Lockheed Martin released a "finalized" design for the production F-35 in the summer of 2002. The F-35 has a nose 12 centimeters (5 inches) longer than the X-35 demonstrator, while the tailplane has been moved back 5 centimeters (2 inches), and the tailfins have been 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 gives the advantage of hydraulic power with easier maintenance and greater combat survivability than traditional hydraulic 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.
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 has been 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, as 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.
Although the P&W F119 engine was selected as the basis for the different engine options of the JSF, in 1995 the US Congress indicated a need 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 are under development by a collaboration of GE, Allison, and Rolls-Royce. Thrust levels will of course be similar to those of the F119. The F136 Alternative Engine program has been controversial, with efforts in Congress to cut the program, but so far it has survived.
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.
* 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 also 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 larger 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 will fire out the top of the left wingroot and will have 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.
* Northrop Grumman is developing the sensor suite for the F-35. 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. This 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 data links with other aircraft to provide greater "electronic domination of the battlespace". If the other aircraft 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 F-35 is also fitted with additional sensor systems, including a 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. It is derived from technology developed for the Lockheed Martin "Sniper" targeting pod.
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 a high-speed datalink, to provide sensor fusion and seamless data display. 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 his or her 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 show which targets are different from each other.
The processing power of the F-35 has presented the electronics system developers with a formidable software challenge. The F-22 Raptor uses about 2.5 million lines of software, but the F-35 will use 5.6 million lines of code. 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 is focusing 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, about 1.1 million lines.
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, this will allow software to be ported to new processors as they become available, requiring only new middleware and maybe a few software tweaks. Interestingly, the code is written in C/C++, strongly suggesting that the military's effort to create Ada as a standardized programming language for defense projects was a dead end.
The current plan is to have a comprehensive but minimal software suite for F-35 operational introduction, and provide improved releases to bring the F-35 up to full combat capability. F-35 electronics system designers hope to leverage off work done for the F-22 Raptor.
* 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)", with dimensions of 20 by 50 centimeters (8 by 20 inches), plus a secondary flight display array. It does not have a "head-up display", however, with this function taken over by a "helmet-mounted display (HMD)" being developed by Visions Systems International, a collaboration of Kaiser Electronics and Elbit of Israel. Symbology and imagery can be displayed on both the PCD and HMD. 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 a spare cockpit control layout.
The "smarts" of the F-35 will be 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 will simply press a "button" on the PCD to convert from vertical to forward flight or the reverse.
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 or impossible to do manually.
The X-35 prototypes are fitted with a Martin-Baker Mk.16E ejection seat. Production F-35s are supposed to use a new seat from the "Joint Ejection Seat Program".
* The NATO air campaign against Yugoslavia over Kosovo in the spring of 1999 revealed a shortfall in electronic warfare (EW) capabilities. EW missions during the Kosovo campaign relied heavily on the venerable EA-6B Prowler, and Prowler crews were stretched to the limit. Although the US Navy is adopting the Boeing EF-18G Growler as its replacement, the US Marines investigated the F-35 as their solution.
The conclusion of the investigation was that the Marines were not going to pursue a two-seat "EF-35" EW variant of the JSF, preferring instead to ensure that the AESA system designed for the F-35B could operate as a useful EW system -- possibly assisted by external conformal EW modules when required by the mission. This would permit a larger and, on a unit price base, cheaper buy of F-35Bs, and more flexible operational use of the aircraft.
