SLS Artemis 1 Aug. 29
NASA officials said the apparent failure of one of the core stage engines to cool to the proper temperature during the Aug. 29 launch attempt may be an artifact of a faulty temperature sensor rather than a problem with the flow of liquid hydrogen into the engine. Credit: Jordan Sirokie

TITUSVILLE, Fla. — NASA has rescheduled the next attempt to launch the Artemis 1 mission for Sept. 3 after concluding that a faulty temperature sensor may be at the root of the problem that scrubbed the first launch attempt.

Agency officials said at an Aug. 30 media teleconference that they’re moving ahead with a second attempt to launch the Space Launch System rocket, carrying the Orion spacecraft, during a two-hour window that opens at 2:17 p.m. Eastern Sept. 3. That is one day later than the agency’s original plan for the next launch attempt.

One reason for the additional delay is to give engineers time to work on a hydrogen leak detected in the tail service mast umbilical that loads liquid hydrogen into the core stage. “We want to do some inspections and want to do some retorques,” said Charlie Blackwell-Thompson, NASA Artemis launch director.

However, that leak was resolved during the Aug. 29 launch attempt and was not the reason the launch was scrubbed. Instead, launch controllers ran into problems with the “kickstart bleed” where liquid hydrogen flows through the four RS-25 engines in the core stage to cool them before launch. One of the four engines, designated engine #3, did not get down to the same temperature as the other three, and efforts to correct the problem failed, prompting the scrub.

John Honeycutt, NASA SLS program manager, said the hydrogen bleed is intended to cool the engines to about –250 degrees Celsius. Three of the engines, #1, 2 and 4, got down to about –245 degrees Celsius, but engine #3 was only at about –230 degrees Celsius, according to temperature sensors in the engines.

He said one possibility for the difference is a problem with the sensor in the engine, rather than the flow of liquid hydrogen into the engine. “We’re a little bit concerned about one of those sensors,” he said, because they’re seeing “good, cold liquid hydrogen” being vented out of the engine.

“We understand the physics about hydrogen performs,” he said later in the briefing. “The way the sensor is behaving doesn’t line up with the physics of the situation.”

For now, the only change planned for the Sept. 3 launch attempt is to start flowing hydrogen into the engines about 30 to 45 minutes earlier, said Blackwell-Thompson, while the core stage hydrogen tank is in the “fast fill” phase of loading. That is what happened during Green Run tests of the core stage at the Stennis Space Center in early 2021, where there were no temperature issues reported.

“The only thing I know to change to replicate the success we had at Stennis is moving the test earlier in the timeline,” Honeycutt said.

If the same temperature problem reappears, it may be possible to proceed with the countdown if engineers believe the sensor is faulty and that engine #3 is properly cooled. “We will have a plan for a go/no-go rather than us sitting around scratching our heads,” he said. “We’ve got to continue poring over the data. We’ve got to put the flight rationale together anticipating we’re not going to get any better results on that engine #3 bleed temp sensor.”

A sensor that is off by tens of degrees, rather than be completely offline, is not unexpected. “Over the course of my career I have seen many sensors on launch vehicles be erratic,” he said, “and go out of calibration.”

Replacing the sensor while the SLS is at the launch pad is “likely not ideal,” Blackwell-Thompson said, and couldn’t be done before the current launch period ends Sept. 6.

Even if technical problems are resolved, weather could be an issue for a Sept. 3 launch. Mark Burger, launch weather officer with the U.S. Space Force’s Space Launch Delta 45, said that the probability of violating weather constraints is “somewhere in the neighborhood of 60%” for that day.

However, because the launch window is two hours long, he said it should be possible to find times within it when conditions will be acceptable as afternoon showers, typical for this time of year in Florida, pass through. “I still think we have a pretty good opportunity, weatherwise, to launch on Saturday.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...