Protecting SLS from Fire and Ice – TPS foam application proceeding at MAF

by Philip Sloss

NASA and prime contractor Boeing are deep into the application of spray-on foam insulation (SOFI) to elements of the first Space Launch System (SLS) Core Stage (CS-1) and their sister test articles, inaugurating one of the later stages of production at the Michoud Assembly Facility (MAF) in New Orleans, with hopes of completing final assembly of the first vehicle by the end of next year.

Elements and sprays:

Foam sprays on the outside of the forward skirt and intertank are complete, allowing integration work to begin on both elements. Elsewhere at the facility, the first liquid hydrogen tank to go through the foam spray process is nearly ready for foam applications to start, after pathfinding previous phases of manufacturing.

There are three different types of foam applied to different parts of the Core Stage; although similar, each has its own formulation.

For large areas of the vehicle, also called “acreage,” a robotically-sprayed foam is used.  The “auto spray” acreage foam has strict environmental requirements for application; for smaller areas of the vehicle or for areas that may need to be applied in “room temperature” environments with little or no air-conditioning, a manually-sprayed foam is used.

“For the manual spray, the foam is more forgiving on the temperature and humidity environment that it is used in, so that’s what it was designed for — it was designed to do closeouts out in the VAB and out on the pad, with no real environmental control, so it’s better in hot and humid environments,” Amy Buck, thermal protection system expert in the Materials and Processes Lab at Marshall Space Flight Center in Huntsville, explained.

“The acreage foam doesn’t like humidity, so it’s got to be really low humidity — I think it’s like twenty to thirty percent humidity — and it has to be really hot, so it has to be a hundred degrees for that.  So it’s a hundred degrees and then twenty, thirty percent humidity, where the manual spray foam has a much wider box — it can go from 65 degrees to 100 degrees Fahrenheit for the application temperature.”

For SLS, as in the past with Saturn and Space Shuttle, there two VABs — the Vertical Assembly Building or Building 110 at MAF and the iconic Vehicle Assembly Building at the Kennedy Space Center in Florida — and foam application work will be done in both buildings.

The third type is a pour foam that is used for hardware with more complex shapes, like propellant feedlines and valves.  All the hardware that the cryogenic propellant flows through will be insulated.

All three types of foam have two elements that are brought together at the time of application.  The spray foam elements are mixed in the spray gun, whereas the pour foam elements are mixed by hand and then go into molds that are set up around the hardware.

The foam is sprayed on the metal surface (the substrate) until the desired thickness is reached.  “It basically exotherms and puffs out for lack of a better word within seconds and then when it gets to the full thickness that’s when you leave it alone for a couple of hours before you do any tests or anything to it,” Michael Frazier, branch chief for the Non-Metallic Materials Division in Marshall’s Materials and Processes Lab explained.

The “net spray” leaves a textured surface on top called the “rind”.

“Net spray is ‘as applied,’ so it’s got that rind material on it,” Buck also noted.  “That bumpy surface, that’s the rind and so ‘net spray’ just means that rind is still left on.  And either trimmed or shaved would be where we take that rind and we remove that rough section of it.”

The Core Stage consists of five major elements: the forward skirt, the liquid oxygen (LOX) tank, the intertank, the liquid hydrogen (LH2) tank, and the engine section.  The LH2 and LOX tanks which hold the extremely cold, cryogenic propellant, are called the “wet” structures.

The other three are “dry” structures.  With the exception of the engine section, spray-on foam insulation is used for thermal protection up and down the Core Stage, and the application of the foam to the elements comes up in different parts of the production process.

The propellant tanks go through a similar process; after welding, internal outfitting of a few components, and proof pressure testing, they are covered with primer for corrosion protection and foam to maintain the cryogenic condition of the propellant as efficiently as possible.

The dry structures have a more elaborate internal configuration that includes computers, electronics boxes for guidance and navigation and communications, lots of electrical wiring, hydraulic and pneumatic controls, and propellant lines that run between the tanks, the engine section, and the four RS-25 rocket engines below.  The forward skirt and the intertank are sprayed with foam first, then go through internal outfitting.

