An MIT team has come up with two cost-efficient orbital fuel depot designs that do not require long-term commitment. Both designs take advantage of the fact that each lunar mission carries a supply of “contingency propellant” — fuel that’s meant to be used only in emergencies. In most cases, this backup fuel goes unused, and is either left on the moon or burned up as the crew re-enters the Earth’s atmosphere.
Instead, the MIT team proposes using contingency propellant from past missions to fuel future spacecraft. For instance, as a mission heads back to Earth, it may drop a tank of contingency propellant at a depot before heading home. The next mission can pick up the fuel tank on its way to the moon as its own emergency supply. If it ends up not needing the extra propellant, it can also drop it at the depot for the next mission — an arrangement that the team refers to as a “steady-state” approach.
A depot may also accumulate contingency propellant from multiple missions, part of an approach the researchers call “stockpiling.” Spacecraft heading to the moon would carry contingency propellant as they normally would, dropping the tank at a depot on the way back to Earth if it’s not needed; over time, the depot builds up a large fuel supply. This way, if a large lunar mission launches in the future, its rocket wouldn’t need a huge fuel supply to launch the heavier payload. Instead, it can stop at the depot to collect the stockpiled propellant to fuel its landing on the moon
The researchers came up with a basic mission strategy to return humans to the moon, one slightly different from that of the Apollo missions. During the Apollo era, spacecraft circled close to the lunar equator — a route that required little change in direction, and little fuel to stay on track. In the future, lunar missions may take a more flexible approach, with the freedom to change course to explore farther reaches of the moon — such as the polar caps, for evidence of water — a strategy that would require each spacecraft to carry extra fuel to change orbits.
Working under the assumption of a more global exploration strategy, the researchers designed a basic architecture involving a series of stand-alone missions, each exploring the surface of the moon for seven to 14 days. This mission plan requires that a spacecraft returning to Earth must change its orbital plane when needed. Under this basic scenario, missions could operate under existing infrastructure, without fuel depots, meaning that each spacecraft would carry its own supply of contingency propellant.
The researchers then drew up two depot designs to improve the efficiency of the basic scenario. In both designs, depots would be stationed at Lagrange points — regions in space between the Earth, moon, and sun that maintain gravitational equilibrium. Objects at these points remain in place, keeping the same relative position with respect to the Earth and the moon.
Hoffman says that ideally, transferring fuel between the depot and a spacecraft would simply involve astronauts or a robotic arm picking up a tank. The alternative — siphoning fuel from tank to tank like you would for your car — is a bit trickier, as liquid tends to float in a gravity-free environment. But, Hoffman says, it’s doable.
• An affordable on-orbit propellant depot framework without needs of depot refill missions is introduced.
• Contingency propellant in every mission is shown to be an efficient substitute of depot refill missions.
• The proposed steady-state strategy can be applied to the sequence of missions with payloads of similar sizes.
• The proposed stockpiling strategy can be applied to the sequence of missions with payloads of various sizes.
This paper introduces new concepts of on-orbit propellant depots for human space exploration based on contingency propellant. The proposed architecture is useful in that it does not require separate depot filling missions, whereas conventional depot architectures require large “prior investment” type missions for depot filling before gaining the returns. Two concepts for this type of depots are shown: “steady-state” architecture and “stockpiling” architecture. In the “steady-state” mode, the depot always keeps the contingency propellant in orbit as well as the reused habitat module. In each mission, the vehicles collect the habitat and the contingency propellant from the depot in orbit on its way to the destination, perform the maintenance for the habitat, and leave the habitat and the unused contingency propellant in orbit on its way back. In the “stockpiling” mode, on the other hand, the habitat module is reused in the same way, but the depot accumulates propellant so that a later mega-mission can carry larger payload. Numerical results show the usefulness of the proposed architectures.