NASA is advancing state-of-the-art technology for thermal protection with more environmentally friendly materials and 3D printed molds for smaller parts. The agency says that extreme temperatures—ranging from -423°F to more than 200°F—call for novel thermal protection systems on the Space Launch System (SLS), its new heavy-lift rocket.
Spray-on foam insulation, along with other traditional insulation materials such as cork, will provide thermal protection for every rocket part, large and small. The insulation is flexible enough to move with the rocket but rigid enough to take the aerodynamic pressures as SLS accelerates from 0 to 17,400 miles per hour and soars to more than 100 miles above Earth in just 8 minutes. The cryogenic fuel, made up of liquid hydrogen at -423°F and liquid oxygen at -298°F, that powers the rocket has to stay extremely cold to remain liquid. If temperatures rise too high, the fuel would become a gas.
“As the SLS flies, it builds up tremendous heat. Without insulation, heat from launch would affect the stability of the cryogenic propellants and the rocket’s structural integrity would be compromised,” says Michael Alldredge, who leads the thermal protection system team for the SLS core stage at NASA’s Marshall Space Flight Center in Huntsville AL. “NASA is asking this unique foam material to do the incredible job of protecting critical rocket systems, which vary from large structures to electronics and fuel lines, in an unforgiving launch environment with extreme temperatures and pressures.”
Materials engineers qualified the third-generation, orange-colored spray-on foam insulation to meet the harsh environments that the SLS will experience. At the same time, they made the foam more environmentally friendly, according to NASA. The foam insulation is composed of two liquids—isocyanate and a special polyol blend—that stay separate in the pumping system and mix in the spray gun before releasing and rising into foam, similar to hair mousse. When the foam is applied, it gives the rocket a light-yellow color that the Sun’s ultraviolet rays eventually “tan,” giving the SLS core stage its signature orange color.
Spray the Big Stuff
Foam will protect the larger of the hardware, including the entire SLS core stage—the 212-foot-tall backbone of the rocket. The foam is applied with robotic or hand-held spray guns, and, much like painting walls in a home, hardware has to be primed and taped off before spraying begins. Primer serves as corrosion protection from the environment and enhances the bond between the insulation and the rocket.
Engineers will use a robotic system to apply both primer and foam to the cryotanks at NASA’s Michoud Assembly Facility in New Orleans where the core stage is being built. The manually-sprayed foam will cover the domes, or bottoms, of both cryotanks. The largest piece of SLS hardware built at Marshall, the launch vehicle stage adapter, which serves as a connector between the core stage and the interim cryogenic propulsion stage will have manually-sprayed foam.
The original plan was to use cork for the SLS launch vehicle stage adapter, according to Amy Buck, Marshall’s launch vehicle stage adapter thermal protection systems lead, but the team determined that foam would be more efficient. “The foam is lighter,” she says. “And since we have the resources to spray it by hand at Marshall, we are saving time and money because we don’t have to ship it to Michoud. We spray on the foam at Marshall at the same time the core stage pieces for the first SLS mission get their foam applied at Michoud.”
It will take about three months for the entire foam application process, according to Buck, with the prep work taking longer than the actual spraying. The hand-spraying, she says, will only take about 30 minutes for each 4-foot-wide section.”
3D Printed Molds Help Protect Smaller Stuff
Engineers will use insulation to protect many small parts of the rocket that play big roles. One is the avionics, or “brains” of the rocket, that are located throughout the vehicle. But other small parts like the intertank’s exterior pockets, the engine section’s internal ducts and close-out areas of hardware—where two major pieces connect—will require manually-sprayed foam or foam cast with 3D printed molds.
“NASA is using a novel 3D printing process to make customized molds for certain parts,” says Alldredge. “Some parts have unique geometries or are in locations in the rocket where it is difficult to cover them with spray foam. The 3D printed molds allow us to shape insulation to protect specific parts.”
Small hardware like internal fuel systems and brackets on the feedline that run along the outside of the core stage and connect it to the engine section need pour foam. The foam is mixed and poured into a mold before it expands to fill the shape it enters.
Put Some Cork on It
Cork will also be used. It is heavier than foam but provides even stronger protection for certain applications. Cork comes in sheets and is applied to areas that have high predicted heat loads, like the core stage engine section, that houses four RS-25 engines to produce 2 million pounds of thrust. Engineers will apply cork under the solid rocket boosters that provide 75 percent of thrust at liftoff and on the fairings, the areas where feedlines come out of the intertank and run down the rocket to connect the intertank to the other hardware.
After thermal protection material density and adhesion are verified for both foam and cork, engineers take thickness measurements to ensure the required amount of thermal protection has been applied. Overall thermal protection systems thickness for SLS ranges from about a half-inch to two inches. The launch vehicle stage adapter requires 0.7 inches of foam while the hydrogen tank requires around 1.2 inches because of its extremely cold temperature. The final system-level test of the insulation, prior to flight, will be when the entire core stage will be tested with all four RS-25 engines firing, and the foam and cork guarding the hardware as hot and cold collide.