Lockheed Martin F-35 Lightning II

The F-35 was the product of the Joint Strike Fighter (JSF) program, which was the merger of various combat aircraft programs from the 1980s and 1990s. One progenitor program was the Defense Advanced Research Projects Agency (DARPA) Advanced Short Take-Off/Vertical Landing (ASTOVL) which ran from 1983 to 1994; ASTOVL aimed to develop a Harrier Jump Jet replacement for the U.K. Royal Navy and the U.S. Marine Corps (USMC). Under one of ASTOVL's classified programs, the Supersonic STOVL Fighter (SSF), Lockheed Skunk Works conducted research for a stealthy supersonic STOVL fighter intended for both U.S. Air Force (USAF) and USMC; a key technology explored was the shaft-driven lift fan (SDLF) system. Lockheed's concept was a single-engine canard delta aircraft weighing about 24,000 lb (11,000 kg) empty. ASTOVL was rechristened as the Common Affordable Lightweight Fighter (CALF) in 1993 and involved Lockheed, McDonnell Douglas, and Boeing.[15][16]

In 1993, the Joint Advanced Strike Technology (JAST) program emerged following the USAF's Multi-Role Fighter (MRF) and U.S. Navy's (USN) Advanced Fighter-Attack (A/F-X) programs cancellations. MRF, a program for a relatively affordable F-16 replacement, was scaled back and delayed due to post-Cold War defense cuts easing F-16 fleet usage and thus extending its service life as well as increasing budget pressure from the F-22 program. The A/F-X, initially known as the Advanced-Attack (A-X), 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 was cancelled due to 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 A-X, which was then renamed A/F-X. Amid increased budget pressure, the Department of Defense's (DoD) Bottom-Up Review (BUR) in September 1993 announced MRF's and A/F-X's cancellations, with applicable experience brought to the emerging JAST program.[16] JAST was not meant to develop a new aircraft, instead developing requirements, maturing technologies, and demonstrating concepts for advanced strike warfare.[17]

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.[17] JAST was subsequently renamed the Joint Strike Fighter (JSF) in 1995, with STOVL submissions by McDonnell Douglas, Northrop Grumman, Lockheed Martin,[N 2] and Boeing. The JSF was expected to eventually replace large numbers of multi-role and strike fighters in the inventories of the US and its allies, including the Harrier, F-16, F/A-18, A-10, and F-117.[18]

International participation is a key aspect of the JSF program, starting with United Kingdom participation in the ASTOVL program. Many international partners requiring modernization of their air forces that deployed the F-16 and F/A-18 were interested in the JSF. The United Kingdom joined JAST/JSF as a founding member in 1995 and thus became the only Tier 1 partner of the JSF program;[19] Italy, the Netherlands, Denmark, Norway, Canada, Australia, and Turkey joined the program during the Concept Demonstration Phase (CDP), with Italy and the Netherlands being Tier 2 partners and the rest Tier 3. Consequently, the aircraft was developed in cooperation with international partners and available for export.[20]

Boeing and Lockheed Martin were selected in early 1997 for CDP, with their concept demonstrator aircraft designated X-32 and X-35 respectively; the McDonnell Douglas team was eliminated and Northrop Grumman and British Aerospace joined the Lockheed Martin team. Each firm would produce two prototype air vehicles to demonstrate conventional takeoff and landing (CTOL), carrier takeoff and landing (CV), and STOVL.[N 3] Lockheed Martin's design would leverage the work on the SDLF system conducted under the ASTOVL/CALF program. The key aspect of the X-35 that enabled STOVL operation, the SDLF system consists of the lift fan in the forward center fuselage that could be activated by engaging a clutch that connects the drive shaft to the turbines and thus augmenting the thrust from the engine's swivel nozzle. Research from prior aircraft incorporating similar systems, such as the Convair Model 200,[N 4] Rockwell XFV-12, and Yakovlev Yak-141, were also taken into consideration.[22][23][24] By contrast, Boeing's X-32 employed direct lift system that the augmented turbofan would be reconfigured to when engaging in STOVL operation.

X-35B flying over Edwards Air Force Base

Lockheed Martin's commonality strategy was to replace the STOVL variant's SDLF with a fuel tank and the aft swivel nozzle with a two-dimensional thrust vectoring nozzle for the CTOL variant.[N 5] This would enable identical aerodynamic configuration for the STOVL and CTOL variants, while the CV variant would have an enlarged wing in order to reduce landing speed for carrier recovery.[25] Due to aerodynamic characteristics and carrier recovery requirements from the JAST merger, the design configuration would settle on a conventional tail compared to the canard delta design from the ASTOVL/CALF; notably, the conventional tail configuration offers much lower risk for carrier recovery compared to the ASTOVL/CALF canard configuration, which was designed without carrier compatibility in mind. This enabled greater commonality between all three variants, as commonality goal was still very high at this stage of the design.[25] Lockheed Martin's prototypes would consist of the X-35A for demonstrating CTOL before converting it to the X-35B for STOVL demonstration and the larger-winged X-35C for CV compatibility demonstration.[26]

