Propellant depots: an idea whose time has (almost) come

by Jeff Foust Monday, May 12, 2008

One of the key factors driving the design of space projects of all shapes and sizes is propellant. The choice of propellants, and their quantities, drive everything from the size and performance of launch vehicles to the lifetimes or orbiting spacecraft. One of the reasons why this is such a critical factor is that, today, there's no way to refuel spacecraft and upper stages once in space (an exception being the International Space Station, whose thrusters can be refueled by visiting spacecraft). As any mission designer will remind you, there are no gas stations in space.

But what if there were? What if it was possible to top off the tanks of an orbiting satellite, or a vehicle headed towards the Moon or beyond? Not having to carry all of the propellant needed for the entire mission at the time of launch would have a major effect on the mission's design, capabilities, and cost. For example, the Mars Direct architecture first promulgated in the early 1990s revolutionized concepts for human Mars missions by proposing to produce the propellant needed for the trip home on Mars, rather than carrying it all the way from Earth, permitting smaller and less expensive missions. Today, one of the key factors in siting a future human lunar base is access to any deposits of water ice that could be used not just for life support but also for propellant.

In situ resource utilization isn't an option for Earth orbit, but creating orbiting depots for propellants hauled up from Earth is an alternative. Such proposals have, in the past, been stymied by a lack of a clear market and business plan, as well as numerous technical issues. However, backers believe that conditions today, both technical and economic, merit a reexamination of the concept.

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