NASA tried again at a third attempt on April 14 to complete the Wet Dress Rehearsal (WDR) test on its Artemis 1 vehicle. The integrated Orion spacecraft and Space Launch System (SLS) rocket for Artemis 1 will be launched by the agency’s Exploration Ground Systems (EGS) program from the Kennedy Space Center (KSC) in Florida on a mission to the Moon.
After two attempts to load the cryogenic stages on the SLS launch vehicle with liquid oxygen (LOX) and liquid hydrogen (LH2) were scrubbed on April 3 and 4, the third attempt was called off on April 14 after gaseous nitrogen, liquid oxygen, and liquid hydrogen issues prevented more than a partial fueling of the Core Stage.
Fueling the upper stage, called the Interim Cryogenic Propulsion Stage (ICPS), was already ruled out due to the failure of helium check valve on that stage.
The final issue on the April 14 attempt was a confirmed leak of liquid hydrogen on the Tail Service Mast umbilical. The area is accessible at the pad for inspection, investigation, and repair.
The WDR was originally intended to be a full, launch-ready demonstration of the vehicle, but the test is also meant to decrease the risk of first-time issues before attempting a real launch, and NASA is now planning to conduct a “modified” countdown for WDR without fully loading the ICPS.
Helium check valve failure prevents second stage fueling, full launch-ready test demonstration
The check valve, which allows gaseous helium to be supplied from ground facilities at KSC’s Launch Pad 39B to a set of composite overwrapped pressure vessel (COPV) bottles on the ICPS, stopped working correctly after some ground systems maintenance was performed following a scrub of the second WDR attempt on April 4. “During that time, they reduced the COPV pressure as a part of that [maintenance],” Artemis 1 Launch Director Charlie Blackwell-Thompson said in a media teleconference on April 11.
“When they had that [ground system repair completed] and they brought pressure back up, they failed to see the expected pressure on the COPV helium bottles onboard the [ICPS]. The team did troubleshoot that, and on Friday [April 8] they were able to determine that it looked like the issue was on the flight side of the interface.”
Mission management team provided a "go" for tanking. Weather still looks favorable. Team is working issues with GN2 pipeline and will begin switching to purge oxygen from the rocket. Gaseous nitrogen is used to displace air to create a safe environment for tanking.-JP
— NASA's Exploration Ground Systems (@NASAGroundSys) April 14, 2022
The three-inch long and one and 1/8-inch diameter check valve allows helium gas to flow from ground storage tanks into the COPV bottles but not in the other direction. After the valve failed, testing showed it was allowing some backflow.
“We did a reverse flow check,” Dr. John Blevins, NASA’s Chief Engineer for SLS, said in the April 11 teleconference. “A check valve is a one-way valve, so a reverse-flow check should just mean that everything stops. It didn’t stop, and so we knew that check valve was not operating the correct way.”
The check valve cannot be accessed while the vehicle is at the launch pad, so the cause of the failure is not yet known and won’t be until sometime after it can be fully inspected. It will also have to be repaired or replaced when the vehicle is back in the Vehicle Assembly Building (VAB), which is where vehicle maintenance and servicing was planned for a fully-stacked SLS in a launch configuration.
The VAB has extensive work platforms that provide access around the vehicle and is indoors, which provides weather sheltering from high winds and lightning.
Once back in the VAB, external work platforms will be extended around the vehicle and then internal work platforms will be installed inside the Launch Vehicle Stage Adapter (LVSA) to allow personnel to reach the check valve, which is located near the COPVs that are mounted around the mid-body of the stage between the LH2 and LOX propellant tanks.
(Photo Caption: The ICPS as seen suspended on a crane in the Vehicle Assembly Building (VAB) transfer aisle last July 5. From top to bottom are the liquid hydrogen tank, the mid-body, liquid oxygen tank, equipment shelf, and RL10 engine. The black cylinders seen attached around the circumference of the mid-body are the COPV bottles that provide gaseous helium for various functions on the stage. The check valve that failed is also located in the mid-body area.)
Increasing the pressure in the ICPS helium system to support propellant loading and unloading operations during the test would increase the risk to other flight hardware, since the check valve is not stopping back flow and holding pressure as designed and so the system cannot control pressure as designed. “We have a faulty device in the system, and that caused us to use caution because the whole point of Wet Dress is to decrease risk,” Blevins said.
“At the point where you start adding risk to the hardware is when you stop adding functions to the WDR, and you might take them away.”