The X-35A first flew on 24 October 2000 and conducted flight tests for subsonic and supersonic flying qualities, handling, range, and maneuver performance.[27] After 28 flights, the aircraft was then converted into the X-35B for STOVL testing, with key changes including the addition of the SDLF, the three-bearing swivel module (3BSM), and roll-control ducts. The X-35B would successfully demonstrate the SDLF system by performing stable hover, vertical landing, and short takeoff in less than 500 ft (150 m).[25][28] The X-35C first flew on 16 December 2000 and conducted field landing carrier practice tests.[27]

On 26 October 2001, Lockheed Martin was declared the winner and was awarded the System Development and Demonstration (SDD) contract; Pratt & Whitney was separately awarded to develop the F135 engine for the JSF. The F-35 designation, which was out of sequence with standard DoD numbering, was allegedly determined on the spot by program manager Major General Mike Hough; this came as a surprise even to Lockheed Martin, which had expected the "F-24" designation for the JSF.[29]

An F-35 wind tunnel testing model in the Arnold Engineering Development Center's 16-foot transsonic wind tunnel

As the JSF program moved into the SDD phase, the X-35 demonstrator design was modified to create the F-35 combat aircraft. The forward fuselage was lengthened by 5 inches (13 cm) to make room for mission avionics, while the horizontal stabilizers were moved 2 inches (5.1 cm) aft to retain balance and control. The diverterless supersonic inlet changed from a four-sided to a three-sided cowl shape and was moved 30 inches (76 cm) aft. The fuselage section was fuller, the top surface raised by 1 inch (2.5 cm) along the centerline to accommodate weapons bays. Following the designation of the X-35 prototypes, the three variants were designated F-35A (CTOL), F-35B (STOVL), and F-35C (CV). Prime contractor Lockheed Martin performs overall systems integration and final assembly and checkout (FACO),[N 6] while Northrop Grumman and BAE Systems supply components for mission systems and airframe.[30][31]

Adding the systems of a fighter aircraft added weight. The F-35B gained the most, largely due to a 2003 decision to enlarge the weapons bays for commonality between variants; the total weight growth was reportedly up to 2,200 pounds (1,000 kg), over 8%, causing all STOVL key performance parameter (KPP) thresholds to be missed.[32] In December 2003, the STOVL Weight Attack Team (SWAT) was formed to reduce the weight increase; changes included more engine thrust, thinned airframe members, smaller weapons bays and vertical stabilizers, less thrust fed to the roll-post outlets, and redesigning the wing-mate joint, electrical elements, and the airframe immediately aft of the cockpit.[33] Many changes from the SWAT effort were applied to all three variants for commonality. By September 2004, these efforts had reduced the F-35B's weight by over 3,000 pounds (1,400 kg), while the F-35A and F-35C were reduced in weight by 2,400 pounds (1,100 kg) and 1,900 pounds (860 kg) respectively.[25][34] The weight reduction work cost $6.2 billion and caused an 18-month delay.[35]

The first F-35A prototype, AA-1, being towed to its inauguration ceremony on 7 July 2006.

The first F-35A, designated AA-1, was rolled out in Fort Worth, Texas, on 19 February 2006 and first flew on 15 December 2006.[N 7][36] The aircraft was given the name "Lightning II" in 2006.[37]

The software was developed as six releases, or Blocks, for SDD. The first two Blocks, 1A and 1B, readied the F-35 for initial pilot training and multi-level security. Block 2A improved the training capabilities, while 2B was the first combat-ready release planned for the USMC's Initial Operating Capability (IOC). Block 3i retains the capabilities of 2B while having new hardware and was planned for the USAF's IOC. The final release for SDD, Block 3F, would have full flight envelope and all baseline combat capabilities. Alongside software releases, each block also incorporates avionics hardware updates and air vehicle improvements from flight and structural testing.[38] In what is known as "concurrency", some low rate initial production (LRIP) aircraft lots would be delivered in early Block configurations and eventually upgraded to Block 3F once development is complete.[39] After 17,000 flight test hours, the final flight for the SDD phase was completed in April 2018.[40] Like the F-22, the F-35 has been targeted by cyberattacks and technology theft efforts, as well as potential vulnerabilities in the integrity of the supply chain.[41][42][43]

Testing found several major problems: early F-35B airframes had premature cracking,[N 8] the F-35C arrestor hook design was unreliable, fuel tanks were too vulnerable to lightning strikes, the helmet display had problems, and more. Software was repeatedly delayed due to its unprecedented scope and complexity. In 2009, the DoD Joint Estimate Team (JET) estimated that the program was 30 months behind the public schedule.[44][45] In 2011, the program was "re-baselined"; that is, its cost and schedule goals were changed, pushing the IOC from the planned 2010 to July 2015.[46][47] The decision to simultaneously test, fix defects, and begin production was criticized as inefficient; in 2014, Under Secretary of Defense for Acquisition Frank Kendall called it "acquisition malpractice".[48] The three variants shared just 25% of their parts, far below the anticipated commonality of 70%.[49] The program received considerable criticism for cost overruns and for the total projected lifetime cost, as well as quality management shortcomings by contractors.[50][51]

