After nearly three weeks of exhaustive tests and data analysis, NASA managers said Friday they are confident Boeing’s oft-delayed Starliner crew capsule can safely launch “as is” June 1, saying a small helium leak in the ship’s propulsion system does not pose a flight safety concern.
Steve Stich, manager of NASA’s Commercial Crew Program, said even if a suspect shirt-button-size rubber seal in the plumbing leading to one specific thruster failed completely in flight — resulting in a leak rate 100 times worse than what’s been observed to date — the Starliner could still fly safely.
“Should we be wrong about something, we could handle up to four more leaks,” he said. “And we could handle this particular leak if that leak rate were to grow, even up to 100 times in this one (propulsion module).”
What will now be a nearly one-month-long launch delay was required because “we needed to take the time to work through this analysis, and to understand the helium leak and understand the ramifications of that,” Stich said.
And to give the work force time off over the Memorial Day holiday weekend.
The Starliner’s two NASA crew members, commander Barry “Butch” Wilmore and co-pilot Sunita Williams, plan to fly back to the Kennedy Space Center next Tuesday to prepare for launch from the Cape Canaveral Space Force Station atop an Atlas 5 rocket at 12:25 p.m. EDT June 1.
If all goes well, they will dock at the International Space Station the next day and return to Earth on June 10.
Wilmore and Williams were in the process of strapping in for launch May 6 when the countdown was aborted because of problems with an oxygen pressure relief valve in their Atlas 5’s Centaur upper stage. Rocket-builder United Launch Alliance hauled 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 Starliner’s four propulsion modules, known as “doghouses,” that showed up when valves were closed as part of normal post-scrub procedures.
The leak eventually was traced to a flange where propellant lines feeding a specific reaction control system thruster in the port doghouse come together. The Starliner is equipped with 28 RCS jets, and helium is used to pressurize the propellant lines, opening and closing valves in each doghouse as needed.
Because traces of extremely toxic propellants could still be present in the plumbing, the seal could not be replaced or even inspected while the capsule was still attached to the Atlas 5. The Starliner would first have to be hauled back to Boeing’s processing hangar at the Kennedy Space Center for invasive repairs that would trigger a lengthy delay.
Instead, NASA and Boeing ordered tests and analysis to fully understand the leak and what sort of problems it might cause in flight. The observed leak rate did not appear to be a concern, but engineers needed to gain confidence it would not dramatically worsen. They also wanted to make sure no other systems were affected.
Stich said the seal in question likely was crimped or had a tiny defect, allowing helium to slip through. But testing 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 scrub had a “silver lining,” because it brought the helium leak to everyone’s attention and “we now know exactly where it was, we have done all the work to understand the root cause and that’s going to help us with improving the system in the future.”
“Had we launched … it would have been a safe flight and a successful flight,” he said, “but we would have not known as much as we know today.”
That includes one unexpected result, what Stich called “a design vulnerability.” The investigation shows that in the very remote chance of major trouble with two adjacent doghouses, including the one with the helium leak, the Starliner could lose redundancy for the thruster firing needed to drop out of orbit for re-entry.
The Starliner was designed to support three redundant de-orbit capabilities. In one, the braking burn is carried out with four powerful orbital maneuvering and attitude control — OMAC — thrusters. The burn also can be carried out with just two working OMAC jets, or with eight smaller RCS thrusters by firing them longer than planned.
In the right circ*mstances, with adjacent doghouse modules out of action, the Starliner could lose the full eight-thruster RCS deorbit capability.
“We’ve worked with the vendor of the thruster, Boeing and our NASA team to come up with a redundant method to do the orbit burn, to break it up into two burns about 10 minutes each, 80 minutes apart, to come up with a four-RCS-thruster deorbit burn and to regain the capability of the original system,” Stich said.
