Second prevalve issue delays completion of SLS Core Stage Green Run campaign

by Philip Sloss

A problem with another one of the eight prevalves in the Artemis 1 Core Stage for NASA’s Space Launch System (SLS) has postponed a second test-firing to wrap up the Green Run campaign. The SLS Program’s first flight article has been installed in the B-2 Test Stand at the Stennis Space Center in southern Mississippi since January 2020.

After a liquid hydrogen prevalve was repaired in November, all the prevalves worked during tanking and firing tests until final preparations for a second Hot-Fire — when a liquid oxygen prevalve stopped opening correctly. Engineers are back inside the stage’s engine section to troubleshoot the failure and determine what needs to be fixed; currently, NASA doesn’t expect to be ready to test-fire the stage again until mid-March.

Second Hot-Fire test delayed by another prevalve problem

The agency announced in a February 22 blog post that one of the liquid oxygen (LOX) prevalves in the SLS Core Stage for Artemis 1 no longer functioned properly during final preparations conducted over the prior weekend for a second Hot-Fire test that was planned for February 25. The balky prevalve is a part of the propellant feedline that delivers LOX from the stage to engine number one.

In an additional post on February 26, the agency noted that the prevalve was not opening correctly, and troubleshooting would continue over the weekend. “After completion of troubleshooting, which will continue over the weekend, NASA will be in a better position to identify a potential date for the second hot fire test,” the post said.

Sources indicated that an estimated target for the second Hot-Fire test was no earlier than March 16.

This is the second problem with a prevalve during the Core Stage’s Green Run test campaign that began at the beginning of 2020. The first problem was with the liquid hydrogen (LH2) prevalve for engine number four.

That intermittent issue with the operation of one of the LH2 prevalves interrupted the stage’s Green Run test campaign in October, delaying the final two of eight test cases while that prevalve unit was repaired in place inside the cramped quarters of the engine section. In that case, troubleshooting and fixing problem with the LH2 prevalve’s clutch mechanism delayed the Green Run schedule by about a month.

This past week, a team that includes the SLS Program, Core Stage prime contractor Boeing, RS-25 engine prime contractor Aerojet Rocketdyne, and NASA Stennis personnel were finishing preparations to begin the countdown for a second Hot-Fire test when the new prevalve problem occurred.

The LOX prevalve functioned correctly throughout the Green Run test campaign, including the short, one-minute long Hot-Fire test on January 16. The early end to the test left several design verification objectives (DVO) incomplete, and NASA decided that a second, longer firing was necessary.

Fixing the problem with this second balky prevalve will take some time because engineers needed to get “under the hood” access to the vehicle, which was already closed out for the Hot-Fire test when the new problem was discovered. In order to get hands on the prevalve, work access needed to be re-established.

Credit: Brady Kenniston for NSF.

(Photo Caption: The flight article for Artemis 1, Core Stage-1, fires in the B-2 position of the B Test Stand at Stennis Space Center on January 16. The Hot-Fire was cut short after 67 seconds of engine runtime when the stage’s hydraulic performance fell below conservative limits just as the engines were being aggressively gimbaled for the first time in the test.)

“Technicians installed platforms that allow engineers to access the valve inside the core stage engine section while the stage remains in the B-2 stand at NASA’s Stennis Space Center near Bay St. Louis, Mississippi,” the February 26 blog post noted.

There are a total of eight prevalves on the Core Stage, four of which deliver the liquid oxygen oxidizer via separate feedlines to each of the RS-25 engines and four of which deliver the liquid hydrogen fuel to the engines. The valves are opened just before ignition to flow fuel and oxidizer from the tanks to the inlets of the four RS-25 engines until just after Main Engine Cut-Off (MECO).

If this new issue is a recurrence of the similar problem with the Core Stage Main Propulsion System prevalves seen last October, it may have other implications if there’s a generic issue with the prevalve hardware itself. The valves are critical to the safe operation of the stage; they must behave within specifications, fully opening and closing within time limits to ensure proper engine ignition and shutdown conditions.

New test-firing date depends on repair work

NASA said the troubleshooting of the LOX prevalve would help to identify repairs needed. A new date will depend in part on how extensive and invasive the repair work is.

For the first prevalve repair for LH2 side in November 2020, engineers for Boeing and prevalve vendor Vacco had to position themselves on an erector set style work platform installed for access inside the engine section. The cryogenic propellant lines in the engine section are insulated with spray-on foam, including the valves, so the foam had to be removed during troubleshooting to be able to disassemble parts of the valve.

SLS’s flight software steers the Solid Rocket Boosters and four RS-25 to maintain the proper launch trajectory and pitch profile during the initial phase of flight of Artemis 1. (Credit: Mack Crawford for NSF/L2)

In the case of the LOX prevalves, they are located in a different area of the engine section. Whereas the LH2 prevalves are on feedlines that connect directly from the LH2 tank, the LOX prevalves branch off from two long “downcomer” feedlines that run from the LOX tank at the top of the Core Stage, down the outside of the LH2 tank, and into the engine section.

Special tooling was designed, fabricated, and employed to perform the first prevalve repair and could be used again; however, work access to the LOX prevalves may be different than the LH2 prevalves. In addition to the different location within the engine section, the LOX prevalve may be in a different orientation than the LH2 prevalves, which could affect the pace of repair operations.

Once the prevalve is fixed it, will take several days reconfigure the valve and the engine section back into a firing configuration, including the reapplication of foam to the prevalve as well as uninstalling the internal platform kit.

(Lead image credit: NASA/SSC.)

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