The JSF program was expected to cost about $200 billion in acquisition in base-year 2002 dollars when SDD was awarded in 2001.[52][53] As early as 2005, the Government Accountability Office (GAO) had identified major program risks in cost and schedule.[54] The costly delays strained the relationship between the Pentagon and contractors; Program Executive Officer Lt. General Christopher Bogdan highlighted the frayed relationship in 2012.[55] By 2017, delays and cost overruns had pushed the F-35 program's expected lifetime (i.e., to 2070) cost to $1.5 trillion in then-year dollars: $406.5 billion for acquisition plus $1.1 trillion for operations and maintenance.[56][57][58] The unit cost of LRIP lot 13 F-35A was $79.2 million.[6] Delays in development and operational test & evaluation has pushed full-rate production to 2021.[59][60]

The first combat-capable Block 2B configuration, which had basic air-to-air and strike capabilities, was declared ready by the USMC in July 2015.[1] The Block 3F configuration began operational test and evaluation (OT&E) in December 2018, the completion of which will conclude SDD.[61] The F-35 program is also conducting sustainment and upgrade development, with early LRIP aircraft gradually upgraded to the baseline Block 3F standard by 2021.[62]

The F-35 is expected to be continually upgraded over its lifetime. The first upgrade program, called Continuous Capability Development and Delivery (C2D2) began in 2019 and is currently planned to run to 2024. The near-term development priority of C2D2 is Block 4, which would integrate additional weapons, including those unique to international customers, refresh the avionics, improve ESM capabilities, and add Remotely Operated Video Enhanced Receiver (ROVER) support.[63] C2D2 also places greater emphasis on agile software development to enable quicker releases.[64] In 2018, the Air Force Life Cycle Management Center (AFLCMC) awarded contracts to General Electric and Pratt & Whitney to develop more powerful and efficient adaptive cycle engines for potential application in the F-35, leveraging the research done under the Adaptive Engine Transition Program (AETP).[65]

Defense contractors have offered upgrades to the F-35 outside of official program contracts. In 2013, Northrop Grumman disclosed its development of a directional infrared countermeasures (DIRCM) suite, named Threat Nullification Defensive Resource (ThNDR). The countermeasure system would share the same space as the Distributed Aperture System (DAS) sensors and acts as a laser missile jammer to protect against infrared-homing missiles.[66]

Participant nations:

  Primary customer: United States

  Tier 1 partner: United Kingdom

  Tier 2 partners: Italy and the Netherlands

  Tier 3 partners: Australia, Canada, Denmark, and Norway

  Security Cooperative Participants: Israel and Singapore

While the United States is the primary customer and financial backer, the United Kingdom, Italy, the Netherlands, Canada, Turkey, Australia, Norway, and Denmark have agreed to contribute US$4.375 billion towards development costs, with the United Kingdom contributing about 10% of the planned development costs as the sole Tier 1 partner.[67] The initial plan was that the U.S. and eight major partner nations would acquire over 3,100 F-35s through 2035.[68] The three tiers of international participation generally reflect financial stake in the program, the amount of technology transfer and subcontracts open for bid by national companies, and the order in which countries can obtain production aircraft.[69] Alongside program partner countries, Israel and Singapore have joined as Security Cooperative Participants (SCP).[70][71][72] Sales to SCP and non-partner nations are made through the Pentagon's Foreign Military Sales program.[73] Turkey was removed from the F-35 program in July 2019 over security concerns.[74][75][N 9]

Japan announced on 20 December 2011 its intent to purchase 42 F-35s to replace the F-4 Phantom II, with 38 to be assembled domestically and deliveries beginning in 2016.[77] Due to delays in development and testing, many initial orders have been postponed. Italy reduced its order from 131 to 90 F-35s in 2012. Australia decided to buy the F/A-18F Super Hornet in 2006 and the EA-18G Growler in 2013 as interim measures.[78][79]

On 3 April 2012, the Auditor General of Canada Michael Ferguson published a report outlining problems with Canada's procurement of the jet; the report states that the government knowingly understated the final cost of 65 F-35s by $10 billion.[80] Following the 2015 Federal Election, the Canadian government under the Liberal Party decided not to proceed with a sole-sourced purchase and launched a competition to choose an aircraft.[81]

In January 2019, Singapore officially announced its plan to buy a small number of F-35s for an evaluation of capabilities and suitability before deciding on more to replace its F-16 fleet.[82] In May 2019, Poland announced plans to buy 32 F-35As to replace its Soviet-era jets; the contract was signed on 31 January 2020.[83][84]

The F-35 is a family of single-engine, supersonic, stealth multirole fighters.[85] The second fifth generation fighter to enter US service and the first operational supersonic STOVL stealth fighter, the F-35 emphasizes low observables, advanced avionics and sensor fusion that enable a high level of situational awareness and long range lethality;[86][87][88] the USAF considers the aircraft its primary strike fighter for conducting suppression of enemy air defense (SEAD) missions, owing to the advanced sensors and mission systems.[89]

