When NASA originally considered employing composites in manned spacecraft, it had conflicting considerations. On the one hand, there were concerns that composites might have an unacceptable leak rate and insufficient damage tolerance. On the other hand, composites potentially offered lots of benefits, including reduced weight and lower lifecycle costs. The most appealing aspect of applying composites to the crew module primary structure was a potential 10 to 15% reduction in weight on complex shapes compared to its aluminum counterpart. In space travel, where every additional ounce of weight drives costs skyward, this weight reduction would have a profound effect on payload capacity and mission expense.

The potential of advanced composites was compelling, so the NASA Engineering and Safety Center (NESC) at NASA's Langley Research Center (Hampton, VA) was charged by Mike Griffin (then NASA administrator) with putting together a team of government and industry structures experts to gain experience in making use of new composite construction and inspection technologies specifically for manned spaceflight structures. One of the primary goals of the program was to determine which composite materials are best suited for future NASA spacecraft, for such things as lunar landers, habitation modules, and launch vehicles. Another goal of the project was to gain experience putting together an organizationally flat, collaborative and geographically dispersed team that could work together effectively. Nine of the 10 NASA sites around the country contributed to the project as well as a number of significant aerospace companies, including ATK, Lockheed Martin and Northrop Grumman.

The team considered nearly a dozen concepts and decided to develop the Composite Crew Module (CCM), a primary structure, a stiffened honeycomb sandwich of carbon fiber. It is composed of upper and lower pressure shells spliced together to help meet an accelerated schedule and keep non-recurring costs under control (a mass produced pressure shell would likely be one-piece using multi-part extractable tooling). Further strengthening the shell are gussets, panels, and various metallic fittings to distribute point loads. The lower shell is stiffened by the floor backbone forming a unified structure which carries pressure and inertial loads via bending.

“Back-of-the-envelope calculations predicted that this concept would reduce the mass of the lower structure by 20% over a traditional ring frame pressure head design,” said Ian Fernandez, lower structure lead at NASA Ames Research Center (Moffett Field, CA). “The concept was verified by finite element analysis and we went with it.”

To help develop the CCM the team deployed FiberSIM composites engineering software from VISTAGY “It’s a big step to go from what’s in Pro/ENGINEER Wildfire to the manufacturing floor and then layup,” said Mike Kirsch, CCM project manager for the NESC. “The fidelity between what we saw in Pro/E and how it translated in terms of wrinkles, ply angle and flat patterns was the true test of FiberSIM.”

“One of FiberSIM’s strengths is defining individual segments of plies, often referred to as ‘flags,’” said Fernandez. “It calculates how big and what shape the flags can be before they become too difficult to conform to the tool.” FiberSIM can display important features of a ply, such as splices, darts, boundaries, local coordinates, warp angle, etc. to help engineers build the best possible part. “We paid special attention to minimizing overlaps in fit-up critical areas to prevent any issues down the road during assembly,” said Fernandez.

FiberSIM then calculates what shape the flat pattern needs to be and exports that data to manufacturing for the NC cutting machine. Another critical FiberSIM capability is generating data to drive accurate laser projections of the flag boundary. “Without this boundary projection on the tool, determining the location of the flags would be a painfully slow process and quality would be degraded,” said Fernandez. FiberSIM was also used to produce laser projection files to help locate and trim the core (in place on the tool) and also to create an inspection grid on the skins.

“I don’t think it would be economical to construct something like the crew module without FiberSIM,” said Kirsch. “You could build a simplified version for the same price, but mass and quality would take a big hit, driving operational costs way up. For state-of-the-art applications like this where every aspect is critical, FiberSIM was the perfect solution.”

Simulation generated by FiberSIM showing how fibers deviate from the specified orientation as a ply of composite material is draped over a tool for making the NASA Composite Crew Module. The areas highlighted in white indicate fibers whose orientations fall within an acceptable range from specification while the yellow and red areas indicate where fibers mildly (yellow) or significantly (red) deviate. FiberSIM enables users to understand the behavior of continuous fiber reinforced composite materials as they conform to complex curvature, ensuring that stiffness and strength requirements are met and validating that the manufactured part matches the design intent.

Fabricators at ATK (Iuka, MS) laying up plies of composite material to create the inner honeycomb sandwich skin of the NASA crew module. The plies are being laid up with the assistance of a laser projection system driven by FiberSIM software.