By Stephen Trimble
A NASA programme manager confirms aluminium-lithium remains the favoured material to serve as the primary structure for the Orion crew module despite an ongoing, two-year programme to develop a mostly composite alternative.
A switch to a relatively unknown structural material would force NASA to invest in a risky and uncertain certification programme in order to fly a human-rated habitation module in space, says Mike Kirsch, programme manager for the composite crew module programme.
"We don't have that much experience certifying composites," Kirsch says. "So there is some uncertainty in scoping the amount that's required to certify this."
Some NASA officials, including former administrator Michael Griffin, promoted composite materials as a potentially superior materials technology compared to even advanced metal alloys, such as aluminium-lithium.
These supporters launched the composite crew module programme for NASA and its contractors to gain practical experience designing and manufacturing the technology for a human-rated spacecraft.
Last year, Michael Saemisch, a Lockheed Orion team member, blamed the composite programme for delaying the preliminary design review of the aluminium-lithium Orion module by several months.
Composites technology has gained widespread use in the aerospace sector as a stronger material than aluminium that can also be lighter and less expensive to manufacture.
By many measures, the composite crew vehicle designed and manufactured by NASA and an industry team led by ATK has been a success. ATK's composites plant in Iuka, Mississippi, delivered the module to the NASA Langley Research Center on 11 September, says Steve Summitt, ATK's programme manager.
In pressure tests to date, the module has performed "scary close" to analytical predictions, Kirsch says. Structural testing is scheduled to continue until the end of the year.
But the design process also revealed unexpected weaknesses for composites technology. The final design weighs about the same as aluminium-lithium, and is not significantly cheaper, Kirsch says. "I don't think it would be better," he adds.
The NASA design assumed the pressurised structure must remain leak-proof and intact despite sustaining heavy damage, he says. This design requirement added bulk to the structure, negating some of the weight-savings normally expected with composites.
Moreover, the crew module must sustain 31,751kg (70,000lb) loads during the abort sequence, requiring a material that remains strong against loads from multiple directions, whereas composite material is best when needed to be stiff against loads coming from only one direction, Kirsch says."