NASA’s Space Launch System (SLS) Program rescheduled the final two tests that will culminate the Core Stage Green Run campaign for December after successfully repairing one of the liquid hydrogen (LH2) prevalves inside the engine section of the stage.
The Wet Dress Rehearsal is now targeted for the week of December 7, and if that goes well the Hot-Fire test of the stage could occur two weeks later during the week of December 21.
Prevalve repaired in tight confines of engine section
The Artemis 1 core stage flight article is installed in the B-2 Test Stand at the Stennis Space Center in Southern Mississippi for the Green Run design verification campaign.
An intermittent failure of the LH2 prevalve for engine four was discovered during previous Green Run testing and after troubleshooting and partial disassembly, analysis narrowed the problem down to a clutch mechanism on the valve. Following the surgical clutch replacement, the fully-reconstructed valve assembly passed retesting and Core Stage prime contractor Boeing is now working to pick up preparations for the first of the two final critical tests: a Wet Dress Rehearsal (WDR) to prepare for a subsequent full-duration test firing of the stage.
NASA reported on November 18 that in-place repairs of the engine four LH2 prevalve in the Core Stage were successful. An intermittent issue with the operation of the prevalve interrupted the stage’s Green Run test campaign, delaying the final two of eight test cases while the problem was identified and resolved.
“Over the weekend, engineers at NASA’s Stennis Space Center near Bay St. Louis, Mississippi, successfully repaired a valve inside the core stage of the agency’s Space Launch System (SLS) rocket,” Kathryn Hambleton, Public Affairs Officer at NASA Headquarters, wrote in the agency’s Artemis blog.
“The team designed an innovative tool to remove and replace the valve’s faulty clutch while the core stage remained in the B-2 test stand, and without removing the entire valve.”
“Subsequent testing of the repaired valve confirmed that the system is operating as intended.” Torque checks showed the repaired valve responding to specification and the prevalve also passed leak checks.
Preparations to perform the last two tests in late October and mid-November were suspended when the prevalve failed a focused checkout procedure in mid-October. Following a few weeks of troubleshooting, fault tree analysis narrowed down the suspect to the clutch mechanism within the overall prevalve assembly. Engineers worked in cramped quarters inside the Core Stage’s engine section to do the troubleshooting and repairs.
The bottom of the LH2 propellant tank, the aft dome, takes up a significant part of the room inside the engine section, with the prevalves attached to the bottom of the tank in a small space above and around all the Main Propulsion System (MPS) equipment at the bottom of the engine section. The valves are attached to the salad bowl shaped aft manifold of the tank which itself extends out from its aft dome.
The manifold has openings for feedlines to flow propellant to each of the four engines and for a fill and drain line to load the tank from ground supplies during the countdown prior to engine ignition. The prevalves control the flow of LH2 from the tank through feedlines directly to the LH2 inlet of each of the four RS-25 engines.
Credit: NASA/Jared Lyons, Jude Guidry, Eric Bordelon, Steven Seipel.
(Photo Caption: The four LH2 prevalves on Core Stage-1 can be seen sticking out of the LH2 tank aft manifold in the upper right part of this image taken in September, 2019, at Michoud Assembly Facility during operations to mate the engine section (left) to the bottom of the hydrogen tank (right). A suspect clutch mechanism in one of the valves was recently repaired from inside the engine section with the stage installed vertically in the B-2 Test Stand.)
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 foam, including the valves, so that had to be removed during troubleshooting to be able to disassemble parts of the valve.
Working in shifts around the clock, the intricate repair of the valve required disassembling enough of the surrounding mechanism to remove the suspect clutch and replace it with a new unit. The valve was repaired with the overall housing remaining bolted in-place inside the aft compartment, so subassemblies like the actuator had to be removed to get to the clutch. As noted in NASA’s blog post, special tooling was built to help support the prevalve repair work.
Following the part replacement, engineers then had to rebuild the internal prevalve mechanisms with the valve in its operational rocket position and orientation, as opposed to on a work bench in a well-lighted work area. Performing the repair in place helped to limit the scope of necessary rework and time it would take to do all that work.
TVC issue still being discussed but not stopping Green Run test schedule
While the prevalve repairs and retesting were being performed, an issue with the stage’s thrust vector control (TVC) system was still being discussed within the program. Test case five performed a checkout of the Core Stage Main Propulsion System (MPS) and RS-25 engines late in the Summer which included a test of the TVC systems for all four engines.
Credit: NASA/Stennis.
(Photo Caption: One of the RS-25 Core Stage engines is swung outwards by the stage’s hydraulically-powered TVC system in a gimbal test performed on August 31. Data from the TVC testing performed during Green Run test case five is still being discussed, but the issue is not holding up plans to run the final two test cases in December.)
In flight the Core Stage moves the four RS-25 engines for steering using hydraulic power; each engine has two TVC actuators connected to it, a pitch and a yaw actuator. The stage’s hydraulic system provides the power to drive all eight TVC actuators in addition to controlling the position of critical valves in the RS-25 engines for start and shutdown along with controlling engine thrust levels.
There still seems to be a disagreement between different sets of instrumentation and models of the TVC system performance during the tests. There was also some discussion about performing additional TVC testing on the Core Stage and in the TVC lab at the Marshall Space Flight Center in Huntsville, Alabama.
The TVC lab in Building 4205 at Marshall has a full Core Stage hydraulic system set up for qualification testing that was recently completed. In addition to the two TVC actuators on each hydraulic system, the lab also has connections to the two dedicated avionics boxes for a hydraulic system, a Core Stage Auxiliary Power Unit Controller (CAPUC) and a TVC Actuator Controller (TAC). The lab is located near the SLS avionics and software test lab which has operating sets of the Core Stage flight computers and avionics.
With NASA and Boeing moving ahead towards the final Green Run tests in December, the issue doesn’t appear to be a showstopper to the new schedule for finishing the Stennis campaign; however, careful positioning of the engines by the stage TVC system is a crucial element of the Hot-Fire test. While the engines are firing in the static ground test they must be positioned to minimize lateral forces imparted from the stage to the test stand, including the structural attachments and the umbilical connections between the stage and the stand.
Ground control and vehicle computers are both monitoring the position of the engines during the Hot-Fire test and a misalignment could cause the test-firing to be aborted.
Lead image credit: NASA/Karl Wilcox and Charles “Danny” Nowlin.