxpayer. But what about the entrepreneur: Can the move into near space be made more affordable, and show intermediate payoffs along the way?

The External Tank: The Key to Space Exploration and Expansion?

A lot has been written about how human beings might live and work in space by the turn of the century. Some enthusiastic studies have suggested that colonies in orbit or on the Moon could mean the beginning of a new era of prosperity, once the resources of space are exploited by advanced industries.

The problem with these wonderful plans seems to be getting started. Today's politician isn't eager to invest in space development programs that will only begin paying for themselves — maybe — after two decades or more.

The first space colonies will not be made from melt-formed asteroids or carved from lunar caverns, but will have to be pieced together from materials lifted from the Earth. At about $1,000 per kilogram lifted into Low Earth Orbit (LEO), it'll be hard to convince the American taxpayer to stake a thousand ton space-base, let alone a full-fledged colony.

The current space shuttle can put less than 30 tons of cargo into a 200 nautical mile circular orbit. The actual load carried is usually significantly less, because of space limitations in the orbiter cargo bay. With only four shuttles available, booked years in advance for military and communications cargos, how will we ever be able to start building those massive structures a colony will require?

The outlook appears grim.

But what if someone were to offer a way to lift an extra 35 tons of aluminum into orbit, for free? That's thirty-five tons over and above the shuttle's regular cargo, orbited every time a shuttle is launched.

Let's further suppose these extra 35 tons will come already fashioned into two huge, clean, airtight chambers, with built-in access ports easily made with existing airlocks?

Let's dream a little more, and suppose these wonderful bottles could enter orbit with another 5 to 20 tons of liquid hydrogen and oxygen inside them, recoverable and usable as fuel for deep space missions or readily converted into water for many other uses.

Would it be too much to ask that taking this 35 ton wonder into orbit will not only be free, but also make the shuttle's regular mission performance more efficient?

It sounds too good to be true. Even if such a device were possible, one would expect it to cost billions, and take years of intensive development.

But these bottles already exist! In fact, a new one is made for every shuttle launch.

By now the reader may have guessed that I'm talking about the Space Shuttle External Tank (logically, if ironically, called the "E.T." by those who have been working on it) — the biggest part of the Space Transportation System.

Built by the Martin-Marietta Company for about $10 million each, the tanks provide 775 tons of cryogenic propellants to the three reusable Space Shuttle Main Engines (SSMEs) at the rear of the Orbiter. (See Figure 1)

Figure 1
Figure 1.

The External Tank is the "gas tank" for the Orbiter; it contains the propellants used by the main engines. Approximately 8.5 minutes into the flight with most of its propellant used, the E.T. is jettisoned and splashed down in the Indian Ocean. It is the only major part of the Space Shuttle system that is not reused.

During liftoff, the SSMEs are fed liquid hydrogen and liquid oxygen from two separate storage tanks aboard the E.T., providing up to one and a half million pounds of thrust for eight minutes. (For the first two minutes of the flight, the shuttle is assisted by two Solid Rocket Boosters (SRBs) which parachute to sea near Cape Canaveral for pickup.)

Since the solid rockets and the Orbiter itself are refurbished and re-used, the External Tank is the only major part of the Space Transportation System that is not recovered.

After the SRBs have dropped off, the Orbiter plus E.T. continue climbing together until all but about 1% of the propellants within the tanks are consumed, and the combined spacecraft has reached 98% of orbital insertion velocity. For a briefest moment, then, the shuttle is actually turned downward and the Tank is released, to tumble and burn as it falls.

For launches from the Kennedy Space Center, the designated area for impact is a remote spot in the Indian Ocean. The Orbiter proceeds alone into orbit, using stored hydrazine fuel to power the Orbital Maneuvering System engines.

All of NASA's calculations to date have involved tossing E.T. away after a few minutes. But do they have to be discarded? Surely if some additional use could be found for the External Tank, it would only be gravy, wouldn't it?