(From the top) F-35A of the 33rd FW, F-35B of VMFAT-501 and F-35C of VFA-101 near Eglin AFB, 2014

The F-35 has a wing-tail configuration with two vertical stabilizers canted for stealth. Flight control surfaces include leading-edge flaps, flaperons,[N 10] rudders, and all-moving horizontal tails (stabilators); leading edge root extensions also run forwards to the inlets. The relatively short 35-foot wingspan of the F-35A and F-35B is set by the requirement to fit inside USN amphibious assault ship parking areas and elevators; the F-35C's larger wing is more fuel efficient.[90][91] The fixed diverterless supersonic inlets (DSI) use a bumped compression surface and forward-swept cowl to shed the boundary layer of the forebody away from the inlets, which form a Y-duct for the engine.[92] Structurally, the F-35 drew upon lessons from the F-22; composites comprise 35% of airframe weight, with the majority being bismaleimide and composite epoxy materials as well as some carbon nanotube-reinforced epoxy in newer production lots.[93][94][95] The F-35 is considerably heavier than the lightweight fighters it replaces, with the lightest variant having an empty weight of 29,300 lb (13,300 kg); much of the weight can be attributed to the internal weapons bays and the extensive avionics carried.[96]

While lacking the raw performance of the larger twin-engine F-22, the F-35 has kinematics competitive with fourth generation fighters such as the F-16 and F/A-18, especially with ordnance mounted because the F-35's internal weapons carriage eliminates parasitic drag from external stores.[97] All variants have a top speed of Mach 1.6, attainable with full internal payload. The powerful F135 engine gives good subsonic acceleration and energy, with supersonic dash in afterburner. The large stabilitors, leading edge extensions and flaps, and canted rudders provide excellent high alpha (angle-of-attack) characteristics, with a trimmed alpha of 50°. Relaxed stability and fly-by-wire controls provide excellent handling qualities and departure resistance.[98][99] Having over double the F-16's internal fuel, the F-35 has considerably greater combat radius, while stealth also enables a more efficient mission flight profile.[100]

Electro-optical target system (EOTS) under the nose of an F-35

The F-35's mission systems are among the most complex aspects of the aircraft. The avionics and sensor fusion are designed to enhance the pilot's situational awareness and command and control capabilities and facilitate network-centric warfare.[85][101] Key sensors include the Northrop Grumman AN/APG-81 active electronically scanned array (AESA) radar, BAE Systems AN/ASQ-239 Barracuda electronic warfare system, Northrop Grumman/Raytheon AN/AAQ-37 Distributed Aperture System (DAS), Lockheed Martin AN/AAQ-40 Electro-Optical Targeting System (EOTS) and Northrop Grumman AN/ASQ-242 Communications, Navigation, and Identification (CNI) suite. The F-35 was designed with sensor intercommunication to provide a cohesive image of the local battlespace and availability for any possible use and combination with one another; for example, the APG-81 radar also acts as a part of the electronic warfare system.[102]

Much of the F-35's software was developed in C and C++ programming languages, while Ada83 code from the F-22 was also used; the Block 3F software has 8.6 million lines of code.[103][104] The Green Hills Software Integrity DO-178B real-time operating system (RTOS) runs on integrated core processors (ICPs); data networking includes the IEEE 1394b and Fibre Channel buses.[105][106] To enable fleet software upgrades for the software-defined radio systems and greater upgrade flexibility and affordability, the avionics leverage commercial off-the-shelf (COTS) components when practical.[107][108][109] The mission systems software, particularly for sensor fusion, was one of the program's most difficult parts and responsible for substantial program delays.[N 11][111][112]

AN/APG-81 AESA radar antenna

The APG-81 radar uses electronic scanning for rapid beam agility and incorporates passive and active air-to-air modes, strike modes, and synthetic aperture radar (SAR) capability, with multiple target tracking at ranges in excess of 80 nmi (150 km). The antenna is tilted backwards for stealth.[113] Complementing the radar is the AAQ-37 DAS, which consists of six infrared sensors that provide all-aspect missile launch warning and target tracking; the DAS acts as a situational awareness infrared search-and-track (SAIRST) and gives the pilot spherical infrared and night-vision imagery on the helmet visor.[114] The ASQ-239 Barracuda electronic warfare system has ten radio frequency antennas embedded into the edges of the wing and tail for all-aspect radar warning receiver (RWR). It also provides sensor fusion of radio frequency and infrared tracking functions, geolocation threat targeting, and multispectral image countermeasures for self-defense against missiles. The electronic warfare system is capable of detecting and jamming hostile radars.[115] The AAQ-40 EOTS is mounted internally behind a faceted low-observable window under the nose and performs laser targeting, forward-looking infrared (FLIR), and long range IRST functions.[116] The ASQ-242 CNI suite uses a half dozen different physical links, including the Multifunction Advanced Data Link (MADL), for covert CNI functions.[117][118] Through sensor fusion, information from radio frequency receivers and infrared sensors are combined to form a single tactical picture for the pilot. The all aspect target direction and identification can be shared via MADL to other platforms without compromising low observability. Link 16 is present for communication with legacy systems.[119]

