The Lockheed Martin F-35 Joint Strike Fighter (JSF)

v2.1.0 / 01 feb 16 / greg goebel

* While the US military's current first-line aircraft remain formidable weapons, their basic designs are decades old, and the services would like to obtain more modern aircraft to fit their future needs. The US Air Force, the US Navy (USN), the US Marine Corps (USMC), the British Royal Navy (RN), and a long list of Western allies are now obtaining a new, advanced attack aircraft, the Lockheed Martin F-35 "Joint Strike Fighter (JSF)". This document outlines the history of the JSF effort.

Lockheed Martin F-35 Joint Strike Fighters

[2] JSF COMPETITION: X-32 & X-35


* The Joint Strike Fighter program began with defense reviews conducted by the Clinton Administration after taking office in 1992. At the time, several government organizations were working on next-generation strike aircraft.

The US Navy had been working in secret on an advanced stealthy strike aircraft named the "A-12 Avenger II", but the program ran into financial trouble, and was canceled shortly after going public in 1991. The Navy requirement remained open and evolved into a new effort designated "Attack / Fighter - Experimental (A/F-X)". The USAF was also considering a next-generation strike aircraft and a replacement for the F-16, with the designation of "Multi-Role Fighter (MRF)".

On another track, the US Marine Corps had been interested in a follow-on for their AV-8B Harrier II "short takeoff, vertical landing (STOVL)" attack aircraft, while the British Royal Navy wanted a next-generation STOVL fighter to replace their Sea Harrier naval STOVL fighters. The two services collaborated on STOVL research in the late 1980s, with control of this effort finally picked up by the US Defense Advanced Research Projects Agency (DARPA) in 1989.

DARPA conducted studies on new STOVL concepts over the next few years. During this time, the USMC had expanded their vision from a STOVL aircraft to replace the Harrier to one that would also replace their F/A-18 Hornet fighters. DARPA began to see the advanced STOVL fighter as a basis for a "conventional take-off and landing (CTOL)" aircraft as well, and the concept became known as the "Common Affordable Lightweight Fighter (CALF)".

In September 1993, the Clinton Administration killed the Navy A/F-X and Air Force MRF projects. The Pentagon was allowed to set up a research office to investigate "Joint Advanced Strike Technologies (JAST)" applicable to fulfilling these requirements at some unspecified time in the future. A few months later, the US Congress mandated that the DARPA-led CALF research effort be merged into the JAST office. At first, the JAST office was generally regarded as just another inept defense boondoggle, but under the direction of Air Force Major General George Muellner, it quickly attained a critical mass for actually building a next-generation strike fighter. Muellner wanted to build a "universal fighter" that would fulfill the needs of all the participants, and was able to get everyone pulling in more or less the same direction.

One of the primary goals of the JAST effort was "affordability". In the wake of the end of the Cold War, procurement funds for new combat aircraft were hard to come by, particularly because the Clinton Administration was determined to balance the budget while simultaneously conducting expensive military interventions all over the world. Although the US economy was in an extraordinary boom at the time, that ironically led to further constraints on the military budget, since the services had to increase pay to keep their people.

* The JAST concept that emerged did not define a single aircraft, but three different aircraft based on common technology:

The JAST also had to perform a secondary air-defense mission, using air-to-air missiles (AAMs) to defend itself, or to protect fleet assets from airborne intruders. High performance was not a requirement, though of course it was desireable. Performance was specified to be comparable to existing F-16s and F/A-18s operating in the strike role, though any incidental improvements in performance were welcome. Nominal top speed was specified as Mach 1.5, or about 1,600 KPH (1,000 MPH).

The answer to all these requirements was to develop a baseline CTOL aircraft at a base price for the USAF requirement, and variants at incrementally higher prices with the added features needed for CV or STOVL operations. The STOVL variant was to have essentially the same performance as the other two variants. That was an ambitious requirement, but the DARPA STOVL studies had indicated that STOVL technology had finally matured to the point where such a thing was possible.