Well, it turns out to be more complicated than that, of course. Mostly, it's better than anyone ever imagined.

Recall that before the E.T. is dumped into the ocean, it has reached 98% of orbital insertion velocity. According to the analysts at Martin-Marrietta, it costs very little to carry it the rest of the way, especially if some of the maneuvers necessary for accurate discard are omitted. Figure 2 shows the projected cargo lift capacity of an Orbiter alone, an Orbiter plus External Tank, and an Orbiter and tank plus an external Aft Cargo Carrier, added to the E.T. The most interesting result is that the Orbiter can actually carry up to an extra ton in its cargo bay, if it takes the E.T. along with it into orbit! This is because of the omitted maneuvers, and because residual propellants in the Tank can be used for just a few seconds longer, replacing heavier OMS hydrazine.

Figure 2
Figure 2.

It was just this revelation that led former NASA Administrator Dr. James Fletcher, heading an advisory committee investigating low-cost concepts for a space station, to appoint a group to look into ways in which the E.T. might be useful in orbit. Dr. James Arnold, Director of the California Space Institute, chaired two workshops in March and August of 1982. The word out of these workshops may be some of the most encouraging news for space enthusiasts in some years.

structures, materials and propellants

The External Tank can be likened to a great "beer-can." In fact, the ratio of its filled to empty mass is about the same. Two beer cans stacked end to end are of about the right shape.

To someone interested in building big structures in space, that 35 tons of high-quality aluminum sounds like manna from Earth to Heaven. The thin-walled hydrogen and oxygen tanks can hold pressures up to one and a half atmospheres, making them obvious candidates for space-station units. The volumes — 550 cubic meters for the oxygen tank and 1523 cubic meters for the hydrogen tank — are unprecedented. Nothing anywhere near this size could be carried up in the Orbiter's cargo bay.

Think of an eleven story building, and you get a bit of the idea.

Each tank has, at both ends, a removable "manhole" 36 inches across. Thirty-six inches happens to be the minimum diameter for the current generation of airlocks to be used in both the shuttle and the ESA Spacelab.

NASA has recently let contracts for preliminary studies of a space station. The results of the External Tank Workshop are being examined by the contractors, some of whom are now contemplating the use of the E.T. as an integral part of an incremental approach to a space station.

Not every External Tank would become laboratory or dormitory space. Eventually, most would be dismantled and used for raw materials. Manufacturing in orbit could get its start using 35 tons lots of aluminum, plus substantial fractions of copper, carbon, oxygen, silicon, iron, and other metals. Large structures, such as solar power centers and communications platforms, could be built at low cost. Alumina-based laser materials, such as large rubies, might be manufactured in quantity, at low gravities.

A powerful use for the material of the E.T. would be in aluminum engines. A finely drawn aluminum wire can be gathered and burned in a lean oxygen mixture in a high-temperature lance, producing useful levels of specific impulse. For slow orbital transfers, then, we seem limited only by the supply of oxygen.

And there is oxygen, not only in the alloys and ablative coatings of the E.T. itself, but in the 5 to 20 tons of residual propellants even an orbit-inserted E.T. will have left over after launch!

These residuals are what really make the space engineers salivate. The leftover propellants of just one or two E.T.s could fuel the upper stage of a moderate sized LEO-to-Geosynchronous orbital transfer vehicle. Or, the cryogenic oxygen and hydrogen might be used to run fuel-cells, maintaining power to an Orbiter and Spacelab for weeks longer than normal. The resulting water can be stored — up to 250 tons in one leftover oxygen tank alone — and reconverted via solar power to H2 and O2 for later re-use.

Finally, the water itself could alter profoundly our ideas of what is practical to attempt in outer space. Ideas that might have been abandoned because large amounts of water were simply too expensive to lift into orbit would become possible once tons of the stuff were being reclaimed in orbit, essentially for pennies. An example might be the production of pharmaceuticals in low gravities.

