Chair Force Engineer

Wednesday, July 29, 2009

Abort, retry... fail?

In response to my earlier post about man-rating the side-mount shuttle-derived rocket, NASA is dead-serious about putting humans on its side-mount heavy-lift launcher. Thanks to NASA Watch, we can now take a look at the agency's analysis of the side-mount abort situation. An initial study of the problem reveals no show-stoppers. As the days go by, side-mount is looking like a better alternative to Ares (assuming we are forced to accept a NASA-designed, NASA-operated launcher.)

At the same time, analysis of Ares aborts by the Air Force's 45th Space Wing at Cape Canaveral has led many engineers to believe this is a show-stopping issue for Ares I. I’m not ready to throw in the towel on Ares without more detailed analysis of the abort problem, but it's a vivid illustration of why solid rocket boosters pose such a challenge for manned launches.

Most Ares critics are focusing on a catastrophic failure of the Ares SRB which would immolate the Orion spacecraft in a cloud of burning fragments. Solid rocket boosters rarely blow up, but they create spectacular explosions and tragic results when they do. Two Titans suffered SRB explosions in recent memory: one in 1986 and another in 1993. The explosion of Brazil's VLS-1 solid rocket on the launchpad in 2003 killed 21 people and dealt the Brazilian space program a major setback.

The Challenger disaster was not the result of a catastrophic failure, but a similar O-ring burnthrough would still give Ares I’s guidance system fits as it struggles to keep the vehicle on course while being torqued by the hot blowtorch escaping from the failed joint. The most likely scenario for the Ares abort system would be escaping the cloud of hot fragments created if the range safety office had to destroy an off-course Ares. Range safety would probably have the luxury of a few seconds between the time when the capsule’s escape motors fire before sending the destruct command to the booster. Depending on how powerful the abort motor was and how much time they allowed, it’s possible for the Ares escape system to get the capsule high enough and downrange far enough to avoid the hot shrapnel that would doom the crew. Then again, this requirement would already add to the massive rocket which would allow Orion to out-run a thrusting Ares.

Much could be learned from the Challenger disaster in terms of how long range safety could wait before triggering the destruct sequence, and how big the debris cloud would become during the period of time Orion would be passing through its abort and descent sequence. Range safety waited what seemed like a long amount of time between the main vehicle breakup and the ground-commanded destruction of the SRB’s. I can't say if the crew module was subjected to an extreme thermal environment before slamming into the ocean. But this may be irrelevant, since the SRB's were quite some distance removed from the crew module before the SRB's were destroyed.

Taken together, all of the challenges going into the design of the Ares abort system demonstrate the compound problems created by the solid rocket first stage. The escape systems on Mercury, Apollo and Soyuz were far less challenging to design and test. They anticipated a shutdown of the liquid-fuel engines on the booster before the escape rocket fired. On a solid rocket, the only premature shutdowns are the ones caused by catastrophic failure. If there’s a need to abort, the astronauts had better hope their escape rocket can get them out of there in a hurry.