Commentary | Propellant Depots Instead of Heavy Lift?
Considerable recent attention has been devoted to the possible use of orbiting fuel depots for human exploration beyond Earth orbit. In this concept, large propellant tanks are placed in a suitable low Earth orbit (LEO), to be filled by multiple launches of medium-payload-class vehicles, i.e., a few tens rather than a hundred or more metric tons of payload capacity. These depots are then used to refuel upper stages, which arrive empty in LEO after launch from Earth, after which they are launched outward to the Moon or beyond.
Advocates for this approach believe that the money saved by not building a heavy-lift launch vehicle such as the Space Launch System () will more than compensate for the cost and operational inefficiencies entailed in bringing the required total mass of propellant to orbit in smaller individual packages.
Whether fuel depots make sense in the near term depends upon what question we are trying to answer. If the question is, “What kind of space architecture will generate a high traffic model for private space firms without having to pay for missions that actually go beyond LEO?” then fuel depots are an attractive concept. But if the question is instead, “How can we efficiently create the strategic space transportation capabilities to enable humans to explore beyond LEO?” then they are not.
The fuel depot concept may be — we think will be — valuable when propellant can be harvested from in-space resources, such as water trapped in lunar craters or oxygen extracted from the regolith. Unfortunately, we are not yet in a position to exploit such resources, and so for now fuel depots are an answer to a question that is at best premature. The SLS and the Multi-Purpose Crew Vehicle (MPCV) are needed today. Fuel depots will be needed tomorrow, when a robust space operations infrastructure has been established and operations beyond LEO are common.
The challenge for fuel depots is simply that the marginal specific cost of payload to orbit is generally lower for larger launch vehicles. There may be exceptions, but the trend is clear. Moreover, this same trend is observed for other forms of transportation — road vehicles, trains, ships and airplanes. Without exception, larger vehicles are used whenever possible for long-haul transportation. In evaluating depot concepts, one must then ask: Why will space transportation be an exception? Is it really an exception? Or are we missing one or more crucial points in the analysis?
When depots or nodes are used in transportation architectures, they must be supplied by the least expensive means available rather than the contrary. There is an old joke about clothiers in the New York garment district who professed, it is said, to sell each garment at a loss, but would “make it up on volume.” Fuel depots seem to exemplify that joke. It is very difficult to see how putting propellant in orbit in small quantities at higher marginal cost can be cheaper in the aggregate than putting it up in larger quantities at lower marginal cost, even without factoring in the cost of the depot itself and its own operational requirements.
The economic attractiveness of propellant depots depends strongly upon the price claims of commercial launch companies for fuel delivered to orbit. At this point, such claims should be considered highly suspect. Even a signed contract offers little assurance, because if the supplier requires additional funds to continue service and there is no government capability available as a backstop, the money will be tendered. Thus, price claims made by companies that are not yet conducting routine operations at that price should be regarded with skepticism.
Issues of technical feasibility and practicality also exist. When cryogenic fuel is stored on-orbit, in whatever vehicle, the ability to maintain it in its cryogenic state is crucial. With today’s capability, we might achieve liquid hydrogen boil-off losses of about 0.35 percent per day, or about 10 percent of the fuel each month. At a boil-off rate of 0.1 percent per day — a capability not yet demonstrated — 10 percent of the fuel will be lost in three-and-a-half months. Completely closed-cycle systems, or those that are nearly so, are possible with active refrigeration. This technology absolutely must be pursued, as it is necessary for missions beyond the Moon. But we should be skeptical of unproven claims about extremely low boil-off rates, such as the 0.5 percent loss rate per month assumed in one recent study, until and unless the technology is demonstrated.
The most reasonable claim made in support of fuel depots is that if they are employed to the exclusion of a heavy lifter, one saves the cost of building the heavy lifter. This is certainly true — but then we do not have a heavy lifter! Heavy-lift launch is a strategic capability for a spacefaring society, and its absence severely constrains any plans. The 130-metric-ton SLS capability should be regarded as the floor of space-lift capability for exploration, not the ceiling.
The kind of space program that we need requires transportation of much that is not fuel. While the international space station offers an existence proof that one can build a 400-metric-ton object in space using pieces weighing less than 15 tons each, the time, money and programmatic risk required for assembly offers the clearest possible demonstration that it was not the best approach.
Thus, those who argue that we could save money by using fuel depots and not building a heavy lifter seem willing to ignore a key theme: We need a heavy lifter for reasons going far beyond the transportation of fuel. It may be that in future space architectures the flexibility offered by fuel depots will compensate for their inefficiency. But they are not an appropriate feature of the developmental systems and architectures we need to build now.
Fuel depots as an element of a near-term space architecture are an example of magical thinking at its best, a wasteful distraction supported by the kinds of poorly vetted assumptions that can cause a concept to appear deceptively attractive. We in the space community are especially prone to such behavior. If we actually want to accomplish anything, it must cease. We need to do the right stuff, right now. When we have settlements on the Moon and Mars, the use of fuel depots will make sense. But for today, the last thing we should do is to put one of the hardest problems — long-term cryogenic fuel storage — in series with our next steps beyond LEO.
Michael D. Griffin, a professor at the University of Alabama, Huntsville, was NASA administrator from 2005 to 2009. Scott Pace is director of the Space Policy Institute at George Washington University.