(There are a few problems in recovering the residual propellants from the External Tank. The liquid hydrogen and oxygen must be drawn out of the big tanks and stored in special dewars, or they will evaporate as the temperature rises, requiring venting. This must be accomplished within a few hours of launch. Engineers in the study group did not consider this an insuperable problem, however.)

The External Tank seems, then, to provide a wealth of resources. This brief article only touches on a few of the possibilities.

"lifeboats" and life sciences

No one really wants to think about disasters in space. But Apollo 13 brought home just how important it can be to plan against that rainy day. Currently, nobody knows quite what to do, for instance, in case an Orbiter crew ever finds itself stranded in space with an unusable spacecraft and rescue possibly weeks away.

The E.T. might very well offer a solution. Dr. David Criswell, of Cal Space, has suggested the External Tank could serve as a "space-foxhole" in case of emergencies — especially once shuttle missions start using standard orbits to return to manned or unmanned space utilities platforms.

The requirements for an Emergency Orbital Habitat include: 1) an airtight chamber; 2) simple access by astronauts; 3) air, water, and power; 4) removal or absorption of carbon dioxide and other wastes; 5) communications and modest supply packets; 6) thermal control; and 7) protection from the radiation environment.

The E.T. Workshop came to the conclusion that all but the last of these could be provided simply by slightly modifying an E.T. that was already being used to store and convert propellants and water.

CO2 dilution, for instance, is no great problem in the vast volume of the hydrogen tank. Quiescent astronauts can exhale for more than five man-months before carbon dioxide levels approach the danger point. Small packages of lithium hydroxide can extend this period greatly.

The radiation problem is a bit worrisome, especially for military missions involving polar orbits. During solar maximum an Orbiter passing over the poles has little protection from the high-energy protons of solar storms, which can come streaming in along magnetic field lines with little warning.

All that is needed to make a Lifeboat radiation-secure is mass. If we shingle the outside of an E.T. with enough pates of aluminum, for instance, the interior will be safe in even the worst storms. Fortunately, there will be plenty of mass, in the form of still more E.T.s, which could be cut up for plating in orbit.

It would not take a tremendous investment, then, to provide our astronauts with a degree of safety currently missing from the Space Transportation System.

If an E.T. can offer minimal life support in case of emergencies, can it be used for growing plants and even crops in space?

The Study Group found that the adaptability of the External Tanks, the huge volumes and easy access to water and oxygen, offer unique opportunities in the area of Advanced Life Support and Closed Ecological Systems. In the latter case, in particular, the possibilities seem most promising.

On Earth, closed microcosms of shrimp and algae have been maintained in flame-sealed, two-quart bottles by Dr. Joseph Hanson of the Jet Propulsion Lab. Most experts in the field agree that the larger the microcosm, the more stable it will be, and the more diverse the aggregate of life forms it will be able to support. If water, air and power were introduced to a sealed E.T. including, perhaps, sunlight focused through windows, where the manholes once were, a variety of important experiments on ecological systems in space might be possible. A soil composed partly of ground-up ablative material, plus fertilizers, might be used.

Using tethered counterweights, a small amount of pseudo-gravity might even be provided, increasing the variety of animals and plants that could be included. Waste materials from space stations might be neutralized and, indeed, turned into benefits by ecological conversion.

In the long run, the goal might be a large degree of independence from the Earth for food, air and water. This in turn would reduce the amount of lift budget the shuttle now devotes to expendables, and would be necessary for the future of deep space missions and intensive industrialization of space, thereby loosening the umbilical to the Earth.

tethers and "tricky stuff"

For decades, mass has been the great enemy of spaceflight. Lighter has always meant better, and a pound of payload shaved meant many pounds of propellant earned.

But this truism may fall into disuse soon, at least as far as space above 100 nautical miles is concerned. There may, indeed, be times when "mass is beautiful," and more is better than less. The trick that could make this happen involves using "tethers" in orbit, to swing massive objects about in ways unimagined until a few years ago.

