In commercial aviation, it seems jet engines grow and bypass ratio increase with newer engines. Here is some examples:

The bypass ratio has been multiplied by nearly 2 in 30 years (1970 -> 2015). I understand the need of really high bypass ratio to decrease fuel consumption while increasing thrust. To have big bypass ratio, the high pressure part of the engine must be small compare to the overall size of the engine.

  • What prevent jet engine designer to design smaller high pressure part or bigger jet engines to increase the bypass ratio?
  • Is there someone that has made prediction about the increasing bypass ratio in the next decades and written it in a serious article?
  • 1
    $\begingroup$ Don't forget ground clearance for any of the answer $\endgroup$
    – vasin1987
    Jan 24, 2015 at 13:36
  • $\begingroup$ It's not just about increasing thrust, it's about increasing fuel efficiency. $\endgroup$
    – RoboKaren
    Jan 24, 2015 at 16:37
  • $\begingroup$ I'd like to add one more data point to that list - 1955: bypass ratio: 0.25 (Rolls-Royce Conway). $\endgroup$
    – Vikki
    May 28, 2018 at 15:30

2 Answers 2


For ungeared two-spool designs, bypass ratios cannot be increased further. Since the fan is coupled to the low-speed turbine, increasing it would overload the turbine. Only if the engine is run at a higher core temperature can the power provided by the low-speed turbine be increased. This would require either better materials (which are not available) or lower times between overhauls (which are costly).

Note that the engines of the Rolls-Royce Trent family have the biggest bypass ratios. Rolls-Royce went with three-spool designs which allow for a finer adjustment of the speed of the different engine components.

When a reduction gearbox is placed between fan and turbine, even a two-spool design can support a larger fan, and this solution will be used in the next generation of P&W engines. Smaller engines like the ALF 502 which is used in the BAe 146 rsp. Avro RJ, used this already years ago to fit a fan to a single-spool engine. For bigger engines, transmitting the enormous power in a compact gearbox was too challenging until now. Just the heat generated by the gearbox of a 200 kN thrust class engine running at 98% efficiency would be almost 400 kW - this needs to be shed somehow!

Another problem remains, however. Bigger fans need bigger nacelles, and their drag increases with the square of the fan diameter increase. Increasing the bypass ratio of a high-bypass engine even further yields diminishing returns, which tend to be compensated by the increased nacelle drag once the bypass ratio grows beyond 12 to 15. To make bigger bypass ratios economical, active laminarisation on the nacelle will be needed.

  • $\begingroup$ Are we reaching an asymptote yet? Or do you think we'll see bypass ratios past 15 in the next decade? $\endgroup$
    – RoboKaren
    Jan 24, 2015 at 16:45
  • 1
    $\begingroup$ How (using what formula or assumption) did you translate "200 kN thrust" into Watts? $\endgroup$
    – ChrisW
    Jan 24, 2015 at 19:10
  • 3
    $\begingroup$ @ChrisW: Good question: I assumed that the engine has half as much thrust at Mach 0.8 than static (100 kN), assumed that 80% of this is from the fan (80 kN) and multiplied that with speed (240 m/s). Gives 19.2 MW, and 2% of that is 384 kW. I'd like to know myself if this is correct, but I have heard such values being bandied about for fan gearboxes before. $\endgroup$ Jan 25, 2015 at 6:11
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    $\begingroup$ @RoboKaren: I do not expect 15 in the next decade. Active laminarisation would be needed, and progress there is slow. Eventually, 15 could be conceivable, but maybe in 20 or 30 years. Who knows - predictions are hard, especially those about the future. $\endgroup$ Jan 25, 2015 at 6:13
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    $\begingroup$ @raptortech97: You suck away the slower parts of the boundary layer through small slits or rows of holes. Once is not enough, you basically make a sieve out of the skin which should be kept laminar. Would make a good question on StackExchange! $\endgroup$ Jan 26, 2015 at 7:08

The only way to increase by-pass ratio is to accelerate the high pressure turbine and avoid energy degradation. To accelerate the high pressure turbine you need higher higher pressure and higher temperature. A jet engine work like this : Imagine this : the air have these values (these values are totally arbirtary) 1, 15, 200 (Pressure, temperature, speed), the jet engine Increase pressure in compressor (10, 5, 10) after this the flow pass in combustion chamber (10, 300, 10) with the turbine you transform temperature and pressure into speed (1, 20, 2000) and you pick some energy to rotate the fan. The fan increase air speed.

So you can increase temperature but all the parts are in metal it's why the manufacturers develop specific alloys and use ceramic foils.

The other thinks to do is avoid leak, the manufacturer develop new abradable material to avoid pressure leak. The technology venture to reduce mechanical leak during energy transmission to the fan blade. The new blades profiles (fan blades are twisted) reduce energy leak due to friction and avoid air stall.

In the next year we will see new engine with open rotor and certainly new temperature resistant materials, thus the by-pass ratio will increase (the predictions are 30 or 40).

For open rotor and by-pass ratio see here (sorry i don't have english source)

  • $\begingroup$ I'm not as confident as you for the openrotor. There is a noise problem that is easily solved by putting the fan inside the engine. For me, it seems to combine inconvenients of both turboprop and jet engine without combining advantages. $\endgroup$
    – Manu H
    Jan 26, 2015 at 9:38
  • $\begingroup$ Yes the open rotor is not sale for the moment because of noise but big companies like boeing make work on this, so i believe that this will soon work properly ! $\endgroup$
    – Kromen
    Jan 26, 2015 at 13:30

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