§ 5.16 Re-using Today's Rockets' Spent Fuel Tanks in Orbit

A potential building block for early space stations is spent fuel tanks which are normally discarded in space.

This is not a product from lunar or asteroidal materials, but is well worth discussing in this chapter, as these resources could be valuable for missions to the Moon and asteroids, and for space industry.

The most attractive of all spent fuel tanks is the Space Shuttle's main external tank (MET, or often called just ET for external tank). To fully understand this issue, it is important to understand how the Space Shuttle uses this tank, and how it is currently disposed of during each launch.

How the space shuttle works

The U.S. Space Shuttle is a -partially- reusable vehicle.

It consists essentially of 3 parts :

1. the reusable winged Orbiter,

2. the throwaway Main External Tank (MET),

3. the reusable pair of Booster Rockets.

As shown in the picture above, the Main External Tank is the centerpiece of the launch vehicle.

The fuel in the MET fuels the engines on the tail of the Shuttle Orbiter (a few pipes run from the MET to the Orbiter).

The two Boosters lift the entire vehicle off the ground and speed up the aircraft until it can go without the Boosters, at which time the Boosters detach themselves and parachute to splashdown in the ocean, where they are recovered for reuse on later Space Shuttle launches.

After the Boosters are detached early in the flight, the Main External Tank fuels the Shuttle Orbiter tail engines at full thrust to gain the high speed required for orbit. When more than 97% of orbital speed is attained, the MET is detached from the Shuttle Orbiter and directed to cross Earth's atmosphere to burn up (Skylab-like) with remnants falling into a remote section of the Indian Ocean. The MET cannot be returned to Earth for reuse on later launches because it cannot be returned without burning up in Earth's atmosphere, unlike the Boosters which detach themselves early before high speeds are attained. Currently, the Main External Tank is just thrown away.

Analogous situations exist for non-Shuttle launch systems, too, e.g., the Ariane throws away all of its spent fuel tanks.

In fact, there are quite a few spent fuel tanks out in orbital space.

This wasteful procedure will eventually change. The questions are "when?" and "by who?"

The external tank represents a potential payload which weighs more than the maximum launch capacity in the Space Shuttle's cargo bay.

Some of the ways we can use these tanks in orbit include:

• Each MET offers an 11-story habitable volume wider than a townhouse, and useful for housing as well as space stations and industrial facilities
• Alternative structures and components built from melted down tanks (e.g., using solar ovens in space, with thin foil mirrors in the vacuum)

Compare the mass and volume of the external tank to the cargo capacity of the Space Shuttle:

Shuttle cargo bay: 29 tons maximum, 1,160 cubic meters (41,000 cubic feet)

External fuel tank: 35 tons, 1,144 cubic meters (51,000 cubic feet)

Each Main External Tank is actually composed of two tanks :

a large tank for liquid hydrogen (fuel) and

a smaller tank on top for liquid oxygen (the oxygen to burn the fuel).

The hydrogen tank can be used as an 11-story building, each level having 56 square meters (600 square feet) -- 610 square meters (7000 sq. ft.) per tank

Artificial gravity can be produced centrifugally by attaching two tanks by a beam or cable and rotating like a ferriswheel. Tanks could be bunched and connected by doors and tunnels. Such a design was adopted by the General Dynamics Convair report on SPSs from lunar material, as pictured below, as an 11-level building.

The external tanks can also be landed on asteroids or the Moon and used for habitats there, with the tanks used horizontally with no artificial gravity, as pictured below. In this case, they are used with two long levels.

Meltdown of tanks (e.g., the oxygen tanks) could be done by a solar furnace. In the zero gravity environment of orbit, immense mirror ovens which keep pointed at the sun are readily achievable. In space's vacuum, foil mirrors can be used (no wind, no gravity).

The tanks are made of aluminum, a metal with many applications, including structures, electrical and heat conductors, and solar oven mirrors (aluminum is one of the best reflector materials). Aluminum has a low melting and boiling point for a metal, so it is easy to work with in a variety of ways.

Notably, the MET, when separated from the Shuttle, still has some hydrogen and oxygen in it. This leftover fuel is abot 1000 kilograms (roughly 2000 pounds) or 1 ton of hydrogen, and about 6,000 kilograms (13,000 pounds) or 6 tons of oxygen, which is 1% of its liftoff fuel. It also has 500 kilograms (245 pounds) of nitrogen (a refrigerant).

The hydrogen and oxygen are useful combined as water (7,000 kilograms, or 15,000 pounds per tank). Humans and plants need water, nitrogen, oxygen, and hydrogen.

Notably, if the tank is saved, the Space Shuttle can lift MORE payload up from Earth, or it can go up to a higher orbit. Why? Saving the tank would eliminate a manuvering operation required to send the tank to burn up safely in a small target area at a remote spot in the Indian Ocean. This requires use of fuel on board the Shuttle, which is a compact but relatively heavy fuel (hydrazine), which is also used to propel the Shuttle to its final orbital velocity. Saving the tank would also allow more of the tank's leftover fuel to be used (by a slow burn at lower tank pressure). An engineering study by the tank's manufacturer, Martin Marietta, shows that the Shuttle can take an extra ton of cargo to orbit if the tank is saved.

What does NASA have to say about this ?

