Spinoff is NASA’s annual publication featuring successfully commercialized NASA technology. This commercialization has contributed to the development of products and services in the fields of health and medicine, consumer goods, transportation, public safety, computer technology, and environmental resources.

Spacecraft fuel tanks have always been round because they need to hold as much fuel as possible under as much pressure as possible with a minimum of material, and the best shape to balance those factors in pressure vessels is a cylinder. But even though round containers are stronger than square ones, which have weaknesses at their corners and edges, they’re still a less efficient use of space. What if you could build a cube that could maintain the same pressure as the cylinder, without adding much mass?

Ted Swanson, a retired NASA Goddard Space Flight Center senior technologist, explained that it can be done with 3D printing. “You can maintain the pressure by building internal struts and it’s all built at once so there’s no welds.”

And there is another benefit to 3D printing: portability. “When we’re up in space, we don’t want to carry a storehouse of stuff. We’d like to be able to make anything we want.” But getting to that point will take some work. Compared to traditional manufacturing, much less is known about the materials, vendors, equipment, and potential pitfalls of the process.

NASA isn’t the only federal agency looking at 3D printing. The Air Force is interested in having a reliable source of spare parts in remote locations without waiting for them to be flown in. Recognizing that widespread interest, in 2012, the Obama administration established a public-private partnership called America Makes to advance research in and applications for additive manufacturing — NASA was an original partner.

At a kickoff event for America Makes, Swanson met Larry Varholak, then head of Oxford Performance Materials (OPM) Aerospace and Industrial, who was interested in a thermoplastic called polyether-ketoneketone (or PEKK) for 3D printing. Swanson immediately thought the material could be great for space “because it’s a very high-grade polymer. It can be used for selective applications — if we know its printed properties and we can be assured that it’s manufactured in a reproducible and predictable fashion.”

Varholak teamed up with Northrop Grumman, which was already working on adding PEKK to the materials it uses for printing airplane parts. This led to Northrop Grumman’s current formulation of PEKK mixed with discontinuous carbon fibers that help dissipate electrostatic charge. Northrop Grumman proposed research and testing of printed PEKK, with the material supplied by OPM and another company to America Makes. The proposal included testing for some 3,000 data points; of those, nearly 1,000 were suggested by NASA to qualify the material for space including cryogenic tests.

Another benefit of additive manufacturing is that it allows engineers to combine parts together into a single component rather than joining multiple pieces. Hexcel Corporation printed this piece using the NASA-tested PEKK material and was to be used for ducting.

The division at OPM has since been sold to Stamford, CT-based Hexcel Corporation. The printed material performed extremely well: among other results, it showed almost no outgassing and was resistant to deterioration from radiation, which made it a good material for space. It’s also a 600-degree melt plastic, which means it is able to withstand temperatures from 300 °F below zero to 300 °F above, and in a fire, it is flame-retardant with very low smoke generation and toxicity.

Although the printed parts are not quite as strong or durable as ones machined from a solid using traditional manufacturing, the results are very close and the benefits of 3D printing outweigh the difference for many applications.

The comprehensive test results enabled the company to demonstrate to customers they could have confidence in the reliability and properties of the final product. Said Varholak, “We won a major contract from Boeing to build parts for the Starliner,” a new commercial crew spacecraft. Henkel built 600 parts for that spacecraft including important structural elements such as brackets to hold fuel lines, as well as components of the life support systems inside the crew capsule.

Northrop Grumman, with Hexcel as a key supplier, has also been using the PEKK-carbon fiber composite for parts for ground systems and then expanded to parts for military aircraft. The company is also developing it for space applications. Commercial aircraft will be next, with Hexcel already currently qualifying its proprietary HexPEKK powder or alloy and the associated proprietary HexAM part fabrication process for commercial airliners. The process will replace existing parts or combine multiple processes into a single part. Because the material is safe in a fire, it could be used inside the cabin of commercial aircraft to protect passengers.

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Tech Briefs Magazine

This article first appeared in the September, 2020 issue of Tech Briefs Magazine.

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