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Aviation History
1973
1973 - 0102.PDF
68 FLIGHT International, II January 1973 propulsion TF34 versus F102 By KEN FULTON TWO VERY DIFFERENT TURBOFANS, the Avco Lycoming F102 and General Electric TF34, are competing keenly for orders totalling an eventual 1,500 or so units (installed and spares) to power 600 A-X close-support air craft for the United States Air Force. While the stakes for the two compet ing airframe entries, the Northrop A-9A and Fairchild Industries A-10A, are relatively obvious, the differing significance to Lycoming and GE of the contest is not so immediately apparent. There is also the twist that both Northrop and Fairchild have been asked by the USAF to submit pro posals using each other's engine in their respective airframes. This is to allow the Air Force to have the final decision on the power unit, regardless of which aircraft gains the winning decision, planned to be announced in March. The F102-LD-100 is a new two-shaft geared turbofan of more than 7,2001b, 32-7kN thrust and is specifically adapted to suit the requirements of the A-9A. As Lycoming's first "jet" engine to fly, the turbofan combines the gas-generator section of the 3,750 s.h.p., 2,786kW T55-L-11A turbo- shaft as core engine with a 6:1 bypass ratio fan based on Lycoming's pioneer ing PLF-1 series, the world's first high- bypass turbofans, which were tested some nine years ago. There is also a single-stage axial "booster" behind the fan. Its commercial counterpart, 'the ALF-502, is soon to fly in the Dassault- Breguet Mystere 30 and has been chosen by Hawker Siddeley as the powerplant for the company's new HS.146 feederliner project. A contract for development of the F102 was awarded in December 1970, and the engine first ran in June of the following year. Flight-testing in a podded nacelle under a North American AJ-2 test-bed began in January 1972. The pace of develop ment has of necessity been fast, to enable flight-testing of the first of two A-9As to start at the end of May last year; the second A-9A flew on August 23. By early October the two aircraft had flown the equivalent of 120 engine hours and, together with the two A-10A prototypes, were handed over to a team of USAF pilots later that month for a 60-day fly-off evaluation which ended early last month. The definitive F102, qualification testing of which is planned for next year, is rated at 7,8601b, 35-6kN and will be able to grow to meet the needs of either the A-9A or A-10A. The in creased rating is achieved by utilising the higher-power, 4,600 s.h.p., 3,430kW T55-L-11B as core engine, and by the addition of a second fan booster stage. Air mass flow is in the region of 2751b/ sec-2801b/sec, and the bypass ratio is somewhat lower, at 5-7:1. Lycoming's contract for the engine is such that, if the F102 becomes the chosen power unit for the A-X, development can pro ceed through directly to the military qualification test. The 7,2001b, 32-7kN version completed its preliminary flight-rating test in March last year. While Lycoming's A-X entry is based on a new low-cost, minimum- risk geared turbofan embodying a US Army service-proven gas-generator, General Electric's entry is a four-year- old US Navy turbofan now in an ad vanced stage of development for the twin-engined Lockheed S-3A Viking ASW aircraft. As the 9,2751b, 42-lkN TF34-GE-2, the A-10's turbofan has also been chosen to power the forth coming eight-en gined Boeing E-3A Awacs (Airborne Warning and Con trol System) aircraft, and Lockheed's winning design of a four-engined Quiet Experimental Stol transport in Nasa's recent Questol contest. On the basis of this it may appear almost greedy of GE to be promoting its TF34 so hard for the A-X competi tion, against Lycoming's efforts simply to get its first foot in the "fixed-wing" door. With likely eventual require ments for about 190 S-3As and 42 E-3As, however, and no decision by Nasa until later this year actually to build the Questol, the total potential business for the TF34 currently stands at some 716 installed engines, plus a possible further 358 spares. This is equivalent only to some 70 per cent of the probable A-X commitment for engines. Just how energetically GE is pushing the TF34 for this role can be seen from its current A-X advertising cam paign in America. Highlights are that: (1) the TF34 is fully qualified, is in production (for the S-3A), and has already accumulated 2,500hr flight experience; (2) the fan for the current production TF34 is the same fan as for the A-X engine; and (3) the TF34 has already demonstrated its durability in three consecutive 150hr endurance tests, which the company says left the test engine in an excellent after-run condition. Consequent claims by GE for its A-X engine are low risk, low development cost and long life with low support costs. First run of the TF34-GE-2 under USN contract for the S-3A took place in May 1969, and 200hr of flight test ing in a modified Boeing B-47 were initiated in January 1971. The S-3A flew 12 months later, and the aircraft is scheduled to enter service with the Fleet early next year. Production, on present plans, extends to about 1976. In support of the Awacs programme, the losing aircraft in the evaluation of competing radars using modified Boeing 707-320s is being re-engined with eight TF34s in four paired nacelles in place of the four Pratt & Whitney JT3D turbofans. The first Awacs development aircraft is scheduled to be rolled out in May 1975, and production deliveries will not start until late 1976. The first of two A-10A prototypes, with TF34s, flew on May 10 last year, followed by the second prototype on July 21. By early October the two aircraft had flown the equivalent of 298 engine hours without incident. In contrast to the F102 the TF34 is a much newer design, although em bodying features based on GE's con siderably larger TF39 engine. The main differences in configuration compared with the Lycoming engine are the lack of a reduction gearbox between the 1-p turbine and fan, and use of a higher-mass-flow fan (3381b/ sec) and 6-23:1 bypass ratio. What are the relative chances of the two competing turbofans in the A-X contest, and what would be the signi ficance of a win for either company? On price alone the F102 is to be pre ferred, being some $140,000 (£58,000) cheaper than the TF34. For this differ ence it also supplies some 1,4151b, 6-42kN less thrust but has the advan tage of being some 44in shorter— 56-8in long compared with lOlin for the TF34. To help reduce the price differential —which is important in view of the deliberate interchangeability of the two engines between the A-9A and A-10A—GE has discussed with Fair- child the possibility of a lighter and less expensive version of the TF34. This would incorporate less advanced subsystems than those used in the TF34-GE-2 for the S-3A which, among other features, requires ' fairly large air bleeds from the core engine to drive various aircraft auxiliary systems. This call on the TF34's air supply is an aspect of the engine which is understood to have incurred problems in meeting Navy perform ance specifications at low forward speed. Except for price and the F102's use of a proven core engine, the TF34 could well be regarded as the better bet in view of its broad range of other installations, which bring the benefits of prior development financing plus a greater commitment by GE and the two US Services to make a success of the engine. By contrast, Lycoming only has two military units, the T53 and T55, plus two private-venture engines, the PLT-27 and LTS-101. The significance to the company of victory by the F102 is considerable, particularly in view of the present decline in sales of Its two older turboshafts/turboprops and the lack of immediate applications for the two new, smaller turboshafts. Also the F102, unlike the TF34, has the attraction to the USAF of a parallel civil development which, if successful, would probably result in a more rapid build-up in flight hours. •
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