Getting into space is an expensive business where every little bit of extra weight, which includes the fuel powering the spacecraft, can add thousands of dollars to the cost of a mission. A team of researchers at MIT proposes establishing gas stations in space as a possible way to help cut the cost of future missions to the Moon.
Before you point out that establishing a "gas station" in space would require fuel to be hauled up to it in the first place, rendering such a concept moot, the MIT team points out that lunar missions carry a supply of "contingency propellant". This is essentially the equivalent of a can of extra gas thrown in the boot that can be pulled out in the case of an emergency.
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Rather than just leaving this backup fuel on the Moon or burning it up on re-entry as is usually the case, the MIT team suggests spacecraft could drop it off at a depot positioned between Earth and the Moon on their way home.
This way, following missions could dock at the depot to pick up the fuel tank for use as its own contingency propellant before continuing on to the Moon. Then, on it's return journey, it could drop off the fuel again if it wasn't used. The team calls this a "steady-state" approach.
Additionally, the depot would be able to stockpile fuel from multiple missions that could be used to top up a lunar mission on its way to the Moon, thereby keeping the amount of fuel carried at launch to a minimum.
'Whatever rockets you use, you’d like to take full advantage of your lifting capacity," says Jeffrey Hoffman, a professor of the practice in MIT’s Department of Aeronautics and Astronautics. "Most of what we launch from the Earth is propellant. So whatever you can save, there’s that much more payload you can take with you."
The team says such an approach would be more cost-effective than other proposals that involve sending tankers to refill orbiting depots or building fuel-manufacturing stations on the Moon like those being pursued by the Shackleton Energy Company.
The fuel depots would be positioned at Lagrange points where they would maintain gravitational equilibrium, allowing them to keep the same relative position with respect to the Earth and Moon.
Hoffman says the ideal approach would involve astronauts or a robotic arm picking up a tank, but that siphoning furl from the depot to the spacecraft would also be doable. However, the siphoning approach would pose greater difficulty because the fuel would float in the gravity-free environment.
Both approaches would also require the depots to be maintained so they remained within the Lagrange point and to ensure the fuel was kept at temperatures cold enough that it didn't boil away. But if these challenges could be overcome, Hoffman says fuel depots in space could provide an efficient way to support future missions to the Moon.
"One of the problems with large space programs is, you invest a huge amount in building up the infrastructure, and then a program gets canceled," Hoffman says. "With depot architectures, you’re creating value which is robust against political uncertainty."
The depot architecture proposed by Hoffman and his students, Koki Ho, Katherine Gerhard, Austin Nicholas, and Alexander Buck, is outlined in the journal Acta Astronautica.