NASA engineers detailed a problem with a temperature sensor on engine 3 that contributed to the scrubbed launch of Artemis I on Monday and said another launch attempt is planned for Saturday afternoon starting at 2:17 p.m. EDT.
There is currently no plan to replace the apparently errant sensor before the Saturday launch, but engineers plan to rely on other temperature sensors on the ground and in the other three engines, as well as other data, to decide that engine 3 has been cooled sufficiently to about the minus-420 degrees needed for its normal operation at liftoff.
The sensor in question has likely not been calibrated since it was shipped to NASA from the contractor long ago and could be faulty and need to be replaced. It was giving a reading about 30 to 40 degrees warmer than the temperature sensors on the other three rocket engines, showing a temperature of about minus-380 degrees, a NASA official said. The other three engines were reading about minus 410 degrees, nearly to the level to go ahead with the flight.
“Over the course of my career, I have seen many sensors be erratic and work and not work and go out of calibration,” said John Honeycutt, Space Launch System Program Manager. ”It’s not terribly unusual.”
Replacing the engine 3 temperature sensor would likely involve moving it indoors to the massive Vehicle Assembly Building near launch pad 39B at Kennedy Space Center in Florida, a process which could require weeks of delays.
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Crews will start the vital process of chilling down engines by about 30 to 45 minutes earlier on Saturday than occurred on Monday to feel sure engine 3 is cooled sufficiently. Crews supply liquid hydrogen from tanks to the engines in a bleeding process that brings them to the right temperature at minus-420 F. During a launch, liquid hydrogen acts as the main source of fuel that is combined with liquid oxygen used as an oxidizer. Each engine has its own bleed system to take heat out of the engines as they are chilled down.
“I think we understand the physics of how hydrogen performs,” Honeycutt said, in describing how crews will evaluate the engine 3 temperature sensor alongside data from other sensors to determine if it is ready to fly.
Before Saturday, crews are working to repair a hydrogen leak on the tail service mast umbilical quick disconnect, sometimes called the purge can, on the launch pad and will be monitoring the weather. There’s a 60% chance of rain but there could be gaps in rain sufficient to allow a launch within a two-hour window.
Should the launch encounter problems, the next available launch window is Monday, Sept. 5.
Artemis 1 is a test mission without any crew that will circle the Moon and return to Earth 42 days later. Successive missions approach the Moon and then land in 2025.