History Of The F-35B Swivel Duct
Interest in designs for vertical takeoff and landing, or VTOL, fighter aircraft began in the 1960s at the height of the Cold War when NATO bases were seen as vulnerable to preemptive attacks. Such aircraft, secured in hardened shelters, could still take off and land from bases with damaged runways.
The United States, United Kingdom, Germany, and France all built and tested multiple VTOL fighter designs. However, only the British Kestrel/Harrier family made it into service. Meanwhile, only the Yak-38, which used a similar engine and nozzle arrangement to the Harrier, saw service on the Russian side.
US Navy studies in the 1960s evaluated Sea Control VTOL aircraft designed to operate from ships with smaller decks than from decks on traditional aircraft carriers. These proposed fighters would take off vertically with full loads. The Navy’s concept of operations would also require these aircraft operate as traditional carrier-launched fighters, which necessitated afterburning engines.
This dual operational approach led to larger, heavier aircraft designs that needed more vertical thrust than could be provided by just the primary engine or engines. The most popular solution was to add small lift engines just aft of the cockpit to provide vertical thrust forward of the aircraft center of gravity. These designs were called Lift Plus Lift/Cruise. Allison, Rolls-Royce, and other engine manufacturers developed compact turbojet engines specifically for such applications. Various combinations of numbers and locations of engines were built and flown on several VTOL prototypes and experimental aircraft.
Three-bearing swivel nozzle designs were studied by virtually all of the engine companies in the mid 1960s. The US Patent Office received applications for many variations of the 3BSD from Pratt & Whitney, General Electric, and even from Boeing Military Aircraft of Wichita, Kansas.
By the late 1960s, Pratt & Whitney was designing and testing a three-bearing swivel nozzle for use on the Convair Model 200 Sea Control fighter. Design drawings dated 1967 show detail design layouts. The first nozzle was built and tested on a Pratt & Whitney JT8D in the mid 1960s. The tests included operating the nozzle in full afterburner with the nozzle deflected ninety degrees. The test rig was positioned to exhaust upward to avoid heating the ground under the test stand, though subsequent tests positioned the nozzle downward at the ground to assess the effects of ground proximity back pressure on nozzle performance.
The Convair Model 200 was proposed in June 1972 to respond to the US Navy request for designs for a fighter/attack aircraft for the Sea Control Ships. The VTOL aircraft would have used a PW401 engine with an afterburning 3BSD plus twin Allison XJ99 lift engines located behind the cockpit for added vertical lift forward of the center of gravity to balance the aft nozzle thrust. To deal with ground environment generated by the combination of the afterburning rear nozzle and the high temperatures and pressures of the lift engines, the ships would be equipped with special vertical landing areas with metal grates to allow the hot air flow to pass through.
This same request for designs led to the development of the Rockwell XFV-12 ejector augmented lift design. The Rockwell design was selected for prototyping but proved unable to produce enough thrust for vertical flight. Some have reasoned that the Navy selected the ejector design knowing that it would fail, thus eliminating a potential threat the smaller Sea Control Ships posed to the large Nimitz class carriers with conventional catapult and trap equipment. Whatever the case, the three-bearing swivel nozzle design was relegated to the file cabinets at Convair in San Diego, California.
DARPA ASTOVL And Beyond
Studies continued through the 1970s and 1980s on STOVL fighters to replace the Harrier. The studies usually added supersonic performance and multimission avionics and radar that the Harrier did not have in its original design.
The United States and the United Kingdom collaborated on studies of propulsive lift systems for the next generation of VTOL and STOVL aircraft. The Defense Advanced Research Projects Agency, or DARPA, started the STOVL Strike Fighter studies in the late 1980s. Lockheed, General Dynamics, McDonnell Douglas, and Boeing all developed concepts. The studies led to the Advanced STOVL competition that Lockheed won. In 1993, Lockheed purchased the General Dynamics Fort Worth operation, which by then was the only GD division involved with aircraft manufacturing and design.
