The NESC Composite Crew Module (CCM) team is chartered to develop a Crew Module (CM) design tailored to composites and to characterize the design drivers such as geometry, mass, manufacturability, inspectability, repairability, damage tolerance, crashworthiness, micro-meteoroid and orbital debris, and radiation shielding. The CCM team has constrained their scope to retain the reference design outer mold line, maintain the inner mold line within 1.5 inch of the reference design, and to maintain the interface points at the Launch Abort System and the Service Module. This is a paralell effort to the NASA and Lockheed Martin metallic crew module (CM) referred to as Orion will be launched and interfaced with other hardware modules. pictured in Fig. 2. The composite crew module (CCM) is being designed to the same loading environment, baselined mid year 2007, as the metallic crew module. A primary intent by NASA is to gain experience designing, analyzing, and testing flight weight composite structures for potential future space missions [1,2,3,4].

Fig. 2, Left - the metallic crew module with the aeroshell and heatshield shown. Right - a cutaway view of the metallic pressure whell and heat shield carrier panel.

This team has developed three concepts: geometrically stiffened laminate, stiffened sandwich (utilizing the aluminum-lithium aeroshell) and monocoque, design analysis, and sizing iterations, but none were optimized with respect to mass or manufacturability. Each concept had a different level of design maturity, and all three had less definition than the reference Crew Exploration Vehicle Project aluminum-lithium design. Comparing estimated mass values at this stage yield rough order of magnitude values at best. The design did not include analysis of landing or dynamic loads, reusability, thermal loads, subsystem packaging and integration, or development costs and schedule implications of incorporating a composite solution.

Fig. 3, The Composite Crew Module project is an alternate design of the pressure shell and maintains the essential design intent and interfaces of the baseline crew module illustrated in Fig. 2. The figure insert in red shows how the pressure shell fits into the existing aeroshell.

The analysis methods are performed with HyperSizer® software. This tool has resulted in significant design-cycle time reduction from software integration and analysis automation that has resulted in the ability to analyze a large number of design configurations of the CCM. There are several benefits of HyperSizer integration on the CCM design and analysis process, along with risk reduction from the use of final analysis methods earlier in the design process.

This project, like any other aerospace design, is making use of composite material’s strength and weight efficiency, and flexibility of fabrication. To gain the most benefit with composites, engineers perform many trade studies to explore the design space and find an optimum set of panel concepts, dimensions, and layup stacking arrangements, referred to as sizing optimization. The need in their set of analysis tools is to evaluate many design alternatives very rapidly and with enough analysis fidelity to discern true differences in performance in competing vehicle configurations and design features.

Fig.5, Different composite aft dome floor architectures shown in gray color: a) conventional OML shape, b) plastic coke bottle geometrically stiffenend shape, and c) the selected lobed membrane shape which attaches to the backbone configuration . Composite materials permit the convenient fabrication of these geometrically efficient shapes.

To accomplish this level of composite specific analysis automation, NASA is using a two part combination of software tools. The first software is FEA. The FEA packages used on this project are NEi Nastran, NX/Nastran, MSC/Nastran, and Abaqus. The other type of software, HyperSizer, is used to perform most of the composite analysis and sizing optimization. First developed by NASA in the late 1980’s, HyperSizer has been commercially developed and sold by Collier Research Corporation since 1995.

HyperSizer incorporates almost all composite analyses required for aerospace structures in a comprehensive user interface that couple very tightly the individual analyses and their corresponding margins-of-safety stress reporting. Starting with importing FEA computed internal element unit forces from the global finite element model of the vehicle’s panels and beams, HyperSizer solves for hundreds of different failure modes very rapidly using material allowables and its failure criteria that are specifically correlated to test results. Its rapid analyses allows full vehicle models to be analyzed to hundreds of load cases while also including stress/strain gradients from local detail effects.

Fig. 7, Areas of sandwich panels (green) and solid laminates (pink). The dark lines define the ply drop offs.

During the progression of design and analysis maturity, three major classifications of analyses were defined:

• Analysis for sizing optimization
  • Architectural trade studies
  • Optimum honeycomb sandwich design
  • Optimum composite layups
• Analysis for failure margins-of-safety for acreage areas
  • Panel buckling
  • Composite strength failure and damage tolerance
  • Sandwich specific: facesheet wrinkling and core shear
• Analysis for fabrication/manufacturing features
  • Those planned early: Cutouts, sandwich ramp downs and laminate ply drops
  • Those unplanned that become known later: Fabric ply overlap regions, fiber angle alignment

The following failure analyses for the design of the full scale NASA Composite Crew Module (CCM) were provided by the HyperSizer software with automated preliminary design sizing and final margin-of-safety stress reporting:

• Composite Material Strength
• Sandwich Specific Analyses
• Ply Drop-off Transitions
• Bonded Joints
• Bolted Joints

The CCM team has recommended that NASA continue the composite CM structural design and include fabrication manufacturing and tooling expertise in a collaborated environment.

The figures below are photographs taken early spring 2008. The CCM full-scale fabrication has begun.