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ULA propelland depot illustration
United Launch Alliance is studying concepts for propellant depots that would be derived from upper stages, their contents kept cold by sunshields. (credit: ULA)

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

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 were 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.

The benefits of propellant depots

The concept of the depot is straightforward: rather than launching a spacecraft with all the propellant it needs to carry out its mission, the spacecraft can, after launch, rendezvous with the depot and top off its tanks. Launching a spacecraft with empty tanks means that its “dry” mass can be heavier, increasing its useful payload, or it can use a smaller and therefore less expensive launch vehicle. Refueling existing spacecraft can, all other things being equal, extend their useful lifetimes.

“Two sites in one mission: that’s what a depot enables,” said Boeing’s Bienhoff.

The depots, in turn, generate launch demand of their own, since their stocks have to be replenished from Earth. This, depot advocates argue, can help spur the development of new low-cost launch systems to meet this demand. Hauling propellant might be an ideal early market for new vehicles that are still demonstrating their reliability, or for vehicles that trade off reliability for lower costs, since propellant is easily replaceable and costs orders of magnitude less per kilogram than a typical satellite (see “Low-cost launch and orbital depots: the Aquarius system”, The Space Review, January 30, 2006).

Jonathan Goff, an engineer with Masten Space Systems and a proponent of the propellant depot concept, draws parallels between depots and naval coaling stations established by the US in Hawaii, Guam, and elsewhere in the late 19th and early 20th centuries to support its fleets in the Pacific. “Basically, the idea was by having propellant prepositioned in places like that, and supplying it on a regular basis, it allowed a lot more flexibility than if you tried to operate everything out of San Francisco or San Diego,” he said during a session on propellant depots he moderated at the Space Access ’08 conference in Phoenix in late March. “I think propellant depots do the same thing for transportation in cislunar space.”

While propellant depots aren’t part of NASA’s current exploration architecture, the Exploration Systems Architecture Study (ESAS), they could be added in such a way to dramatically increase the performance and capabilities of its key components. The current approach, involving the Ares 1 and 5 launch vehicles, the Orion crew exploration vehicle, Earth Departure Stage (EDS), and Altair lunar lander, can put about 18 tons on the lunar surface. That sounds like a lot, but most of that is taken up by the mass of the lander itself leaving “one, maybe two tons of useful surface payload,” according to Dallas Bienhoff of Boeing.

Adding a propellant depot in Earth orbit that could fill the tanks of the EDS and Altair could dramatically change the architecture and improve its performance. In such an architecture, the EDS could perform not just the translunar injection (TLI) burn but also the lunar insertion burn, something that under ESAS would be performed by the descent stage of Altair. That greatly increases the amount of mass that can be landed on the Moon: 51 tons, according to Bienhoff, virtually all of it in the form of additional useful payload. “The other thing that it does is enable a two-sortie opportunity for a single mission,” he said, if the full lander—both descent and ascent stages—returns to lunar orbit from one site and refuels from the EDS before landing at another location on the lunar surface, rather than carry a heavier cargo payload. “Two sites in one mission: that’s what a depot enables.”

Adding a depot can also have other effects on the ESAS architecture. Since the EDS and Altair would be refueled in Earth orbit, it reduces the concerns about boiloff of their cryogenic propellants—liquid hydrogen and liquid oxygen—during the period between the Ares 5 launch of the EDS/Altair and the Ares 1 launch of Orion carrying the crew—particularly if the Ares 1 launch is delayed for any reason. Bienhoff added it may be possible to get rid of the dual-launch architecture entirely, shifting instead to one where Orion, along with the EDS and Altair, are all launched on an Ares 5. An alternative tradeoff would be to reduce the performance of the Ares 5, making it potentially less expensive to develop and operate.

Depending on the type of mission, the revised architecture would need a depot with between 150 and 175 tons of liquid oxygen and liquid hydrogen. Bienhoff said he envisioned having two depots in orbit for redundancy. A small fraction of the depot’s capacity could be used for other missions, ranging from boosting payloads into geosynchronous orbit to interplanetary missions.