The JAST concept that emerged envisioned a stealthy, single-seat aircraft, with a high degree of cockpit automation to make life easier for the pilot. It would accommodate sensors and avionics adequate for its mission. It would be fitted with an advanced radar system, working in conjunction with other electronic and infrared systems for defense and all-weather attack, but would not feature a highly optimized integration of systems in order to lower costs and preserve flexibility.

The stealth requirement meant that JAST would be able to accommodate a small warload internally, consisting of two guided munitions and a pair of AAMs, along with a much larger warload on underwing stores pylons. The JAST would operate strictly with internal weapons during initial phases of an air campaign, allowing it to perform stealthy strikes to suppress air defenses or hit heavily defended targets, and then carry heavier external loads in later phases of a conflict. The JAST office referred to this operational concept as "first day stealth".


[2] JSF COMPETITION: X-32 & X-35

* Major aircraft manufacturers began to consider JAST designs in 1994. The JAST program office issued a request for proposals in March 1996. A short time later, the project name was changed to "Joint Strike Fighter (JSF)" to indicate that they were working on real flying hardware, not blue-sky design concepts. Given that the program envisioned production runs of thousands of aircraft, there was intense interest from the US aerospace community. Three companies offered proposals to meet the JSF request:

The STOVL designs from all the manufacturers had the same infrared signatures in normal flight as the CTOL and CV variants. That was an improvement from the Harrier, whose four rotating exhaust nozzles arranged around the aircraft's center of gravity were regarded by critics as providing a perfect "bulls-eye" for heat-seeking missiles.

* In 1996, the Boeing and Lockheed-Martin concepts were selected as finalists, and both companies then began formal development of demonstrator aircraft. The elimination of McDonnell Douglas from the competition was a major blow to that company, and one of the contributing factors to the company's subsequent buyout by Boeing. Boeing planned two "X-32" demonstrators, while the Lockheed Martin planned one "X-35" demonstrator.

Boeing X-32A demonstrator

In 1998 Boeing performed a major redesign of its JSF concept to reduce cost and weight, changing its pure delta-wing configuration by adding a conventional tailplane. The Boeing demonstrator were not changed to the new configuration. First flight of the Boeing "X-32A" CTOL demonstrator was on 18 September 2000. First flight of the "X-32B" STOVL demonstrator was in March 2001, with vertical flight testing beginning in June 2001.

Lockheed Martin X-35A demonstrator

First flight of the Lockheed Martin "X-35A" CTOL demonstrator was on 24 October 2000, with first flight of the "X-35C" CV variant on 16 December 2000. The X-35A was then updated the "X-35B" STOVL demonstrator by addition of the lift fan system and other hardware, with initial vertical flight testing in June 2001.

In October 2001, the Lockheed Martin X-35 was selected as the winner of the competition. Boeing was perceived as having the edge in management, while both companies were rated equally on cost and support. However, the Lockheed Martin design was seen as involving lower risk, with the lift-fan concept for the STOVL variant scoring particular points on the win. Lockheed Martin released a "finalized" design for the production F-35 in the summer of 2002.

The British ended up at loggerheads with the Americans over the program for a time. The British wanted to have "operational sovereignty" over their F-35s, allowing them to modify and upgrade their machines as they wished. The sticking point was that this meant they would be supplied with the source code for the F-35's elaborate computer software, and the Americans were reluctant to give it up, fearing that security would be compromised. In late 2006, an agreement was hammered out; the details were secret, but it was clear that the UK was not going to pull out of the program.

The British had a long discussion over the quantities of VTOL versus CV machines, but finally settled on an F-35B VTOL fleet. Two new British carriers are being built to go into service in the next decade. The F-35 was given the official name of "Lightning II" in the summer of 2006, as a tribute to both the US Lockheed P-38 Lightning and the British English Electric Lightning. As for the two Boeing X-32 demonstrators, they are both now museum pieces.

