External Tanks in Orbit

Recently , I was very pleased to receive a report from a gentleman over at Nasa Johnson on one scheme for the utilization of an Shuttle external tank as a part of a space station. The idea of utilizing Shuttle external tanks has fascinated me for some time. It fits with my overall view of Space development. Also today, I had conversations with Gene Meyers (a primary ET promoter), my brother (a test pilot and an Aero-Astro engineer), and Ashley Shuler (a space enthusiast friend). Each conversation was filled with enthusiasm for these external tanks as important components of a re-vitalized American space program.

Clearly there has been alot of design and redesign of the International Space Station (ISS). This project, after eating up alot of money and design effort has yet to put hardware in orbit. And, as currently designed, it is nothing more than Mir II. I will not argue here against the ISS, but it seems to be clear to many that there is a strategic lack-of-vision concerning its development. In fact, I would argue that it is important to continue with its initial deployment, in that something flawed is far superior to a never-ending search for conceptual perfection without deployment. It is necessary for America's future in space that we build and deploy manned hardware.

What I would like to discuss here is the short and mid-term utilization of the Shuttle's external tank in orbit, along with a philosophy which emphasizes aquiring mass-in-orbit and a general ambition toward the development of an off-planet economy.

Instead of thinking of a space station as a tin can orbiting above us, let us think of most of us as sitting deep inside a gravity well, and that those in a space station are the first of us to get out of the gravity well. Once out that gravity well, mankind is much closer to expanding out into the solar system.

Instead of thinking of space-related expenditures as tribute to a glorified past of manned exploration, let us think of investment in space as the development of the off-planet economy. It is important to remember that one day we will open a Wall Street Journal and find the price of a kilogram of aluminum in low-earth orbit listed.

The Golden Asteroid

Let us make believe that there arrived, in Earth orbit, an asteroid made up of Al, C, H, and O, and having 2x10^6 kg. Wouldn't the aerospace community want to go out and mine it? Wouldn't it make a nice source of raw materials?

Well, that is the approximate mass of the roughly 75 Shuttle external tanks that have been denied a chance at low-earth orbit. Each tank weighs in at 66,000 lbs (29,000 kgs), and consists of refined aluminum, a foam insulation (carbon, etc.) exterior, and residual LH2 and LO2 inside the tanks. In addition, consider that the tanks are immediately capable of being pressurized for potential use by humans. Orbital gold.

Engineering details are subject to debate and refinement. Plans for the use of the external tank range from simple (widen access hatches, attach basic attitude control units) to complex (GEODE, modified station designs). There are, however, no engineering show-stoppers. Arguments against the concept generally wind up asking the question "why would you want to place these tanks in orbit?"

In order to answer this, the basic argument needs to be made in favor of having control of refined mass outside Earth's gravity well, as well as the argument in favor of the development of the off-planet economy.

Perhaps it is a belief instead of a reasoned position, but I believe that any mass in Earth orbit is very precious, especially if you can control how it behaves. This includes old satelites, fuel tanks, and various forms of junk and waste. I believe that to return almost any of this mass to the surface of the Earth is wrong and wasteful. I understand that that mass in orbit has a tendency to fall out of orbit (a la Skylab), and thus it is important to shepherd the junk that one collects to keep it from falling.

Concerning the external tanks, they are most of the way into orbit, and with a few operational changes to the Shuttle's procedures, they could achieve a semi-stable low-earth orbit without detrimentally affecting normal Shuttle operations. In order to bring them into stable, higher, and more useful orbits, manuevering units / thrusters would need to be attached. Similarly, the 900 lbs of explosives currently attached to the tank to help it break up upon re-entry should be removed. Modifications to make the tanks more useful for exploitation can be considered, but should not infringe upon the central concept that every tank becomes extremely valuable when boosted into orbit.

It is obvious to most space-enthusiasts that at some point in time Man will develop a significant presence off-planet, and that that presence will be encouraged and hastened by commercial interests. So far those commercial interests have been mainly interested in satellites and contracting on government directed projects. Because of the enormous capital involved and the marginal returns on manned missions, commercial interest in these projects has been slim to nil.

The existence of external tanks in orbit has the possibility of changing this situation. For the first time it becomes possible to have much larger operations in space, both because of the amount of pressurized volume and because of the amount of available mass. Other elements, including power, consumables, climate control, and guidance do need to be addressed. However, it is possible to now consider having Shuttles ferrying dozens of workers into orbit to work on projects of significant size, because with each flight another 29 metric tons of refined mass goes into orbit, to be supplemented by various supplies and cargo also conveyed.

The change is based on the belief that mass in orbit is good. And that additional pressurized volume also doesn't hurt. The external tank is a excellent and cost-effective way to implement these beliefs.

A recent idea by Phil Fraering, concerning the ET space station: instead of doing a HLLV based on shuttle hardware, do a smaller launch vehicle based on the external tank and the SSME's only. It'll reach orbit with 30K pounds of material (which could be used in much the same way Geode's packet could). This could provide a starting point for attaching modules launched by a SSTO or whatever. You could use no SRB's, or smaller non-segmented solids. Or decide to use different, hopefully cheaper engines.

Another idea proposed by Joseph Strout jstrout@ucsd.edu is that investors might wish to buy one or several ETs from Nasa, as future investments. Assuming that the orbits could be maintained, these could become valuable structures for later use.