NASA working through wet dress rehearsal, final major pre-flight test for moon rocket

With the arrival of SLS and Orion at Pad-B on March 18, engineers and technicians were busy making final preparations for Sunday’s critical last design verification test: the Wet Dress Rehearsal, or WDR.

The test was scrubbed on April 3 before fueling due to an inability to provide positive pressure to the enclosed areas within Mobile Launcher -1 (ML-1) and was rescheduled for April 4 and then scrubbed that day at T-31 minutes 36 seconds due to a stuck gaseous hydrogen vent valve o the 18 meter (160 ft) level of ML-1.

At this time, NASA has not announced when they will try again to fuel the SLS.

The WDR will validate the vehicle’s readiness for flight as well as all elements of the new Mobile Launcher and pad propellant system upgrades for LC-39B’s liquid hydrogen storage and disposal systems — a multi-year process undertaken by NASA’s Exploration Ground Systems team.

Wet Dress Rehearsal

The test marks a significant milestone as NASA works towards launching SLS on Artemis 1 and is the first (and last) time Launch Director Charlie Blackwell-Thompson and the launch team have to take an actual SLS through a full-duration countdown ending just before the start of the Core Stage engine ignition.

The final part of the wet dress countdown will see the launch team pick up the terminal count at T-10 minutes with a plan to count to T-90 seconds and then hold for three minutes to verify the vehicle’s hold capability constraints that exist after the T-6 minute mark in the countdown.

Overall, according to NASA personnel in across interviews with NASASpaceflight’s Philip Sloss, the team plans two runs through the terminal count, with the first incorporating the three-minute verification hold at T-90 seconds before continuing down to T-33 seconds.

This is the point where the ground computers would hand off control to SLS’s onboard computers.

Here, the count will be stopped and recycled to T-10 minutes to allow the launch team the ability to practice these procedures with the fully fueled rocket.

After recycling to T-10 minutes, the terminal count will begin again and proceed down to T-9.34 seconds — just a few hundredths of a second prior to the start of the start sequence of the RS-25 engines on the Core Stage.

The goal for the team with this second run is to take the count as far as possible without starting the engines.

At this time, a hold will be called, with teams first recycling to T-10 minutes and then officially “scrubbing” the count.

Propellants will then be drained from the Core Stage and ICPS and the Wet Dress Rehearsal concluded — barring any unforeseen issues identified during the test.

NASA will then analyze all the data from the test, and if all is found to be good, the agency may be ready to discuss a launch target for SLS during a scheduled press conference on the day after the wet dress is concluded. 

Current schedules show a no earlier than possible launch date for Artemis 1 of June 2022.

Pad-B’s liquid hydrogen readiness for SLS

While the entire 45-hour 40-minute SLS countdown has been undertaken for the WDR, the most serious part of the test is the complete fueling of the rocket’s Core Stage and Interim Cryogenic Propulsion Stage (ICPS) — a modified Delta Cryogenic Second Stage from United Launch Alliance (ULA) — with liquid oxygen and liquid hydrogen.

While liquid hydrogen (LH2) was used at LC-39B for the Saturn family of rockets and the Space Shuttles, the amount of liquid hydrogen and how it is used differs from the previous two programs, and significant work was needed for Pad-B’s fueling systems to accommodate the SLS.

Originally built in the 1960s, LC-39B’s LH2 storage sphere is capable of holding 850,000 gallons of propellant. But from the beginning, there was a problem: the tank was losing more LH2 per day then design specifications.

By the end of the Shuttle program, it was losing 1,200 gallons of LH2 to boil-off a day.

SLS on LC-39B for its Wet Dress Rehearsal. The pad’s original 850,000-gallon liquid hydrogen storage sphere can be seen to the left. (Credit: Nathan Barker for NSF)

“I happen to be the guy who found the void back in 2003, and it had been a problem with that storage tank from the beginning, from the start with Apollo,” said Mark Berg of NASA’s Cryogenic Propulsion Branch in an interview with NASASpaceflight.

The void referenced was the ultimate cause of the excessive LH2 evaporation in the tank — which was supposed to only lose 640 gallons of LH2 a day per design.  For comparison, Pad-A’s LH2 tank averaged just 317 gallons of LH2 lost per day over its entire 60-year careeer.

The excessive loss on Pad-B’s tank coupled with the “normal loss” of LH2 to boil-off in the storage spheres and the amount loss during replenishment of the storage tanks and launch fueling operations resulted in nearly half the total LH2 purchased for the Shuttle program being lost to evaporation across both the Pad-B and Pad-A systems and tanks. 

Over the entire life of the Shuttle program, 91,749,760 gallons of LH2 were purchased, of which 50,106,880 gallons (54.6% of the total purchased) flew with the vehicles while 41,642,880 gallons were lost to evaporation from both tanks at Pad-A and Pad-B.

The cause of the excessive evaporation from the Pad-B tank — which was not seen on the Pad-A tank — found by Mr. Berg ultimately related to a perlite bulk-fill insulation void in the storage sphere.

Liquid hydrogen use v. loss during the Shuttle program.

(Image credit: “NASA Experience with Large Scale Liquid Hydrogen” presentation at the Hydrogen Liquefaction and Storage Symposium, University of Western Australia, September 26, 2019. Presented by William Notardonato NASA KSC)

“I did some work with an IR camera, and we did ultimately find, when we took the tank out of service [after the end of the Shuttle Program], that there was a large void there. And so we replenished that missing perlite.” 