Although NASA doesn’t want to fully load the ICPS with propellant or fully put it in a launch-ready configuration while the check valve is broken, the agency decided late on April 8 to continue with a modified Wet Dress Rehearsal test. The modified test will continue to test the SLS Boosters and Core Stage, along with Orion spacecraft systems according to the original plan; however, the test will only chill down the ICPS.
The ICPS propellant tanks will not be filled during tanking operations and they will not be pressurized for flight during the terminal countdown.
Teams with EGS and prime launch processing contractor Jacobs were preparing the Artemis 1 vehicle, Mobile Launcher-1, and Pad 39B systems for another Wet Dress Rehearsal attempt with a two-hour, terminal countdown test window on the afternoon of April 11 when the latest issue was first publicly reported on April 7. Following the decision late the next day, the countdown for the modified test was rescheduled to begin on the afternoon of April 12, with the two-hour test window on the afternoon of April 14.
(Photo Caption: The ICPS is almost in position as it was being lowered in the VAB last July 5 during lift and mate activities. The barrel of the ICPS liquid hydrogen tank is the only part of the stage in the vehicle’s outer moldline. During ascent to Earth orbit most of the ICPS is encapsulated inside the conical Launch Vehicle Stage Adapter.)
The WDR was originally planned as a full, launch-ready demonstration, running through the planned launch countdown timeline and automated sequences all the way down inside of T-10 seconds before stopping. The final major test planned before the Artemis 1 lunar test flight would verify that the ground facility and ground control systems can automatically configure the vehicle for launch, keep it in that configuration, and simultaneously satisfy all the launch commit criteria for those ground and flight system parameters up to the point of ignition.
Although that demonstration was one of the big picture goals, another over-arching goal for WDR was risk reduction. NASA decided it was better to maximize the current opportunity they have for risk reduction while the vehicle is at the pad right now by attempting to complete a modified test plan.
A modified test might not be able to uncover all the unknowns going into a launch attempt, but the flight and ground systems are currently in a configuration to run the test after a considerable amount of time and effort was made to set up for it. Even if it isn’t a perfect opportunity, it is an immediate opportunity to gain more operational experience and for more testing of predictive models with actual system behavior in launch conditions.
The SLS Core Stage and Boosters have both been ground tested in the past, but WDR is the first opportunity to conduct a high-fidelity launch test of them at KSC with the EGS facilities and ground control system.
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“We sat down on Friday [April 8] afternoon as a team and we looked at what tests make sense to go do while we’re at the pad,” Blackwell-Thompson said. “We certainly believe that there is great benefit in getting the Core Stage loaded all the way to replenish and buying down that risk associated with the stage and then went and looked at what are those functions that we get in terminal count.”
“When you go look at the very specifics of that, it really depends on how you count some of these, but just [as] a rough order of magnitude, if you look at your WDR critical events from T-10 [minutes and counting] and down to just inside of 10 seconds, there’s about 25 — I would say — critical events that we would list as a part of these test objectives for terminal count alone, not including the things that happen before you get to terminal count. Two of those are very specific to ICPS, so you’re getting 23 of 25.”
“Then when we went and looked at our GLS commanded events, we got some of them in our Countdown Sequencing Test, but that’s not in a cryogenic environment,” she continued. “But when we went and looked at them from T-10 [minutes and counting] down to just inside of 10 seconds, we have somewhere on the order of about 75 to 80 functions that we are commanding, and three out of those 75 to 80 are ICPS unique.”
“So there is a significant amount of testing and data and risk buy down that you get relative to Core Stage, that you get relative to the ground systems, [and] that you get relative to the Boosters.”
[image of Axiom and SLS, KSC-20220406-PH-JBP01-0001~large.jpg; NASA/Jamie Peer.]
(Photo Caption: A view from the air of both Launch Complex 39 pads as occupied on April 6. In the foreground at Pad A, the Axiom-1 vehicle stands ready for launch on April 7. The Artemis 1 vehicle in the background at Pad B was being serviced for its next Wet Dress Rehearsal attempt.)
The gaseous helium system where the check valve is located is used by the United Launch Alliance (ULA) ICPS stage for multiple functions. Blevins said that the helium system is relatively simple.
“[It has] got the [quick disconnect] that connects to [the] ground side, then it’s got the check valves, then it has the [COPV] bottles,” he said. “Now when it leaves those bottles, it goes to various functions.”
