US Army successfully flies a hybrid electric drone with 3D-printed heavy-fuel rotary engine — 400v motor-generator uses advanced engineering to ditch a bulky liquid cooling system

Researchers at U.S. Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory (ARL) recently successfully flew the VICTOR hybrid electric drone with a heavy-fuel rotary engine, rapidly developed thanks to metal 3D printing — allowing the drone to extract more performance out of the denser but less volatile military fuel source.

The heavy fuel (JP8) used in the drone is a denser kerosene-based fuel source compared to regular jet fuel, making it desirable to reduce total fuel load or increase mileage. The JP8 fuel is part of the military’s initiative to power as much of its gas equipment on a single fuel source. However, JP8 is less volatile and harder to atomize, making it a challenge to achieve peak efficiency. Furthermore, the fuel’s flash point is 140℉, compared to gasoline’s -40℉, requiring the engine to have a much higher compression to ignite it, making pressure leaks a common issue.

JP-8 fuel on aircraft and drones is not uncommon; however, the ARL researchers wanted to further optimize the heavy-fuel rotary engine to extract more performance and improve its design. The researchers used computational fluid dynamics (CFD) and a clean-sheet design (designing from scratch, essentially), allowing the engineers to come up with more innovative solutions to get more out of the engine with less.

"The use of advanced manufacturing, including 3D metal printing, allowed us to rapidly redesign engine components based on lessons learned through experimentation,” Dr. Kweon, one of the ARL researchers, said to Army.mil. Dr. Kweon further explained that 3D printing allowed them to design an air-cooled engine and ditch a bulky liquid cooling system, reducing the size of the propulsion system. The rapid iteration eventually allowed the researchers to improve fuel atomization and decrease compression leaks.

The heavy-fuel rotary engine doesn’t actually drive the drone’s propellers directly; it uses the engine to spin a 400-volt motor-generator to generate electricity to drive the propeller motors, allowing the engine to spin independently at its most optimal RPM — similar to some hybrid EVs on the road today, except that the engine doesn’t have a mechanical link to the propellers. This system may include a battery for the drone to fly without the engine running; however, the US military has not disclosed much about the VICTOR drone’s battery configuration.

The VICTOR drone is a vertical take-off and landing (VTOL) unmanned aircraft, able to fly without a runway. The US Army has shared a video of its first flight, and the drone is able to land and take off in terrain that would otherwise be impossible for other non-VTOL two-winged aircraft. With a battery system, the drone could cover large distances on a single fuel-up, and air-cooling gives it another layer of robustness, avoiding maintenance on a liquid cooling system.

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