The astronauts who rode Boeing's Starliner spacecraft to the International Space Station last month still don't know when they will return to Earth.
Astronauts Butch Wilmore and Suni Williams have been in space for 51 days, six weeks longer than originally planned, as engineers on the groundwork through problems with Starliner's propulsion system.
The problems are twofold. The spacecraft's reaction control thrusters overheated, and some of them shut off as Starliner approached the space station June 6. A separate, although perhaps related, problem involves helium leaks in the craft's propulsion system.
On Thursday, NASA and Boeing managers said they still plan to bring Wilmore and Williams home on the Starliner spacecraft. In the last few weeks, ground teams completed testing of a thruster on a test stand at White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft's thrusters in orbit to check their performance while docked at the space station.
“I think we’re starting to close in on those final pieces of flight rationale to make sure that we can come home safely, and that’s our primary focus right now," Stich said.
The problems have led to speculation that NASA might decide to return Wilmore and Williams to Earth in a SpaceX Crew Dragon spacecraft. There's one Crew Dragon currently docked at the station, and another one is slated to launch with a fresh crew next month. Steve Stich, manager of NASA's commercial crew program, said the agency has looked at backup plans to bring the Starliner crew home on a SpaceX capsule, but the main focus is still to have the astronauts fly home aboard Starliner.
“Our prime option is to complete the mission," Stich said. "There are a lot of good reasons to complete this mission and bring Butch and Suni home on Starliner. Starliner was designed, as a spacecraft, to have the crew in the cockpit."
Starliner launched from Cape Canaveral Space Force Station in Florida on June 5. Wilmore and Williams are the first astronauts to fly into space on Boeing's commercial crew capsule, and this test flight is intended to pave the way for future operational flights to rotate crews of four to and from the International Space Station.
Once NASA fully certifies Starliner for operational missions, the agency will have two human-rated spaceships for flights to the station. SpaceX's Crew Dragon has been flying astronauts since 2020.
Tests, tests, and more tests
NASA has extended the duration of the Starliner test flight to conduct tests and analyze data in an effort to gain confidence in the spacecraft's ability to safely bring its crew home and to better understand the root causes of the overheating thrusters and helium leaks. These problems are inside Starliner's service module, which is jettisoned to burn up in the atmosphere during reentry, while the reusable crew module, with the astronauts inside, parachutes to an airbag-cushioned landing.
The most important of these tests was a series of test-firings of a Starliner thruster on the ground. This thruster was taken from a set of hardware slated to fly on a future Starliner mission, and engineers put it through a stress test, firing it numerous times to replicate the sequence of pulses it would see in flight. The testing simulated two sequences of flying up to the space station, and five sequences the thruster would execute during undocking and a deorbit burn for return to Earth.
"This thruster has seen quite a bit of pulses, maybe even more than what we would anticipate we would see during a flight, and more aggressive in terms of two uphills and five downhills," Stich said. “What we did see in the thruster is the same kind of thrust degradation that we're seeing on orbit. In a number of the thrusters (on Starliner), we're seeing reduced thrust, which is important.”
Starliner's flight computer shut off five of the spacecraft's 28 reaction control system thrusters, produced by Aerojet Rocketdyne, during the rendezvous with the space station last month. Four of the five thrusters were recovered after overheating and losing thrust, but officials have declared one of the thrusters unusable.
The thruster tested on the ground showed similar behavior. Inspections of the thruster at White Sands showed bulging in a Teflon seal in an oxidizer valve, which could restrict the flow of nitrogen tetroxide propellant. The thrusters, each generating about 85 pounds of thrust, consume the nitrogen tetroxide, or NTO, oxidizer and mix it with hydrazine fuel for combustion.
A poppet valve, similar to an inflation valve on a tire, is designed to open and close to allow nitrogen tetroxide to flow into the thruster.
"That poppet has a Teflon seal at the end of it," Nappi said. "Through the heating and natural vacuum that occurs with the thruster firing, that poppet seal was deformed and actually bulged out a little bit."
Stich said engineers are evaluating the integrity of the Teflon seal to determine if it could remain intact through the undocking and deorbit burn of the Starliner spacecraft. The thrusters aren't needed while Starliner is attached to the space station.
"Could that particular seal survive the rest of the flight? That's the important part," Stich said.