An external tanks space station was a direct competitor to funding for the NASA space station Alpha, which was eventually abandoned anyway after several years and billions of dollars in designs. An external tanks space station was also a competitor to funding for the International Space Station (ISS) effort that has gained so much political support and money for NASA. There were many interest groups who opposed use of the MET.

After some political pressures were applied, NASA offered to deliver its tank to orbit for free to any entity capable of handling it properly. NASA operates the Shuttle, so NASA is the authority/boss.

Prodded by outside inquiries, NASA eventually supported a study into retaining the tank in orbit, and the result was more positive than expected at first.

However, NASA has taken on a policy of relying on business to take the initiative in this area instead of government, which most of us will probably agree with.

NASA doesn't want to spend its limited budget on the infrastructure required to bring these tanks to orbit and handle them. NASA has offered to deliver the tank to orbit for free, but at the same time has pointed out a number of complications and costs involved to the Shuttle program and established understandable conditions for delivery, mainly for a third party to be waiting to collect it for at least safety reasons.

There is no system in orbit to collect these tanks, and NASA can't be expected to modify its clients' launch schedules and orbits to accommodate putting all the tanks in orbits close to each other.

Nonetheless, NASA has stated what is needed to utilize the tanks, e.g., a system to collect the tanks and control them so that they don't become a hazard, a way to pump the residual fuel out of the tanks, a way to outfit the tanks with the desired contents (by teleoperated robots or human extravehicular activity), and various infrastructure. NASA is not willing to launch the material to be moved inside the tank, but is willing only to give an empty tank which anyone can dock with later, on their own. NASA is not willing to devote much shuttle astronaut time or resources on behalf of the tank client, and any client requests to the manufacturer of the tank to redesign the tank must not entail any risk to the mission at all, i.e., probably no significant redesigns of the tank will be acceptable.

Fair enough. NASA has done its part; now it's up to business to come up with a best scheme to capitalize upon such an opportunity, without using U.S. taxpayers' dollars and without interfering too much with the Space Shuttle's agenda.

Out of all the creative ideas on how to use the external tank, one of the most interesting concepts is to fill the external tank with the desired floors, desks, etc., on the ground before filling it with fuel. This has been dubbed a "wet launch" of a habitat. It solves most of the problems and expense of needing lots of robotic or human extravehicular activity in space to outfit the tank with its desired contents. However, since NASA has said that any use of the external tank can't have any effect on launch performance, and this design has a more massive tank with a resultant loss in payload capacity, it doesn't look as if NASA will accept this. NASA doesn't like any redesign of the manned Shuttle system due to potentially lowering the safety to the crew by any mistakes due to redesign, e.g., structural dynamics.

Notably, using a spent fuel tank for a habitat or space station will not be the first time this has been done. Skylab (the 1970s U.S. space station that fell back down to Earth a few years after it was abandoned) was made from a fuel tank which was outfitted with all its equipment and launched dry (i.e., never filled with fuel).

Further, the concept of "wet launch" is not new. In fact, the original designs for Skylab were for a wet launch - launching a Saturn V rocket (the same as was used for Apollo) with the the upper stage fuel tank filled with floors, desks, etc. Once in orbit, the fuel tank would be vented and then sensitive things like computers and laboratory items would be moved in. However, as the program progressed, the wet launch concept was eventually replaced with a dry launch concept, as the agency decided not to engage in any extra-vehicular activity (EVA) to outfit the tank in orbit, but instead to launch the tank already equipped with everything inside. Also, the thermal insulation was on the inside of Skylab instead of on the outside like the MET, and NASA felt poisonous outgassing from the absorbed hydrocarbon propellant was a problem not easily solved. (This is not a problem with the MET since it uses just hydrogen.) Relocating the insulation was not something seriously considered, either.

The Space Shuttle's MET is manufactured by the Martin Marietta Corporation. Two employees of the Martin Marietta Manned Space Systems Division in New Orleans, Louisiana, who reviewed various external tank on-orbit concepts by others, A.R. Pagano, Jr., and E.M. Enright, have come up with a scheme of their own to launch the tank without the Space Shuttle, and redesign the inside of the tank along the lines of a wet launch. The main issues are covered in a paper presented at an SSI/AIAA Princeton conference entitled "Ground Integrated ET-Derived Space Platforms for On-orbit Applications" (reference) In this paper, they recommend that everyone else who has developed a design for MET on-orbit applications engage in a redesign along these lines.

Notably, the Space Shuttle (and its above derivative system) isn't the only vehicle that can take a tank from an upper stage into orbit along with other cargo. However, it is the biggest.

Links to others' pages on fuel tank habitats

A very well written page with graphics on this issue has been created by Cris A. Fitch at his excellent Orbit6 website.

Another good external tank graphics page with sponsors is on the web. When you get to this page, click on their "Projects" and "Ring Station" icon. Sponsored by Symantec (Norton), Apple, Specular and a few other visionary supportive companies.

For some real pictures of external tanks on the ground, in flight, and being wasted in space, see Frank Williams' page.

Special thanks to Tom Abbott for pointing out the three links above. He also suggests that a virtual tour through a converted external tank would help internet users appreciate its size. Volunteers, anyone?

Breaking through the external tank psychological, institutional and public relations barriers would be a good practice run to breaking through the same barriers for implementing PERMANENT.

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