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Variable Cycle Engines being studied for advanced commercial supersonic transports show potential for significant environmental and economic improvements relative to 1st generation SST engines. The two most promising concepts are: a Variable Stream Control Engine and a Variable Cycle Engine with a rear flow-control valve. Each concept utilizes variable components and separate burners to provide independent temperature and velocity control for two coannular flow streams. Unique fuel control techniques are combined with cycle characteristics that provide low fuel consumption, similar to a turbojet engine, for supersonic operation. This is accomplished while retaining the good subsonic performance features of a turbofan engine. A two-stream coannular nozzle shows potential to reduce jet noise to below FAR Part 36 without suppressors. Advanced burner concepts have the potential for significant reductions in exhaust emissions.

In a search for potential methods of reducing future air transport fuel consumption, advanced conventional turbofan and more unconventional propulsion systems were examined. Early results encouraged a more detailed evaluation of an advanced Brayton cycle gas generator in a turbofan engine or geared to an advanced multi-bladed, small diameter propeller. The fuel savings potential and operating costs are estimated for the turbofan and turboprop systems to assess both the energy conservation possibilities and the airline economic benefits.

The application of digital electronic technology to the control of advanced commercial aircraft engines is expected to reduce fuel consumption by approximately 1% and hot section maintenance costs by as much as 50% because of the ability of electronic controls to operate more precisely than their hydromechanical counterparts. Other advantages, which will accrue from the use of electronic digital controls, will be the virtual elimination of engine control trimming, a greatly simplified interface with aircraft condition monitoring equipment, and more rapid and less costly control maintenance procedures. Digital electronic control is still an emerging technology, but it offers potential benefits which should justify the investment to bring it into commercial service.