Joint Strike Fighter

Last revised May 23, 2020


The Lockheed Martin F-35 Lightning II was the end product of the Joint Srike Fighter (JSF) program that began in 1995, which had the goal of eventually replacing multi-role and strike fighters in the inventories of the USA and its allies. These included the Harrier, the F-16, the F/A-18, A-10, and F-117. It was intended that the JSF would have variants that would be be capable of vertical takeoffs and landings, aircraft carrier operations, as well as conventional runway takeoffs and landings. Stealth was an important concern. International participation was a key part of the JSF program, with the United Kingdom, Italy, the Netherlands, Denmark, Norway, Canada, Australia, and Turkey all participating, and it was indended that the JSF would be available for export.

The JSF prgram originated as a merger of several combat aircraft programs. One of these was the Defense Advanced Research Projects Agency (DARPA)-sponsored Advanced Short Takeoff and Vertical Landing (ASTOVL) project, which a joint US/UK project that was designed to develop a Harrier replacement. Under one of ASTOVL's classified programs, the Supersonic STOVL Fighter (SSF), Lockheed Skunk Works had conducted research for the STOVL fighter that would be intended for both U.S. Air Force (USAF) and the US Marine Corps; a key technology explored was a shaft-driven lift fan (SDLF) system for vertical takeoffs and landings. The fan was to be installed in the fuselage so that its axis was vertical. The forward fan stream was separate from the core engine airflow at all times, and the fan was shut down during crusing flight, It was imagined that the fan-boosted STOVL aircraft could be modified into a conventional fighter by removing the vertical fan and replacing it with a fuel tank. Lockheed's concept was a single- engine canard delta aircraft weighing about 24,000lb (11,000kg) empty. The ASTOVL project was rechristened as the Common Affordable Lightweight Fighter (CALF) in 1993 and involved participation by Lockheed, McDonnell Douglas, and Boeing.

Due to development problems and budgetary pressues, the USAF's Multi-Role Fighter (MRF) and U.S. Navy's (USN) Advanced Fighter-Attack (A/F-X) programs were both cancelled in September 1993. The MRF, which was a program for a relatively affordable F-16 replacement, was scaled back and delayed due to post-Cold War defense cuts, and it was imagined that there would be an easing F-16 fleet usage and thus extending its service life as well as increasing budget pressures from the F-22 program. The Navy's A/F-X program, initially known as the Advanced-Attack (A-X) program, began in 1991 as the USN's follow-on to the Advanced Tactical Aircraft (ATA) program for an A-6 replacement. The resulting A-12 Avenger II had been cancelled due to development problems and cost overruns in 1991. In the same year, the termination of the Naval Advanced Tactical Fighter (NATF), an offshoot of USAF's Advanced Tactical Fighter (ATF) program, to replace the F-14 resulted in additional fighter capability being added to the A-X, which was then renamed A/F-X. Amid increased budget pressures, the Department of Defense's (DoD) Bottom-Up Review (BUR) in September 1993 announced MRF's and A/F-X's cancellations

The Joint Advaced Advanced Strike Techology (JAST) project was intended to create a family of aircraft that would cover all of the operational neads that were to have been met by the cancelled CALF, MRF, and A/F-X. The JAST was not meant to develop a new aircraft, instead was meant to develop requirements, maturing technologies, and demonstrating concepts for advanced strike warfare. As JAST progressed, the need for concept demonstrator aircraft by 1996 emerged, which would coincide with the full-scale flight demonstrator phase of ASTOVL/CALF. Because the ASTOVL/CALF concept appeared to align with the JAST charter, the two programs were eventually merged in 1994 under the JAST name, with the program now serving the USAF, USMC, and USN. The notion of developing one aircraft that could work for all services evoked some skepticism, with memories of the F-111 debacle, which had tried and failed to produce an aircraft for both the Navy and the Air Force. JAST was subsequently renamed the Joint Strike Fighter (JSF) in 1995, with STOVL submissions by McDonnell Douglas, Northrop Grumman, Lockheed Martin, and Boeing. Northrop Grumman agreed to collaborate with McDonnell Douglas and British Aerospace. An important part of JSF program was the requirement to incorporate stealth technology with the help of radar absorbent materials.

A competition was opened, and several aircraft corporation submitted entries. The McDonnell Douglas team was eliminated and Boeing and Lockheed Martin were selected in early 1997 to participate in the Concept Definition Phase, with their concept demonstrator aircraft being designated X-32 and X-35 respectively. The JSF office avoided using "YF" designations to emphasize that there was no "fly-off", so the aicraft were designated in the X-series. Northrop Grumman and British Aerospace joined the Lockheed Martin team. Each firm would produce two prototype air vehicles, one to demonstrate conventional takeoff and landing (CTOL) as well as carrier takeoff and landing (CV), and one to demonstrate vertical takeoff and landing. McDonnell Douglas merged with Boeing in 1997.