“Every one of those functions has filters; they go through regulators in order to supply helium gases to different operations. It can be valve operations on the vehicle, it can be… when we evacuate the oxygen tank on a Wet Dress Rehearsal we’re using helium, and that increased demand is why we’re not adding that particular propellant in this test to any substantial amount.”
“And then it also goes continuously in our current state to the RL10s to keep them dried out as purge,” Blevins added.
Blevins also explained that the check valve failure is not blocking helium from being added to the stage tanks, but is allowing some helium already in the bottles to escape back through the valve. “The check valve is not failed closed, [the failure is that] the check valve will not stop reverse flow,” he said.
— Nathan Barker (@NASA_Nerd) April 14, 2022
“And so we have decided to quit adding additional pressure if you will; we’re adding a consistent flow rate through that check valve to maintain COPV pressure sufficient enough to purge our engines and to do other functions. We do have pressure in the COPVs and in fact that pressure in the COPVs is providing purge gases to the RL10 engine in the ICPS at this very moment.”
During the tanking operations planned for the third attempt on April 14, the launch team plans to flow propellants into the ICPS main propulsion system (MPS) lines to chill them down. They won’t be able to flow LOX and LH2 into the propellant tanks, but they expect the chilldown to provide useful information.
“We are going to flow cold temps into the vehicle, monitor those temperatures, and when they hit cryogenic temperatures that’s when we’re going to stop,” Blevins said. “So we’re going to get a lot of data about flowing the cold gas across the interfaces right up to the point where we would start what we call fast fill.”
“I think that’s a really important piece of data to get because historically when you see leaks, especially with hydrogen, it is usually when you get down to cryogenic temperatures, and so we’re excited to see the data we get from chilling down to cryogenic temps.”
Initial attempts stopped by first-time issues, weather, and KSC support outages
Prior to rolling the Artemis 1 vehicle out to Launch Pad 39B for the first time the night of March 17 into the early morning of March 18, NASA had planned to conduct the WDR from April 1 through April 3. After completing engineering tests and loading hydrazine onto the SLS Booster hydraulic systems, the WDR countdown began on time on the afternoon of April 1.
(Photo Caption: The countdown clock display at the Launch Complex 39 Press Site on April 3 shows the Artemis 1 Wet Dress Rehearsal countdown in the “pre-tanking” hold at T-6 hours and 40 minutes. The Mission Management Team holds their pre-tanking briefing to review current vehicle status and the current weather forecast before a poll is taken whether or not to proceed into the tanking process of loading the SLS liquid-fueled stages with cryogenic propellant.)
Preparations for tanking the integrated SLS vehicle for the first time on April 3 continued until severe weather halted work on the afternoon of April 2. “Our launch weather officer from the 45th Space Delta forecast that had a 90 percent chance of precipitation, an 80 percent chance of lightning, and a 20 percent chance of hail,” Mike Sarafin, NASA’s Artemis 1 Mission Manager, said in an April 3 media briefing. “He was right on all but the hail and classified the weather setup as strong to severe.”
“It turned out that we had not one, but we had four lightning strikes inside the pad perimeter that afternoon. This was in addition to the heavy rain that was predicted and high winds that were predicted.”
“One of the lightning strikes was, according to our weather expert, fairly rare; it was a positively-charged, cloud-to-ground strike and it was much stronger than the other ones and it hit the catenary wire that sits between the towers,” Sarafin explained.
“So that set us behind by several hours, I would say about four hours or so,” Blackwell-Thompson said. “The team did a great job overnight. It is a really hard job to get everything finished up, I like to refer to it as ‘the big steel moves.'”
“[It includes the] engine service platform dropped and pulled back to the launch position, side flame deflectors into position, extensible columns jacked. All that work was completed after the weather had passed and in and around the time that we were about ready to send the teams back into the pad to begin that work is when we were notified that we had had a confirmed lightning strike.”
Teams will have a maximum two-hour long launch window to work with on a particular launch day, so the weather challenges may have factored into a tanking decision differently for a launch. But with more time to work with, the Mission Management Team (MMT) chaired by Safarin and the launch team headed by Blackwell-Thompson gave a go to proceed into tanking operations at 6:45 am Eastern time on April 3, and the countdown resumed at T-6 hours and 40 minutes.
Sometime after that an issue was reported with maintaining positive pressure on the Mobile Launcher, and an unscheduled hold occurred at T-6 hours. After the attempt was scrubbed, problems were reported with both supply fans providing that positive pressure to the Mobile Launcher. The fans are not on the Mobile Launcher but are a part of the Pad 39B infrastructure away from the pad surface.