The F-35 was designed from the outset to incorporate improved processors, sensors, and software enhancements over its lifespan. Technology Refresh 3, which includes a new core processor and a new cockpit display, is planned for Lot 15 aircraft.[120] Lockheed Martin has offered the Advanced EOTS for the Block 4 configuration; the improved sensor fits into the same area as the baseline EOTS with minimal changes.[121] In June 2018, Lockheed Martin picked Raytheon for improved DAS.[122] The USAF has studied the potential for the F-35 to orchestrate attacks by unmanned combat aerial vehicles (UCAVs) via its sensors and communications equipment.[123]

Note the sawtooth design on the landing gear door and access panels

Stealth is a key aspect of the F-35's design, and radar cross-section (RCS) is minimized through careful shaping of the airframe and the use of radar-absorbent materials (RAM); visible measures to reduce RCS include alignment of edges, serration of skin panels, and the masking of the engine face and turbine. Additionally, the F-35's diverterless supersonic inlet (DSI) uses a compression bump and forward-swept cowl rather than a splitter gap or bleed system to divert the boundary layer away from the inlet duct, eliminating the diverter cavity and further reducing radar signature.[92][124][125] The RCS of the F-35 has been characterized as lower than a metal golf ball at certain frequencies and angles; in some conditions, the F-35 compares favorably to the F-22 in stealth.[126][127][128] For maintainability, the F-35's stealth design took lessons learned from prior stealth aircraft such as the F-22; the F-35's radar-absorbent fibermat skin is more durable and requires less maintenance than older topcoats. However in April 2020 the Pentagon indicated that F-35B and C can only fly at supersonic speeds for short bursts of time before there is a risk of structural damage and loss of stealth capability. This impacts not only the F-35B and C’s airframe and the low-observable coating that keeps it stealthy, but also the myriad antennas located aft that are currently vulnerable to damage. The limitations on supersoic flight could may make it impossible for the F-35B and C to conduct supersonic intercepts.[129] [130] The aircraft also has reduced infrared and visual signatures as well as strict controls of radio frequency emitters to prevent their detection.[131][132][133] The F-35's stealth design is primarily focused on high-frequency X-band wavelengths;[134] low-frequency radars can spot stealthy aircraft due to Rayleigh scattering, but such radars are also conspicuous, susceptible to clutter, and lack precision.[135][136][137] To disguise its RCS, the aircraft can mount four Luneburg lens reflectors.[138]

Noise from the F-35 caused concerns in residential areas near potential bases for the aircraft, and residents near two such bases—Luke Air Force Base, Arizona, and Eglin Air Force Base, Florida—requested environmental impact studies in 2008 and 2009 respectively.[139] Although the noise level in decibels were comparable to those of prior fighters such as the F-16, the sound power of the F-35 is stronger particularly at lower frequencies.[140] Subsequent surveys and studies have indicated that the noise of the F-35 was not perceptibly different from the F-16 and F/A-18E/F, though the greater low-frequency noise was noticeable for some observers.[141][142][143]

F-35 cockpit simulator

The glass cockpit was designed to give the pilot good situational awareness. The main display is a 20- by 8-inch (50 by 20 cm) panoramic touchscreen, which shows flight instruments, stores management, CNI information, and integrated caution and warnings; the pilot can customize the arrangement of the information. Below the main display is a smaller stand-by display.[144] The cockpit has a speech-recognition system developed by Adacel.[145] The F-35 does not have a head-up display; instead, flight and combat information is displayed on the visor of the pilot's helmet in a helmet-mounted display system (HMDS).[146] The one-piece tinted canopy is hinged at the front and has an internal frame for structural strength. The Martin-Baker US16E ejection seat is launched by a twin-catapult system housed on side rails.[147][148] There is a right-hand side stick and throttle hands-on throttle-and-stick system. For life support, an onboard oxygen-generation system (OBOGS) is fitted and powered by the Integrated Power Package (IPP), with an auxiliary oxygen bottle and backup oxygen system for emergencies.[149]

The F-35's helmet-mounted display system

The Vision Systems International[N 12] helmet display is a key piece of the F-35's human-machine interface. Instead of the head-up display mounted atop the dashboard of earlier fighters, the HMDS puts flight and combat information on the helmet visor, allowing the pilot to see it no matter which way he or she is facing.[150] Infrared and night vision imagery from the Distributed Aperture System can be displayed directly on the HMDS and enables the pilot to "see through" the aircraft. The HDMS allows an F-35 pilot to fire missiles at targets even when the nose of the aircraft is pointing elsewhere by cuing missile seekers at high angles off-boresight.[151][152] Each helmet costs $400,000.[153] The HMDS weighs more than traditional helmets, and there is concern that it can endanger lightweight pilots during ejection.[154]