* The initial "system design & development (SDD)" phase of the F-35 program involved the construction of 15 flight-test aircraft and seven static-test airframes. The F-35B ran into problems with "weight creep", with Lockheed Martin performing substantial redesign of structural elements and other features to trim well more than a tonne off the fighter's weight, allowing it to return to a carrier without having to dump heavy munitions.

First flight of an SDD aircraft, an F-35A CTOL variant, was on 15 December 2006, with company test pilot Jon Beesley at the controls. An F-35B performed its first (conventional) takeoff on 11 June 2008, with CTOL flights from early 2009. The first F-35C was rolled out on 29 July 2009, with first flight in November 2011. By early 2014, over 100 F-35s had been built. The US Marines were the first to announce operational capability, with the F-35B going into formal service with the Marines in the summer of 2015.



* The production F-35 has a nose 12 centimeters (5 inches) longer than the X-35 demonstrator, while the tailplane was moved back 5 centimeters (2 inches), and the tailfins were rearranged a bit. All the flight controls, except for the electromechanically-driven leading-edge flaps, are driven by an interesting "electro-hydrostatic actuation system (EHAS)", in which the actuators are all hydraulic -- but are self-contained and driven by electrical signals, not operating off a central hydraulic system. This combines the power of hydraulic systems with the greater reliability of electrical systems. Although the individual weight of each of the EHAS actuators is more than that of a traditional actuator, the overall system weight is less.

Compared to the USAF F-35A CTOL variant, the USN F-35C CV variant has a larger wing and tail, giving it better range and good low-speed carrier landing characteristics. The wing features folding wingtips. Of course, the F-35C has stronger landing gear and an arrester hook. The F-35B STOVL version has shorter tailfins, implemented as part of the weight-reduction redesign.

F-35A versus F-35B versus F-35C

The Air Force F-35A has a refueling-boom socket behind the cockpit, while the F-35B and F-35C have a retractable refueling probe on the right side of the nose. The tricycle landing gear, with a forward retracting nosewheel and inward-retracting main gear, has single wheels on all assemblies in the F-35A and F-35B. The F-35C differs in having twin wheels on the nose gear to handle hard carrier touchdowns.

The F-35's airframe makes heavy use of composite materials, with much work placed on reducing the cost of composite assemblies, which have traditionally been extremely expensive. In fact, the F-35 was designed to be as cheap to manufacture as possible, using the latest computer-aided design and manufacturing tools.

The F-35 is powered by a modified version of the P&W F119 engine, designated the "F135". While it is as powerful as the original F119, it is much cheaper, since it uses lower-cost components at the expense of greater weight. It has the same thrust levels as the F119, with 151 kN (15,420 kgp / 34,000 lbf) dry thrust and up to 222 kN (22,675 kgp / 50,000 lbf) afterburning thrust. The engine intake ducting is arranged in a "serpentine" fashion to eliminate radar reflections from the compressor blades. The shaft-driven lift fan for the STOVL F-35B is built by Rolls-Royce / Allison, and provides up to 80 kN (8,150 kgp / 18,000 lbf) of lift thrust.

Even before the flight of the X-35 demonstrators, the US Congress had been pushing for an "Alternate Engine" as a backup plan. The GE F120, originally designed for the F-22 Raptor program in competition with the P&W F119, was selected as the Alternate Engine, and modifications to the F120 as the "F136" for the F-35 were pursued by a collaboration of GE, Allison, and Rolls-Royce. The program proved controversial, critics arguing that there was no need for the F136 proportional to its cost; given funding constraints, the effort was finally killed off in late 2011.

Lockheed Martin F-35B

* The F-35 has two weapons bays, each of which can accommodate a single Joint Direct Attack Munition (JDAM) GPS-guided bomb and an AIM-120 Advanced Medium-Range Air-to-Air Missile (AMRAAM). The F-35A and F-35C can carry two 900 kilogram (2,000 pound) JDAMS internally, while the STOVL F-35B is limited to internal carriage of two 450 kilogram (1,000 pound) JDAMs. The F-35A and F-35C variants have bulged weapons bays to accommodate the larger munitions; the F-35B's weapons bays have less internal volume. All variants of the F-35 can carry up to eight 112 kilogram (250 pound) Small Diameter Bombs (SDBs) internally. The two bays have two doors each, with the AMRAAM fitted on a launch rail on the inner door.

