Chair Force Engineer

Friday, September 28, 2007

Fuel for a Revolution

Rand Simberg's PopMech article on orbital propellant depots really resonated with me, particularly the part about propellants being a low-cost payload.

Let's look at the economics of the space business for a second. While launch vehicles are big-ticket items, the reality is that the satellite payloads are where the money is at. The launcher's payload is usually worth several times what the launcher costs. For that reason, most satellite operators are reluctant to build and fly space assets unless they absolutely have to; and when they have to build a space asset, they try to make it as lightweight and compact as possible.

Under the current paradigm of high-cost, low-mass payloads, there are two consequences for the launch vehicle industry. The first is that the flight rate is too low to justify the development of reusable launchers. The second is that there is no demand for heavy lifters such as Ares V.

Now let's assume that NASA adopted an orbital-refueling strategy for going to the moon or Mars, and offered commercial contracts to anybody who could deliver propellants to orbit. All of a sudden, there's a need for multiple launches of payloads that cost thousands, rather than millions or billions, of dollars. There's also a demand for large propellant masses to be delivered over a short span of time (due to propellant boil-off concerns.) And because propellant is such a cheap payload, the only thing lost in the event of a failure is the time and money that went into the launch campaign for the vehicle that just failed (which is substantial at this point in space history.)

The need to deliver low-cost propellants to orbit creates an intriguing niche in which either an RLV or a heavy-lifer could be economically justified. Studies have shown that an RLV would need to fly fifty times per year (or more) to be economically viable. Assuming the technologies to build such a reliable RLV were available, and assuming 20 metric tons of propellant being delivered in a single launch, there must exist a need for 1000 metric tons of propellant on-orbit per year. Likewise, the same propellant mass could be delivered with nine launches of the Ares V (a number that is in line with the shuttle's historic maximum yearly flight rate.)

Assuming that NASA and private industry move beyond a "two moon landings per year" mentality, the need for 1000 metric tons of propellant per year can be justified. Imagine a propellant depot in low earth orbit, serving as a waystation between the earth and moon. A reusable launcher would deliver crews and/or propellants to a small space station equipped with a large propellant depot. The crews would use the space station as a point to board and refuel the craft that will take them to the moon and back.

Taking that concept one step further, a similar space station & tank farm could be set up at one of the Earth-Moon Lagrange points. A crew would transfer from their in-space transfer craft to a dedicated, reusable lunar lander. The landers would take on oxygen and perhaps hydrogen at the Lagrange Point space station. Some of the landers would be dedicated to ferrying oxygen and perhaps hydrogen extracted from the moon to the tank farm.

If humans are to become a space-faring society, it seems that we should embrace and perfect techniques like orbital refueling, instead of running away. Nobody is foolish enough to drive cross-country without making a few stops for gas along the way. The exploration of space should be no different.