Boeing's Strainer spacecraft is seen docked with the International Space Station in this photo taken July 3.
Enlarge / Boeing’s Strainer spacecraft is seen docked with the International Space Station in this photo taken July 3.
The astronauts who boarded Boeing’s Starliner spacecraft to the International Space Station last month still don’t know when they’ll return to Earth.
Astronauts Butch Wilmore and Suni Williams have been in space for 51 days, six weeks longer than originally planned, while engineers on the ground troubleshoot problems with Starliner’s propulsion system.
The problems are twofold. The spacecraft’s jet thrusters overheated and some of them shut down as Starliner approached the space station on June 6. Another problem, though perhaps related, involves helium leaks in the spacecraft’s propulsion system.
On Thursday, NASA and Boeing officials said they still plan to return Wilmore and Williams aboard the Starliner spacecraft. In recent weeks, ground teams have completed testing a thruster on a test stand in White Sands, New Mexico. This weekend, Boeing and NASA plan to fire the spacecraft’s thrusters in orbit to check their performance once docked with the space station.
“I think we’re starting to get closer to those final pieces of the flight logic to make sure we can get home safely, and that’s our primary focus right now,” Stich said.
Those issues have led to speculation that NASA might decide to bring Wilmore and Williams back to Earth aboard a SpaceX Crew Dragon spacecraft. One Crew Dragon spacecraft is currently docked at the station, and another is scheduled to launch with a new crew next month. Steve Stich, NASA’s commercial crew program manager, said the agency has studied contingency plans to bring the Starliner crew home aboard a SpaceX capsule, but the primary goal remains to get the astronauts home aboard Starliner.
“Our first option is to terminate the mission,” Stich said. “There are many good reasons to terminate this mission and bring Butch and Suni back aboard Starliner. Starliner was designed, as a spacecraft, to have the crew in the cockpit.”
The Starliner launched from Cape Canaveral Space Force Station in Florida on June 5. Wilmore and Williams are the first astronauts to fly to space aboard Boeing’s commercial crew capsule, and the test flight is intended to pave the way for future operational flights to shuttle four-person crews to and from the International Space Station.
Once NASA fully certifies Starliner for operational missions, the agency will have two human-ready spacecraft for flights to the station. SpaceX’s Crew Dragon has been carrying astronauts since 2020.
Tests, tests and more tests
NASA extended the duration of Starliner’s test flight to conduct tests and analyze data to build confidence in the spacecraft’s ability to safely bring its crew home and to better understand the root causes of the thruster overheating and helium leaks. Those problems are inside Starliner’s service module, which is jettisoned to burn up in the atmosphere upon reentry, while the reusable crew module, with the astronauts inside, parachutes in for an airbag-cushioned landing.
The most significant of these tests was a series of test firings of a Starliner thruster on the ground. The thruster was taken from a hardware package slated to fly on a future Starlink mission, and engineers stress-tested it, firing it repeatedly to replicate the pulse sequence it would see in flight. The tests simulated two flight sequences to the space station, and five sequences the thruster would execute during undocking and a deorbit burn for return to Earth.
“This thruster has undergone a number of pulses, perhaps even more than we would anticipate in a flight, and more aggressive in terms of two ascents and five descents,” Stich said. “What we saw in the thruster is the same type of thrust degradation that we see in orbit. In a number of thrusters (on Starliner), we see reduced thrust, which is important.”
Starliner’s flight computer shut down five of the spacecraft’s 28 reaction control system thrusters, manufactured by Aerojet Rocketdyne, during rendezvous with the space station last month. Four of the five thrusters were recovered after overheating and losing thrust, but officials declared one of the thrusters unusable.
The ground-tested thruster exhibited similar behavior. Inspections of the thruster at White Sands showed a bulge in a Teflon seal on an oxidizer valve, which could restrict the flow of the nitrogen tetroxide thruster. The thrusters, which each generate about 85 pounds of thrust, consume nitrogen tetroxide oxidizer, or NTO, and mix it with hydrazine fuel for combustion.
A flapper valve, similar to an inflation valve on a tire, is designed to open and close to allow nitrous oxide to flow into the propellant.
“This valve has a Teflon seal at its end,” Nappi explained. “Under the effect of the heat and the natural vacuum that occurs when the propellant is fired, this seal has deformed and even bulged slightly.”
Stich said engineers are evaluating the integrity of the Teflon seal to determine if it could remain intact during the undocking and deorbiting of the Starliner spacecraft. Thrusters are not needed while Starliner is attached to the space station.
“Could this particular seal survive the rest of the flight? That’s the important part,” Stich said.
.jpg)
.webp)
0 Comments