General Electric YF120 / F120
General Electric began developing the YF120 for the ATF competition in the early 1980s. Unlike competitor Pratt & Whitney, GE elected against developing a conventional low bypass turbofan and instead chose to design a variable cycle engine. This decision was made as a result of the challenging ATF requirement of supercruise. This meant the engine had to produce a large amount of dry thrust (without afterburner) and therefore have high off-design efficiency ("design" being standard cruise conditions).
The core technology used in the YF120 was developed during two industry-government programs, the Advanced Technology Engine Gas Generator (ATEGG) and Joint Technology Demonstration Engine (JTDE) programs.
The YF120's variable cycle system worked by varying the bypass ratio of engine for different flight regimes, allowing the engine act like either a low bypass turbofan or nearly a turbojet. As a low bypass turbofan, the engine performed similar to comparable engines. When needed, however, the engine could direct more airflow through the hot core of the engine (like a turbojet), increasing the specific thrust of the engine. This made the engine more efficient at high altitude, high thrust levels than a traditional low bypass turbofan.
An expected disadvantage of this variable cycle system would be increased complexity and weight. GE claims to have combated this by using simple pressure driven valves rather than complex mechanically actuated valves to divert airflow. GE stated that this system resulted in the variable cycle system adding only 10 lb to the engine. Additionally a production F120 engine was expected to have 40% fewer parts than the F110 engine.
The YF120 engine featured a two-dimensional thrust vectoring nozzle. The nozzle allowed for vectoring in the pitch direction. This capability gave the aircraft it was installed in a serious advantage in pitch agility by greatly increasing the amount of nose pitching moment available to the aircraft. The pitching moment is traditionally generated by the horizontal stabilizer (and/or canard, if applicable), but with a thrust vectoring nozzle that moment can be agumented by the thrust of the engine. Furthermore, the pitching moment can be maintained even at low airspeed when the reduced dynamic pressure on the control surfaces makes them relatively ineffective.
GE lost the engine competition for this aircraft to Pratt & Whitney F119. While the YF120 engine never went into production, it was installed in the YF-22 used for the high angle of attack demonstration program as part of the ATF competition. During this demonstration, the YF120 powered aircraft flew, trimmed, at 60 degrees angle of attack at 82 knots. At this attitude the aircraft was able to demonstrate controlibility. Later analysis revealed that the aircraft could have maintained controlled, trimmed flight up to 70 degrees angle of attack.
The YF120 was also proposed as the basis for a more exotic engine, the Turbine-Based Combined Cycle (TBCC) engine that was to be used in demonstrator aircraft like the X-43B and future hypersonic aircraft. Specifically, the YF120 was to be the basis for the Revolutionary Turbine Accelerator (RTA-1). The variable cycle technology used in the YF120 would be extended to not only turn the engine into a turbojet but also into a ramjet. In that mode all airflow would bypass the core and be diverted into the afterburner-like "hyperburner" where it would be combusted like a ramjet. This proposed engine was to accelerate from 0 to Mach 4.1 (at 56,000 ft) in eight minutes.