Detailed analysis of the evolution of drone propulsion systems, from the first models to the advanced technologies of the SR-72.

A retrospective of the first drone propulsion systems

The first drones, developed during the First World War, mainly used piston engines. These engines, similar to those of manned aircraft of the time, offered moderate power suitable for short flights. For example, the “Kettering Bug”, designed in 1918, was powered by a 37 kW piston engine, enabling it to reach a speed of around 80 km/h.

During the Second World War, technological advances led to the use of more efficient engines. The German drone “Fieseler Fi 103”, better known as the V-1, was equipped with an Argus As 014 pulsejet engine, delivering a thrust of around 3 kN and propelling the aircraft at an average speed of 640 km/h. This innovative propulsion system for the time allowed the V-1 to travel up to 250 km.

In the 1950s and 1960s, the rise of jet engines transformed the landscape of military drones. The United States developed the “Ryan Model 147”, a reconnaissance drone powered by a Continental J69 turbojet engine, reaching speeds of over 800 km/h and a range of over 1,500 km. This period marked a transition to faster drones capable of prolonged missions.

A transition to hybrid and advanced propulsion

With the evolution of military and technological needs, drones have incorporated hybrid propulsion systems. The “MQ-1 Predator”, introduced in the 1990s, used a Rotax 914 turbocharged piston engine of 86 kW, allowing an endurance of more than 24 hours at a cruising speed of about 130 km/h. This combination of long endurance and low speed was ideal for extended surveillance missions.

At the same time, supersonic drones emerged. The “Lockheed D-21”, developed in the 1960s, was powered by a Marquardt RJ43-MA-11 ramjet, enabling it to reach speeds close to Mach 3 (approximately 3,700 km/h). However, the challenges associated with propulsion at these speeds limited its operational use.

The modern era has seen the emergence of electrically powered drones, particularly for civilian applications. Models such as the DJI Phantom use brushless electric motors, offering high energy efficiency, reduced maintenance and increased maneuverability, although their range is limited by battery capacity.

The development of the SR-72 and its advances in propulsion

The “Lockheed Martin SR-72”, often nicknamed the “son of the Blackbird”, represents a significant advance in drone propulsion. Designed to succeed the legendary “SR-71 Blackbird”, the SR-72 aims for hypersonic speeds of up to Mach 6 (approximately 7,400 km/h). To achieve such performance, it incorporates a turbine-based combined cycle propulsion system (TBCC), combining a turbojet for low speeds and a scramjet for hypersonic speeds.

The major challenge lies in the transition between these two modes of propulsion. The turbojet engine works efficiently up to about Mach 2.2, while the scramjet becomes optimal above Mach 3. The design of the SR-72 must therefore ensure a smooth transition between these systems to cover the entire speed spectrum. Lockheed Martin, in collaboration with Aerojet Rocketdyne, has been working on this complex integration since 2006. In December 2014, NASA awarded a contract worth 892,292 dollars (around 1.19 million euros) to study the feasibility of this hybrid propulsion system.

The SR-72 is envisaged as a strategic reconnaissance drone, capable of penetrating defended airspace thanks to its exceptional speed. It is planned to be commissioned in the 2030s, with a first test flight scheduled for around 2025. If these objectives are achieved, the SR-72 will redefine the speed standards for drones, offering unprecedented capabilities in terms of surveillance and rapid strike.

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