Years of Army scientific research culminated in the successful flight demonstration of a 3D-printed heavy-fuel rotary-engine series-hybrid system, advancing power and propulsion science for future tactical unmanned aircraft systems.
Researchers at the U.S. Army Combat Capabilities Development Command, known as DEVCOM, Army Research Laboratory, designed the advanced drone propulsion through its Versatile Tactical Power and Propulsion essential research program.
“The program addresses critical power and propulsion challenges by advancing fundamental science that could enable drones to fly longer and faster, operate at higher altitudes, carry heavier payloads and perform in extreme environments while reducing logistics burdens,”
said Dr. Chol-Bum “Mike” Kweon, DEVCOM ARL program manager.
“For warfighters, the program has transitioned research findings to industry partners for maturation and integration into capabilities that could enable mid-sized drones to operate beyond line of sight, extending the reach of ground commanders and reducing risk to Soldiers.”
ARL researchers used advanced computational fluid dynamics modeling and a clean-sheet design approach to address the challenges of efficiently burning heavy fuel in a rotary engine. The work improved fuel atomization and vaporization while minimizing compressed-air and fuel-vapour leaks.
“The use of advanced manufacturing, including 3D metal printing, allowed us to rapidly redesign engine components based on lessons learned through experimentation,”
Kweon said.
“It enabled us to design an air-cooled engine that reduces propulsion-system size and eliminates the need for liquid coolant.”
Integrated with a novel 400-volt motor-generator system, the engine forms a heavy-fuel rotary-engine series-hybrid system designed to support vertical takeoff and landing operations in contested environments.
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.
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.
To demonstrate the propulsion system in flight, ARL worked with industry partners, including Griffon Aerospace, to develop an aircraft platform around the propulsion system. The novel series hybrid system powers the Group 3 midsized, unmanned aircraft system, Versatile Intelligence Controlled Tactical Operational Reconnaissance and Attack, or VICTOR UAS.
The VICTOR essential research program generated many additional outcomes for the Army, industry and academia, including:
- Collaborative research with more than 30 universities, more than 13 federal laboratories and more than 20 industry partners that resulted in hundreds of scientific journal publications in diverse areas including combustion chemistry, sensing and controls, material design, batteries and hybrid electric technology
- Professionally developed hundreds of scientists and engineers now working in government, industry and academia
- Research advances that address energy and propulsion challenges relevant to tactical-level energy independence
- Technical data packages, design methods and reliability lessons applicable to future propulsion and drone design and integration efforts
- Tens of patents and patent applications
Lessons from the essential research program will inform follow-on ARL research and industry development efforts, including the Persistent Unmanned Aircraft System for Logistics, Strike, Autonomy, and Reconnaissance, or PULSAR, an advanced drone-operation concept that enables multi-mission capabilities through a swarm of VICTOR drones.
The VICTOR UAS and the PULSAR concept will be made available to military partners for further experimentation and development. Industry partners will continue refining VICTOR UAS for other applications.
DEVCOM ARL is the Army’s sole fundamental research laboratory serving as the nexus of science between the military, academia and industry. Operating under the U.S. Army Futures and Autonomous Systems Command, ARL executes globally recognized research to accelerate delivery of war-winning, disruptive technologies for tomorrow’s Army.
Sources: U.S. Army; Tom’s Hardware