Maximus_Meridius said:
I tried finding this the other day, but does anyone know what kind of thrusters are used in the OMAC? I saw some pictures that make me think they're mono-propellant, but I can't get confirmation.
They are derivatives of the MR-104 from Aerojet, used on everything since the original Voyager. The ones on Starliner use an oxidizer with the hydrazine, as it is a lot more efficient than a monopropellant. Starliner has 3 different types of thrusters, optimized for different purposes. The valve moisture issues occurred on the oxidizer side of the mid-range thrusters (nitrous tetra oxide), not the fuel side (hydrazine).
The root cause determination found that moisture entered the oxidizer valve housing through the electrical connectors, which have now been sealed. The housings are also now enclosed in a hood with dry nitrogen gas purge.
The analysis found that corrosion could begin in as little as 7 days after oxidizer loading. Due to various launch delays, Starliner had ended up being loaded 44 days before launch. So there was more than enough time for the problem to develop.
The requirements for corrosion were weeping of the oxidizer past the Teflon valve seals, ambient moisture in the air, and the aluminum valve materials. The solution addressed the moisture, but they may choose to address the other two with a valve redesign, as a more permanent fix.
There are 12 OMAC thrusters distributed between 4 doghouses. In the port doghouse, 2 of the thrusters had early shutdowns, but the 3rd was ok. Starliner has levels of redundancy that include the loss of any one doghouse, with the others compensating. So even if the remaining thruster went out in that one doghouse, it would be ok.
From the post-docking Starliner media conference, the two OMAC thruster problems were both low chamber pressure, which triggered a shutdown. Happened a second time today so they just mapped those thrusters out, since they have plenty of redundancy. Two RCS thrusters also failed during maneuvering today, but again there is ample redundancy so they just turned them off.
Also had a problem develop in the two cooling loops, which developed excess pressure and low flow rates. They resolved this by duty-cycling the radiators to boost flow and lower pressure, which has worked well as again there is ample capacity. Thought to be related to the system possibly freezing as there was a very low heat load without the crew.
There was an intermittent problem with one of the Starliner directional antennas, that caused momentary LOS to ground stations, but communication was continuous to ISS. Also the docking system got into an incorrect configuration when the ring was extended, but stowing it and resetting the system fixed the issue. Lastly there was small glitch in the computer graphics drawing, that showed a small offset in the Starliner location. They confirmed the offset was not real and proceeded.
Leuders mentioned that all the effort, planning, and time invested in the spacecraft pays off, when unexpected problems occur but the team quickly works around them. That is an excellent point, and exactly right.
The RCS thrusters can also suffice in a pinch. It was not necessary to invoke those scenarios. I disagree with '08 and AA in concluding that this is a demonstration Boeing is really going to put lives at risk over the two thrusters shutting down, and I also think NASA leadership and Boeing alike would desperately want to avoid the scrutiny such a serious incident would lead to, if it is a real risk.