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Introduction
The Defense Advanced Research Projects Agency (DARPA) has unveiled an ambitious new initiative—the Series Hybrid Electric Propulsion AiRcraft Demonstration (SHEPARD) program, designated as the XQ-73. This “X-prime” program builds upon previous advancements from the Air Force Research Laboratory (AFRL) and Intelligence Advanced Research Projects Activity (IARPA) under the Great Horned Owl (GHO) project, which led to the development of the XRQ-72. With its first flight slated for the end of 2024, the SHEPARD program is set to revolutionize unmanned systems by leveraging a series hybrid electric propulsion architecture that promises enhanced performance, reduced emissions, and superior operational flexibility.SHEPARD aims to accelerate the integration of hybrid electric propulsion systems into a mission-ready, long-endurance unmanned aircraft, with the first flight expected by the end of 2024.
Series Hybrid Electric Propulsion in Aircraft: Architecture and Advantages
A series hybrid electric propulsion system in aircraft operates by using a gas turbine or internal combustion engine solely as a generator to produce electricity, rather than directly providing thrust. This electricity powers electric motors that drive the propellers or fans, with any excess energy stored in batteries for later use. Unlike parallel hybrids, where both the engine and electric motors can provide mechanical power, the series configuration decouples the engine from the thrust system, allowing it to run at optimal efficiency regardless of flight conditions.
This architecture offers several key advantages for aircraft. This separation of power generation and propulsion allows the engine to operate at its most efficient point, regardless of the aircraft’s speed or altitude. As a result, the architecture offers superior fuel economy, reduced emissions, and quieter operation compared to conventional turboprop or jet engine systems.
First, it enables greater fuel efficiency since the engine operates at a constant, optimized speed to generate electricity, reducing fuel consumption compared to traditional propulsion. Second, it allows for quieter operation, as electric motors produce less noise than combustion engines, making it ideal for stealth and reconnaissance missions. Additionally, the system provides flexible power distribution, enabling enhanced endurance and redundancy—critical for long-duration missions. The ability to store excess energy in batteries also supports peak power demands, such as during takeoff or high-speed maneuvers, without requiring an oversized engine.
This configuration also simplifies engine control and maintenance, while offering the potential for rapid scaling and integration with renewable energy sources. Overall, series hybrid electric propulsion represents a transformative approach to aircraft design, promising significant improvements in efficiency, environmental performance, and mission versatility.
Program Aims and Objectives
The primary goal of SHEPARD is to mature hybrid electric propulsion technology while reducing integration risks for future military applications.
One key objective is demonstrating a reliable, efficient series hybrid-electric propulsion system for unmanned aircraft. This technology could enable long-endurance missions with reduced thermal and acoustic signatures, making the aircraft harder to detect.
Additionally, the program focuses on providing a scalable power architecture adaptable to various DoD platforms. By leveraging prior research from the XRQ-72 program, DARPA intends to accelerate the transition to operational use.
While DARPA has disclosed limited specifications, the agency confirmed the XRQ-73 will be a Group 3 Unmanned Aerial System (UAS), weighing approximately 1,250 pounds (567 kg). The first flight is anticipated by the end of 2024. The concept artwork suggests a stealth-optimized flying wing design, featuring tapered and clipped wingtips, along with two upper fuselage air inlets—likely for engine cooling or intake. A prominent fairing between the inlets may house sensors, communications equipment, or other mission payloads, though DARPA has not elaborated on its function.
Although exact performance metrics remain undisclosed, the DoD’s Group 3 UAS classification provides some insight into potential capabilities. Aircraft in this category typically have a maximum takeoff weight between 55–1,320 lbs, operational altitudes of 3,500 ft to FL180 (18,000 ft), and cruise speeds between 100–250 knots (115–288 mph). Known Group 3 platforms—such as the RQ-7 Shadow and RQ-21 Blackjack—are primarily used for reconnaissance, suggesting the XRQ-73 may fill a similar role but with enhanced endurance and reduced detectability thanks to its hybrid-electric propulsion. The flying wing configuration further implies an emphasis on low radar cross-section (RCS), aligning with DARPA’s focus on survivability in contested environments.
