You can do a lot with 3D printing

—even create a working model jet engine. As a final project for a jet engine manufacturing course at the University of Virginia, students led by Professor David Sheffler constructed a one-quarter-scale working replica of the Rolls-Royce AE3007 turbofan jet engine (which is found on the Air Force's RQ-4 Global Hawk UAV) that spins and whirs furiously, and that was built from plastic using 3D printing.

Powered by compressed air rather than jet fuel, the turbofan jet runs at the same idle speed as the real engine. "We put a strobe light up to it, and the core was spinning 1500 to 2000 rpm," Sheffler says. Though the students' replica engine can fit on a tabletop, Sheffler estimates that if an engine this size were built from the same materials as a true jet engine— titanium and cobalt alloys —it would be able to produce between 500 and 1000 pounds of thrust.

To start the project, Sheffler gave the students CAD (computer-aided design) files for the 43 parts of the replica engine. But it wasn't as easy as just plugging those instructions into a 3D printer. The students modified and refined those designs to ensure that the engine would run smoothly once printed, and Sheffler also tasked his students with building their engine parts within real aircraft tolerances. That meant that all the parts were printed in layers measuring 0.010 of an inch at a time. And the students had to manually machine down those parts that required even tighter tolerances, such as the fan and compressor blades, within 0.002 of an inch. "They essentially treated these plastic parts like they were real metal parts," Sheffler says.

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Finally, the class spent more than 150 hours assembling the engine. Though Sheffler, a 20-year veteran in aerospace engineering, did most of the design work, students made the crucial modifications to make their creation into a working replica. That included creating a gearbox like the one used to start up the actual Rolls engine. To start such a turbofan jet engine, the entire engine must be spinning before ignition can begin. To do this, bleed air is pumped from the aircraft's auxiliary power unit to a gearbox, which spins the engine's high-pressure compressor, fan and then low-pressure compressor. Since the University of Virginia students didn't have an airplane and its auxiliary power unit, they substituted an air compressor to power their gearbox. "They designed that whole gearing mechanism on their own, and then integrated it into the design," says Sheffler. (The class left the Rolls ignition system out of its replica, though, since actually firing a jet engine would have melted the plastic into a fiery mess.)

Not only did 3D printing allow the team to create parts within such tight specifications, but the technique also made this project financially viable for a college class. "If you could get the time and design all this stuff, by my estimate it would cost a quarter-million dollars to fabricate what we did," he says. The class's fabrication costs using 3D printing? Fifteen hundred dollars for the plastic and another $300 for the bearings, nuts, and bolts.

And besides creating an attractive and operational turbofan jet, students also gained valuable experience in the process. "It was a lot of hands-on that you don't get in other classes," senior Oscar Leon says. Classmate Justin Sinaguinan agrees: "What I really appreciated was knowing a popular machine, and learning how it works from the ground up."

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