The DARPA program later evolved into the tri-service Joint Advanced Strike Technology, or JAST effort, which evolved into the Joint Strike Fighter concept, then the X-35B prototype, and finally to today’s F-35B.
The DARPA program included construction and wind tunnel tests of a Large Scale Powered Model, or LSPM, used to measure the aerodynamics and propulsion interaction of the shaft-driven lift system developed by Lockheed.
The original design for the primary nozzle on the LSPM was a two-dimensional Single Expansion Ramp Nozzle, called SERN. On this design, one nozzle flap is longer than the other. The nozzle vectors the primary thrust by deflecting the upper flap through at least ninety degrees. To control the nozzle exit area in hover, the lower flap was designed as a sliding panel that would retract as needed to adjust the backpressure on the engine – a critical control needed to make the shaft-driven lift fan turbine work. Rolls-Royce was contracted to build the LSPM nozzle to run behind a Pratt & Whitney F100 engine and to design the X-35 prototype nozzle.
As Lockheed began small-scale wind tunnel tests of the nozzle under the DARPA program and as Rolls-Royce began building the LSPM hardware, the shortcomings of the design became more apparent.
However, trying to turn the flow with the upper flap and getting the flow to turn around the sharp lower lip produced a poor thrust coefficient. In effect, the engine flow was running into a wall (the upper flap in the deflected position) and separating across the lower lip. The nozzle was also gaining weight. The flat sides and large upper flap did not make a good pressure vessel. Thicker material and significant amounts of external stiffening were needed to hold the nozzle shape and to permit the flaps to seal. Moving a six-foot-long upper flap against full engine thrust required a very large and heavy actuator.
The tests of the LSPM in the NASA-Ames wind tunnel and on a hover test stand proved the shaft-driven lift fan system could operate. A better solution was needed for the nozzle of the X-35B, however.
At this same time, Lockheed was integrating parts of the former General Dynamics team into the ASTOVL effort. Engineers from Fort Worth had access to the archives from Convair that were transferred to Fort Worth when the San Diego operation closed.
Included in these archives were the Model 200 documents – particularly the description of the nozzle. In October 1994, Pratt & Whitney funded the Lockheed Fort Worth team to perform a study of the 3BSD for the ASTOVL configuration. This effort evaluated ground clearance of the nozzle in vertical lift position, calculated aft body drag of the nozzle, and predicted overall performance of the installed propulsion system.
The results of that study showed that the 3BSD design was significantly lighter than the SERN nozzle. Moreover, the design also showed superior propulsion performance in all modes. The 3BSD was subsequently included in the ASTOVL Configuration 141 – the original canard delta design of what evolved into the X-35.
The 3BSD was scaled to match the PW611 engine being designed for the X-35. The weight saved by incorporating the 3BSD in place of the square SERN was estimated to be more than 1,800 pounds. Moreover, the weight savings occurred at the far aft end of the aircraft and thereby helped the overall balance of the X-35 design. Furthermore, the 3BSD provided built-in yaw capability that the SERN did not have. The original ASTOVL design would have incorporated yaw vanes in the lift fan at some additional weight. Their position at the bottom of the lift fan would have produced unwanted rolling moments when yaw was commanded.
The 3BSD provides yaw control through the first swivel bearing. The resulting yaw thrust force is applied through the centerline of the engine – very near the overall aircraft vertical center of gravity. In this position, the thrust force results in no added rolling moment. The axisymmetric nozzle provided better thrust coefficients in both horizontal flight and vertical lift mode. The 3BSD moved the vertical thrust location of the nozzle farther forward relative to the SERN, resulting in a better hover balance between the forward lift fan and the rear nozzle.
The 3BSD was then combined with low-observable, or LO, axisymmetric nozzle designs that had been recently flown on the US Air Force F-16 fighter. Pratt engineers also dug into their archives and found much of the original design and test data on the 1960s development of the 3BSD. They also found designs for moving duct liner cooling air across the bearings.
