General Electric TF39

The United States Air Force opened the "CX-X Program" in 1964, intending to produce a next-generation strategic airlifter. Of the several airframe and engine proposals returned for consideration, Lockheed's aircraft and General Electric's engine were selected for the new design in 1965.

The high-bypass turbofan was a huge leap in engine performance, offering a thrust of 43,000 pounds, while improving fuel efficiency by about 25%.[2] The TF39 had an 8-to-1 bypass ratio, a 25-to-1 compressor pressure ratio, and a 2,500 °F (1,370 °C) turbine temperature made possible by advanced forced-air cooling. The first engine went for testing in 1965. Between 1968 and 1971, 463 TF39-1 and -1A engines were produced and delivered to power the C-5A fleet.

TF39s on a C-5 Galaxy, from the rear

The TF39 was a revolutionary 1960s engine rated from 41,000 to 43,000 lb_f (191 to 205 kN) of thrust. It introduced use of a large by-pass ratio which, together with advances in core technology, contributed to a significant improvement in fuel efficiency over engines available at the time.

The engine included features developed from previous GE engines:

• Variable stator vanes (used in the J79/CJ805[3])
• Turbine cooling techniques (advanced from the J93 used in the XB-70[3])
• Cascade-type thrust reverser (from the CJ805[3])
• Snubbered first stage fan blades (snubbers, or mid-span shrouds, had been introduced by GE on the YJ93 first stage compressor blades[4])

Today, a conventional turbofan featuring a T-staged fan would have an overhung fan rotor (without any inlet guide vanes), followed by one or more T-stages supercharging the core stream alone.[5] The name T-staged fan is derived from the overall appearance of the LP compressor when the side elevation is shown diagrammatically.

GE took a different approach with their first high bypass ratio turbofan, the TF39. It is a unique, very complex design.[6] The T-stage, which supercharges the core stream, is located ahead of the main fan rotor. The T-stage itself comprises an overhung mini-rotor followed by a set of outlet guide vanes; the main fan rotor is located immediately behind these OGVs. Outboard of the T-stage rotor are the main inlet guide vanes, which only affect the bypass stream. The T-stage rotor extends to about half the main rotor stage annulus [4][7] and runs in a tip shroud. The main fan stage has a mid-span platform/flow splitter which separates the single-stage outer annulus from the two-stage inner annulus.[7] These two stages mainly supercharge the 16-stage high-pressure compressor.[8] However, a fair proportion of the air entering the T-stage is bled off into the bypass duct, there being two annular passages leading to the bypass duct. The nominal bypass ratio of 8:1 refers to the ratio of total bypass mass flow to HP compressor entry mass flow.

The rotor blades are snubbered. 'Snubbers' are protuberances that stick out at right angles to the fan aerofoil at mid[4] to 2/3 span.[9] At speed, the snubbers on adjacent fan blades butt-up against each other to prevent blade failures due to flutter.[9] They, together with the second-stage inlet guide vanes in the outer half of the duct, are visible when looking into the engine intake.[10][11]

The high-bypass ratio of 8:1 for the TF-39 had its origins in the lift-fan technology demonstrated by GE in the XV-5 Vertifan aircraft.[3] This aircraft had two X353-5 engines, each consisting of a 62.5-inch-diameter[12] lift-fan driven by a gas generator (J85). The bpr in VTOL operation was 12.3.[13] This tip-turbine driven lift-fan concept was turned 90 degrees and developed as an 80-inch-diameter "cruise fan" demonstrator, driven by a J79 gas generator.[3] For the CX-X program GE demonstrated a half-scale engine, the GE1/6, with 15,830 lb thrust and an sfc of 0.336.[14] This was developed into the TF39 with a 97 in.[7] fan.

• Lockheed C-5A/B/C Galaxy

A Boeing B-52E (Serial Number 57-0119) testing a TF39 on the right inboard engine pod. The TF39 has more than double the thrust of the twin Pratt & Whitney J57s that were originally in that spot.

Data from [15]

General characteristics

• Type: Turbofan
• Length: 312 in (792 cm)
• Diameter: 97 in (246 cm)
• Dry weight: 8000 lb (3630 kg)

Components

• Compressor: Axial, 2-stage fan, 16-stage high-pressure compressor
• Combustors: Annular
• Turbine: Axial, 2-stage high-pressure turbine, 6-stage low-pressure turbine

Performance

• Maximum thrust: 43,300 lbf (193 kN)
• Overall pressure ratio: 25:1
• Fuel consumption: ~3.7646 lb/s (1.7075 kg/s)
• Specific fuel consumption: 0.313 lb/lbf-hr
• Thrust-to-weight ratio: 5.4

Related development

• General Electric CF6
• General Electric LM2500

Comparable engines

• Pratt & Whitney JT9D
• Progress D-18T
• Rolls-Royce RB211

Related lists

• List of aircraft engines

• Gunston, Bill (2006). World Encyclopedia of Aero Engines, 5th Edition. Phoenix Mill, Gloucestershire, England, UK: Sutton Publishing Limited. ISBN 0-7509-4479-X.

Wikimedia Commons has media related to General Electric TF39.

• General Electric TF39 website