Hydrogen leak troubleshooting will resume inside the VAB
Once the decision was made, EGS and Jacobs suspended troubleshooting to find and fix the hydrogen leak and began preparations to roll the vehicle and Mobile Launcher back to the VAB. The leak was observed during operations to fill the SLS Core Stage liquid hydrogen tank on the April 14 attempt.
“We had been very anxious to load hydrogen. As folks know, the hydrogen molecularly is very small and at cryogenic temperatures sometimes in the beginning stages of loading is when you will see a leak,” Blackwell-Thompson said in a media teleconference on April 15. “We got through our slow fill operations, everything was nominal, [we] had no issues.”
“When we transitioned to fast fill… you will see an increase in pressure as part of that load operation, [and] we did detect a leak on the Tail Service Mast on something that we call the purge can or the purge canister. It’s actually on the back of the ground-side plate. We did see a leak there.”
(Photo Caption: The two tail service mast umbilical (TSMU) connections from the Mobile Launcher to the SLS Core Stage engine section can be seen from this overhead image taken last September. The liquid oxygen TSMU is on the left, the LH2 TSMU on the right. A white purge canister assembly can be seen enclosing the two biggest lines in the middle of the LH2 umbilical connection.)
“The indication that we saw in the purge can exceeded our ground safety limit and immediately caused a stop flow to take pressure off that line. So it’s isolated to the purge can area, and [there are] a number of suspect areas within that purge can that we’re going to go explore once we get back to the VAB,” Horvath also noted.
There are two, 30-foot-tall Tail Service Masts on the deck of the Mobile Launcher’s launch platform, one for liquid oxygen (LOX) and one for liquid hydrogen (LH2) that supply propellant and other fluids to the Core Stage through umbilical lines connected to the stage’s engine section. A ground-side and a flight-side plate are mated to provide a leak-tight seal around quick disconnect lines that run from the ground into the vehicle and from the vehicle out to ground facilities.
The purge canister where the leak was detected is on the outside of the ground-side plate on the LH2 tail service mast umbilical (TSMU). It encloses the two main lines that carry LH2 into and out of the Core Stage engine section, an eight-inch diameter LH2 fill and drain line used for propellant loading and a four-inch diameter bleed line that allows hydrogen used to chilldown the Core Stage Main Propulsion System (MPS) and RS-25 engines to be dumped overboard.
“It’s a two-piece can made of stainless steel, and it’s bolted together and then it’s bolted to the backside of the plate,” Horvath explained. “What we do is we [constantly] purge that volume with an inert gas and our [hazardous] gas sensing team monitors that volume for hydrogen because of the flammability and explosive nature of hydrogen in oxygen with an ignition source.”
After the vehicle was drained of propellant and inerted following the April 14 attempt, initial leak troubleshooting started at Pad 39B before standing down for rollback preparations. “Prior to the decision to roll back to the VAB we had set up access at the pad to go prepare for our troubleshooting plan,” Horvath noted.
“We did get to a leak check without having to disassemble that can, we put a GN2 purge in that can and we sensed the helium in the fill and drain line and the bleed line, that pressure. We took a reading out at the pad, and we did not see any kind of indication of leakage with an ambient condition there.”
Horvath said the team has continued to work the troubleshooting plan while they wait for the vehicle and Mobile Launcher to be brought back inside VAB High Bay 3. “Part of that plan is going to be to disassemble the can,” he said.
The two tail service mast umbilicals that connect to the outside of the Core Stage engine section via a ground plate and a flight plate carry more than just the LOX and LH2 propellants to load the tanks. “They have various commodities going through it, primarily liquid hydrogen for the fill and drain and then we have a bleed that comes off the Core Stage engine section that bleeds the cold gas back to the facility drain,” Horvath explained. “We also have electrical and pneumatic functions that go through that interface as well.”