With the perlite issue fixed, the original LH2 tank at Pad-B was put back into service — after other refurbishment work — with a new loss rate of less than 640 gallons a day, better than the original estimates from 60 years ago when it was first designed and built.

But the storage of LH2 and how much was being lost per day to storage boil-off wasn’t the only item with the pad’s LH2 systems that needed upgrading and reconfiguring for SLS.

In the Shuttle program, a small amount of LH2 pumped into the vehicle was fed into the Space Shuttle Main Engines (RS-25s) to help properly condition them for ignition.

This LH2 was then recirculated back into the External Tank for fuel for the engines. 

To the left, the Tail Service Masts (TSMs) reach out to the bottom of the Core Stage. The TSMs will fuel the stage with LH2 and liquid oxygen. Seen here, the LH2 TSM is visible while the LOX TSM is perfectly hidden behind it. Both commodity TSMs connect to the same side of the Core Stage. (Credit: Nathan Barker for NSF L2)

For SLS, this recirculation ability has been removed to save mass and reduce complexity, so the LH2 that will be fed to the four RS-25 engines of the Core Stage to condition them during the countdown will now need to be safely drained away from the rocket in its liquid form.

This is different from how LH2 in the Core Stage will boil off into a gas and then be vented away from the SLS in secure transfer lines that will move the gas to the hydrogen flare stack where it will be safely burned off. 

The LH2 that can’t be pumped back into the Core Stage also can’t be sent directly to the flare stack — which can only handle gaseous hydrogen. 

To solve the issue of how to safely dispose of the liquid hydrogen after its fed through SLS’s engines, NASA added a 60,000-gallon liquid hydrogen separator tank to the overall hydrogen disposal system. 

“We have a different process than what we used for Shuttle,” said Mr. Berg. “And as a result, we will have more hydrogen than we will need to dispose of through the flare stack, and the separator helps with that process.”

“We’ll have a lot more hydrogen that will be obviously not going into the propellant tanks. It’s going to have to be disposed of, and we cannot send liquid directly to the flare stack based on a specification it has. We can only send gaseous hydrogen up to a specified rate.”

“So the separator allows for that; it allows the liquid hydrogen to pool inside of there and then as it boils off in the separator, it can proceed on to the flare stack.”

And this adds to an issue surrounding the original LH2 tank and fueling systems at Pad-B: the 850,000-gallon tank is, in general, not large enough to support a 24-hour turnaround between launch attempts for SLS.

This is because the amount of hydrogen lost during a full fueling and countdown does not leave enough LH2 for a second full attempt the next day.

The two LH2 storage spheres at LC-39B. The new 1.25 million gallon tank is on the right while the original 850,000 gallon tank is on the left. (Credit: Nathan Barker for NSF)

Instead, NASA will have to replenish the LH2 lost during a fueling attempt before trying again — though this is somewhat dependent on when, after fueling begins, a launch attempt is scrubbed.

The replenishment plan for re-filling the LH2 tank will involve bringing LH2 already stored at ULA’s SLC-37B pad on the Cape Canaveral Space Force Station where the Delta IV Heavy is launched.

The LH2 that will be brought from ULA’s storage area is propellant NASA has purchased and made an agreement with ULA to store in their tank.

To solve this LH2 capacity issue going forward, NASA is in the process of constructing a larger, 1.25 million gallon LH2 tank at Pad-B — 47% larger by usable volume than the original tank — that will work together with the existing 850,000-gallon tank.

“It’s very far along, and we’re pulling the vacuum on the tank,” said Mr. Berg. “There’s a bit more electrical work that needs to be done for the infrastructure that goes along with the tank. We need to have the new vaporizers installed. And ultimately we’re going to need to tie in the transfer line to the existing transfer line.”

(Image credit: “NASA Experience with Large Scale Liquid Hydrogen” presentation at the Hydrogen Liquefaction and Storage Symposium, University of Western Australia, September 26, 2019. Presented by William Notardonato NASA KSC)

But that will be something that will have to wait until after the Artemis 1 launch.

While the new tank won’t be ready in time for Artemis 1, it will greatly aid SLS launch opportunities for Artemis 2 and beyond.

In all, the amount of LH2 that will be handled during an SLS countdown and for the Artemis Program, in general, is greater than during Shuttle and Apollo operations. But the safety and mitigation strategies in place are largely the same. 

As Mr. Berg explained, “We’ll have the same types of mitigations. We’ve got an excellent hazardous gas detection system. And with hydrogen, we try to limit the number of locations where there’d be like a flange, any kind of mechanical joint like that.”

“But anywhere we do have that, we do have a very good hazardous gas detection system.”

The three human orbital launch pads in the United States. Left to right: ULA’s SLC-41 for Atlas V/Starliner, SpaceX’s LC-39A for Falcon 9/Dragon, and NASA’s LC-39B for SLS/Orion. (Credit: Nathan Barker for NSF)

These systems will help the launch team ensure that hydrogen isn’t leaking out into the ambient atmosphere around the rocket and launch pad.

Overall, the Wet Dress Rehearsal will be the first time since December 9, 2006, that the pad’s LH2 systems have been used to fuel a rocket, though the system remained fully operational through May 2009 when it would have been needed to launch Shuttle Endeavour on a rescue mission for Atlantis’ STS-125 Hubble crew.

The upgraded LH2 systems were last tested with the Mobile Launcher — but not with a rocket — in 2019.

(Lead image: SLS on LC-39B shortly after the start of Wet Dress Rehearsal countdown operations on April 1, 2022. Credit: Nathan Barker for NSF)

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