Astronauts Butch Wilmore and Suni Williams climbed into their seats inside Boeing's Starliner spacecraft Monday night in Florida, but trouble with the capsule's Atlas V rocket kept the commercial ship's long-delayed crew test flight on the ground.
Around two hours before launch time, shortly after 8:30 pm EDT (00:30 UTC), United Launch Alliance's launch team stopped the countdown. "The engineering team has evaluated, the vehicle is not in a configuration where we can proceed with flight today," said Doug Lebo, ULA's launch conductor.
The culprit was a misbehaving valve on the rocket's Centaur upper stage, which has two RL10 engines fed by supercold liquid hydrogen and liquid oxygen propellants.
“We saw a self-regulating valve on the LOX (liquid oxygen) side had a bit of a buzz; it was moving in a strange behavior," said Steve Stich, NASA's commercial crew program manager. "The flight rules had been laid out for this flight ahead of time. With the crew at the launch pad, the proper action was to scrub.”
The next opportunity to launch Starliner on its first crew test flight will be May 17 at 6:16 pm EDT (22:16 UTC), NASA announced late Tuesday. Managers spent Tuesday reviewing data from the faulty valve and determined it should be replaced. This will require ULA to roll the Atlas V rocket back into its hangar about a third-of-a-mile south of the launch pad, eliminating any chance to launch the mission later this week.
Work ahead
Everything else was going smoothly in the countdown Monday night as Wilmore and Williams strapped into their seats for the first crew flight on the Starliner spacecraft and Atlas V rocket from Cape Canaveral Space Force Station, Florida. This flight is the culmination of nearly a decade-and-a-half of development by Boeing, which has a $4.2 billion contract with NASA to ready Starliner for crew missions, then carry out six long-duration crew ferry flights to and from the International Space Station.
The crew test flight will last at least eight days, taking Wilmore and Williams to the space station to verify Starliner's readiness for operational missions. Once Starliner flies, NASA will have two human-rated spacecraft on contract. SpaceX's Crew Dragon has been in service since 2020.
After officials called off the launch attempt, the Boeing and ULA support team helped the astronauts out of the capsule and drove them back to crew quarters at the nearby Kennedy Space Center to wait for the next launch attempt.
"I promised Butch and Suni a boring evening," said Tory Bruno, ULA's CEO. "I didn’t mean for it to be quite this boring, but we’re going to follow our rules, and we’re going to make sure that the crew is safe."
The valve in question vents gas from the liquid oxygen tank on the Centaur upper stage to maintain the tank at proper pressures. This is important for two reasons. The tank needs to be at the correct pressure for the RL10 engines to receive propellant during the flight, and the Centaur upper stage itself has ultra-thin walls to reduce weight, and requires pressure to maintain structural integrity.
Around an hour before ULA scrubbed the launch attempt, the launch team noticed oscillations in measurements at the top of the Centaur liquid oxygen tank. The ground crew at the launch pad, a few feet from the rocket, reported an unusual "chattering" sound at the same time.
This chattering or buzzing sound was associated with rapid movement of the relief valve. In this case, it was cycling about 40 times per second.
"Every now and again, on rare occasions, a valve like that can get into a position where it’s just off the seat," Bruno said. "Its temperature, its stiffness, everything is just right and it will flutter, or it’ll buzz in this base. It will cycle. We’ve seen that before."
On previous occasions, such as on an Atlas V launch in 2015, ULA's launch team commanded the valve closed and reopened the valve, and the buzzing motion stopped. This allowed the launch to proceed successfully. For safety reasons, ULA didn't want to take that step with the astronauts inside the Starliner spacecraft atop the rocket, so they waited until the crew departed the launch pad Monday night.
"If this was a satellite, that is our standard procedures, and the satellite would already be in orbit," Bruno said in a press conference Monday night. "But that changes the state of the fueled Centaur, and we don’t do that when people are present. So our flight rules called for us to scrub and take the crew off before we cycled that valve."
Once the astronauts were safely away from the rocket, ULA's launch team temporarily dampened the valve oscillations, but then the rapid movement resumed twice as engineers drained propellants from the Atlas V. Bruno said the valve is capable of functioning normally after at least 200,000 cycles, and the valve exceeded that limit.
"After evaluating the valve history, data signatures from the launch attempt, and assessing the risks relative to continued use, the ULA team determined the valve exceeded its qualification and mission managers agreed to remove and replace the valve," NASA said in a statement Tuesday night.
Teams will roll the Atlas V and Starliner back to ULA's vertical hangar on Wednesday. There, technicians would change out the valve, a process that Bruno said would take a few days. "The ULA team will perform leak checks and functional checkouts in support of the next launch attempt," NASA said.
This story was updated at 8:15 pm ET after NASA announced an additional delay until May 17.