Four stores pylons can be attached to all variants to provide a much heavier warload, at the expense of stealth. The inner pylon on each wing is rated for up to 2,270 kilograms (5,000 pounds), while the outer pylon is rated for up to 1,135 kilograms (2,500 pounds).

Only the USAF F-35A has a built-in gun. Early plans were for carriage of a variant of the Mauser BK-27 27-millimeter revolver-type cannon, but the final decision was to go with the GE GAU-12/U 25-millimeter five-barrel Gatling-type cannon, like that used on the US Marine AV-8B Harrier. It fires out the top of the left wingroot and has a store of 182 rounds. Maximum rate of fire is 3,300 rounds per minute. The other variants do not have a built-in gun, but can carry a "stealthy" cannon pod for the GAU-12/U between the weapons bays, with the pod accommodating 220 rounds of ammunition.

As for the future, Lockheed Martin has discussed the possibility of the F-35 carrying laser weapons. There's been a great deal of progress in combat laser systems in this century, but when they actually become fully operational on the battlefield remains to be seen.



* The sensor suite for the F-35 was developed by Northrop Grumman. The initial design assumption was that the JSF would be a consumer of sensor data, obtaining information from specialized intelligence-gathering aircraft, satellites, and other sources. This approach promised to keep costs down. However, as the pieces began to fit together, something different emerged. That was partly due to the "bottom-up" realization that the new technologies being developed for the JSF were far more powerful than had been considered; and to the "top-down" realization that the numbers of expensive specialized intelligence-gathering aircraft would be small, while there could be thousands of JSFs.

Now the F-35 is seen more as a producer of sensor data, with each aircraft interacting through high-speed datalinks with other platforms to provide greater "electronic domination of the battlespace". If the other platforms are F-35s, they will be able to cooperate to provide a capability greater than the mere sum of the parts.

The heart of the F-35's sensors is the Northrop Grumman AN/APG-81 radar, based on the AN/APG-77 "active electronically scanned array (AESA)" developed for the Lockheed Martin F-22 Raptor. An AESA consists of an array of "transmitter-receiver (T/R)" modules linked by high-speed processors. Different T/R modules in the array can be allocated to different tasks, with more modules allocated to tasks that require greater power or sensitivity.

The F-35's AN/APG-81 provides a range of functions, acting as a multimode radar; active jamming system; passive electronic defense system; and communications system. The system generates signals over a wide range of frequencies and pulse patterns in an unpredictable fashion to ensure "low probability of intercept", allowing the F-35 to "see but not be seen." The AN/APG-81 uses improved technology compared to the F-22's AN/APG-77, but airframe constraints mean that it has fewer T/R modules, limiting it to about two-thirds the range (165 kilometers / 90 nautical miles) of the AN/APG-77.

The US Marines considered a specialized two-seat electronic warfare (EW) derivative of the F-35, the "EF-35", but decided that the F-35B and the AN/APG-81 would provide a useful EW system -- with software to allow a single-seat aircraft to do the job, and possibly help from external conformal EW modules when required by the mission. That would permit a larger and, on a unit price base, cheaper buy of F-35Bs, and more flexible operational use of the aircraft.

* The F-35 is fitted with other sensor systems, including an "infrared search and track (IRST)" system for defense and air-to-air combat, and a targeting system for precision attack on ground targets. The IRST system is known as the "distributed aperture infrared system (DAIRS or DAS)". DAS includes six IR sensors mounted on different points of the fuselage to provide full-sphere IR detection and tracking. DAS can identify and pinpoint both incoming missiles and airborne targets.

