by After nearly three weeks of tests and exhaustive data analysis, NASA managers said Friday that they have confidence in Boeing Starliner crew capsule often delayed able to launch safely “as is” on June 1, saying a small helium leak in the ship’s propulsion system is not a flight safety concern.
Steve Stich, manager of NASA’s Commercial Crew Program, said even if a suspected shirt-sized rubber seal in the plumbing failed that caused one specific thruster to completely collapse in flight – resulting in a leak rate 100 times worse than observed so far – the Starliner could still fly safely.
“If we were wrong about something, we could handle up to four more leaks,” he said. “And we could handle this particular leak if that leak rate grew, even up to 100 times in this one (drive module).”
The nearly one-month launch delay was necessary because “we need to take the time to work through this analysis, and understand the helium leak and understand the consequences of that,” Stich said .
It also gives workers time off over the Memorial Day holiday weekend.
Two NASA Starliner crew members, commander Barry “Butch” Wilmore and co-pilot Sunita Williams, plan to fly back to Florida’s Kennedy Space Center next Tuesday to prepare for launch from the Cape Canaveral Space Force Station on top Atlas 5 rocket at 12:25 pm EDT June 1.
If successful, they will dock with the International Space Station the next day and return to Earth on June 10.
Wilmore and Williams were strapping in for a May 6 launch when the countdown was halted due to problems with an oxygen pressure relief valve in their upper Atlas 5’s Centaur. Rocket builder United Launch Alliance towed the booster back to a processing facility and replaced the valve without incident.
At the same time, Boeing engineers began a detailed investigation of a small helium leak in one of the four Starliner propulsion modules, known as “doghouses,” which showed when valves were closed as part of normal post-screw procedures.
Ultimately, the leak was traced to a flange where drive lines that feed a separate reaction control system thruster in the port house meet. The Starliner is equipped with 28 RCS jets, and helium is used to pressurize the drive lines, opening and closing valves in each doghouse as needed.
Because the plumbing could still contain traces of highly toxic propellant, the seal could not be replaced or even inspected while the capsule was still attached to the Atlas 5. The Starliner would have to be towed back to Boeing’s processing hangar at Kennedy first. Space Center for invasive repairs that would induce longer delays.
Instead, NASA and Boeing ordered tests and analysis to fully understand the leak and the problems it could cause during flight. The observed leak rate did not appear to be a cause for concern, but engineers needed to gain confidence that it would not go too far. They also wanted to ensure that no other systems were affected.
Stich said the seal in question was likely crimped or had a small defect, allowing helium to slip through. But the test showed that even if the seal was removed from the flange, the Starliner could still fly safely. The helium manifold in question could be isolated and the Starliner’s many other thrusters could easily compensate.
Mark Nappi, Boeing’s Starliner program manager, said the May 6 launch scrubbing had a “silver lining” because it brought the helium leak to everyone’s attention and “we now know exactly where it was, the work is We have done everything to understand the root cause, and that will help us improve the system in the future.”
“If we had launched … it would have been a safe flight and a successful flight,” he said, “but not as much as we know today.”
That includes one unexpected result, something Stich calls a “design vulnerability.” The investigation shows that the Starliner could lose redundancy for thruster firing needed to drop out of orbit for re-entry if there is a very remote chance of major trouble with two nearby doghouses, including the which has the helium leak.
The Starliner was designed to support three redundant deorbit capabilities. In one, the braking burn is done with four powerful Orbital Maneuvering and Attitude Control (OMAC) thrusters. The burn can also be done with two working OMAC jets, or with eight smaller RCS thrusters, by firing them longer than intended.
Under the right circumstances, with adjacent doghouse modules out of service, the Starliner could lose the full eight-thruster RCS deorbit capability.
“We worked with the thruster vendor, Boeing and our NASA team to come up with a redundant way to do the orbit burn, to break it up into two burns about 10 minutes each, 80 minutes apart, to catch up . four-RCS-thruster deorbit burn and to regain the capability of the original system,” said Stich.
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