What drives and/or allows the move to higher-bypass turbofans on jetliners? Note: I'm by no means an expert on any of this, I'm just curious.

There seems to be a general trend toward higher and higher bypass ratios in commercial jetliner engines. (I'm thinking here of 737/A320-size and larger aircraft.)

I understand why this is happening (fuel-efficiency &amp; noise). But what has dictated the odd pace of the development?

There was obviously a drive toward higher bypass early on. E.g. P&amp;W JT8D engines introduced in 1962 ran at 0.96:1, while - just a few years later in 1966 - the JT9D ran at 5:1. Quite a leap.

But then it seems to me that it settled around 5-6:1 for 25 years. Next big leap seems to be the GE90 in which runs at 9:1. That was 1992 and then it largely settled again at 9-10:1 for another couple of decades. The latest (smaller) leap appears to be the PW1000G at 12:1.

What has caused such sudden jumps and plateaus? Why hasn't there either been a more linear progression from the 1960s to now? Or a much faster progression toward the ratios we see today? They're obviously desirable.

I can just imagine P&amp;W engineers looking at the JT9D in the mid-1960s, and thinking "well, we went from 1:1 to 5:1 in one go. Let's go for 10:1 next!". But that didn't happen. It seems their next major airliner engine was the PW2000 - fully 20 years later - at 6:1.


Was it due to technical/manufacturing limitations?
Has the emphasis on certain performance characteristics shifted? E.g. settling more permanently on economy over speed.
Is it simply that fleets stabilized around certain aircraft, and no new engines were needed/wanted for a long time?


The last point seems very plausible. Even so, the 747-8, 737-MAX and A320neo, while being comparable to their ancestors in size and role, have made sudden jumps from 5-6:1 to 10-12:1 engines. That feels like something that could conceivably have happened earlier or more gradually.

Of course I know that bypass ratio isn't the only interesting metric for an engine, but it's a noticeable one, even to the untrained eye; engines are simply becoming bigger and bigger. But that's also why I'm asking about the pace of development, since "making the fan bigger" seems like an obvious step to take (I know there's more to it but still). It also seems like something that'd be more likely to happen gradually, rather than in spurts.

Again, I'm no expert, I've just been googling. But I may have missed something basic.
For a clue look at the development in the price of oil:



Historical oil price chart (source)

While the first development into the 5:1 region predates the first oil crisis (1973) and was driven by fuel economy, better range and lower airport noise, the next step was kicked off by the second crisis in the early 1980s. It took one decade to develop the engines and the airplanes which could fit them, so the Boeing 777 - GE90 combination came out in 1995 in a time of falling oil prices. 

The next round was only kicked off after the US invasion of Iraq which made oil prices shoot up from 2003 on. Again it took a decade for the engines to become available, but now the increase in diameter is less dramatic and existing designs can be fitted with them (A320neo, 737MAX).

Before the early 1960s the bypass ratio could not grow more because


jet engines were developed for military aircraft, and more emphasis was placed on top speed than on efficiency. Airliners used what was available (note that the Convair 880 was powered by a simplified, non-afterburning version of the J-79, which powered the F-104 Starfighter, the F-4 Phantom II and the B-58 Hustler).
early airliners (De Havilland Comet, Tupolev 104) had their engines in the wing roots, so little space was available for an increased bypass ratio. When Rolls-Royce was developing the first turbofan engine, the Conway, they limited the bypass ratio to 0.25 so it could fit into the Comet and the HP Victor bomber, which also had its engines in the wing roots.


The first oil crisis of 1973 kicked off the development of the unducted propfan (UDF) engine, which has been held back by noise problems. Again, it took about a decade until it could take to the air.



GE36 UDF engine fitted to the MD-81 (picture source)

The development of a new engine takes longer than that of an aircraft, so every time a big push, triggered by a substantial jump in oil prices, was needed to start the development of a new generation with significantly higher bypass ratio. Once the engine technology was ready and certified, it would be quickly incorporated into many models, because the better fuel economy would force everyone to upgrade.