Leak in service module
Stich:
Boeing engineers traced the leak to a flange on a single reaction control system thruster in one of four doghouse-shaped propulsion pods on the Starliner service module. The spacecraft has 28 of these small control jets to point the spacecraft and make minor adjustments to its orbit. Helium, an inert gas, is not combustible and is used to pressurize the propulsion system and force hypergolic hydrazine and nitrogen tetroxide propellants through pipes leading to the craft's maneuvering thrusters.
The most likely culprit for the leak is a defect in a rubber seal about the size of a button on a shirt, at a point where two metal parts of the flange connect. “This particular leak, I don’t think it implicates the design of the seal or the flange, it’s just maybe a defective part," Stich said.
"This is a high-pressure system, and helium is a very small, tiny molecule, and it tends to leak," he said.
In order to repair it, Boeing would need to remove the Starliner spacecraft from its Atlas V rocket and take it back to a nearby facility, where technicians would drain the service module of toxic propellants. There, workers could safely access the leaky seal and install a replacement. Boeing would then have to refill the spacecraft with propellant, reinstall it on the Atlas V rocket, and run the entire vehicle through another series of integrated mechanical and electrical tests. "It would be quite involved," said Mark Nappi, Boeing's vice president and program manager for Starliner.
All this would probably take at least several months and could delay downstream flights to the International Space Station and future ULA missions stacked up behind the Starliner test flight.
Instead, managers decided to take an "analysis-based approach" to understand the leak, Stich said. Part of this approach included a three-day pressure decay check on Starliner's propulsion system. Engineers detected no helium leaks elsewhere on the spacecraft.
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Five of the capsule’s 28 thrusters on the service module failed after Starliner arrived at the ISS. NASA said all but one thruster restarted, and they were found to be working during a later test firing.
Officials suspect heat from increased thruster use on arrival at the ISS caused the problem. Boeing said the one problematic thruster had been turned off and would not be an issue for the return trip.
The capsule launched with one small helium leak and four more were discovered by the time it reached the ISS. Helium is used to pressurize fuel for the thrusters. A rubber seal was suspected of failing, causing the problem.
Hardware or software
The fifth will stay deactivated for the rest of the mission, but the glitch — which may actually be in Starliner's software and not the thrusters themselves — won't pose a risk for the return to Earth.
Reference
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