Due to the HMDS's vibration, jitter, night-vision and sensor display problems during development, Lockheed Martin and Elbit issued a draft specification in 2011 for an alternative HMDS based on the AN/AVS-9 night vision goggles as backup, with BAE Systems chosen later that year.[155][156] A cockpit redesign would be needed to adopt an alternative HMDS.[157][158] Following progress on the baseline helmet, development on the alternative HMDS was halted in October 2013.[159][160] In 2016, the Gen 3 helmet with improved night vision camera, new liquid crystal displays, automated alignment and software enhancements was introduced with LRIP lot 7.[159]

To preserve its stealth shaping, the F-35 has two internal weapons bays with four weapons stations. The two outboard weapon stations each can carry ordnance up to 2,500 lb (1,100 kg), or 1,500 lb (680 kg) for F-35B, while the two inboard stations carry air-to-air missiles. Air-to-surface weapons for the outboard station include the Joint Direct Attack Munition (JDAM), Paveway series of bombs, Joint Standoff Weapon (JSOW), and cluster munitions (Wind Corrected Munitions Dispenser). The station can also carry multiple smaller munitions such as the GBU-39 Small Diameter Bombs (SDB), GBU-53/B SDB II, and the SPEAR 3 anti-tank missiles; up to four SDBs can be carried per station for the F-35A and F-35C, and three for F-35B.[161][162][163] The inboard station can carry the AIM-120 AMRAAM. Two compartments behind the weapons bays contain flares, chaff, and towed decoys.[164]

F-35A with all weapon bay doors open

The aircraft can use six external weapons stations for missions that do not require stealth.[165] The wingtip pylons each can carry an AIM-9X or AIM-132 ASRAAM and are canted outwards to reduce their radar cross-section.[166][167] Additionally, each wing has a 5,000 lb (2,300 kg) inboard station and a 2,500 lb (1,100 kg) middle station, or 1,500 lb (680 kg) for F-35B. The external wing stations can carry large air-to-surface weapons that would not fit inside the weapons bays such as the AGM-158 Joint Air to Surface Stand-off Missile (JASSM) cruise missile. An air-to-air missile load of eight AIM-120s and two AIM-9s is possible using internal and external weapons stations; a configuration of six 2,000 lb (910 kg) bombs, two AIM-120s and two AIM-9s can also be arranged.[151][168][169] The F-35A is armed with a 25 mm GAU-22/A rotary cannon mounted internally near the left wing root with 182 rounds carried; the gun is more effective against ground targets than the 20 mm cannon carried by other USAF fighters. The F-35B and F-35C have no internal gun and instead can use a Terma A/S multi-mission pod (MMP) carrying the GAU-22/A and 220 rounds; the pod is mounted on the centerline of the aircraft and shaped to reduce its radar cross-section.[170][171] In lieu of the gun, the pod can also be used for different equipment and purposes, such as electronic warfare, aerial reconnaissance, or rear-facing tactical radar.[172][173]

Lockheed Martin is developing a weapon rack called Sidekick that would enable the internal outboard station to carry two AIM-120s, thus increasing the internal air-to-air payload to six missiles, currently offered for Block 4.[174][175] Block 4 will also have a rearranged hydraulic line and bracket to allow the F-35B to carry four SDBs per internal outboard station; integration of the MBDA Meteor and Brimstone is also planned.[176][177] The USAF and USN are planning to integrate the AGM-88G AARGM-ER internally in the F-35A and F-35C.[178] Norway and Australia are funding an adaptation of the Naval Strike Missile (NSM) for the F-35; designated Joint Strike Missile (JSM), two missiles can be carried internally with an additional four externally.[179] Nuclear weapons delivery via internal carriage of the B61 nuclear bomb is planned for Block 4B in 2024.[180] Both hypersonic missiles and direct energy weapons such as solid-state laser are currently being considered as future upgrades.[N 13][184] Lockheed Martin is studying integrating a fiber laser that uses spectral beam combining multiple individual laser modules into a single high-power beam, which can be scaled to various levels.[185]

The USAF plans for the F-35A to take up the close air support (CAS) mission in contested environments; amid criticism that it is not as well suited as a dedicated attack platform, USAF chief of staff Mark Welsh placed a focus on weapons for CAS sorties, including guided rockets, fragmentation rockets that shatter into individual projectiles before impact, and more compact ammunition for higher capacity gun pods.[186] Fragmentary rocket warheads create greater effects than cannon shells as each rocket creates a "thousand-round burst", delivering more projectiles than a strafing run.[187]

The single-engine aircraft is powered by the Pratt & Whitney F135 low-bypass augmented turbofan with rated thrust of 43,000 lbf (191 kN). Derived from the Pratt & Whitney F119 used by the F-22, the F135 has a larger fan and higher bypass ratio to increase subsonic fuel efficiency, and unlike the F119, is not optimized for supercruise.[188] The engine contributes to the F-35's stealth by having a low-observable augmenter, or afterburner, that incorporates fuel injectors into thick curved vanes; these vanes are covered by ceramic radar-absorbent materials and mask the turbine. The stealthy augmenter had problems with pressure pulsations, or "screech", at low altitude and high speed early in its development.[189] The low-observable axisymmetric nozzle consists of 15 partially overlapping flaps that create a sawtooth pattern at the trailing edge, which reduces radar signature and creates shed vortices that reduce the infrared signature of the exhaust plume.[190] Due to the engine's large dimensions, the USN had to modify its underway replenishment system to facilitate at-sea logistics support.[191]

