NASA’s Space Launch System (SLS) and Exploration Ground Systems (EGS) programs are working with Core Stage prime contractor Boeing and prime test operations and support contractor Jacobs to finish repairs to thermal protection system (TPS) spray-on foam insulation on the Artemis 1 Core Stage at the Kennedy Space Center in Florida.
The TPS foam, cork, and foil performed well during the final, critical test cases of the Green Run campaign in mississippi, which culminated in three propellant loading cycles, two test-firings, and one full duty cycle — all firsts for the SLS program. Now that the stage is out of the test stand and indoors in the transfer aisle of the Vehicle Assembly Building, teams can reach all the spots on the exterior where the foam insulation needs repairs; once those repairs are complete, EGS and Jacobs can move into preparations for stacking the Core with the two SLS boosters.
TPS tested during Green Run tanking and test-firings
The Boeing-built Core Stage is a ground-started sustainer stage that fires for over eight minutes and is lifted out of the low atmosphere for the first two minutes of launch with the high-thrust assistance of two large Northrop Grumman Solid Rocket Boosters (SRB). Spray-on foam insulation (SOFI) covers equipment in the Core that stores and delivers the cryogenic liquid oxygen (LOX) and liquid hydrogen (LH2) propellants used by the stage’s four Aerojet Rocketdyne RS-25 engines.
The recently-completed Green Run campaign in the B-2 Test Stand at Stennis was a unique chance to “fly” the expendable stage in place on the ground and gather an extended data set beyond what could be collected during launch. The test campaign was also the first opportunity to fuel a Core Stage flight article under countdown conditions and fire it under launch conditions, which provided a first opportunity to evaluate the performance of systems like the TPS.
“We were very happy with how the TPS performed for the vehicle, both internally and externally,” Michael Alldredge, NASA SLS TPS Subsystem Manager, said in an April 29 interview. “Overall it was great.”
“Any time you do a first-time test, you learn some things about the design, you learn some things about how your assumptions matched up to reality, and so we did pick up a couple of things that we had to go do some repairs on overall. But in the grand scheme of things, that’s minor compared to the [performance across the] rest of the vehicle.”
Credit: NASA/SSC.
(Photo Caption: As seen from a NASA drone, Core Stage-1 fires during the second Green Run Hot-Fire test on March 18. The 500-second firing was the final test of the campaign that saw Boeing take the vehicle through three full tanking cycles and two test-firings, which also provided data on the actual performance of the stage’s thermal protection system to compare with analytical predictions.)
The SOFI that covers the outside of the propellant tanks and the upper four-fifths of the stage provides a distinctive orange look, but the foam also insulates all the cryogenic LOX and LH2 propellant lines running inside, from ones that fill and drain the vehicle during countdowns to ones that feed the propellant from the tanks to the engines during launch.
“We were very happy with that as well,” Alldredge said. “We saw a couple of little things [where] we probably were a bit overconservative with our models.”
“But again, when you’re doing the math up front, you have to be conservative because if you don’t, you wind up setting yourself up for problems down the road. So it’s much easier to take things away than to say ‘oh gosh we didn’t put enough on there’ to begin with.”
Refurbishment work best done while horizontal in the VAB
Part of the overall SOFI TPS repair work stems from the three full tanking cycles on the Core Stage during the Green Run at Stennis. The cryogenic propellants cause the structures to contract when they are very cold and then expand again after the tanks are drained and they warm back up to ambient temperature.
“The first tanking cycle is where we had the full duration Wet Dress [Rehearsal], and then we cold soaked for two and a half, three hours, which is a long time to sit,” Alldredge explained. “We did see a number of cracks that did show up [then], some of which didn’t get any worse as we got into Hot-Fire 1. And then, Hot-Fire 2, some of [them] did get a little bit worse.”
Most of the areas that need to be addressed are at the flange between the intertank and the LH2 tank. The flanges are where the five major structural elements of the Core Stage are bolted together during assembly.