Targeting is performed by the "electro-optical targeting system (EOTS)", featuring a forward-looking infrared (FLIR) imager; a CCD TV camera; a targeting laser; and a laser spot tracker. Unlike typical contemporary targeting systems, EOTS is not turret-mounted. It has a wide aperture that is blended into the aircraft's nose contours, covered by a window that is opaque to radar, and remains operational through the entire mission. EOTS is derived from technology developed for the Lockheed Martin "Sniper" targeting pod; it has been refined, particularly to provide better imaging resolution, since entering service. Other avionics include a Northrop Grumman "communication, navigation, and identification (CNI)" system and a countermeasures suite provided by Sanders.

* The F-35's software collects the inputs from all the sensors, as well as inputs relayed over high-rate datalinks, to provide sensor fusion and seamless data display -- pilots describing the capability of the sensor system as "mind-blowing".

There are two datalink systems -- the "Link16", which is a standardized high-bandwidth datalink, and the "Multifunction Advanced Data Link (MADL)", which is a "stealthy" datalink intended to hook up cooperating teams of F-35s. The software is executed on an "integrated core processor (ICP)". The ICP serves as a central "brain" for the aircraft, integrating all the other electronics systems and coordinating them for display to the pilot, and also executing the pilot's commands. This system is vitally important, since the F-35 is a single-seat aircraft, and the pilot needs help to carry out the mission. The processor system is linked to the aircraft subsystems over a triple-redundant MILSTD 1394B high-speed serial bus network.

Northrop Grumman selected a "commercial off-the-shelf (COTS)" processor system for the ICP. The F-35 ICP is cheaper than the F-22's "Integrated Core Processor", which was designed a decade ago, but is an order of magnitude more powerful. One of the functions of the central processing system is to provide "automatic target recognition and classification (ATRC)". It can often identify specific targets, and if it can't say exactly what a target is, it can at least categorize the unknown targets into distinct sets.

The processing power of the F-35 has presented the electronics system developers with a formidable software challenge, with the F-35 using millions of lines of code, over twice as much as the F-22. The F-35 not only has a more advanced electronics system, but it operates in both air-to-air and air-to-ground modes, and is being built in three different versions. The software design strategy focused on modularizing the code so that the portions that are unique to each F-35 variant can be isolated, and the remaining code used as-is on all three variants. The portions that are unique to each variant are a minority, less than a fifth of the total.

In addition, the code is largely executed by an interpretive software layer known as "middleware" that isolates the code from the specific details of the processor used. In principle, that will allow software to be ported to new processors as they become available, requiring only new middleware and maybe a few software tweaks. The software is being introduced in a phased fashion, with initial aircraft featuring a "Block 1" core subset to be brought up to full capability in a sequence of software blocks, the "Block 4" being currently in the pipeline.



* The pilot receives inputs from the F-35's electronic systems using an advanced cockpit layout, featuring a full-panel-width "panoramic cockpit display (PCD)" -- made up from two LCD flat panels with a total area of 20 by 50 centimeters (8 by 20 inches), plus a secondary flight display array. The F-35 does not have a "head-up display", however, with this function taken over by a "helmet-mounted display (HMD)", developed by Rockwell Collins. Symbology and imagery can be displayed on both the PCD and HMD; HMD development proved troublesome, but the bugs were worked out.

A first-generation HMD was used in trials, with a second-generation HMD used in initial flight operations. Rockwell Collins has now introduced a third-generation helmet. In maturity, the helmet's HMD system allows the pilot not only to observe symbology, but also "see through" the aircraft, using synthetic vision driven by the aircraft's sensors. The helmet also has night vision imagers that the pilot views through the HMD, a scheme which is much cleaner and more convenient than night vision goggles.

The pilot flies the aircraft with "hands on throttle and stick (HOTAS)" controls; the PCD is touch-sensitive and functions to an extent as a reprogrammable keyboard, resulting in an austere cockpit control layout.