There are multiple additional connections to the fill and drain line and the bleed line through “penetrations” in the purge can. “On the back side of that ground plate we have connections with our ground system; on the 8-inch fill and drain line there’s a flanged connection,” Horvath said.
“We have a debris plate that is sandwiched between the flanges and two gaskets on either side of that debris plate. Likewise on the four-inch bleed line there’s [another] connection there as well, so those are the two main sources of hydrogen going into and out of the vehicle.”
“Now within that configuration as well we have a purge line, called a drain-assist purge line, on both the 8-inch and the 4-inch [lines] and then we have pressure pickup points that are on that configuration as well, on both lines, and there’s [also] a temperature probe on the fill line,” Horvath explained.
None of those secondary connections to the two main lines carry hydrogen, Horvath said, “but all those penetrations into those lines will see hydrogen.” Once they disassemble the purge can that encloses all of that back in the VAB, they will start to look at the different connections to the two LH2 lines.
(Photo Caption: Another view of the LH2 tail service mast umbilical connection taken from the zero level or “deck” of the Mobile Launcher last September. The two, 30-foot-tall service masts are on the left edge of the image, with the LH2 service mast and umbilical connection in the left foreground. The purge canister encloses the two “quick disconnect” lines that carry liquid hydrogen to and from the vehicle.)
“We’re going to isolate each of those areas that we think may be a leak source and we can do a sniff check of those areas individually,” Horvath noted. “The flanged area we will tape around and put a probe in there to see if we can isolate the leak to any one of those joints.”
“Some other work that we’re also talking about doing as well is to verify the torque values on each of those connections to the line and also the flange that mates between the debris plate. If we see something, that data will lead us to what we do next.”
“That’s about where we are right now on the troubleshooting plan,” Horvath said in the April 20 interview. “We’re going to be data-driven, and we’ll talk once we get the data to see what course of action we will take further.”
The structures to perform SLS and Orion servicing and maintenance are predominantly in the VAB, and also allow members of the EGS and Jacobs team to gain access inside the launch vehicle to inspect and resolve the gaseous helium check valve on the Interim Cryogenic Propulsion Stage (ICPS) that failed in between the second and third WDR attempts. Workers will set up platforms and stands outside the Launch Vehicle Stage Adapter (LVSA) and then another set of platforms inside to gain access to the check valve.
The check valve was allowing helium in the onboard ICPS storage bottles to escape, which prevented the stage from being safely loaded with its LOX and LH2 propellants. The third WDR attempt would have omitted fully loading and preparing the second stage for liftoff, but assuming that the check valve is repaired or replaced, the full set of countdown demonstration test objectives should be possible on the next attempt.
NASA management reviewing options for next trip to the pad
The Exploration Systems Development (ESD) division and the EGS, Orion, and SLS Programs it includes and integrates are discussing a range of options for the next trip the Artemis 1 vehicle makes from the VAB to Pad 39B. Shortly after the decision to roll back, the first few, tentative options were being developed. “Once we get back, we are currently working on a couple of different schedule options that we’ll be reviewing with the management team over the course of the week,” Blackwell-Thompson said on April 18.
“We’ll look at what I call a ‘VAB quick turn option,’ which is we come back to the VAB, we take care of a minimal amount of things, upper stage check valve, the TSMU leak, maybe one or two other items that are determined to be highly desirable or required before the next Wet Dress attempt, and then we would look to roll back out and get that Wet Dress attempt behind us.”
“There’s a second option that looks at doing a greater amount of work in the VAB, maybe getting closer to your rollout-for-flight configuration and then you would roll out, do your Wet Dress, come back, take care of your FTS (Flight Termination System) testing, and then roll out for launch,” she added. “And then a third option that looks at whether or not you could do a Wet Dress and launch in one rollout campaign.”
By the end of the week, the third option looking at launching after Wet Dress without rolling back was ruled out; a NASA blog released late on April 22 noted: “Following completion of the test, SLS and Orion will return to the VAB for the remaining checkouts before rolling back out to the pad for launch.”
