IN Brief:
- The US Army Research Laboratory has demonstrated a heavy-fuel rotary engine developed using metal additive manufacturing aboard the VICTOR unmanned aircraft.
- The engine uses air cooling and a 400V motor-generator system in a series-hybrid propulsion architecture.
- Research findings will support further unmanned aircraft development, including the proposed PULSAR multi-aircraft concept.
The US Army Research Laboratory has completed a flight demonstration of a heavy-fuel rotary engine developed using metal additive manufacturing and integrated into a series-hybrid propulsion system. The powerplant was installed aboard the VICTOR unmanned aircraft, bringing together an air-cooled combustion engine and a 400-volt motor-generator architecture developed for tactical aviation applications.
The demonstration follows research conducted through the Army’s Versatile Tactical Power and Propulsion programme. Scientists and engineers at the Combat Capabilities Development Command Army Research Laboratory developed the engine and worked with industry partners, including Griffon Aerospace, to produce an aircraft capable of flight testing the propulsion system.
The resulting unmanned aircraft, known as Versatile Intelligence Controlled Tactical Operational Reconnaissance and Attack (VICTOR), is a Group 3 platform designed around the experimental propulsion technology. Its successful launch, flight, and landing near Huntsville, Alabama, provided an opportunity to assess the engine and associated electrical equipment within an airborne system.
Heavy-fuel operation has been a central part of the research because tactical aircraft can benefit from using fuels compatible with established military logistics arrangements. Compact rotary engines, however, present particular difficulties when operating on heavier fuels whose combustion behaviour differs from that of petrol. Reliable operation depends on the fuel being prepared, mixed with air, and burned effectively within the engine’s operating cycle.
Army researchers used computational fluid dynamics to examine the movement of air and fuel inside the engine. Their work concentrated on improving fuel atomisation and vaporisation while reducing leakage of compressed air and fuel vapour. Those factors influence the quality of combustion and the energy that can be extracted from the fuel.
Atomisation divides liquid fuel into smaller droplets, increasing the surface area available for evaporation. Vaporisation allows fuel to mix more effectively with air before combustion. Temperature, pressure, fuel characteristics, and internal airflow influence both processes, making the arrangement of the fuel system and combustion chamber important to engine performance.
Rotary engines use a moving rotor within a specially shaped housing to create the changing volumes required for intake, compression, combustion, and exhaust. The relationship between the rotor and housing must be controlled to maintain suitable compression. Leakage at the sealing interfaces can reduce combustion effectiveness, contributing to the engineering challenges examined through the Army’s modelling and experimental work.
The programme used a clean-sheet engine design, allowing the geometry and supporting components to be developed around the heavy-fuel operating requirement. Simulations were used to examine possible changes, while experimental testing provided information about how those changes affected actual engine operation. The process enabled successive design iterations before the system was installed aboard the aircraft.
Metal additive manufacturing supported that development cycle by allowing researchers to produce revised engine components as the design evolved. Building parts through successive layers can accommodate complex geometries and internal features that may require more extensive processing with conventional manufacturing methods. The technique also allows design modifications to be incorporated into new hardware during iterative development.
Manufactured parts still require appropriate finishing, inspection, and testing before use in demanding mechanical applications. Component geometry, dimensional accuracy, material properties, and surface condition influence their behaviour under load and temperature. The research programme combined additive manufacturing with the engineering and experimental work needed to establish a functioning propulsion system.
One outcome was an air-cooled engine that eliminates the need for a liquid coolant circuit. Conventional liquid-cooling arrangements require additional equipment such as pumps, hoses, and coolant reservoirs. Removing those components can reduce the space occupied by the propulsion installation and simplify aspects of its mechanical arrangement.
Air cooling transfers heat from the engine through its structure and into the surrounding airflow. The effectiveness of the process depends on the available cooling surfaces, airflow, and thermal conditions during operation. These factors must be considered when positioning the engine within an aircraft, where surrounding structures and flight conditions influence the cooling environment.
The engine is connected to a 400-volt motor-generator system in a series-hybrid propulsion architecture. In this type of arrangement, the combustion engine supports electrical generation, while propulsion equipment receives energy through the electrical system. The architecture allows engine operation and electrical power delivery to be managed through separate but connected functions.
Electrical generation, power conversion, and propulsion demand must be coordinated during aircraft operation. Different phases of flight place changing requirements on the propulsion system, while onboard electrical equipment needs a suitable supply. The voltage architecture provides a basis for connecting those functions, with power management and control determining how the complete system operates.
The Army is developing the propulsion technology to support future vertical take-off and landing applications in contested environments. The VICTOR demonstration established flight operation of the integrated hybrid powerplant, while the broader research objectives include improving range, endurance, payload capability, and operation under demanding environmental conditions.
Griffon Aerospace and other industry participants contributed to development of the aircraft used for testing. Designing the airframe around the experimental propulsion equipment allowed the mechanical, electrical, and control interfaces to be considered together. The aircraft provided a practical installation in which the researchers could evaluate the combined propulsion architecture rather than testing its components only in isolation.
The wider research programme involved more than 30 universities, 13 federal laboratories, and 20 industry partners, producing work across combustion, materials, controls, batteries, and hybrid-electric technology. Its results include technical data, engineering methods, and lessons intended for further development by Army researchers and industrial organisations.
One proposed follow-on application is the Persistent Unmanned Aircraft System for Logistics, Strike, Autonomy, and Reconnaissance (PULSAR). The concept envisages multiple VICTOR aircraft operating together across different missions, extending the propulsion research into a broader autonomous aviation architecture. Further experimentation will be required to develop that concept and assess its practical capabilities.
The Army intends to make VICTOR and the PULSAR concept available to military partners for additional research and experimentation. Industry participants will continue refining the aircraft for potential applications, with future work drawing on the completed heavy-fuel engine flight demonstration and the wider findings of the propulsion research programme.