Illustration of the STOVL swivel nozzle, lift fan, and roll-control posts

The F135-PW-600 variant for the F-35B incorporates the SDLF to allow STOVL operations. Designed by Lockheed Martin and developed by Rolls-Royce, the SDLF, also known as the Rolls-Royce LiftSystem, consists of the lift fan, drive shaft, two roll posts, and a "three-bearing swivel module" (3BSM). The thrust vectoring 3BSM nozzle allows the main engine exhaust to be deflected downward at the tail of the aircraft and is moved by a "fueldraulic" actuator that uses pressurized fuel as the working fluid.[192][193][194] Unlike the Harrier's Rolls-Royce Pegasus engine that entirely uses direct engine thrust for lift, the F-35B's system augments the swivel nozzle's thrust with the lift fan; the fan is powered by the low-pressure turbine through a drive shaft when engaged with a clutch and placed near the front of the aircraft to provide a counterbalancing thrust.[195][196][197] Roll control during slow flight is achieved by diverting unheated engine bypass air through wing-mounted thrust nozzles called roll posts.[198][199]

An alternative engine, the General Electric/Rolls-Royce F136, was being developed in the 2000s; originally, F-35 engines from Lot 6 onward would competitively tendered. Using technology from the General Electric YF120, The F136 was claimed to have a greater temperature margin than the F135.[200] The F136 was canceled in December 2011 due to lack of funding.[201][202]

In 2016, the Adaptive Engine Transition Program (AETP) was launched to develop and test adaptive cycle engines, with one major potential application being the re-engining of the F-35. Both GE and P&W were awarded contracts to develop 45,000 lbf (200 kN) class demonstrators, with the designations XA100 and XA101 respectively.[65] In 2017, P&W announced the F135 Growth Option 1.0 and 2.0; Growth Option 1.0, which had finished testing and was production ready in May 2017, was a power module upgrade that offered 6–10% thrust improvement and 5–6% fuel burn reduction. The power module could be retrofitted onto older engines and seamlessly added to future engines at low cost rise and no impact on delivery. Growth Option 2.0 would be the adaptive cycle XA101.[203][204] In June 2018, Pratt & Whitney changed its development plan for the F135, and instead offered an adaptive three-stream fan as Growth Option 2.0 that's separate from the XA101, which would instead have a new engine core.[205][206]

The F-35 is designed to require less maintenance than earlier stealth aircraft. Some 95% of all field-replaceable parts are "one deep" — that is, nothing else need be removed to reach the desired part; for instance, the ejection seat can be replaced without removing the canopy. The F-35 has a fibermat radar-absorbent material (RAM) baked into the skin, which is more durable, easier to work with, and faster to cure than older RAM coatings; similar coatings are currently being considered for application on older stealth aircraft such as the F-22.[207][208][130] Skin corrosion on the F-22 led the F-35's designers to use a less galvanic corrosion-inducing skin gap filler and to use fewer gaps in the airframe skin needing filler and better drainage.[209] The flight control system uses electro-hydrostatic actuators rather than traditional hydraulic systems; these controls can be powered by lithium-ion batteries in case of emergency.[210][211] Commonality between the different variants allowed the USMC to create their first aircraft maintenance Field Training Detachment to apply the USAF's lessons to their F-35 operations.[212]

The F-35 was intended to be supported by a computerized maintenance management system named Autonomic Logistics Information System (ALIS). In concept, any aircraft can be serviced at any F-35 maintenance facility and for all parts to be globally tracked and shared as needed.[213] Due to numerous problems, such as unreliable diagnoses, excessive connectivity requirements, and security vulnerabilities, program officials plan to replace ALIS with the cloud-based Operational Data Integrated Network (ODIN) by 2022.[214][215][216]

The first F-35A, AA-1, conducted its engine run in September 2006 and first flew on 15 December 2006.[217] Unlike all subsequent aircraft, AA-1 did not have the weight optimization from SWAT; consequently, it mainly tested subsystems common to subsequent aircraft, such as the propulsion, electrical system, and cockpit displays. This aircraft was retired from flight testing in December 2009 and was used for live-fire testing at NAS China Lake.[218]

