Chair Force Engineer

Thursday, September 06, 2007

Fly Away Home

One of the challenges in building a fully reusable launcher is the recovery of the spacecraft's lower stage (or stages.) Analytically, it's easily demonstrated that a reusable launcher will probably have two or three stages. (A single-stage vehicle would place a horribly tiny fraction of its liftoff mass into orbit, and probably isn't even feasible with contemporary structures and propulsion technology.) But the booster recovery problem is much more difficult than it initially seems.

The most mechanically simple technique is to recover the boosters with parachutes. They'd either splash down in the ocean, or make an airbag-assisted landing on terra firma. The space shuttle SRB's have demonstrated this technique over a hundred times. Our operational experience with this method has taught us much about why we need to find an alternative. The cost of the recovery fleet is considerable. So is the cost of tearing each booster down between flights, shipping it to Utah and back, and reassembling it.

Parachute recovery shouldn't be completely rejected, although there's admittedly a lot riding against it. Rocketplane-Kistler hopes to avoid a lot of the shuttle SRB's problems by bringing the first stage of their K-1 to a landing on dry ground. Assuming that you have a large, open area in your predicted drop zone, I can see the Kistler approach working fairly well. Instead of relying on NASA's SRB recovery flotilla, Kistler could lift the booster with mobile cranes and haul it away on a flatbed truck. It also solves the problems associated with protecting the delicate parts of a liquid-fuel rocket from corrosive seawater.

The next best approach is the flyback booster. Buzz Aldrin's company, Starcraft Boosters Inc, has done a lot of work in this field. One of their important conclusions is that a booster staging at speeds under Mach 3.3 has enough energy to glide back to the launch site; a booster burning out at Mach 6 would require turbofan engines to return to base, but its aluminum heat-sink airframe could survive the expected heating.

In the 1960's, Max Faget's "DC-3 Shuttle" concept made use of a single, very large, winged booster. His proposed craft would have a manned crew, and retractable turbofan engines. The booster would exceed Mach 10 before burning out and falling away. It would have required heavy heat shielding based on its mission profile. The booster could be recovered downrange, then refueled with kerosene and flown back to the launch site like a conventional aircraft.

One idea I've been toying with is a very large booster with a very high staging velocity. How high, you ask? Well, high enough to fly an "Abort Once Around" trajectory and return to the launch site after a single revolution of the earth. The structural requirements and payload inefficiencies of such a booster would be nearly as bad as for a rocket that went to orbit with a single stage. The associated second stage/orbiter would be very tiny in comparison to the orbiter. Yet it has some operational advantages, such as the ability to land without turbines.

How do we proceed with the development of an RLV from here? I think that the StarHawk concept from Starcraft Boosters is a logical first step. The Air Force, under the banner of "Responsive Space Access," should take the lead on this program. While Aldrin's company doesn't have the ability to take lead in building the booster, the Air Force should keep them onboard in a consulting role, and contract the detail design and construction of the booster to the established aerospace firms. StarHawk should even be designated as an "X-Plane."

Building a modest flyback booster like StarHawk will give the US government and industry a lot of experience in designing, building, and operating a reusable lower stage booster. That experience can be shared with the industry and leveraged to build even bigger flyback boosters for rockets that will truly open the space frontier.

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