Some aspects of the Core Stage have their heritage in the Space Shuttle External Tank.  Although similar in application and appearance, the foam composition and application have evolved with the vehicle.  The foam insulates the cryogenic fuels in the propellant tanks, which are hundreds of degrees colder than room temperature, from the outside.

This helps to maintain the propellant temperature in the required range and also keeps the outside of the tank from getting too cold, which would condense and freeze the water vapor in the air into ice; ice is a debris hazard and would add additional weight to the vehicle, reducing overall performance.  During the early stages of ascent, the foam also provides thermal protection from aerodynamic heating.

Forward skirt:

Each of the different types of Core Stage elements goes through its own sequence of foam sprays in different areas at MAF.

Although all the elements are approximately 27 feet in diameter, the forward skirt is the shortest of the elements.  Like the other pieces, the forward skirt was coated with primer first.  The ring panels were primed prior to welding and then rings were circumferentially welded to the top and bottom of the skirt at the end of last year.  Primer was then applied to all the weld lands using rollers.

“For some of the dry bay structures they did go ahead and do the pre-panel priming, like LVSA did and they’re rolling the primer in those weld lands as well,” Buck noted.

The Launch Vehicle Stage Adapter (LVSA) which sits between the Core Stage and the Interim Cryogenic Propulsion System (ICPS) upper stage is now going through this same process of using paint rollers for the primer at Marshall ahead of its foam sprays.

The welded rings at the top and bottom of most of the elements form flanges that will be bolted to the other parts of the Core Stage, which are little trickier to roller paint.  “Yeah, the flanges are universal on these pieces, so everybody’s got to deal with those flanges,” she said.  “The flanges are a little more challenging than flat acreage, but we’re rolling it on them, too.”

The forward skirt was then moved into Cell Q at MAF, where the robotically applied “auto spray” was then applied.  The forward skirt was set up vertically on a turntable for this process.  Buck explained that the auto sprays are done with a single spray gun that is fixed while the outside of the article is rotated in front of it.

“All the SLS sprays are single gun sprays right now,” she said.  “They’ve got the option to switch to a second gun, but it’s in the same location.  If that one clogs, they can put another gun on, but it’s in the same location, there’s no dual gun sprays on this program.”

Information on L2 notes that the foam application was completed in late August, allowing the forward skirt to be moved to its integration area.  “Application and final trimming of the SLS Core Stage 1 Forward Skirt Thermal Protection System (TPS) was completed at MAF Aug 21.

On Aug 24, the FS was successfully moved onto the Final Assembly Jig to begin the mechanical integration of the shelves, harnesses, fluid tubing, vent ducts, avionics, and all the miscellaneous support structure.”


Application of foam for the intertank is more complicated.  The intertank is the heftiest structural element of the Core Stage; the barrel panels are much thicker than the other elements, and are bolted together rather than welded.  As with the forward skirt, the intertank panels were “pre-painted” with primer before being assembled.

Following completion of structural assembly, the flight intertank was moved across Building 103 to Cell G in Building 114 at MAF in mid-June, where it was placed on a turntable in a similar set up as the forward skirt.

Although similar in function to the Shuttle ET intertank, the structure is different.  Buck explained that among the differences is that the SLS intertank has circumferential ribs and the outside where the foam is applied has different geometries.

The intersecting ribs in most of the intertank panels create rectangular pockets, and those are filled first.  “In between the ribs and the pockets is going to be filled with the manual foam and then it’s going to be over-sprayed with the acreage foam,” she said.

As with the LVSA foam work at Marshall which uses the same manual spray foam, Buck said that the same process is used to fill the intertank pockets, with two technicians using a single, portable spray gun in one area with foam.

The two components of the foam are fed from different containers and mixed at the gun when it is sprayed on the substrate.

