Engineers at Rockwell International Corporation's Space System Division have developed a new method of insulating composite structural material that will stand up to the harsh environment of space. The new liquid-hydrogen cryogenic tankage proposed for advanced launch systems — such as the Reusable Launch Vehicle and the X-33 — will be made from a graphite/epoxy material. Although this composite material will produce a lighter-weight cryogenic insulation tank, current cryogenic insulation materials did not endure rigorous stress testing.
&Conventional cryogenic tankage (such as the external tank of the space shuttle, and those used on the Saturn SII, SIVB, and Atlas Centaur) are commonly made of aluminum or stainless-steel alloys. Cryogenic insulation materials for these types of tanks consist of monolithic forms of polyurethane foam or an organic spray-on or bonded insulation. These rigid insulation materials proved to be incompatible with graphite composite material at –423 °F ( –253 °C) — the cryogenic temperature of liquid hydrogen.
Rockwell engineers called upon previous experience with bonding honeycomb cores to graphite fiber/resin matrix composites, knowing that honeycomb core materials will allow for subtle shifts in the composite and insulation material without bond or shear failure. This "flexibility" is possible because of the accordionlike construction of the honeycomb core and good adhesion to the composite material.
Cryogenic-tank insulation systems for future launch vehicles also called for insulation that could withstand a maximum 400 °F (204 °C) at the thermal-protection-system interface. Additionally, the core/foam/composite interfaces at the tank wall must be sealed to prevent cryo-pumping.
Because no current pour or spray-in-place insulation materials were available to meet these requirements, engineers decided to combine a 2.0-lb/ft3 (32-kg/m3) polymethacrylimide foam sheet (Rohacell 31A or equivalent) with a 2.0-lb/ft3 (32-kg/m3) polyurethane spray foam (PDL 1034-141B or equivalent) for temperatures less than 300 °F (150 °C).
This foam combination is accomplished by pressing the Rohacell 31A sheet into the knife edges of one side of the honeycomb core like a cookie cutter. The foam is pressed to the predetermined 300 °F (150 °C) interface. The partially filled core is then inverted, with the bare cells facing up. Next, the Rohacell 31A foam surface is edge-sealed and vacuum-chucked to a curved tool surface representing the curvature of the tank wall. The open cells are then filled with polyurethane spray foam using standard Gussmer pumps and Binks mixing gun equipment. The excess foam material is machined flush with the core cells to the tank contour while still under vacuum from the spray-foam operation. This insulation assembly is then inverted, vacuum chucked to the contour of the convex tool surface, and machined to the final interface contour and thickness.
The completed insulation panels are bonded to the tank-wall surface with a polyurethane adhesive that cures at room temperature (Crest 212 modified with silane Z6040 coupling agent or equivalent). The bonding process involves vacuum-bagging the insulation panel directly to the tank wall and curing the adhesive for at least eight hours prior to releasing the pressure. The total cure time of the adhesive at room temperature is 48 hours.
The process of forming and testing this advanced cryogenic insulation system has been recorded on a videotape, titled "Composite Hydrogen Tank Foam Process for SSTO," tape number 1-9506-09.
This work was done by Jeff D. Eichinger, Richard G. Jackson, John (Jack) S. Jones, Conley S. Thatcher, and Dave Wittman of Rockwell International Corporation for the Marshall Space Flight Center.