(Photo Caption: The three lightning towers and the water tower at Launch Pad 39B are seen on April 21 surrounding the Artemis 1 vehicle and the Mobile Launcher. Also in the foreground are two flare stacks to safely burn off hydrogen that is vented during propellant loading operations; the one on the left was installed during the Shuttle-era and the one on the right was installed with a recently added second liquid hydrogen sphere that will support future Artemis launches.)
Managers will be monitoring the progress of the work in the VAB and at the GN2 plant over the next several weeks. “As we work our way through this, we’ll be looking at what that work looks like in the VAB, we’ll be looking at how this capability is being accomplished out at the GN2 plant, how their schedules are coming along, and we’ll be looking at those two [efforts] in parallel swim lanes and we’ll be making decisions,” Blackwell-Thompson said.
“Certainly, we wouldn’t just do the minimum amount if we had additional time in the VAB, we would take advantage of that additional window in the VAB while we were waiting on the additional capability to come online at the plant, we’d be doing other work that gets us closer to launch.”
With the third option already ruled out and delays to the Cape Canaveral launch calendar already delaying and stretching out the planned work schedule at the GN2 plant, it is unlikely that Artemis 1 will be ready to launch until July at the earliest. It’s possible that the Wet Dress Rehearsal still won’t be completed by the time Launch Period 22 arrives in early June and given the time it takes to move the vehicle back and forth between Pad 39B and the VAB, Launch Period 23 in late June and early July will also be challenging.
The next lunar launch window would be Launch Period 24, which opens in late July and closes in mid-August; however, that opportunity would overlap with a planetary launch window for NASA’s Psyche spacecraft that begins on August 1 and runs through most of the month. If the opportunities were to overlap, not only would the launches need to be prioritized, but Psyche will also launch on a SpaceX Falcon Heavy from nearby Pad 39A, which would also require more close coordination between operations on both pads.
NASA is still planning to perform a full WDR. “We will absolutely go back out, we’re absolutely going to do a Wet Dress Rehearsal,” Whitmeyer said on April 18. “[We will] demonstrate cryo loading…and we [will] also demonstrate terminal countdown.”
The primary objective of the full countdown demonstration test is to demonstrate that the brand new EGS launch control system, from ground infrastructure and support equipment to launch control computers and software, can prepare the fully integrated Orion spacecraft and SLS multi-stage launch vehicle for liftoff all the way down to T-9 seconds. The launch control system has to command and control the vehicle and ground systems, maintain all the critical countdown parameters within specific launch commit criteria, and then safely command the ground systems after liftoff.
The Core Stage was thoroughly tested at Stennis during its Green Run campaign, but that was conducted at a different space center than KSC, with a different ground control system. Green Run systems and software also didn’t take into account all the other dynamic elements for a launch; there were no Solid Rocket Boosters, no ICPS, and no Orion spacecraft involved at Stennis.
While in the VAB, NASA will consider that additional work that puts the vehicle in the rollout-for-flight configuration that Blackwell-Thompson described. “When we rolled out for Wet Dress we had accomplished all of the work that was required for Wet Dress Rehearsal, but there was a little bit of remaining work that we knew that we were going to have to perform once we got back to the VAB in addition to the [Flight Termination System] verifications that were required [for launch],” she said on April 18.
“So, when I talk about that middle option it really is finishing out all of the work required for flight, any non-conformances that we picked up out at the pad, any non-conformances that were listed against the VAB work schedule that we didn’t require for Wet Dress but that we knew that we needed to complete for flight.”
Examples of some of that final pre-launch work after the vehicle rolls back from WDR are removing test instrumentation on the exterior of the vehicle that collected data during the rollout and rollback to and from the pad, installing a newer ICPS flight computer, final equipment stowage in the Orion crew module for the mission, and any post-WDR thermal protection system (TPS) maintenance.
Lead image credit: NASA/Ben Smegelsky.
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