Steve Komadina, SHEPARD Program Manager, emphasized the program’s purpose: “The idea behind a DARPA X-prime program is to take emerging technologies and burn down system-level integration risks to quickly mature a new missionized long endurance aircraft design that can be fielded quickly.”
Key Technologies and Innovations
The SHEPARD program integrates several cutting-edge technologies to achieve its objectives. By integrating advanced energy storage solutions, power management systems, and high-efficiency electric motors, the program is poised to set new benchmarks in fuel efficiency and operational endurance.
A central innovation is the series hybrid electric propulsion system, which combines a gas-powered generator with electric motors for efficient power distribution. This architecture allows for optimized energy use during different flight phases.
The aircraft incorporates a low-observable design to reduce thermal and acoustic signatures, enhancing stealth capabilities. Advanced energy storage systems, supplied by EaglePicher Technologies, provide the necessary power for sustained operations.
Lightweight composite structures developed by Scaled Composites and Cornerstone Research Group help optimize weight and durability. Meanwhile, PC Krause and Associates and Brayton Energy contribute to thermal and electrical system optimization, ensuring efficient power management throughout missions.
The innovative propulsion architecture is being rigorously tested to ensure that it can meet the demanding performance requirements of modern military operations, including extended mission durations and rapid deployment scenarios. This technology not only reduces logistical burdens but also minimizes the thermal and weight constraints typically associated with conventional propulsion systems.
Military Impact and Operational Advantages
The XQ-73 SHEPARD is expected to deliver significant operational benefits to U.S. military forces.
One major advantage is extended mission endurance, potentially enabling days-long operations without refueling. The hybrid-electric system also reduces fuel consumption compared to traditional propulsion methods, lowering operational costs.
The aircraft’s reduced detectability, thanks to quieter electric propulsion and thermal management, makes it suitable for sensitive reconnaissance missions. Furthermore, its flexible payload integration capabilities allow for various configurations supporting ISR (Intelligence, Surveillance, Reconnaissance) and electronic warfare roles.
These capabilities align with the DoD’s broader push toward more sustainable, efficient unmanned systems capable of operating in contested environments.
Challenges and Risks
Despite its promising potential, the SHEPARD program faces several technical and operational challenges.
Thermal management presents a significant hurdle, as the hybrid system must efficiently dissipate heat generated during extended operations. Power density remains another critical concern, requiring careful balancing of battery weight against performance requirements.
The integration complexity between gas and electric systems demands meticulous engineering to ensure seamless interaction. Additionally, cost and scalability considerations will influence the program’s transition from prototype to potential mass production.
DARPA’s X-prime approach aims to mitigate these risks by emphasizing rapid testing and iterative refinement before committing to full-scale development.
Program Details: Phases and Industry Partners
Prime Contractor & Major Suppliers
Northrop Grumman serves as the lead systems integrator for the SHEPARD program. Scaled Composites is responsible for airframe development, while Cornerstone Research Group contributes advanced materials expertise.
Brayton Energy focuses on thermal and power systems, and PC Krause and Associates provides electrical system modeling support. EaglePicher Technologies delivers specialized energy storage solutions crucial for the hybrid-electric architecture.
Collaborating Agencies
The Air Force Research Lab (AFRL) supports the program by providing research data from the earlier XRQ-72 project. The Office of Naval Research (ONR) also contributes, exploring potential maritime applications for the technology.
Expected Timeline
Development and ground testing of the XQ-73 prototype will continue through 2023-2024. The program anticipates conducting its first flight demonstration by late 2024. Following successful evaluations, the technology could transition to military use as early as 2025.
Conclusion: A Stepping Stone for Future Hybrid Aircraft
The XQ-73 SHEPARD represents a significant advancement in hybrid-electric military aviation, blending lessons from past programs with innovative new technologies. Its success could establish a foundation for next-generation unmanned systems featuring unprecedented endurance and operational efficiency.
With Northrop Grumman leading a strong industry consortium and support from key defense research organizations, DARPA’s SHEPARD program is well-positioned to deliver a transformative platform for U.S. military operations.
References and Resources also include:
- DARPA Official Release
- AFRL Great Horned Owl (GHO) Project Documents
- Northrop Grumman Press Statements
- Defense News & Aviation Week Reports
