When you travel from the earth to the moon, you usually assume it's a non-stop trip where you have to launch with everything you need for the journey. But an MIT team has thought up a way to create orbital fueling stations that don't require expensive, long-term servicing missions and could even survive NASA's budgetary ups and downs.
The idea of building space fuel depots or creating fuel-manufacturing stations on the moon or Mars is nothing new. But the MIT proposal is a bit more modest: cost-efficient space depots that could store extra rocket propellant leftover from the emergency supply of previous lunar missions. Over time, such stations might be able to stockpile enough unused emergency fuel to provide a huge orbiting supply for larger lunar missions in the future.
"Whatever rockets you use, you'd like to take full advantage of your lifting capacity," said Jeffrey Hoffman, a professor of the practice in MIT's Department of Aeronautics and Astronautics, in an MIT news release. "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 manned Apollo lunar missions typically carried "contingency propellant" to use in case of emergency situations. But unused emergency fuel was usually left on the moon or burned up as the human crew reentered the Earth's atmosphere. Either way, it's effectively wasted.
Hoffman and his students laid out two space depot scenarios in the March-April issue of the journal Acta Astronautica. In a "steady state" scenario, lunar missions would drop off their unused emergency fuel for the next lunar mission to pick up and use as its own emergency fuel. In a "stockpile" scenario, lunar missions would keep dropping off unused propellant to build up a large supply for more ambitious lunar missions down the line.
Either astronauts or a robotic arm could do the job of swapping out the extra tanks of propellant between the depot and spacecraft. The Dextre robotic arm aboard the International Space Station already tested such a scenario by conducting a mock refueling mission.
MIT researchers called for such depots to be stationed at the Lagrange points—regions of space where the gravitational forces of the Earth, moon and sun balance out to achieve gravitational equilibrium. Such regions allow spacecraft or other objects to orbit in a constant pattern relative to the larger bodies in space.
Space depots could change future lunar exploration in several ways. First, the possibility of picking up extra propellant on the way to the moon could allow future missions to launch carrying a heavier mission payload rather than propellant, or perhaps reduce the need for heavy-lift rockets. Second, a network of space depots could also give future lunar missions the extra fuel required to change orbits as they explore farther regions of the moon.
The idea of space depots still faces challenges in terms of maintenance, keeping the depots within the Lagrange points, and keeping the fuel cool enough to prevent it from boiling away. But Hoffman pointed out that the space depot plan could prove more cost-efficient and sustainable in the long run compared to more ambitious refueling schemes—no small matter when considering NASA's perpetual budget uncertainty.
Similar stepwise strategies could also aid planning for manned missions to Mars—a far more daunting proposition in terms of both sustaining a human crew for months and having enough rocket fuel for a round trip. The rocket fuel and launch vehicle take up the vast majority of mass launched into space, noted Olivier de Weck, an MIT aerospace engineer, in a previous IEEE Spectrum special report on Mars.
Illustration: Christine Daniloff/MIT