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 & noise). But what has dictated the odd pace of the development?
There was obviously a drive toward higher bypass early on. E.g. P&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&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.