In contrast to the work area at Marshall, she said the foam spray cells at MAF have more amenities:  “I think they’ve got better access than we do for LVSA.  For LVSA we’ve got a guy in a lift, they’ve got platforms.  I think they were talking about having two teams working on kind of opposite sides of the hardware.”

The intertank panels also vary from one to another; the ones on the side where the Solid Rocket Booster (SRB) attach hardware and the thrust beam connect have more crisscrossing ribs, creating different pocket sizes.

“Some of the smaller ones, they’re filled in one [pass] — actually multiple pockets are filled at a time,” she noted.  “There’s a certain area [size] that you can do manually before you need to break and do a tie-coat line, so it depends.”  The tie-coat is an adhesive used in areas that require multiple spray passes.

Buck and Frazier explained that using the auto spray over the top will give the SLS intertank a similar appearance to the propellant tank acreage.

“So those pockets will all be filled in with foam and then it’ll be a flat acreage spray over the top,” Buck said.  “So it should be a smooth exterior.”

“It’ll be the same color orange across the intertank as it is with the cryo tanks,” Frazier added.

Foam applications and final trimming are complete, and the intertank was moved to Area 55 on the Building 103 floor in early November to begin its integration work.

Propellant tanks:

The two propellant tanks are the largest elements of the Core Stage, and due to their size both primer and foam applications are done with the tanks positioned horizontally.  After the tanks have been proof tested and cleaned, each one will go individually into Cell P for a coating of primer and then Cell N for foam applications.  Both cells are located in Building 131.

After development work done at Marshall and MAF, the processes for the big sprays of primer and foam were demonstrated this year using a LOX tank weld confidence article; the first of the longer LH2 tanks to go through the process is the LH2 qualification tank.

SLS is still in development and in addition to the flight structures, a series of nearly identical qualification / structural test articles are going through most of the same manufacturing, assembly, and production processes — in some cases, the qualification articles are the first to go through the different stages of work.

After welding last year, the qualification tank was proof tested in Building 451 early this year and then brought back to Building 103 for a post-proof non-destructive evaluation (NDE) of the welds.  Then in June, it was the first SLS tank to go through the internal clean cell, Cell E in Building 110.  The tank was removed from the cell in early August, leading to final preparations for spraying primer.

The primer sprays were completed in mid-September and the tank has since been moved into Cell N to be prepped for the foam sprays.  The tanks are lined up horizontally for the acreage sprays, during which the tanks rotate, rotisserie-style, in front of the gun applying the acreage foam.

In addition to the environmental temperature and humidity requirements, the surface the foam is being applied to must also be heated.  “The substrate requirement is that it be 130 degrees Fahrenheit, and that’s for better adhesion,” she noted.  The tanks are heated inside and outside during the spraying process.

Prior to use, the two acreage foam components are kept cooled in the “pot room” in Cell M, adjacent to Cell N.  “[The acreage foam is] a polyisocyanurate foam, so [the components are] a polyol component and an isocyanurate component,” Buck explained.

“Both sides have the blowing agent in it and the new blowing agent that’s more environmentally friendly boils at 65 degrees Fahrenheit, so those materials are required to be kept in cold storage in order to keep the blowing agent in solution.  So that’s why the pot rooms are so important. We heat the material in the lines but it’s pressurized at that point and then it meets in the gun and makes the actual foam as it sprays.”

Similar to the forward skirt and the intertank vertical turn-tables, the tanks will rotate at about two revolutions per minute (RPM) for the acreage sprays.  “For a tank that big that’s still moving pretty fast,” Buck said.  “We build [the foam] up in layers so for the hydrogen tank I believe it’s like five layers, so it’ll be five passes over the same area.  It ‘barberpoles’ down where it overlaps and that’s how you build up your thickness.”

“If you’ve ever wrapped a pole with tape, where you overlap the next piece on the piece that you laid down before, it’s kind of like that,” she explained.  When finished, the foam on the LOX tanks will be about half an inch thick; on the LH2 tanks, it will be over an inch thick.

After the acreage sprays of the barrel sections of the tank are complete, foam will be applied to the hemispherical domes on both ends.  NASA and Boeing opted to do these sprays using the manual spray foam.