Boeing X-32

The Boeing X-32 was designed to minimize the variations between the different JSF versions. It was designed around a large one piece carbon fiber composite delta wing. The wing had a span of 9.15 meters, with a 55-degree leading edge sweep and could hold up to 20,000 pounds of fuel. The purpose of the high sweep angle was to allow for a thick wing section to be used, while still providing limited transonic aerodynamic drag, and to provide ample space for fuel as well as a good angle for wing-installed leading edge conformal antenna equipment. The cockpit was mounted above the apex of the wing, and the twin vertical tails were originally to be affixed to the wingtips but were later moved to attachment points on the rear fuselage.

There were two versions, the X-32A and the X-32B. The X-32A was the conventional takeoff and landing version and the X-32B was the VTOL version.

The X-32A was powered by a conventional derivative of the F-22 afterburning turbofan, the Pratt and Whitney F119-PW-614C. It offered a dry thrust of 28,000 lb. st and an afterburning thrust of 43,000 lb.s.t

The STVOL version was powered by a modified version of the F119-PW-614C, the -614S. In the STOVL version Boeing picked a direct-lift thrust vectoring system for the VTOL version, which would require the addition of a thrust vectoring module around the main engine. This required that the engine be mounted fairly forward in the fuselage, just behind the cockpit. In the STOVL version, part of the straight duct was replaced by a lift module, incorporating two retractable vectoring nozzles mounted at mid-fuselage, which were covered by doors when not in use. A blocker was built into the exhaust that would be activated when the aircraft was in STOVL mode. Since the addition of conventional bifurcated intakes would add several feet to the length of the aircraft, a large chin-mounted air intake was provided in order to provide sufficient air to the engine during hover, but this was a problem for stealth, since it exposed the compressor blades to radar. In normal flight, the -614S was configured as a conventional afterburning turbofan. However, in the STOVL mode a butterfly valve diverted the core stream exhaust gases to a pair of thrust vectoring nozzles located close to the aircraft's center-of-gravity. Forward of these nozzles, a jet screen nozzle provided a sheet of cool bypass air to minimise hot gas recirculation. A pair of ducts exited from the fan exhause and were directed toward the nose to provide forward-pitch control. The exhaust duct in the tail was blocked by the aft nozzle when in STOVL mode, and there was also a pair of ducts exiting from the aft exhaust duct and leading to roll nozzles near the wing tips to provide roll control during VTOL.

The Navy verison had a vortex flap above the inboard leading edge, to help address the tendency of delta-winged aircraft to assume nose-high attitudes at low speeds.

Weapons bays were to be provided on the fuselage sides underneath the high-mounted delta wing.

The first flight of the X-32A (designed for CTOL and carrier trials) took place on 18 September 2000. The X-32B STOVL version made its first flight on March 29, 2001. Some observers have pointed out that the X-32 was the ugliest airplane they ever saw.

The aircraft was 45 feet long, the wingspan was 36 feet, the wing area was 590 square feet. Maximum takeoff weight was 38,000 pounds. Maxium speed was Mach 1.6 (1200 mph) at altitude, and the ranges were 850 nm (USAF mission profile), 750 nm (USN mission profile), and 600 nm for the VSTOL version)

Lockheed Martin X-35

Unlike the X-32, the X-35 had a pair of conventional side-mounted air intakes. The F-35 layout was fairly conventional, and strongly resembled that of the F-22. The engine was the Pratt and Whitney F119-PW-611 turbofan. There were three versions--X-35A, X-35B, and X-35C. The X-35A was intended to test conventional take-off and landing capability, and was later converted to F-35B to test STOVL capablility. The X-35C was intended to evaluate carrier landing and takeoff capability.

In the X-35B STOVL aircraft, instead of lift engines or using a direct lift engine like the Rolls-Royce Pegasus in the Harrier Jump Jet, the X-35B was powered by the F119-PW-611 which used a shaft-driven lift fan system that was developed by Rolls Royce. In normal wing-borne flight, the F119-PW-611 was configured as a normal medium-bypass reheated turbofan. The turbofan acted somewhat like a turboshaft engine embedded into the fuselage (but with a much smaller percentage of total heat energy being extracted by the turbine stage), a portion of engine power was extracted via a turbine, and used to drive a shaft running forward via a clutch-and-bevel gearbox to a vertically mounted, contra-rotating lift fan located forward of the main engine in the center of the aircraft. There were large doors covering the lift/cruize nozzle and the fan inlet and exhaust which had to open and close on every flight. The rear exhaust of the engine was fitted with a "three-bearing" vectoring nozele, with the tail pipe incorporating two angled segments connected by rotating bearings. The rear exhaust of the engine tilted downward to provide lift during vertical takeoffs and landings. Bypass air from the turbofan engine compressor stages exhausted through a pair of roll-post nozzles mounted in the wings on either side of the fuselage, in orer to provide roll control during STOVL operation.

