Here is a brief list of materials that NASA will not be using to construct spacecraft: wood, adobe, fiberglass, bone. While it might be obvious why these materials would not make for safe space travel, they do share a common characteristic with materials that may well be the future foundation of spacecraft design: They all are composites. Formed of two or more unlike materials—such as cellulose and lignin in the case of wood, or glass fibers and plastic resin in the case of fiberglass—composites provide enhanced mechanical and physical properties through the combination of their constituent materials. For this reason, composites are used in everything from buildings, bathtubs, and countertops to boats, racecars, and sports equipment.
NASA continually works to develop new materials to enable future space missions—lighter, less expensive materials that can still withstand the extreme demands of space travel. Composites such as carbon fiber materials offer promising solutions in this regard, providing strength and stiffness comparable to metals like aluminum but with less weight, allowing for benefits like better fuel efficiency and simpler propulsion system design. Composites can also be made fatigue tolerant and thermally stable—useful in space where temperatures can swing hundreds of degrees.
NASA has recently explored the use of composites for aerospace applications through projects like the Composite Crew Module (CCM), a composite-constructed version of the aluminum-lithium Multipurpose Crew Capsule. The CCM was designed to give NASA engineers a chance to gain valuable experience developing and testing composite aerospace structures.
Composites are an increasingly significant part of terrestrial applications, from transportation (the largely plastic composite airframe of the new Boeing 787 Dreamliner, for example) to medicine (composite prostheses among other uses). As the composites industry has matured, says Brian Jensen, materials scientist at Langley Research Center, NASA “started an effort to not only improve the performance of composites, but also to lower the cost.” Cost is a major challenge to composite manufacturing. Composites are traditionally fabricated by layering prepreg materials—sheets of reinforcement material, such as carbon fiber, already pre-impregnated with the bonding resin—in a mold, either by an automated process or, often, by hand. The mold is then placed in an autoclave, a pressurized oven, to cure. Large composite structures like parts of rockets, airplane wings, and wind turbine blades require massive, extremely expensive autoclaves for curing.
To help develop less costly, out-of-autoclave composite manufacturing techniques, Langley partnered with Accudyne Systems Inc., of Newark, Delaware, through the Small Business Innovation Research (SBIR) program. Through this partnership, Accudyne applied its expertise in creating custom automation equipment for composite manufacturing to develop an automated thermoplastic tape laydown head. Unlike a thermoset composite, which once cured cannot be reshaped or reprocessed, a thermoplastic can be reheated for remolding or to heal flaws in its structure. The Accudyne device deposits preheated strips of the thermoplastic materials, welding the strips together through heat and pressure to build up structures layer by layer—eliminating the need for an autoclave.
“It’s been a dual effort between Langley and Accudyne, where Langley is trying to develop the thermoplastics that work well, and Accudyne has been fine-tuning the heated head to make this process happen in a reasonable time and yield a high-quality part,” Jensen says.