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For subsonic operation, high-bypass turbofans seem to have essentially only advantages for efficiency, noise and takeoff thrust, at least until they reach the size where ground clearance or LP/fan speed mismatch become major issues. Yet early jetliners first used turbojets then only very low-bypass turbofans, despite the existence of turboprops meaning that using gas turbines for indirect propulsion was evidently understood.

Did they not fully realise the potential benefits at the time/thought they wouldn't work well, or was it just very difficult to build a jet with useful excess turbine power (to drive the fan) at the time?

Talisker
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  • Related: How was the high-bypass concept invented? –  May 23 '18 at 21:20
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    Because a turbofan requires excess energy, and that means a high temperature core, and the temperature capability of early turbine blade material and combustion sections was insufficient. That's the reason why it was difficult to build an early gas turbine with excess power. And it is still a limiting reason stopping higher bypass ratios, which increase propulsion efficiency. – Penguin May 24 '18 at 09:12
  • Difficult when taking off excess power at the shaft, or also when taken from a power turbine? – rackandboneman May 24 '18 at 20:00
  • In addition to the Germans appreciating the benefits of turbofans by WW2 (see Peter Kampf below), so did the British. Frank Whittle understood by 1936 that it is more efficient to give a small acceleration to a large mass of air than a large acceleration to a small mass of air. See this Stack Exch answer https://aviation.stackexchange.com/questions/35648/how-was-the-high-bypass-concept-invented?noredirect=1&lq=1 – Flynn May 24 '18 at 20:26
  • @rackandboneman. Both, when taking power at the shaft, or from a power turbine. In either case, power is needed, and the thermodynamics is the same; both situations require a core that can accommodate the temperature required. – Penguin May 25 '18 at 10:05

3 Answers3

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Production turbofans just weren't available. Once they came on the market and were mass-produced, jetliners switched to them.

While turboprops did exist at the time, they were (and are) a turbine engine driving a propeller in place of a piston engine, a straightforward combo of two techs. Turbojet engines were fairly complex machines for the era on their own.

A turbofan involves a large diameter fan, similar in design to an axial compressor, which is more difficult to build than simpler centrifugal compressors used in the first jets and most turboprops. It spins at higher rpm than a prop, and needs to be built and balanced to jet engine tolerances, with the fan tips very close to the casing, but not quite touching it.

The fan also needs to be driven by a core that has enough net power left to produce both thrust and torque, and to do so at low enough rpm to keep the fan (mostly) subsonic. This combination only brings considerable efficiency gains with a two-spool core, using a low-speed shaft to match the fan's optimum rpm, fitted inside a hollow high-speed shaft driving the compressor (unless an aft fan is used). A two-spool with its extra seals is a much more complex engine than a single-spool turbojet or centrifugal flow turboprop.

All of this was done and turbofans quickly became the norm, but it took a few years to develop and ramp up production.

Therac
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  • I wonder if turboprop showed up any time earlier than turbofan. Neither the turbine nor the gear box of a turboprop is anywhere simpler than a small-medium BPR turbofan. – user3528438 May 23 '18 at 21:34
  • The shaft and the seals of a turboprop can be simpler - a gearbox (mature tech already) makes a single shaft design more reasonable. The fan isn't complicated, but it's got more blades that are thinner and have tighter tolerances than a propeller. – Therac May 23 '18 at 21:58
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    @user3528438, a gearbox isn't simple, but even back when the first turboprop engines came out, it was a mature, well-understood technology. In contrast, a two-spool jet engine used highly experimental technologies. – Mark May 23 '18 at 23:11
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    @user3528438 the Vickers Viscount, the first turboprop airliner, first flew in July 1948, just over a year before the first flight of the dH Comet – Talisker May 23 '18 at 23:53
  • @Therac But to make a small BPR turbofan you need neither a gearbox nor two spools, (nor a large diameter fan). I guess RR chose not to do it that way because performance wise that will be too similar to a turbojet and simply doesn't worth the resources. – user3528438 May 24 '18 at 00:19
  • @Therac conceptually, the fan isn't mechanically complex, but designing an efficient fan which doesn't stall, doesn't have aerodynamic flutter and/or vibration problems, and can withstand bird strikes, is far from simple - plus the requirement for a containment system if it does fail. The early fan designs using relatively simple solid blades, and snubbers between the blades to control vibration, wouldn't scale up to modern large turbofan engines. A propeller was much simpler to design than a fan, and experience with props dated right back to the Wright brothers. – alephzero May 24 '18 at 09:13
  • @alephzero: In general I agree with your comment, but prop experience is much older: Lots of powered airships had them way before the Wrights. – Peter Kämpf May 26 '18 at 20:15
  • No need for a two-spool engine if you're willing to go with a pusher turbofan. – Vikki Dec 19 '18 at 23:25
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Did they not fully realise the potential benefits at the time/thought they wouldn't work well?

Turbofans and turboprops were already studied in Britain and Germany during WW II, but at that time were technically not yet possible. Their benefit was indeed clear, even though the lower noise of turbofans was less of a consideration than it is now.

or was it just very difficult to build a jet with useful excess turbine power (to drive the fan) at the time?

Yes, that was the reason for the first jets. The pressure ratios of early jet engines did not allow to add a fan, and also the added complexity of a second spool was seen as not economical.

However, the next generation of jets could have supported a fan but didn't because they had been developed for fighters. Here the goal was to fly as fast as possible, and that needs low frontal areas and high exit speeds. Both would have suffered if a fan had been added. Civilian use of jet engines was initially restricted to the Comet, which buried its engines in the wings and could not accommodate a fan. Same for the V-bombers. Only Boeing's bombers could have benefitted from turbofans, but aerial refuelling made their low engine efficiency less obvious. Note that the Pratt&Whitney J-57, which was one of the first two-spool engines, was a pure turbojet for the first eight years of its life.

Only when the market volume of jet transport aircraft grew to a sufficient size in the late Fifties did the engine companies start to add fan stages to their existing engines. The first was the RR Conway, which had been developed for the V-bombers and had a bypass ratio of only 0.25 due to the diameter restrictions of the existing designs. The J-57 turbojet was developed further into the JT-3D turbofan in 1958 when the range of the first, turbojet-powered, Boeing 707s and DC-8s turned out to be marginal for transatlantic flights.

Yet early jetliners first used turbojets then only very low-bypass turbofans, despite the existence of turboprops

There were civilian turboprops for short range flights, but the speed advantage of jets meant that all new types developed for more than regional flying had to be jets in order to be competitive. Note that the latecomer in the trio of Boeing 707, Douglas DC-8 and Convair 880 used a smaller fuselage diameter and four Starfighter engines in order to gain a speed advantage over the other two.

Federico
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Peter Kämpf
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Because, when the first jetliners appeared, there weren't any turbofans available to power them. The first jetliner, the de Havilland Comet 1, first flew in July 1949; the first turbofan, the Rolls-Royce Conway RCo.5, wasn't available in a production-ready version until August 1955 (although, as it turned out, the first version of the Conway to power a jetliner was actually the RCo.10, which was itself rapidly superseded by the RCo.12).

Vikki
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