Availability and development cost (rather than technical) considerations dictate that large drones are powered by off-the-shelf turbine engines. Examples include the 36.9-kN Rolls-Royce AE3007 turbofan used in the Northrop Grumman Global Hawk, the 520-kW Honeywell TPE331-10T turbo-prop of the General Atomics Predator B and the 478-kW Pratt & Whitney Canada PW200/55 turboshaft used in the Bell TR918 Eagle Eye.
Aside from those used for small-arms practice, target drones are generally powered by simple turbojets to provide realistic speeds. Safran/Microturbo produces the 1.6-kN TRS-18-1 for the Meteor Mirach 100/5 and the 4.4-kN TRI-60-5 for the Raytheon MQM-107E.
In the 'sub-kN' category, the Pentagon is developing (relatively) low-cost turbojets under its Small Business Innovative Research programme. The principal result to date is the 13.3-daN Technical Directions J45 (see title photograph) for the Lockheed Martin Loitering Attack Missile. It has an SFC of 0.13 kg/N.hr, around 25% less than for the best aeromodeller's engine.
Cheaper hobbyist turbojets in the thrust range 4.5 to 23 daN are available. JetCat Germany's engines are used in the Eads-Dornier twin-engined targets: 16dan P160s in the Do-DT35, and 20 daN P200s in the Do-DT25. AMT Netherlands is working on a 67-daN unit, while Spain's Aries Jet Microturbines is developing the 35-daN A J-307, both for military use.
South Africa's Baird Micro Turbines (BMT) has produced 12-daN and 16 daN units featuring advanced electronic controls. BMT is now developing the 20-daN BMT-200-KS (kerosene starting) for possible target applications.
The Wankel-type rotary is widely used in drones weighing up to one tonne. It has far fewer parts than a conventional piston engine, is cheaper to produce and generates a lot of power for its size. Less prone to hot spots, it is more tolerant of low-octane fuels.
A rotary engine is vibration-free, but runs at high revs, increasing the need for silencing and a reduction gearbox. Its slender chambers result in less complete combustion, and consequently higher fuel consumption and emissions.
UAV Engines (UEL), owned by Silver Arrow/Elbit Systems, has provided mogas/avgas-burning rotaries for over 30 drone types, based on developments by Norton Motors between 1969 and 1992.
The bottom of the UEL range is the air-cooled AR731, which produces 28.3 kW at 7800 rpm. Designed for limited life, it powers various target and attack drones, including the IAI Harpy. Weighing only 9.9 kg, it probably has the highest power/ weight ratio (2.86 kW/kg) in its class...