General Electric CJ805

Impetus

Turbojet engines consist of a compressor at the front, a burner area, and then a turbine that powers the compressor. In order to reach practical compression ratios, compressors typically consist of multiple "stages", each further compressing the intake air. In the case of an axial flow design, the stages resemble fan disks or pinwheels, situated behind each other along a common shaft.

One common problem in early jet engine designs was the phenomenon of "surging" or compressor stall. This occurs mostly at low speeds when the airflow through the engine is small, or "off-design", conditions. It may also occur when the airflow is not directly onto the front of the engine. As the air reaches the higher compression stages of the compressor, instabilities can form that, in extreme cases, can damage the engine. One common solution found on early engines was the use of "bleed air", openings near the middle of the compressor stages that feed the air around the high-pressure section and into the burner. These are closed as the engine RPM increases towards operational speeds.

Another solution to this problem is the use of variable stators or inlet vanes. In these designs, the angle of incidence of the stators at the front of the engine can be changed, allowing the degree of compression to be adjusted. At low speeds, the vanes are angled to lower the compression. This has the advantage of being more efficient than a bleed system because the entire compressor is used across the entire power range, but the disadvantage is significant mechanical complexity as each stator blade has to be independently rotated to the desired angles.

Rolls-Royce considered the variable stator idea in the 1940s, but abandoned it.^[2] They began development of multi-spool designs, a concept that was also selected by Pratt & Whitney. The variable stator was only selected by GE after a long internal debate on the best way to design an engine that would operate at speeds low enough for easy landings on an aircraft carrier all the way to Mach 2 (2,450 km/h; 1,522 mph). The J79 emerged as a powerful, lightweight design, and GE began considering it as the basis for a high-power engine for commercial use.^[3]

CJ805 program

RB-66A test aircraft powered by two GE CJ805-3 engines, on the ramp at Edwards AFB

In 1952, Chapman Walker's design team at GE built a one-off prototype of a jet engine designed specifically for transatlantic airliners. It used a single-stage fan powered by the same turbine shaft as the main engine compressor, as opposed to the Pratt & Whitney designs that were using a separate power shaft to run the fan. The GE design proved to be difficult to start and operate and was not developed further.^[2]

In 1955 Jack Parker took over GE's Aircraft Gas Turbine division. He hired Dixon Speas to begin interviewing executives at airlines to try to get a sense of the future market. Parker asked Speas to interview not the CEO's, but executives that might be the CEO by the time GE was ready to enter the civilian jet engine market. Parker, Speas and Neil Burgess, who ran the J79 program, spent a month meeting with AA, Delta, United, KLM, Swissair and SAS. The meetings demonstrated that those airlines that were flying propeller aircraft across the Atlantic were all looking to replace them with jets.^[4]

CJ805-3

Around the same time, Convair was canvassing US carriers and found demand for a smaller jet aircraft for medium-range domestic routes. They began development of what would become the 880, and approached Burgess to see if GE could develop a version of the J79 for this role. Burgess responded by quickly sketching a version of the J79 with the afterburner removed and replaced by a thrust reverser, giving them an estimated unit price of $125,000 per engine.^[2]

The 880's primary sales feature over the competing Douglas DC-8 and Boeing 707 was a higher cruise speed. This demanded more engine power from a lighter design, which naturally led to a design like the J79. To gain experience with the engine in a civil setting, GE equipped a Douglas RB-66 with the new engine and flew simulated civil aviation routes out of Edwards Air Force Base.^[5]

As development progressed, the 707 began to enter service, and noise complains became a serious issue. There was already a lawsuit, by residents around Newark airport, concerning the noise from existing propeller-driven aircraft such as the Super Constellation, Stratocruiser and DC-7C.^[6] One way to reduce this problem is to mix cold air into the jet exhaust, which was accomplished on early engines with the addition of scalloped nozzles.^{[lower-alpha 1]} This solution was also adopted for the CJ805.