The "smarts" of the F-35 are particularly appreciated by pilots flying the F-35B STOVL version. Short takeoffs in the Harrier are a troublesome affair that require the pilot to have "three hands": one for the throttle, one for the stick, and the third for the lever that controls the direction of the Harrier's swiveling exhaust nozzles. An F-35B pilot, in contrast, flies the plane with stick and throttle, with the software handling the fine details of short takeoff: the pilot simply pressed a "button" on the PCD to convert from vertical to forward flight, or the reverse. In vertical flight, the left-hand stick controls rate of ascent or descent; just let it go, the aircraft will sit in hover, and as has been said the pilot can eat a sandwich and take a nap until the aircraft runs out of fuel.

While the Harrier has reaction control thrusters driven by engine bleed to provide low-speed maneuverability, the F-35B simply modulates the four points of its vertical-lift system -- the pivoting exhaust, the two wing exhaust ducts, and the lift fan -- to provide control. This trick would be difficult to impossible to do manually. The pilot sits on a Martin-Baker Mk.16E ejection seat; a next-generation ejection seat was considered for production aircraft, but not proceeded with.

* Britain is anxiously awaiting to get its F-35Bs in service, for operations off the new carriers HMS QUEEN ELIZABETH and HMS PRINCE OF WALES. Both will have "ski jump" prows to allow the F-35Bs to perform short rolling takeoffs. Trials using a land-based ski jump ramp were performed in 2015, and revealed no difficulties. The British had actually opted for F-35Cs at first, but found that adding a catapult system to the carriers made them much more expensive.

Along with the UK, other foreign partners in the program include Australia, Denmark, Italy, Japan, the Netherlands, Norway, South Korea, and Turkey, with Israel obtaining the F-35 as well; the Israelis plan to install their own countermeasures systems. Canada was involved, but withdrew in 2015.

Purchase quantities also tend to be a moving target and are not discussed here; there's been concern over cost increases, with some estimates claiming the cost is doubled, though advocates of the program insist these estimates are gross exaggerations. The program did suffer enough problems to lead to a change in management, and there have been steady cutbacks in the number of projected aircraft to be built.

Exactly how many F-35s are to be built and of which variants is in flux at present. For the moment, the F-35 remains on track, but given a combination of program troubles and general military cutbacks, any figures of quantities of aircraft to be purchased by the various players in the exercise have to be regarded as made of straw, and likely to stay that way until operational squadrons start going online. The military has suffered some embarrassment over the F-35, and not without fair cause; however, given how long the F-35 is likely to remain in service, its painful birth may not seem so significant some decades on.



* In the summer of 2009, some clever people at Lockheed Martin came up with a set of images of the F-35 rendered in USAF Thunderbirds flight demonstration team colors. It was a snappy exercise that attracted considerable attention online.

The F-35 may well be the last of its breed. The expense of development new piloted combat aircraft has become increasingly unbearable, and the increasing availability of deadly air defense technologies appears to be outpacing improvements in aircraft survivability. In 2009 Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff commented: "There are those who see [the F-35] as the last manned fighter-bomber. And I'm one who's inclined to believe it, whether it's right or not." One wonders what the Thunderbirds will do when the piloted jet fighter goes out of fashion. Jet trainers will then be the better option.

Lockheed Martin F-35A JSF

* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.

* Sources include:

The JSF Program Office website was a very useful source of many details.

* Revision history:

   v1.0   / 01 dec 99 / Merged into F-22 document in May 2000.
   v2.0.0 / 01 jan 02 / Restoration & rewrite. 
   v2.0.1 / 01 nov 02 / Review & polish.
   v2.0.2 / 01 nov 04 / Review & polish.
   v2.0.3 / 01 nov 06 / Review & polish.
   v2.0.4 / 01 nov 08 / Review & polish.
   v2.0.5 / 01 aug 09 / Added Thunderbirds F-35.
   v2.0.6 / 01 may 10 / Review & polish.
   v2.0.7 / 01 apr 12 / Cancellation of F136 Alternate Engine.
   v2.1.0 / 01 mar 14 / Document reorganization, update on program status.
   v2.1.1 / 01 feb 16 / Review & update.