NASA and Boeing readied a gargantuan Space Launch System rocket for a second test firing in Mississippi Thursday to clear the way for a long-delayed maiden flight late this year or early next to kick off the space agency's Artemis moon program.
Two months after glitches cut short an initial attempt on January 16, the 21-story SLS core stage's four shuttle-heritage RS-25 main engines were expected to ignite one after the other, at 120 millisecond intervals, throttling up to a combined thrust of 1.6 million pounds. The two-hour test window opens at 3 p.m. EDT.
Firmly locked down on a massive test stand at NASA's Stennis Space Center just east of New Orleans, the core stage's Aerojet Rocketdyne engines were expected to fire for up to eight minutes — the duration of an actual climb out of the atmosphere — consuming more than 700,000 gallons of liquid oxygen and hydrogen in the process.
The test run called for throttling the engines down and back up as they will be during an actual flight, testing the rocket's thrust vector control, or steering, system and monitoring how propellant tank pressures are maintained as the fuel level drops.
Data from hundreds of sensors will reflect a wide variety of parameters, including critical vibration levels at different thrust settings. The data will feed into 10 "detailed verification objectives," or DVOs, required to demonstrate the rocket is ready for flight.
Mission managers said they expected to collect all the data needed in the first four minutes, but planned to let the engines run twice that long to simulate an actual climb to space and to monitor performance when the tanks are nearly empty and the rocket weighs a fraction of its initial 2.3 million pounds.
"This is not by any means a vanilla firing the core stage," said Boeing SLS manager John Shannon. "Since it's a highly instrumented vehicle, we're taking that opportunity to do some very aggressive maneuvers of the thrust vector control system. ... It's going to stress the vehicle.
"Our primary goal is to not only get the hot fire data, but also to protect the stage (for use as) the first flight article. So we're kind of threading a needle here between keeping the vehicle safe and gathering this data."
NASA is counting on the Space Launch System rocket to propel the agency's Lockheed Martin-built Orion crew capsules on flights back to the moon, using commercially built landers to carry astronauts down to the surface for the first time in 50 years.
For an actual launch, the Space Launch System first stage will be attached to a pair of already tested Northrop Grumman solid-fuel boosters, each one generating 3.6 million pounds of thrust, and a hydrogen-fueled upper stage topped off by an Orion crew capsule.
The fully assembled SLS rocket will weigh 5.75 million pounds, stand 322 feet tall and generate 8.8 million pounds of thrust at liftoff, making it the most powerful operational rocket in the world.
It also will be one of the most expensive ever built. NASA's inspector general reported last March that total SLS program costs were expected to climb above $18 billion by the time the Artemis 1 rocket finally takes off.
The high price tag has prompted critics to argue in favor of less powerful, but much less expensive, boosters like SpaceX's Falcon Heavy. But NASA managers say the SLS is the only rocket available in the near term that is capable of accommodating the Orion crew capsule and other large components envisioned for the Artemis program.
During the initial test run in January, the booster's flight computer shut down the engines about one minute into the firing after detecting low pressures in the hydraulics used by the thrust vector control steering system. That problem was blamed on software, which was modified for the second attempt.
Other adjustments were made, along with repairs of a liquid oxygen valve that combined to push the second attempt to Thursday.
If analysis of telemetry confirms the rocket performed properly the second time around, engineers will spend about a month carrying out detailed inspections and readying the core stage for shipment by barge to the Kennedy Space Center.
Once in Florida, the stage will be bolted to a pair of already-assembled solid-fuel boosters, followed by attachment of its waiting second stage and an Orion crew capsule.
Assuming no other major problems develop, NASA could be ready to launch the first SLS on an unpiloted maiden flight — Artemis 1 — before the end of the year, sending the Orion capsule on a long, looping flight around the moon and back.
Working on a schedule imposed by the Trump administration, NASA planned to launch the Artemis 2 mission in the 2023 timeframe, followed by a piloted moon landing by the end of 2024.
But Congress has not yet provided the full funding needed for a new lunar lander and while the Biden administration has indicated support for the Artemis program, it's not yet clear when a landing could be attempted.