MIT Team Proposes Plan for Realistic Space Refueling Depot

The idea of a spacecraft en route to a distant world docking at an orbital fueling station to refuel is an old idea long kept from becoming a reality because of the huge price tag and long-term investment required to complete such a project. But now, a team of scientists say they've come up with a way to make an outer space fueling depot an object of science fact rather than science fiction.

Writing in the journal Acta Astronautica, the team of Massachusetts Institute of Technology scientists offers their suggestions on viable options for orbital fueling stations.

The plan is based around the fact that all lunar missions carry a supply of what's known as "contingency propellant" - reserve fuel that is used only in emergencies. Most of the time, this backup fuel goes unused left behind or burned upon reentry to the Earth's atmosphere.

The MIT researchers suggest that this contingency propellant could be put to better use if it is stockpiled in an orbital supply depot.

While on the way back to Earth, a mission that did not need to use its backup fuel could drop it off at the depot, and a later mission could pick up the backup fuel tank at the orbital depot on the way to the Moon. If this mission does not need to use the backup fuel, it can restock it at the depot, effectively creating a steady stream of fuel supply.

If enough fuel is stocked at the depot, it could pave the way for a large-scale lunar mission that would normally not be able to take place because so much of a rocket's payload is dedicated to fuel.

"Whatever rockets you use, you'd like to take full advantage of your lifting capacity," said Jeffrey Hoffman, a professor 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 orbital supply depots would be stationed at Lagrange points - regions in space between the Earth, moon, and sun that maintain gravitational equilibrium. Objects at Lagrange points remain in place, keeping the same relative position with respect to the Earth and the moon, the researchers said. 

Ideally, Hoffman said, the transfer of fuel from an Earth-returning spacecraft to the supply depot would be as simple as a robotic arm grabbing the fuel tank and bringing it in for storage. But he said that siphoning liquid fuel from a spacecraft to the waystation could also be done.

"In building the International Space Station, every time a new module is added, we've had to hook up new fluid connections," Hoffman said. "It's not a trivial design problem, but it can be done."

The main drawbacks for both the fuel canister-transfer and fuel-siphoning scenarios are the same. Maintenance of the system will be challenging, keeping the depots orbiting within Lagrange points is tricky, and preventing the stored fuel from heating up to a point where it evaporates is also a challenge. But Hoffman thinks these hurdles can be overcome.

"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 idea for this design was spawned from a series of classes Hoffman teaches on redesigning a lunar lander and evaluated different approaches to landing on the moon. Hoffman's students Koki Ho, Katherine Gerhard, Austin Nicholas, and Alexander Buck contributed to the journal article.