What prevents jet engine designs with larger bypass ratios?
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&amp;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.