Chair Force Engineer

Saturday, July 11, 2009

You aren't going to stick people on that thing, are you?

During the Augustine Commission hearings, the side-mount, shuttle-derived vehicle has emerged as a surprising dark horse. Compared to Ares and even DIRECT, a side-mount can be developed quicker and cheaper than its shuttle-flavored competitors. It also does the most complete job of preserving more of the STS workforce within the first few years after the orbiters retire.

The most startling aspect of the side-mount SDV presentation is the option to mount a crew capsule and escape system. Because the crew capsule would be located laterally to the external tank, most observers feel it would not be a great improvement over the shuttle when it comes to launch aborts. Precise guidance and thrust-vectoring would be required in the escape system to pull the capsule away from the ET, even if the abort was triggered while the ET was structurally intact. In a Challenger-like situation where the ET rapidly disintegrates, there may be little or no chance of protecting the capsule from ET-produced shrapnel.

Another abort scenario worth considering is the shuttle's Return to Launch Site maneuver. The shuttle stack would flip end-over-end and fire the engines in the opposite direction to cancel out the forward velocity and head home. The orbiter would then separate and glide in for a landing. On a side-mount crew launcher, the capsule doesn't have the same cross-range as the shuttle orbiter. Hopefully the escape tower would be able to pull it away from the stack and set it down somewhere in the Atlantic ocean for recovery. The bigger question is the point at which the escape tower is going to be jettisoned for a side-mount crew launcher. Will the service module engine have the thrust and steering necessary to pull free from the ET during late-boost aborts?

The side-mount SDV has been given extensive study since the days before the first shuttle launch, and it remains a valid approach for transporting large unmanned payloads to space. But is it suitable as a crew launcher? It's probably no less safe than the existing shuttle, but it would still give me a high pucker-factor if I was an astronaut. The cynic in me suspects that NASA doesn't take the side-mount crew launcher seriously, but is pitching it as a means of undermining the rationale for EELV or DIRECT. After all, DIRECT may be a more difficult and expensive development than a side-mount SDV, but it's much more suitable for manned aborts during all phases of flight.

The approach I favored during the early days of Project Constellation was a Delta IV crew launcher and side-mount SDV for unmanned cargo. It's the cheapest crew launcher paired with the cheapest heavy lifter design. I would not be surprised if the Augustine Commission seriously considers this combination.