Somehow I missed what out of this world happened to result in some people stranded in space, and I thought it’d be fun/interesting to see how others describe what led to it.
Somehow I missed what out of this world happened to result in some people stranded in space, and I thought it’d be fun/interesting to see how others describe what led to it.
The gist of it is that to reenter the atmosphere safely, you need to point the heat shield at the oncoming air. To do that, you need working reaction control thrusters.
Boeing Starliner capsule apparently has some kind of latent failure mode with its thrusters where they start degrading and can fail in a few hours of operation. On the way up, the spacecraft was in transit for around 26 hours, and recorded five thrusters disabled, of which four relit in subsequent testing. One thruster is apparently really dead. The capsule has about fifty thrusters, so being down five doesn’t really compromise steering.
Ground testing of these thrusters was performed at White Sands, NM, and apparently NASA really did not like the results. But I haven’t heard anything about what those tests found.
There’s also a helium leak (active only when the RCS is warmed up), but it’s not clear if this is a critical factor. Compressed helium is used to force the fuel to go out of the thrusters.
As far as I know, the actual estimated risk of failure for this capsule is still pretty low, but now believed to be high enough that they might as well not take the risk. (And remember these are test astronauts… They have a pretty high risk tolerance).
Also, even if the risk of one reentry is maybe low enough to try it, the White Sands and orbital data probably revealed a systemic design problem with the RCS that now precludes operational certification. Since neither Boeing nor NASA is willing to pay for a second test flight, this capsule is effectively dead as an ongoing space program. So why take any kind of risk on a reentry if you don’t have to?
It’s actually a lot worse than just thrusters not working any more. At least according to the unofficial “word on the street” about what’s going on, the details haven’t been officially released yet.
It appears that the cause of the failures was because the thrusters are housed inside compartments that are containing their waste heat. The thrusters were tested individually, but apparently were never tested once installed inside the capsule. The heat is causing teflon valves to fail, which clogs the thruster plumbing and disables them. But the scary thing is that the heat necessary to do that to teflon would also potentially be enough to boil the hypergolic fuel itself inside the fuel lines.
When you heat hypergolic fuel up enough it will spontaneously ignite. It’s got its own oxidizer in it, essentially. Which means those thrusters could well be bombs that could go off if they’re fired too long.
The way they’re talking about moving the unmanned capsule away from ISS, slowly and gently, it sounds like they’re concerned Starliner could literally explode next to the station. That would be, to put it mildly, very very bad.
If any of this is true then this is going to be a colossal scandal. This is Starliner’s third test flight, it’s absolutely incredible that Boeing wouldn’t have figured this out by now and that NASA let them get away with such a shoddy program.
Kinda. The RCS is fueled by two fluids that spontaneously combust when mixed: N2H4 and N2O4, aka hydrazine and nitrogen tetroxide (or dinitrogen tetraoxide).
If the N2H4 gets hot enough, it will exothermically decompose into a large volume of hot gas. Its sometimes used as a monopropellant - squirt some of it onto a catalyst and direct the gas out a rocket nozzle, and you’ve got a simple, reliable thruster. Mixing it with N2O4 produces hotter gas, resulting in a thruster that gets about 35% higher fuel efficiency.
Problems with the helium system could make hydrazine decomposition in the fuel lines more likely to happen. One of the ways the hydrazine is kept cool while its in the fuel lines is via high flow rate, but that requires the fuel tank to be pressurized by helium - clearing the fuel lines after thruster shutdown may also require helium. Low pressure could lead to lower flow rate and possibly cause cavitation (which can cause tiny spots of very high heat), which could in turn cause the hydrazine to decompose in the fuel lines/tank. At that point, mixing with N2O4 would be overkill - Starliner would already be destroyed before the oxygen started burning.
Thanks for the clarification. Wasn’t sure how down into the weeds of why Starliner would go boom I should go, but this is clear and I should have been more specific about the “hypergolic” term.
Is it really that incredible, though? It’s Boeing after all.