External Tanks in Orbit

The High Frontier of Gerry O'Neill

Tom Abbott: Dr. O'Neill's timetable was based on extensive use of space shuttle External Tanks to put the basic space development infrastructure in orbit such as a low-Earth orbit space station and a Moonbase. Using External Tanks was calculated to reduce development costs significantly and Dr. O'Neill's plan devised a way for each step of the space development path to pay for itself.

Gerard O'Neill - "The High Frontier" In the "Low Profile" plan, the tanks would be carried into orbit, at a very small cost in shuttle payload. We would set up a storehouse of empty tanks in orbit; some would be fitted out as living quarters, each tank providing about twenty comfortable, private apartments for as many workers. (snip) Most of the external tanks would end up as reaction mass, in pelletized or powdered form. (snip)

Moonbase

Tom Abbott: Instead of using the entire External Tank, as in the spacestation designs, the Moonbase design would use only the smaller oxygen tank of the ET as a habitation module. An intertank ring would be attached to the bottom of the oxygen tank to carry the engines and fuel tanks necessary for flying to and landing on the Moon. The ET Moonbase would be large enough to house 12 astronauts. It would be assembled and outfitted on the ground before launch and would be launched into Low-Earth orbit (LEO) just like the Option C spacestation: connected to the side of a fuel-carrying ET and solid rocket boosters.

Four RL-10 rocket engines would be attached to the intertank ring to propel the Moonbase once in orbit. The RL-10's are some of the most reliable rocket engines ever produced and are the same engines the DC-X is using. The DC-X flight test data can be applied directly to the ET Moonbase since it will use the same engines and will land on the Moon in the same manner as the DC-X lands on Earth.

To put the ET Moonbase on the Moon requires the launching of 80 tons of oxygen and hydrogen fuel to LEO to fuel the ET Moonbase for the trip. The 80 tons of fuel is enough to transport the ET Moonbase, along with 70 days worth of supplies for 12 people, to the Moon's surface.

Launching the 80 tons of fuel into orbit will cost about $150 million? for one launch of a Shuttle-Derived HLV (or its Energia equivalent) or $1.5 billion if Titan IV vehicles (5 Titan IV's at $300 million each) are used (see how cost-effective developing a Heavy-Lift vehicle would be. Without a Heavy-Lift vehicle it would cost ten times as much to launch 80 tons of fuel to LEO: $150 million versus $1.5 billion. An HLV would pay back its development costs in short order).

Total cost to implement the ET Moonbase will be about $7 billion: $5 billion for the oxygen tank/intertank conversion; $500? million for RL-10 developments (although DC-X development should help this cost); and about $1.5 billion (at most) to orbit the fuel.

Use One as a !Fuel Tank!

fcrary@rintintin.Colorado.EDU (Frank Crary): One "additional task" occurred to me, which wouldn't have these disadvantages. Use the ET as a fuel tank for a lunar or Mars mission. Refuel it by sending up fuel on many launches by the unmanned launch vehicle of your choice, and you have a very low mass fuel tank for the actual mission. (The tank to fuel mass ratio goes as, roughly, the cube root of volume, so the shuttle ET would have half the mass fraction as eight smaller tanks. That's why this would be better than using many, individually launched, tanks.) Unfortunately, the ET is just too big: To use it, you'd have to launch a _big_ mission; about two to three times the size of currently suggested ones. If someone was willing to pay for a ten man mission to Mars, it would be a viable idea. But that's getting into the size (and cost) range of the old, SEI $500 billion missions...

greason@ptdcs2.intel.com (Jeff Greason) The crucial technology for deep-space missions is neither docking nor assembly, but on-orbit propellant transfer and storage. At least 90% of the mission mass for a Mars mission will be propellant -- which doesn't particularly care what size package it comes in. Two shuttle experiments on on-orbit propellant transfer and storage could therefore cut the size of the vehicle needed by 10X or so, trivially.

Note that that if you had a mission which needed 80 ton "Magnum Lifter" capability, for example, that one trick cuts it to 8 ton payload size. If you are willing to dock smaller tanks together rather than "one big tank", you can probably cut it to 4 ton -- and if you use inflatables, there's not a volume problem either. Now you can do missions formerly requiring an HLV with Delta/Atlas class payloads -- and payload sizes in the upper end of the burgeoning "small RLV" market.

If we're going to work on something, on-orbit propellant transfer/depot is the thing to work on. And the *most* simple use of a Shuttle ET is to serve as a permanent propellant depot -- propellant tanks require minimal modification to serve as propellant tanks :-)