“They switched to the manual spray foam because they didn’t want to apply an extra heat cycle to the [acreage] foam,” Frazier explained.

“To get away from that they spray this [manual spray] foam where you don’t have to heat the tank up before you spray onto it, so you could spray the dome without heating the tank article up and you don’t have to expose the barrel to any additional heat.  [The manual spray foam] gives them the ability to not double heat the foam that’s going to be on the barrel so they’re going to manually apply foam to the domes.”

Other thermal protection:

The one major element of the Core Stage that isn’t using foam on its exterior is the engine section; instead, cork will be applied to the outside for thermal protection.  “The cork goes on in sections,” Buck noted.  “[For] the big sheets of cork they apply the adhesive to [it] and then vacuum bag down [to the structure] to cure that adhesive.”

Cork will also cover the base heat-shield and boat-tail elements that will attach to the bottom of the engine section and around the powerheads of the four RS-25 engines.  The cork will be covered with a top coat of white paint to prevent moisture intrusion into the cork.  Buck also noted that the aerodynamic fairings that cover the two sixteen-inch diameter liquid oxygen feedlines (also known as “downcomers”) as they exit the intertank will be covered with cork and white paint.

Although not used on the outside of the engine section, there will be a lot of foam inside of the engine section, covering the maze of Main Propulsion System (MPS) cryogenic lines and valves that help distribute the fuel from the ground spheres to the vehicle tanks and to the stage engines.

“All the cryo lines are insulated, that’s to keep the bulk temperature inside the engine section actually high enough so that the fuels stay at the right temperatures for the engines to be able to start,” she noted.  “It’s very tight temperature control in the engine section for the engine start box.”

Pour foam will be used to cover the lines in the same thicknesses as for the other liquid hydrogen and liquid oxygen elements, including the same primer coat that goes on first.  Although rarely seen, this is also similar to the boat-tail of the Shuttle orbiters, where the engines were attached to the MPS lines, which were also foamed for launches.

“For the pour foam, MAF is actually using a lot of 3D printed molds, so they’ve got the design for those and then they 3D print them,” Buck said.  “It’s a two component foam again, so you mix that up and then pour it into the mold and then it’ll rise and cure up in that mold.”

The MPS lines in the engine section will be welded together in places inside what will eventually be tight confines in the engine section, and so the complex geometries of the winding pipes and valves will be foamed as much as possible outside first.  “As much as they can, because it’s a lot easier to [apply] foam if you’re not in an engine section,” she added.

During final assembly, a number of elements will be attached to the outside of the Core Stage.  Like the engine section propellant lines, the downcomers that run along opposite sides of the hydrogen tank will be covered with pour foam before they are attached and then the joints will be closed out with manual sprays or pour foam where necessary.

The cover for the systems tunnel, which runs the length of the Core Stage and houses electrical and data cabling to connect sensors, electronics, avionics boxes, and computers in the different parts of the vehicle, will be covered with manually sprayed foam before it is placed over the tunnel itself.  “They are trenching out the foam for the systems tunnel,” Buck said.

“So they’ll come in and trim where they’re going to put the systems tunnel and then bolt the systems tunnel in place and then cover the system tunnel with covers.”

Color changers:

Although the foams used for Shuttle External Tank and the SLS Core Stage are different formulations, they are similar in chemistry and both visibly change color over time as they are exposed to ultraviolet (UV) radiation.  “It’s the poly-urethane chemistry, it just discolors when it’s exposed to [UV],” Frazier explained.

“In the past we’ve put some type of UV blockers in [the foam] to try to limit that, but it doesn’t cause any damage to the foam, it just changes the color of it.”

Immediately after application, the foam is a light yellow; however, after long enough exposure to UV radiation, it becomes a dark orange, something seen on many External Tanks that were outdoors for a long time.

Buck noted that at some point, the foam reaches a point where the color won’t change much more.  “It saturates, so once all the surface has turned that orange it won’t turn more orange,” she said.