Credit: NASA/Isaac Watson.
(Photo Caption: Scaffolding is set up around Core Stage-1 in the VAB low-bay transfer aisle on May 5. The stands and environmental enclosures provide access to perform repairs to areas of the spray-on foam insulation (SOFI) on the sides and top of the exterior of the stage. Work to install Flight Termination System components was also performed in parallel as another task that needed to be completed before the stage can be stacked vertically with its boosters.)
A specific formulation of SOFI is manually applied to cover the area to keep as much ambient heat from the outside away from the cryogenic propellants in the tanks and feedlines. “The LH2 [tank] to the Intertank flange is where we saw the bulk of the damage,” Alldredge said.
“What you’ve got there is a very dynamic situation because you have this pressure vessel, the hydrogen tank, [which] wants to bulge, it wants to push out [when pressurized]. It’s also contracting from the LH2, but then you’ve got the intertank right there which is a very stiff structure.”
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“So you kind of get this ‘heel-toe’ action at the flange, and so you see a lot of dynamic energy right there in and around that flange. And so we did pick up a good bit of cracks there, which is not uncommon,” he explained. “We expected that to be a potential, and it showed itself. But it was only on one side of the vehicle and not all the way around — which we attributed to the weep holes.”
During tanking operations, the forward skirt, intertank, and engine sections are purged with nitrogen gas. When the LH2 tank is filled, some of that gas in the bottom of the intertank is liquified by the forward dome of the LH2 tank which takes up almost half of the intertank’s internal volume.
The weep holes allowed that condensation to drain out of the narrow area between the outer edge of the LH2 dome and the intertank wall where it would otherwise collect. “We had eight weep holes around the flange,” Alldredge said.
“It’s very cold in there, so you get the cold LN2 (liquid nitrogen) that’s condensing in that volume. And it sits down in that crotch where you have the dome of the hydrogen tank and the wall of the intertank, and so you start getting liquid in there. The weep holes were there to allow that to flow out instead of sitting in there.”
Credit: NASA/SSC
(Photo Caption: The top of the Core Stage is seen in the B-2 Test Stand at Stennis Space Center during removal operations in late-April. The lighter-colored rings denote foam closeouts of the flanges between the major elements of the stage. The bottom-most flange seen in the image, between the intertank (above) and LH2 tank (below), is where a lot of cracks were seen during Green Run tankings.)
The cold liquid nitrogen condensate coming out of the weep holes would then run over the closeout foam sprayed over the intertank-LH2 flange. “You had this constant flowing of liquid, and [it was] more pronounced on the south side of the rocket, the zero-degree side of the rocket than [it was] on the north side of the rocket, the 180-degree [side],” Alldredge noted.
“So we saw the bulk of our cracks in that flange on the south side on the zero degree side, which we attributed to that constant flowing of liquid nitrogen out of those weep holes running across that foam. It’s a cold-cold state, so you don’t give the foam anywhere to really relax to. So you had a very cold, very rigid, set of conditions for that foam, so we saw a lot of cracks there.”
In addition to making the repairs to the foam around that flange, NASA and Boeing decided to plug the weep holes going forward. “So the [Green Run] Hot-Fire 2 is the last time you’ll see a weeping Core Stage,” Alldredge said.
“When we get out here to do the Wet Dress [Rehearsal] at Kennedy and we go to fly, that joint will fill up with ice, which means that you won’t have that much of a thermal dynamic condition. So that should mitigate the cracking problem we saw with the flange TPS.”
Now that the Core Stage is in the Vehicle Assembly Building (VAB) at the Kennedy Space Center (KSC), TPS refurbishment is the remaining major task before it can be rotated to vertical, lifted into the High Bay 3 integration cell, and mated to the SLS boosters that are already stacked on the Mobile Launcher. The primary areas of TPS refurbishment/repair are to the SOFI.
