In 1996, Williams teamed with NASA to develop a small and light turbofan engine, the FJX-2, for general aviation. The General Aviation Propulsion (GAP) program was part of NASA’s 1992 Advanced General Aviation Transport Experiments (AGATE) program; a joint NASA/industry venture to revitalize general aviation.

The FJX-2 is a high-bypass-ratio turbofan engine that produces 700 pounds of thrust, yet weighs only 85-100 lb, about one-fourth the weight of piston engine propulsion systems with similar capabilities. To keep costs low, the FJX-2 team applied many lessons learned from research of automotive gas turbine engines. Emphasis was placed on simplifying design and reducing the number of parts. Low-cost design techniques and advanced automated manufacturing methods have led to the first turbine engine that is cost competitive with piston engines.

The Williams F121 (company designation WR36-1) is a small turbofan engine designed for use in the AGM-136 Tacit Rainbow anti-radiation cruise missile.

The F121 engine had a rare set of design parameters as it is designed to be used only once. As a cruise missile engine, it was designed to have a long shelf life (be able to sit around unused for long periods of time) and then operate when needed for several hours. It was designed to power the AGM-136 Tacit Rainbow, which was to be a stand-off anti radiation missile. Its first flight was on July 30, 1984. The AGM-136 program was canceled several years later.

Another unique feature of the engine is that it was started with an explosive cartridge because it couldn’t start while still mated to its aircraft.

In the late 2000s, the engine was being used by the Naval Air Warfare Center at Naval Air Weapons Station China Lake to test fuel performance and additives.

Specifications:
Type: Single Spool Turbofan
Length: 40 in (1.01 m)
Diameter: 8.5 in (0.22 m)
Dry weight: 49 lb (22.22 kg)
Compressor: 1 stage axial fan, 6-stage axial compressor
Combustors: Turboméca Piméné-type
Turbine: 2-stage axial
Maximum thrust: 70 lbf (0.31 kN)
Bypass ratio: 1.7:1
Thrust-to-weight ratio: 1.43:1

Williams International had been building small turbofan engines for cruise missile applications since the 1960s, and had successfully entered the general aviation market in 1992 with the FJ44 engine. That same year, NASA initiated a program, Advanced General Aviation Transport Experiments (AGATE), to partner with manufacturers and help develop technologies that would revitalize the sagging general aviation industry. In 1996, Williams joined AGATE’s General Aviation Propulsion (GAP) program to develop a fuel-efficient turbofan engine that would be even smaller than the FJ44 and designated the FJX-2 engine.

Initially, Williams contracted with Burt Rutan’s Scaled Composites to design and build the Williams V-Jet II, a Very Light Jet (VLJ) to use as a testbed and technology demonstrator to showcase the new engine. The aircraft, powered by two interim FJX-1 man-rated version of Williams’ cruise-missile engine, debuted at the 1997 Oshkosh Airshow. Development of the FJX-2 engine progressed, most of the design work was completed during 1998 with initial prototype parts being delivered in the second quarter of that year. The FJX-2 engine was designed with many experimental systems and manufacturing processes to minimize parts count and lower production costs and having a bypass ratio of 4:1. As a result, result there were many technical difficulties and failures of the initial prototype hardware. However, subsequent re-designs and the incorporation of more conventional systems resulted in the engine eventually meeting the NASA requirement of 700 lbf (3,100 N) thrust. The program ultimately culminated with altitude testing at the NASA Glenn Research Propulsion Systems Laboratory from March – April 2000.

In 2000, Williams joined with Eclipse Aviation to develop an FAA-certified version of the FJX-2, designated the EJ22, to be used on the Eclipse 500 VLJ due for first flight in June 2002. This would be an unprecedentedly short period of time to develop a new man-rated turbofan engine. The new EJ22 powered the Eclipse 500 prototype on its first flight in the Summer of 2002.

Eclipse initially required the engine to produce 770 lbf (3,400 N) thrust, exceeding the 700 lbf (3,100 N) rating of the FJX-2 by 10%. This was only the first of a large number of continuously changing requirements Eclipse would demand. Ultimately, numerous technical problems with the EJ22, significantly compounded by the frequently changing requirements of Eclipse Aviation, grounded the plane and prompted Eclipse Aviation to terminate its relationship with Williams International in late 2002. Following termination of the contract, development work and FAA certification was halted shortly thereafter.

To achieve the required TSFC, the EJ22 turbofan was designed as a three spool engine having a fan, two axial compressors and three expansion turbines. As a result, the engine was significantly more complicated than any prior Williams International engine. While very impressive on the test stand, the EJ22 proved quite temperamental during the two years of its development process and it was frequently subject to problems starting, overheating, part failures and various subsystem issues. While most of problems may have eventually been resolved during a normal development program, the shortened development period, and the frequent changes by Eclipse, proved to be unsurmountable obstacles.

The engine is a 700 lbf (3,100 N) thrust class medium-bypass ratio (4:1 bypass ratio) turbofan with a fan diameter of about 15 inches. Length is 41 inches (1,000 mm), and basic engine weight was 85 pounds. At the time of testing in March 2001 of an early EJ22 prototype the engine weighed 96 lb (44 kg) and demonstrated a thrust-to-weight ratio of 7.52. The main compressor has 6 stages, and weighs only 1.22 lb (0.55 kg). Engine layout was a three-spool arrangement, with all three compressors and turbines being axial. This was a departure from previous Williams engines, which had all used centrifugal high-pressure compressors followed by axial turbines. A reverse flow combustor and a mixed exhaust were other features.