CJ805-23

Cutaway of a CJ805-23, the turbofan version of the engine, with fan at the rear

The turbofan CJ805-23 powered the Convair 990 airliners

Several airlines asked Convair for a larger version of the 880 with potential transatlantic range. Such a design would be larger to hold more seating, as well has having to carry more fuel. To power it, a more powerful engine would be needed. By this time, the Rolls-Royce Conway was entering service, and the Pratt & Whitney JT3D was following close behind. These designs both had twin-spool compressors, as opposed to using variable stators, and the lower speed of the front, low-pressure, spool make it easy to power a fan.^[8]

The problems RR and P&W had addressed with the two-spool system had been solved on the J79 with the variable stators, so in relative terms, the single compressor rotational speed was much faster than the low-pressure stage of these other engines. This meant it was not suitable for direct connection to a fan stage. Instead, GE solved this problem with the addition of a completely separate fan system at the rear of the engine, powered by a new turbine stage. The system was essentially a bolt-on extension to the existing design and had almost no effect on the operation of the original engine.^[9]

Each turbine blade was an integral part of a blucket, the outboard section of which was a fan rotor blade.^[10] Running freely on a stub shaft, a series of buckets, mounted on a disc, made up the aft rotor assembly. The efflux from the turbojet expanded through the (inner) turbine annulus, thus providing power directly to the fan blades located in the outer annulus. A full-length cowl, an annular exhaust system and a bucket thrust-reverser were fitted for the Convair 990.^[11]

The unique feature of the -23 was the transonic single stage fan.^[12] NACA had done a lot of research on multistage transonic compressors during the 50s. Using this data, GE decided to design and test a high-pressure ratio single stage transonic fan. Much to their amazement the unit more than met the design target, including that of high efficiency. A modified version of this research unit was subsequently incorporated into the CJ805-23 aft fan. With no experience of transonic fan design and little time available, Pratt & Whitney had to resort to using 2 fan stages to produce a similar pressure ratio for their JT-3D turbofan. Although not an overhung design, the -23 transonic fan did not require any inlet guide vanes. There was, however, a series of structural vanes to help support the fan casing.^[13]

Production ends

With additional changes, fuselage stretches, and the addition of anti-shock bodies, the new airliner emerged as the Convair 990. However, by this time the project had suffered several delays, allowing new versions of the DC-8 and 707 to lock up major sales. In the end, Convair sold only 102 880s and 990s in total, losing $600 million on the program.^[14]

There was only one other customer for the 805-23. In 1961, Sud Aviation approached GE to pitch them on the idea of adapting the Rolls-Royce Avon powered Caravelle to the 805-23, producing a flying technology showcase for both companies.^[15] For this role they introduced a new version with a relatively short fan cowl and thrust reverser, compared to the full-length cowling on the 990.^[16] Rolls-Royce quickly built and tested an aft-fan demonstrator Avon to compete with the greater thrust and lower specific fuel consumption of the CJ805-23. In the end, the Caravelle was instead re-engined with the P&W JT8D turbofan.^[17]

The CJ805 program was not a commercial success, and GE lost approximately $80 million on the program with only a few hundred engines produced in total.^[15] In service, the design proved fragile, but these problems led to the programs ultimate success for the company.^[18]

During the time they were talking to airline CEOs, in 1956 the company hired the former head of American Airline's maintenance department, John Montgomery, to run the production lines. Montgomery gathered comments from the industry on the state of the engine market, and found that many were complaining about the unreliability of the large piston engines then being used, notably the Wright R-3350. Wright management refused to put more money into the program to improve the engine, leading to a serious backlash from the customers.^[19]

Montgomery hired Walter Van Duyan away from Wright to set up GE's service department, and they provided excellent service in spite of the engine's problems. GE quickly gained a reputation for standing behind their products that endures to this day.^[19]

The work on the 805 also had several spin-off products. Among them was another aft-fan design, the General Electric CF700 used in the Dassault Falcon 20 business jet, which was developed from the General Electric J85 in the same way as the J79 was adapted to the 805.^[20] Their fan technology was also used in the XV-5 Vertifan.^[21]

Citations

Bibliography