The X-35A first flew on 24 October 2000 and tested air vehicle performance and handling characteristics. After 28 test flights, the aircraft was converted to the X-35B, which added the shaft-drive lift fan, aft swivel nozzle, and roll posts to provide VTOL capability. On 20 July 2001, to demonstrate the X-35's STOVL capability, the X-35B took off in less than 500 feet (150 m), went supersonic, and landed vertically.

The X-35C first flew on 16 December 2000 and tested simulated carrier recovery and power approach. The X-35C was slightly longer than the X-35A/B (50.8 feet vs 50.5 feet). The main difference was that the X-35C had a longer wing span (40 feet vs 33 feet) and a correspondingly larger wing eara (540 square feet vs 450 square feet).

X-35A specifications Length: 50.5 ft Wingspan: 33 ft Height: 13.3 ft Wing area: 450 sq ft Empty weight: 26,500 lb Max takeoff weight: 50,000 lb Fuel capacity: 15,000 lb internal Powerplant: 1 Pratt & Whitney JSF119-PW-611 augmented turbofan, 25,000 lbf thrust dry, 40,000 lbfwith afterburner Performance Maximum speed: Mach 1.5+ at altitude Range: 1,400 mi or more Combat range: 690 mi Service ceiling: 50,000 ft

The Winner

On 26 October 2001, the Department of Defense announced that the Lockheed Martin X-35 had won the JSF competition. Pratt & Whitney was separately awarded a contract to develop the F135 engine for the JSF. The X-35 would be developed into the production aircraft for the USAF, the US Marine Corps, the US Navy, as well as for several foreign airforces. The nine major partner nations, including the U.S., plan to acquire over 3,100 JSFs through 2035, which, if delivered will make the JSF one of the most numerous jet fighters.

The assignment of the designation F-35 to the winning JSF competitor is sort of curious. Ordinarily, the designation chosen would have been the next entry in the US fighter designation system. The previous fighter in the sequence was the Northrop YF-23A, so the JSF should have been designated F-24 or perhaps F/A-24. But during the press conference announcing the winner of the JSF contest, the Pentagon acquisition chief was questioned about what the designation would be. The Pentagon moderator relayed the question to the program director. Confused, he said "X-35". The acquisition chief misheared him and said "F-35". The mistake stuck, and the designation of F-35 was officially confirmed in June 2002.

Manufacture of the F-35 was distributed among several different aircraft companies. Northrop Grumman Corporation in Palmdale and El Segundo, California will manufacture the center-fuselage. The aft fuselage and tails will be manufactured by BAE Systems in Samlesbury, England. Lockheed Martin in Fort Worth will manufacture the forward fuselage and wings. Final assembly of the F-35 will take place at Lockheed Martin Aeronautics Company in Fort Worth, Texas.

There would be three different versions of the F-35, the F-35A, F-35B, and F-35C. The F-35A would be a conventional takeoff and landing aircraft and would have the US Air Force as its primary customer. The F-35B would be the vartical takeoff and landing version, and would have the US Marine Corps as its primary customer. The F-35C would be the carrier-capable version, and would have the US Navy and the US Marine Corps as primary customers. The F-35 program evolved to become an international effort beyond the commitments of the United States and the United Kingdom and have included Australia, Canada, Denmark, Italy, Netherlands, Norway, and Turkey to varying degrees. Each nation is expecting to operate the F-35 platform in number at some point and each player is rated by partner levels in the development (the UK is a top partner as a 'Level 1' contributor while Italy and the Netherlands are 'Level 2' partners and Canada, Turkey, Australia, Norway, and Denmark are 'Level 3' partners. Israel and Singapore signed on as "Security Cooperative Participants" (SCP).

Very early on, the F-35 program became quite controversial, becuae of ballooning cost overruns, numerous technical problems, and lengthy delays.

The X-32A was sent to the National Museum of the USAF, near Dayton, Ohio in 2005. The X-32B was sent to the Patuxent River Naval Air Museum in Maryland. The X-35B is now at National Air and Space museum Steven F. Udvar-Hazy Center, and the X-35C went to the Patuxent River Naval Air Museum in Maryland

Sources:


  1. Boeing X-32, Wikipedia, https://en.wikipedia.org/wiki/Boeing_X-32

  2. Lockheed Martin X-35, Wikipedia, https://en.wikipedia.org/wiki/Lockheed_Martin_X-35

  3. Ultimate Fighter--Lockheed Martin F-35 Joint Strike Fighter, Bill Sweetman, Zenith Press, 2004 Orion, 1987.

  4. Lockheed Martin F-35 Lighning II, https://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=23