The General Electric J85 is a small single-shaft turbojet engine. Military versions produce up to 2,950 lbf (13.1 kN) of thrust dry, afterburning variants can reach up to 5,000 lbf (22 kN). The engine, depending upon additional equipment and specific model, weighs from 300 to 500 pounds (140 to 230 kg). It is one of GE’s most successful and longest in service military jet engines, with the civilian versions having logged over 16.5 million hours of operation. The United States Air Force plans to continue using the J85 in aircraft through 2040. Civilian models, known as the CJ610, are similar but supplied without an afterburner, while the CF700 adds a rear-mounted fan for improved fuel economy.
Design and development
The J85 was originally designed to power a large decoy missile, the McDonnell ADM-20 Quail. The Quail was designed to be released from a B-52 Stratofortress in-flight and fly for long distances in formation with the launch aircraft, multiplying the number of targets facing the SA-2 surface-to-air missile operators on the ground. This mission demanded a small engine that could nevertheless provide enough power to keep up with the jet bomber. Like the similar Armstrong Siddeley Viper being built in England, the engine on a Quail drone had no need to last for extended periods of time, so therefore could be built of low-quality materials.
The fit was a success on the Quail, but again like the Viper it was later built with normal grade materials and subsequently used to power small jet aircraft, including the Northrop T-38 Talon, Northrop F-5, Canadair CT-114 Tutor, and Cessna A-37 Dragonfly light attack aircraft. More recently, J85s have powered the Scaled Composites White Knight aircraft, the carrier for the Scaled Composites SpaceShipOne spacecraft, and the Me 262 Project.
The basic engine design is quite small, about 18 inches (46 cm) in diameter, and 45 inches (110 cm) long. It features an eight-stage axial-flow compressor powered by two turbine stages, and is capable of generating up to 2,950 lbf (13.1 kN) of dry thrust, or more with an afterburner. At full throttle at sea level, this engine, without afterburner, consumes approximately 400 US gallons (1,500 L) of fuel per hour. At cruise altitude and power, it consumes approximately 100 US gal (380 L) per hour.
Several variants were produced. The J85-21 variant added a stage ahead of the base 8-stage compressor for a total of 9 stages, improving thrust.
More than 12,000 J85 engines had been built by the time production ended in 1988.
- Type: Turbojet engine (with or without afterburner)
- Length: 45.4–51.1 in (115–130 cm) without afterburner (depending on accessory equipment installed)
- Diameter: 17.7 in (45 cm)
- Dry weight: 396–421 lb (180–191 kg) (depending on accessory equipment installed)
- Compressor: 8 stages (9 in J85-21) axial
- Combustors: annular
- Turbine: 2 stages
- Fuel type: jet fuel
- Maximum thrust: 2,850–3,100 lbf (12.7–13.8 kN) (dry)
- Overall pressure ratio: 8.3
- Air mass flow: 45 lb (20 kg) per second
- Turbine inlet temperature: 1,470°F (977°C)
- Specific fuel consumption: 0.96 – 0.97 lb/(lbf·h) (27 g/kN·s)
- Thrust-to-weight ratio: 7.5 (-21), 6.6 (-5), 6.8 (-13), 7 (-15)
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