The good news is that the seal, although degraded, remained intact after firing the thruster on the ground five times more than it will need to fire for Starliner to return to Earth, he said. Managers also decided to forego a manual flying demonstration that was planned after Starliner undocks from the space station. Instead of the astronauts taking manual control for tests, the spacecraft will fly in an automated mode.
Fundamentally, engineers believe the thruster problems are caused by overheating, particularly on rear-facing thrusters exposed to more sunlight as Starliner orbits the Earth. Starliner's power-generating solar arrays are on the rear of the spacecraft, so it typically flies with its aft end facing the Sun.
An additional factor also leads to overheating. The 28 reaction control system thrusters are located in four doghouse-shaped propulsion pods around the circumference of the service module. Each "doghouse" contains seven reaction control system thrusters for relatively minor orbit-adjust maneuvers or pointing of the spacecraft, plus three more powerful orbit control engines used for larger impulses.
“The doghouse has a lot of insulation on it," Stich said. "It’s designed to protect the thrusters from cold temperatures while we’re docked. But I think we’re now seeing an integrated effect that we maybe didn’t uncover on the ground during some of the early testing of Starliner, that if you command too many pulses on a thruster, and have those thrusters pointed at the Sun... we can have some crosstalk in the thermal (environment) and have the temperatures increase. So that’s where we see the thruster degradation.”
Nappi likened the doghouse to a thermos. "It’s likely that the heat is not just coming from the one thruster, but it’s coming from the thermos effect of that entire doghouse," he said. "That was modeled, but I think we’re learning about the actual flight conditions (and) we’ll have to go back and adjust that model.”
If it goes well, a hot-fire test of Starliner's reaction control system thrusters this weekend will give NASA managers confidence that the ship's thrusters are ready for the trip back to Earth. These thrusters will be used to guide Starliner safely away from the space station and then control the spacecraft's orientation during a critical deorbit burn with a different set of larger engines. The small thrusters could also be used as a backup for the deorbit burn itself if there's a problem with the primary engines.
As part of the hot-fire test, the Starliner will open valves in its propulsion system to allow helium to flow and pressure tanks and propellant lines in the service module. This will allow engineers to see if the helium leak rate has changed since they last checked in June. Since then, the helium valves have remained closed, isolating the helium system and closing off the leak. The helium valves must be open during undocking and the deorbit burn, but officials say the spacecraft has ample helium for the return journey, even if the leaks became more significant.
NASA and Boeing officials believe degradation in a different set of seals is responsible for the leaky helium system on Starliner.
"It’s a very important set of tests over the weekend that we’ll do," Stich said. "The hot-fire test this weekend will give us confidence in all the thrusters," Stich said.
Assuming good results this weekend, NASA managers could convene a flight readiness review at the end of next week to discuss the health of the Starliner spacecraft. If NASA's leadership signs off on the plan, Starliner could be cleared to return to Earth with Wilmore and Williams as soon as early August. NASA would like to have the spacecraft back on the ground before the launch of SpaceX's next Crew Dragon mission, currently slated for no earlier than August 18.
Another Starliner delay
For future Starliner missions, Stich said engineers are looking at several solutions to the thruster overheating problem. Boeing and its propulsion supplier, Aerojet Rocketdyne, may need to modify the thruster design or use a new type of seal. There may also be a way to overcome the overheating problem by changing insulation on the doghouses or by software adjustments to fire the thrusters less often.
Boeing may also need to replace seals in the helium system to prevent leaks there.
Whatever fixes are necessary, they will take time to implement. NASA hoped the next Starliner flight, which will ferry a crew of four astronauts on a six-month expedition at the space station, could be ready for liftoff in February.
Stich ruled that out in a press conference on Friday. The four astronauts who will start their stint on the space station in February will now ride to orbit on a SpaceX capsule. The following long-duration crew would head to the station in August 2025, and that's now the earliest opportunity for Starliner to become part of the lab's regular cadence of crew rotations.
NASA signed contracts with Boeing and SpaceX in 2014 to complete the development of their Starliner and Crew Dragon spacecraft. The initial contracts included six operational crew rotation flights for each company. SpaceX has received a contract extension for additional flights to pick up the slack as Boeing has failed to deliver.
The next SpaceX crew launch in mid-August will be the company's ninth astronaut mission for NASA and 14th astronaut mission overall, including fully commercial flights with private passengers.