The Lockheed ASTOVL/JAST team formally changed from the SERN nozzle to the 3BSD in 1995 with a compact axisymmetric convergent/divergent nozzle for the STOVL version and a longer set of nozzle flaps for better performance on the CTOL and CV variants. Other changes included planform trades (canards versus aft tails), inlet designs (caret inlets versus a diverterless bump inlet), landing gear arrangement, and weapons integration.
But a critical ingredient of meeting the STOVL weight and performance had already been put in place by combining a thirty-year-old vectoring approach with a modern engine and LO nozzle. The axisymmetric nozzle provided predictable back pressure control that worked cleanly with the Shaft Driven Lift Fan system. The pieces were falling into place.
Pratt & Whitney and Rolls-Royce built and flight qualified the 3BSD for the X-35 prototype using many of the design concept drawings from the earlier P&W work. The prototype included the liner that directs bypass cooling air through the swivel joints at all deflections, even in afterburner (though afterburner was and is not used in hover on the X-35B and F-35B).
The first flight of the conventional takeoff and landing X-35 occurred in October 2000 with the STOVL X-35B flying in June 2001. The prototype and production engine nozzles closely resemble the designs from the Convair installation. All three variants of the X-35 flew with the short nozzle flaps designed for the STOVL variant. The longer LO nozzle flaps for the production F-35A variant and the F-35C variant were developed later under a production System Design and Development effort.
Russian Swivel Nozzle Designs
A great deal of misinformation has appeared on the Internet regarding the relationship of the Soviet Yak-41 (later Yak-141), NATO reporting name Freestyle, to the X-35 and the rest of the JSF program. The Pratt & Whitney 3BSD nozzle design predates the Russian work. In fact the 3BSD was tested with a real engine almost twenty years before the first flight of the Yak.
Throughout the 1970s and 1980s, the Soviet Navy wanted a supersonic STOVL fighter to operate from its ski jump equipped carriers. At what point the Yakovlev Design Bureau became aware of the multi-swivel nozzle design is not known, but the Soyuz engine company created its own variant of it. The Yak-41 version of the nozzle, from published pictures, appears to be a three-bearing swivel duct with a significant offset “kink.” The Yak-141 also used two RKBM RD-41 lift engines – an almost identical arrangement to the Convair Model 200 design. The aircraft was also re-labeled as a Yak-141 to imply a production version, but no order for follow-on series came from the Russian Navy.
The Yak-141 was flown at the Paris Airshow in 1991. The flight displays of the Yak were suspended when the heat from the lift engines started to dislodge asphalt from the tarmac. At the 1992 Farnborough show, the Yak was limited to conventional takeoffs and landings with hovers performed 500 feet above the runway to avoid a repeat performance of asphalt damage. But the Yak-141 does deserve credit for being the first jet fighter to fly with a three-bearing swivel nozzle – twenty-five years after it was first designed in the United States.
During the early days of the JAST effort, Lockheed (accompanied by US government officials from the JAST program office) visited the Yakovlev Design Bureau along with several other suppliers of aviation equipment (notably also the Zvezda K-36 ejection seat) to examine the Yakovlev technologies and designs.
Yakovlev was looking for money to keep its VTOL program alive, not having received any orders for a production version of the Yak-141. Lockheed provided a small amount of funding in return for obtaining performance data and limited design data on the Yak-141. US government personnel were allowed to examine the aircraft. However, the 3BSN design was already in place on the X-35 before these visits.
The 3BSD was invented in America in the 1960s, proposed by Convair to the US Navy in the 1970s, first flown by the Russians in the late 1980s, re-engineered from the 1960 Pratt & Whitney design for the X-35 in the 1990s, and put into production for the F-35 in the 2000s. Sometimes a good idea has to wait for the right application and set of circumstances to come along. One moral of this story is not to throw out good work done in the past. It just might be needed later on.
Kevin Renshaw served as the ASTOVL Chief Engineer for General Dynamics and was later the deputy to Lockheed ASTOVL Chief Engineer Rick Rezabek in 1994 when the 3BSD concept was incorporated into the X-35B design. Renshaw continues to work in the Advanced System Development branch of Skunk Works where he is currently working on flight demonstration of the DARPA ARES VTOL UAV program.