Thursday, March 22, 2007

SpaceX launch!

Apparently they launched with a known faulty GPS beacon on the first stage, and as a result they did not recover the first stage. That seems like a pretty substantial loss. My guess is that scheduling the range is difficult enough that knowing that they would probably lose the first stage was not enough to scrub the launch. That makes me wonder about their claim that range fees are not going to eat their profit margins.

Also, I'll note that Elon Musk was speculating about a stuck helium thruster causing the second stage wobble. I don't think this would be a roll thruster, since that wouldn't get progressively less controllable. Their roll control is with these cold-gas thrusters, so control authority would be constant relative to the unexpected torque. If they could cancel the torque in the first minute of second-stage burn, they'd be able to cancel it until the helium ran out.

But SpaceX uses axial helium cold-gas thrusters to seperate the tanks and settle the propellant in the second stage tanks. If one of those thrusters was stuck on, you could end up with some torque from the stage seperation that would explain the nozzle hit during the second-stage seperation. I'm not sure exactly how a single stuck axial helium thruster could explain the worsening roll wobble, but some coupling is at least conceivable.

Propellant slosh is an issue for SpaceX because they have a partially pressure-stabilized structure, made with thin skins welded to internal stringers and rings. Their interior surface is a lot cleaner than the isogrid surface of, say, a Delta IV or Atlas V, and so damps sloshing worse. Once they've got a little roll wobble going, it can really build up over time, especially since they'll remove very little rotational inertia from the propellant through the drain, so that you'd get a bit of the ice-skater effect as the propellant drains and concentrates any roll slosh into the remaining propellant.

The Space Shuttle also has welded stringers and so on in it's external tank. I'm not sure how they do anti-sloshing. I think I've seen cutaway pictures of the tank with extra stuff in there just to settle down the propellants.

One other thing I noticed about this launch. Last year, they added insulating blankets to the exterior of the vehicle which were ripped away during launch. The blankets were added after a scrub due to helium loss in turn due to excessive liquid oxygen boiloff. This year, the blankets were gone. My guess is that Elon had them build a properly sized LOX supply on Omelek, so that they would have no more troubles with boiloff. ("That'll be SIX sh*tloads this time!")

As for 90% of the risk being retired...
  • Orbital insertion accuracy is a big deal, and no data on that.
  • Ejecting the payload without tweaking it is... at least somewhat tough, no data on that. Consider problems with Skylab's insulation and solar panels, and the antenna on Galileo.
  • Getting away from the payload and reentering is risky too.
Still, I'm happy to see progress. I sure hope that the OSD/NRL satellite is easy & cheap to replace.