“There are two primary problems with the Boeing Starliner,” says Jonathan Aitken in the department of automatic control and systems engineering at the University of Sheffield, who studies the safety of high integrity systems.
The first and most severe issue is what got the two astronauts into their present predicament. At its root is the reaction control thrusters that are used to manoeuvre Starliner as it travels through space. “On approach to the ISS, five thrusters out of the 28 onboard failed,” says Aitken.
Although the crew was able to successfully dock in spite of the massive failure, it was a major concern that looms large over the remainder of the mission. “Identifying the reasons behind the failure are crucial, because the docking and undocking processes are complex and require significant control inputs to ensure a successful manoeuvre,” says Aitken.
Getting the reaction control thrusters to work correctly is vital because of Starliner’s proximity to the ISS and the SpaceX Crew-8 Dragon spacecraft. “Imagine trying to manoeuvre out of a tight space in a car park when the vehicle might not respond to the inputs you apply, failing to stop or unexpectedly moving left or right – and add in up or down,” Aitken says. “Now amplify the severity of any accident, given the process is operating in orbit.”
Surround yourself not just with expensive cars in that hypothetical car park, but with multi-billion dollar spacecraft and a space station that has been in orbit for more than 25 years, relied on to conduct the world’s missions to better understand space. It quickly becomes obvious why precise movement is needed.
What caused the issue with the thrusters has since been investigated on Earth, as well as a space-based test fire. It appears to involve the thrusters being starved of propellant when they needed it most – an issue believed to be caused by a faulty tightening seal.
The problem vexing Boeing engineers and all those involved with the Starliner mission is that the seal fault appears not to happen all the time: five were non-operational when the astronauts attempted their real manoeuvre, while only one failed during the subsequent space-based test fire. “This could be caused by the seals settling during prolonged usage, or just that they've not been pushed to the extent that they were during the docking process,” says Aitken.
Regardless, that unreliability is an issue in itself. “There is cause for uncertainty in whether the problem will present itself, especially under the more prolonged usage seen during an active manoeuvre,” Aitken says. It is a big unknown that has halted much of the further progress of the mission, and returning the astronauts back to Earth. Those behind the mission cannot be sure whether the seal issue is caused by repeated use, or eased by continual use.
Nor can they “totally prove with certainty what we’re seeing on orbit is exactly what’s been replicated on the ground,” said Steve Stich, NASA’s commercial crew programme manager, at a press briefing earlier this month.
That is not the only thing holding up the astronauts’ safe return. A helium leak is also stymying progress. “This was known about before take-off, and a decision was taken to continue,” says Aitken, who points out that such a decision is not uncommon. “Commercial aircraft will often fly with systems that are not operational. This isn't a problem if they aren't needed for the flight.”
The single helium leak issue was known prior to take-off. But the subsequent two since discovered mid-flight were not, adding to mission control’s headaches. “Careful monitoring of the situation onboard is crucial to ensure helium levels are sufficient to complete the mission,” says Aitken. “Identification of the location and cause of these leaks is essential for the operation of future missions.” It’s also essential for getting Wilmore and Williams back.
Repairing the helium leak is unlikely to be possible without engineers onboard to identify and fix the issue, so the astronauts are currently in a holding pattern. Helium levels are being monitored to check whether they’re likely to remain high enough to get back down to Earth.
If not – in combination with the thruster issues – a fallback option would be to return to Earth using SpaceX’s Dragon capsule. But, as Aitken points out, doing that would be tantamount to Boeing admitting it is not up to the job and a competitor is. Instead, engineers are working on Earth-based facsimiles of the Starliner to see if they can mimic the issue, and find an engineering solution that will work with as close to 100% certainty as possible.
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