Finally, the VC7 was intending to use the Rolls-Royce Conway, the first production bypass engine, which further increased range and improved fuel economy.

In July 1986 IAE confirmed to conduct a preliminary design study for a concept examining a high bypass engine which used the V2500 core components.

Review of these design studies in June indicated the design of such a high bypass engine as a single rotation, geared fan configuration with variable blade pitch.

Both the General Electric Company and Pratt & Whitney had been developing ultra-high bypass engines, which look much like propeller engines facing the back of the plane.

Availability of new Pratt & Whitney JT8D higher bypass engines drove early studies including designs known as Series 55, Series 50 (re-fanned Super Stretch), and Series 60.

Until recently, McDonnell Douglas had been planning to use a new technology known as the unducted fan or ultra-high bypass engine, which is 35 percent more fuel-efficient than traditional engines, and quieter, but more expensive.

Although successful in this role, only sixty-nine 707's and DC-8s were built with the Conway, due largely to the delivery of the first US-built bypass engines, particularly the Pratt & Whitney JT3D.

Alan Arnold Griffith had proposed a number of different bypass engine designs as early as the 1930s while he and Hayne Constant were trying to get their axial-flow jet engines working at the Royal Aircraft Establishment.

These limitations meant that SST designs were not able to take advantage of the dramatic improvements in fuel economy that high bypass engines brought to the subsonic market, but they were already more efficient than their subsonic turbofan counterparts.

Low bypass ratios tend to be favored for military combat aircraft as a compromise between improved fuel economy and the requirements of combat, which values higher power-to-weight ratios, supersonic performance, and the ability to use afterburners which are more compatible with low bypass engines.

The Ivchenko AI-25 is a family of military and civilian twin-shaft medium bypass turbofan engines developed by Ivchenko OKB of the Soviet Union and was the first bypass engine ever used on short haul aircraft in the USSR.