Yes Brett, that is what I heard during one of the televised briefings. "The thrusters were prematurely overheating and causing the Teflon seals to swell and extrude enough to cause the helium to leak.
Norm
The implication is an issue called "soak-back" . When a thruster is firing full-on, lots of nice 65-degree propellant runs through it and the valves stay nice and cool. When you stop, the combustion chamber/reaction chamber is typically white-hot. Most of that heat is supposed to radiate away into space, but some of it conducts back up into the valve. Typically, the highest valve temperatures are reached minutes or 10s of minutes after the firing ends, at least on small radiation-cooled thrusters. When you pulse fire it, usually, at some combination of firing frequency and on-time/duty cycle, there is not enough propellant flow to offset the heat generated, and it slowly heats up. These are operating regions to avoid. There are number of ways to deal with this, mostly, you try to design it so you never need to run in these regions.
There's a lot more to it, as you might expect, but once it does heat up, several bad things can happen. Depending on the propellant, if it gets hot enough, it can "decompose", that is, break the bonds and flash to a gas while generating large amounts of heat. This is the same effect that is used for monpropellant thrusters, where you spray some fuel into a catalyst (like hydrazine or high-grade hydrogen peroxide on to a platinum screen or pebble bed). Glow engines depend on the decomposition of another pair of monopropellants, methanol and nitromethane, onto a red-hot platinum, rhodium, etc, wire. But when it happens in a closed pressure vessel like a thruster valve, at best it spits out gas for very poor performance, or overheats it further and melts or breaks something, or worst case, the entire system blows sky-high, as the shock from the initial decomposition travels back up the pipe and triggers the rest of it to blow. Most spacecraft propellants are not very prone to that. But it is what happened to Rutan when he ran a "oxidizer flow test" on the ramp at the Mojave airport with a crowd of rubberneckers, they opened or closed a valve, the shock started to decompose the oxidizer - nitrous oxide - which then sent a shock wave back up into the lines and the tank, kaboom. 3people dead, completely due to hubris.
Other things that can happen is that the heat can overtemp the materials, in this case, apparently, teflon, and cause leak, clogs, jammed valve mechanisms, etc.
Everybody more-or-less knows about these sorts of issues and addresses them somehow, but sometimes they don't get it right. You might think, perhaps, that burying the thrusters inside other structure and reducing the radiative cooling and effectively insulating the engine makes it run hotter than they thought. Of course, they also knew they were doing that, so you would expect that they had a plan that for one reason or another, didn't work out.
Again, we have precious little information about the specifics - like which thruster, who made it (we are just guessing it was Aerojet, but maybe Moog, or one of the innumerable startups), wht fuel it uses, how it is cooled, what propellant is uses, or more-or-less a thousand other things. Or in fact what the problem and symptoms might be.
So treat that as general information, we still don't have any specifics on the problem and don't expect any will be forthcoming.
Brett
p.s. OK, I spent a few minutes trying to find out, there are *55* engines, total, on the capsule, and they are all made by Aerojet. Here is the 1500 lb engine firing in a test. I guess it was one of these, or the 85 lb attitude control thrusters, since those are the ones that would be used a lot after separation from the booster.
I would also add that Boeing thinks that it is OK the way it is, presumably from simulating the return/reentry and finding that the engines don't run in a bad regime.
Topic: Brett Buck / Starliner (Read 11976 times)