Technical and other obstacles

While propellant depots offer significant benefits to lunar exploration or other missions, critics point to a number of major technical issues with the concept. Storing cryogenic propellants in orbit for long periods of time without boiloff is a critical issue, as is the transfer of those propellants to and from the depot. “There is a problem, and it’s the historic depot paradigm” of large-scale structures requiring technologies that today have low readiness levels, said Frank Zegler of United Launch Alliance (ULA). “It’s created essentially an insurmountable psychological barrier.”

“The key thing here is that it’s really not that difficult, so long as you take small steps,” he offered. The small step he proposed is based on the needs of the exploration architecture. “All this fancy exploration stuff, the lunar landers and all that stuff, those are almost trivialities. The big job we have to do here is moving liquid oxygen, and lots of it.”

Zegler said the depot concept under study at ULA was focused on the simplest possible approach. “What we’re talking about here is a single propellant, liquid oxygen, a single unit launched on a single launch. It’s based on existing stuff,” he said. “We don’t like something new unless it buys us something huge.”

“The key thing here is that it’s really not that difficult, so long as you take small steps,” said ULA’s Zegler.

At the core of the concept is a new upper stage, the Advanced Common Evolved Stage (ACES), currently under study at ULA and based on the Delta 4’s hydrogen tank. The upper stage is protected from the Sun by a conical sunshield that is based on the technology used in the sunshield for the James Webb Space Telescope. That sunshield, Zegler said, is critical for any depot. “Without a sunshield, you will never achieve the boiloff rates that are required,” he said.

An upper-stage-derived depot, launched on an EELV, could place 25 metric tons of propellant into low Earth orbit—about the same amount of propellant required by Altair. “Essentially you could bring up with this thing, on one of our smaller birds, the propellant for the descent to the Moon,” he said.

Much of the same technology could be used for a dedicated depot, replacing the ACES stage with an extended tank that could fit inside the same sunshade but could contain 230 tons of liquid oxygen, well over that’s needed by ESAS. The tank would be launched empty and filled by other vehicles.

Most of the technologies for this approach are in hand, Zegler said. A full-scale sunshield has been built and tested in the lab in the last year, with plans to eventually incorporate the technology into existing upper stages. “We’re trying to on-ramp this technology because we can gain performance in our existing vehicles for GSO [geosynchronous orbit] missions by using these simple sunshields,” he said. On an upcoming launch that uses a Centaur upper stage that will have excess propellant once it deploys its payload, Zegler said they plan to spin up the Centaur to test the rotational settling of the propellants, one solution to the problem of transferring propellants in zero-g.

A bigger challenge than the technology, though, might be to get NASA and others to adopt the concept of propellant depots. While NASA administrator Mike Griffin has been open to the concept, suggesting in public speeches that NASA would be willing to purchase services from commercial fuel depots, right now ESAS doesn’t depend on the concept. That, said Bienhoff, who has briefed a number of NASA officials on his proposals, is an obstacle to gaining acceptance of the concept within NASA. “They’re bound by the architecture, and they can’t spend any money on it because it’s not in the architecture.”

Goff said that the propellant depot is “one of the key technologies for turning our civilization into a spacefaring civilization instead of just a space visiting one.”

Aerospace consultant Rand Simberg said there are three business models for propellant depots to work: through a standard NASA contract; a government-chartered organization, analogous to the early communications satellite industry; and a fully-private model. He was skeptical of the first approach, because of concerns this would be business as usual. “This is one of the reasons there is so much resistance to propellant depots at NASA,” he said. “Because in everyone’s minds, they’re thinking, ‘This is going to be another space station.’” One solution he proposed was a hybrid approach, where NASA developed the basic technology that was implemented by a quasi-government organization that is later transferred to private industry.

The Space Access panelists in general believed that the development of depots was only a matter of time, and that time might be coming soon. Goff cited something he called “Bahn’s Principle”, after Pat Bahn of TGV Rockets: “The best time to discuss an important new idea is when it is almost ready for primetime.” And, perhaps, the sooner the better: Goff said that the propellant depot is “one of the key technologies for turning our civilization into a spacefaring civilization instead of just a space visiting one.”