31 August 2022
NASA will make a second attempt to launch the agency’s giant Space Launch System rocket Saturday on a test flight to send an unpiloted Orion crew capsule around the moon and back, a major milestone in the agency’s.
by trouble cooling one of the rocket’s four shuttle-era engines to the required pre-start temperature, managers said Tuesday engineers have come up with a work-around. Assuming final clearance to proceed, the launch team will start a fresh countdown at 4:07 p.m. EDT Thursday.
That will set the stage foron the Artemis 1 mission at 2:17 p.m. Saturday, one day later than NASA’s original backup launch date. As always, the team will have to work around the weather, with forecasters predicting a 60% chance of stormy conditions during the rocket’s two-hour launch window.
Mike Sarafin, chairman of NASA’s mission management team, said the core stage fueling procedure will be adjusted in an attempt to improve cooling to all four RS-25 engines. In addition, fittings will be tightened around a fuel-line umbilical at the base of the rocket to improve sealing and prevent leaks like one that briefly occurred Monday.
“We agreed on what was called ‘option 1,’ which was to operationally change the (fuel) loading procedure and start our engine chilldown earlier,” Sarafin said. “We also agreed to do some work at the pad to address the leak that we saw at the hydrogen tail service mast umbilical.
“And we also agreed to move our launch date to Saturday. We are going to reconvene the Mission Management Team on Thursday to review our flight rationale and our overall readiness.”
The 322-foot-tall 5.75-million-pound SLS is theever built by NASA, generating 8.8 million pounds of thrust at liftoff using four Aerojet Rocketdyne RS-25 engines left over from the shuttle program and two Northrop Grumman solid rocket boosters attached to a Boeing-built core stage.
Accelerating to 70 mph — straight up — in just seven seconds, the solid rocket boosters and the core stage will boost the Orion capsule, carrying instrumented test dummies and a suite of sensors and experiments, into an elliptical orbit. The rocket’s upper stage, provided by United Launch Alliance, then will propel the capsule out of Earth’s gravity and onto a trajectory to the moon.
After a close flyby, the capsule will whip around the moon and out into a distant orbit that will carry it farther from Earth than any human-rated spacecraft. Then, after another lunar flyby, the ship will head back to Earth for splashdown in the Pacific Ocean west of San Diego on October 11.
The goal of the Artemis 1 mission is to put the SLS rocket and the Orion spacecraft through their paces, including a high-speed, high-temperature re-entry, before launching four astronauts around the moon in late 2024. The firstis planned for the 2025-26 timeframe.
Given the constantly changing positions of the Earth and moon, along with the rocket’s ability to reach the correct trajectory, NASA must launch the Artemis 1 mission within specific “windows.”
Complicating the picture, the battery used by the upper stage’s self-destruct system must be serviced after 25 days, and that can only be done back in NASA’s Vehicle Assembly Building.
That means the Artemis 1 mission must get off the ground by Monday or the rocket will be hauled back to the VAB, delaying another launch attempt until late September at the earliest or, more likely, to October.
The SLS rocket is the key to the Artemis program and NASA managers and engineers want to make sure it works as planned before launching astronauts to the moon.
A full-duration eight-minute core stage engine test firing was carried out at the Stennis Space Center in Mississippi on March 18, 2021. The rocket then was shipped to the Kennedy Space Center for launch processing.
NASA carried out a dress-rehearsal countdown and fueling test on April 3, a key milestone needed to make sure the rocket, launch pad and ground systems work together as planned. But engineers ran into a series of mostly ground-system problems that prevented them from loading propellants,
Two more fueling attempts failed on April 4 and 14 due to a variety of unrelated problems. Engineers were finally able to fully load the core stage on June 20, but only after a leaking quick-disconnect fitting was isolated that prevented the flow of hydrogen coolant to the core stage engines — a requirement for an actual launch.
The quick-disconnect was repaired back in the Vehicle Assembly Building and the SLS rocket was rolled back out to pad 39B on August 16 to ready the vehicle for launch.
During Monday’s launch attempt, the repaired quick-disconnect appeared to work normally. With the core stage tanks filled and topped off, liquid oxygen and hydrogen began circulating through the engine plumbing to condition them to the ultra-low temperatures of the propellants — minus 423 degrees Fahrenheit for hydrogen and minus 297 degrees for oxygen.
But none of the engines reached the target temperature. Engines 1, 2 and 4 got to about -410 degrees while engine No. 3 only reached about -380 degrees. During troubleshooting, engineers diverted all the hydrogen coolant to engine 3 and it still did not reach the planned operating temperature.
John Honeycutt, manager of the SLS program at the Marshall Spaceflight Center, said engineers suspect a faulty sensor might be responsible for the readings from engine 3. Pressure measurements and other data indicate good cooling.
“The way the sensor is behaving, it doesn’t line up with the physics of the situation,” he said. “And so we will be looking at all the other data that we have to use it to make an informed decision whether or not we’ve got all the engines chilled down or not.”
By starting the chilldown procedure about 45 minutes earlier when the engines are near ambient temperatures, engineers believe they can manage to cool all four engines as needed.
A similar procedure was used during the rocket’s test firing last year at the Stennis Space Center. In that case, the engines were properly cooled and started normally for a full-duration “green run.”
“As of today, and based on the data that we’ve got, we think we can do something like what we did at the Stennis Space Center to put ourselves in a better position for launch,” said Honeycutt.
As Sarafin said, the team will review all the data Thursday before giving final clearance to proceed with a launch attempt.
“The team is in the middle of poring through the data and building the flight rationale,” Honeycutt said. “I don’t have that just yet, but I do expect us to be able to get there.”
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Author: William Harwood