CAPE CANAVERAL—NASA is retargeting the launch of its first Space Launch System (SLS) rocket and Orion spacecraft for Sept. 3, following analysis of an engine thermal-conditioning issue that scuttled an initial launch attempt on Aug. 29.
Engineers believe that chilling the SLS’s four Aerojet Rocketdyne RS-25 engines 35-40 min. earlier in the countdown—as demonstrated during the March 2021 green run static fire at NASA’s Stennis Space Center—will set the stage for the inaugural liftoff of the rocket on the Artemis I flight test, which will send an uncrewed Orion spacecraft on a 42-day mission around the Moon.
The engines are brought to their -420F operational temperatures by increasing the pressure on the core stage liquid hydrogen tank to route a portion of the cryogenic propellant to the engines.
The procedure failed during the closing hours of the launch countdown on Aug. 29, prompting a scrub. Three of the four engines were close to the required temperature, reaching about -410F after the liquid-hydrogen flush, but sensors showed one engine was 30-40 deg. warmer.
Engineers suspect the sensor may be faulty. “The way the sensor is behaving does not line up with the physics of the situation,” SLS Chief Engineer John Honeycutt told reporters Aug. 30.
NASA is developing alternative means to assess the temperature of the engines, Honeycutt added. “I’m confident where the team is headed. We have a path forward to get where we need to go to support the next launch attempt.”
Countdown activities are due to pick up on Sept. 1, leading to a planned liftoff at 2:17 p.m. EDT on Sept. 3 at the opening of a two-hour launch period.
Prior to committing to the launch attempt, the NASA Mission Management Team plans to reconvene to review the flight rationale and the work ahead, Artemis I Mission Manager Mike Sarafin told reporters Aug. 30.
The weather forecast for a launch attempt on Sept. 3 is not ideal, with just a 40% chance conditions will be suitable for liftoff due to cloud cover and the chance of rain and lightning. ”We expect to have at least some threat of a violation,” Space Force meteorologist Mark Berger said. ”We have two hours to work with. The showers tend to have quite a bit of real estate between them. I still think we have a pretty good opportunity weather-wise.”
NASA had intended to practice the so-called hydrogen kick-start procedure needed to chill the core-stage engines, ahead of launch during a practice countdown in June, but was stymied by a leak in the hydrogen propellant bleed line. The SLS and Orion were returned to the Vehicle Assembly Building for repairs and final launch preparations.
If there is a second launch scrub, NASA could look to Sept. 5 for a third attempt, assuming the delay is not associated with a hardware or system failure requiring more time to address.
If launch occurs on Sept. 3, the flight plan for the Artemis I mission remains a 42-day test of the uncrewed Orion spacecraft around and beyond the Moon and back for a parachute-assisted splashdown and recovery in the Pacific Ocean off the coast of San Diego, California.
Data gathered about the high-velocity re-entry, the habitability of Orion in deep space and the SLS performance will help to determine the next steps. Artemis I is to be followed by a crewed Orion flight test, Artemis II, currently targeted for May 2024, and Artemis III, which is to return human explorers to the surface of the Moon at the unexplored lunar south pole in late 2025.
Teamed with international as well as commercial partners, NASA also plans to build the Gateway—a lunar-orbiting, human-tended outpost—and the south pole Artemis base.
The lunar strategy is to lead to human expeditions to Mars in the late 2030s to early 2040s.