There are slight differences in the color of the different foams and the last sprays to be done will have less time to darken.

All the main elements of the SLS Core Stage will be bolted together and they all have rings on either one or both ends where they will be joined.  Also called flanges, each one has 360 holes around the circumference where bolts will be installed.  After bolting, those areas will be “closed out” with manual spray foam.

The forward skirt and the intertank will be stacked vertically with the liquid oxygen tank in one of the two vertical integration cells in Building 110.  Separately, the liquid hydrogen tank and the engine section will also be stacked in one of those cells.

Those two sub-assemblies will then be rotated to horizontal and rolled into the final assembly area of the adjoining Building 103, where the flange between the aft end of the intertank and the forward end of the liquid hydrogen tank will be bolted and sprayed.

The last large spray on the Core Stage will be done in the VAB at Kennedy; after the LVSA is stacked and bolted on top of the forward skirt, manual spray foam will be applied over that bolted flange.

Lining up:

Meanwhile, the rest of the propellant tanks for flight and pre-launch testing are lining up behind the LH2 qualification tank to get their TPS applied.

After delays due to engineering issues, a hardware handling mishap, and even severe weather, welding of all the major elements of CS-1 and sister qualification / structural test articles (STA) was completed at the end of the summer.

Welding of the LOX qualification tank was completed on July 6.  While that tank then started into the process of inspections in Cell D of Building 110 and plug welding in Area 6 of Building 103, the hardware for the flight LOX tank was moved into the VAC.  Welding of the flight LOX tank was completed on August 18.

The Core Stage (CS)-1 Liquid Oxygen (LOX) tank completed Vertical Assembly Center (VAC) welding operations at MAF.  The flight article was then transferred to Cell D for vertical integration, dimensional check-out, and baffle installation,” contemporaneous notes from L2 said.

The flight LOX tank moving to Cell D cleared the VAC for the replacement LH2 flight tank hardware to move in and begin welding.

The tanks are welded in the VAC from top to bottom, with the forward dome going into the welder first, followed by the barrels and finally the aft dome.  The thicker LOX tanks have two barrels, the longer LH2 tanks have five.  Welding of the replacement hydrogen tank, the third one through the VAC, was completed on September 15.

As with the flight LOX tank, after VAC welding was completed the tank was moved to an adjacent integration cell, with the LOX tanks going to Cell D and the LH2 tanks going to Cell A.  Both flight tanks were outfitted with an anti-vortex baffle assembly in their aft end.  After a breakover operation to rotate the tanks to horizontal, one by one the tanks were rolled around to the nearby Area 6 for plug welding.

A single hole is drilled into the tanks where the VAC performs each circumferential, self-reacting friction stir weld between barrels and domes.  The pin starts and exits through the same hole, which the plug welding has to seal to make the tanks pressure-tight.

Following plug welding the tanks are prepared to be proof tested, which verifies the pressure integrity of the welded tank.  LOX tank welds are proof tested hydrostatically in Cell F in the VAB at MAF, Building 110.  The flight LOX tank was the first article to use Cell F for SLS; proof testing was completed on October 5.

As with the qualification article before it, the flight LH2 tank was outfitted with sensors to measure stresses during its proof testing, which is done in Building 451 well away from other locations at MAF for safety reasons.

The flight tank was moved to Building 451 in early November.  In contrast to the static proof test for LOX tank, proof testing for the hydrogen tanks is more dynamic, involving a series of test cases pressurizing them with gaseous nitrogen while simultaneously imparting loads to the structure.  It is hoped that the LH2 flight tank proof testing will be complete around the end of the year.

After the flight tanks have completed proof testing, they go through “post-proof NDE (non-destructive evaluation)” inspections.  The welds are re-inspected to compare and assess their condition from before and after proof test.

The tanks will then line up to go through Cell E in Building 110, where their insides are washed clean before heading to be painted with primer ahead of their foam applications.

(Images via NASA/MSFC Michoud: Jude Guidry. And Philip Sloss for, L2).

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