A space-borne tether is the little brother of the "beanstalk" or "Skyhook" which has received so much attention from science fiction authors such as Arthur C. Clarke or Charles Sheffield — mammoth cables linking an equatorial mountain to a giant counter-weight beyond geosynchronous orbit. Space elevators may be pie-in-the-sky, beyond our foreseeable engineering ability — and yet, we still may see massive objects in the heavens, whirling about joined by slender ropes tens of hundreds of kilometers long, and less than a centimeter across.

The secret is transfer of momentum. Like two ice skaters who approach, link hands to spin about their center of mass, then let go to fly in opposite directions, two massive bodies in space can play fascinating games of velocity and energy exchange. All that's needed are small rocket impulses and long, strong cables made of Kevlar, or some modern, super-strong material.

Although the details are too complicated to go into here, the External Tank Workshop found the initial studies so promising that vigorous investigations of tethers in space were among its strongest major recommendations to NASA. Specifically, the committee urged pursuit of the Italian-U.S. experimental tethered satellite project, inspired by Professor Giuseppe Colombo of the University of Padua and the Harvard Smithsonian Center for Astrophysics.

If we start putting E.T.s into orbit, NASA wants to be darned sure they stay there. They certainly don't want another Skylab scare! Tethers may provide the way to keep External Tanks in orbit for long periods of time. Using the gradient of Earth's gravitational field to line up the E.T. at the right angle could reduce its drag as it passes through the uppermost reaches of the atmosphere. Figure 3 shows how this might be done.

Figure 3
Figure 3.

We see in Figure 4 that two E.T.s both placed initially in 500 km orbits, will experience greatly different decay times if one is allowed to orient itself, as Skylab did, broad face forward, and the other is tether stabilized as in the left half of Figure 3.

Figure 4
Figure 4.

In this way, the fiery fate of Skylab wouldn't befall External Tanks stored above for future use for a long time, even without reboost.

The E.T. in turn may play a vital role in the future of tethers in space. The slingshot effects discovered by Colombo, and Joseph Carroll of San Diego, become more and more effective the more mass is added as a counterweight. And there is no better source of mass in orbit than all those External Tanks NASA had been planning to throw away!

If the bugs can be worked out, tether maneuvers can provide some fancy tricks to enhance our abilities in space. For instance, payloads at LEO can be flung into somewhat higher orbits, spending virtually no fuel at all. An Orbiter that has completed its missions may be dropped back to Earth with almost negligible retrofire, leaving its E.T. behind and contributing the momentum given up to a "tank farm" gradually accumulating in orbit.

The tethered satellite phenomenon deserves a lot more discussion. In a future article we may have a chance to examine some of these possibilities in more detail.


While it is very well to talk about colonies in high orbits, we must pass through a wide desert before reaching those promised lands. The prospect, may, indeed, prove too forbidding to the taxpayer, who must pay for decades before rewards are reaped. Somehow the intervening steps from a modest space station to L5 must be made much more affordable, and show intermediate payoffs along the way.

We need a key to unlock the vestibule to space.

Ironically, the key to space may be forged from a piece of expendable hardware — a "beer can" that we were planning to throw away, again and again.

Recycling pays, it seems.

Figure 5
Figure 5.

Figures courtesy of Smithsonian Astrophysical Observatory.


The External Tank: The Key to Space Exploration and Expansion?

about this article

Copyright © 1983 by David Brin. All rights reserved.

"The External Tank: The Key to Space Exploration and Expansion?" (published in full here) was first published in L5 News, February 1983, while participating in the External Tank Study Group as coordinator for one of the subgroups. His short story, Tank Farm Dynamo, shows these concepts in action.

articles, films & books cited here

David Baker, International Space Station: An insight into the history, development, collaboration, production and role of the permanently manned earth-orbiting complex (Owners' Workshop Manual) (book)

Gregory Benford and James Benford, eds., Starship Century: Toward the Grandest Horizon (book)

David Brin, Tank Farm Dynamo (short story)

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