The first F-35B, BF-1, flew on 11 June 2008, while the first weight-optimized F-35A and F-35C, AF-1 and CF-1, flew on 14 November 2009 and 6 June 2010 respectively. The F-35B's first hover was on 17 March 2010, followed by its first vertical landing the next day.[219] The F-35 Integrated Test Force (ITF) consisted of 18 aircraft at Edwards Air Force Base and Naval Air Station Patuxent River. Nine aircraft at Edwards, five F-35As, three F-35Bs, and one F-35C, performed flight sciences testing such as F-35A envelope expansion, flight loads, stores separation, as well as mission systems testing. The other nine aircraft at Patuxent River, five F-35Bs and four F-35Cs, were responsible for F-35B and -C envelope expansion and STOVL and CV suitability testing. Additional carrier suitability testing was conducted at Naval Air Warfare Center Aircraft Division at Lakehurst, New Jersey. Two non-flying aircraft of each variant were used to test static loads and fatigue.[220] For testing avionics and mission systems, a modified Boeing 737-300 with a duplication of the cockpit, the Lockheed Martin CATBird has been used.[175] Field testing of the F-35's sensors were conducted during Exercise Northern Edge 2009 and 2011, serving as significant risk-reduction steps.[221][222]

The first delivered USAF F-35 on its delivery flight to Eglin Air Force Base in July 2011.

Flight tests revealed several serious deficiencies that required costly redesigns, caused delays, and resulted in several fleet-wide groundings. In 2011, the F-35C failed to catch the arresting wire in all eight landing tests; a redesigned tail hook was delivered two years later.[223][224] By June 2009, many of the initial flight test targets had been accomplished but the program was behind schedule.[225] Software and mission systems were among the biggest sources of delays for the program, with sensor fusion proving especially challenging.[112] In fatigue testing, the F-35B suffered several premature cracks, requiring a redesign of the structure.[226] A third non-flying F-35B is currently planned to test the redesigned structure. The F-35B and F-35C also had problems with the horizontal tails suffering heat damage from prolonged afterburner use.[227][228] Early flight control laws had problems with "wing drop"[N 14] and also made the airplane sluggish, with high angles-of-attack tests in 2015 against an F-16 showing a lack of energy.[229][230]

At-sea testing of the F-35B was first conducted aboard USS Wasp. In October 2011, two F-35Bs conducted three weeks of initial sea trials, called Development Test I.[231] The second F-35B sea trials, Development Test II, began in August 2013, with tests including nighttime operations; two aircraft completed 19 nighttime vertical landings using DAS imagery.[232][233] The first operational testing involving six F-35Bs was done on the Wasp in May 2015. The final Development Test III on USS America involving operations in high sea states was completed in late 2016.[234] A Royal Navy F-35 conducted the first "rolling" landing on board the HMS Queen Elizabeth in October 2018.[235]

After the redesigned tail hook arrived, the F-35C's carrier-based Development Test I began in November 2014 aboard USS Nimitz and focused on basic day carrier operations and establishing launch and recovery handling procedures.[236] Development Test II, which focused on night operations, weapons loading, and full power launches, took place in October 2015. The final Development Test III was completed in August 2016, and included tests of asymmetric loads and certifying systems for landing qualifications and interoperability.[237] Operational test of the F-35C began in 2018.[238]

The F-35's reliability and availability have fallen short of requirements, especially during early years of testing. The ALIS maintenance and logistics system was plagued by excessive connectivity requirements and faulty diagnoses. In late 2017, the GAO reported the time needed to repair an F-35 part averaged 172 days, which was "twice the program's objective," and that shortage of spare parts was degrading readiness.[239] In 2019, while individual F-35 units have achieved mission capable rates of over the target of 80% for short periods during deployed operations, fleet-wide rates remained below target. The fleet availability goal of 65% was also not met, although the trend shows improvement. Gun accuracy of the F-35A remains unacceptable.[227][240]

Operational test and evaluation (OT&E) with Block 3F, the final configuration for SDD, began in December 2018.[241]

An F-35B launches off the "ski-jump" of HMS Queen Elizabeth in 2018.

The F-35A and F-35B were cleared for basic flight training in early 2012.[242][243] However, lack of system maturity at the time led to concerns over safety as well as concerns by the Director of Operational Test & Evaluation (DOT&E) over electronic warfare testing, budget, and concurrency for the Operational Test and Evaluation master plan.[244][245] On 10 September 2012, despite problems remaining in the operational testing plan, the USAF began an operational utility evaluation (OUE) of the F-35A, including logistical support, maintenance, personnel training, and pilot execution.[246][247] OUE flights began on 26 October and were completed on 14 November after 24 flights, each pilot having completed six flights.[248] On 16 November 2012, the USMC received the first F-35B at MCAS Yuma, although Marine pilots had several flight restrictions.[249] During the Low Rate Initial Production (LRIP) phase, the three U.S. military services jointly developed tactics and procedures using flight simulators, testing effectiveness, discovering problems and refining design.[250] In January 2013, training began at Eglin Air Force Base with capacity for 100 pilots and 2,100 maintainers at once.[251]

The USMC declared Initial Operational Capability (IOC) for the F-35B in the Block 2B configuration on 31 July 2015 after operational trials. However, limitations remained in night operations, communications, software and weapons carriage capabilities, as highlighted by DOT&E J. Michael Gilmore's criticisms of the trials.[252][253] USMC F-35B participated in its first Red Flag exercise in July 2016, though operations were limited with 67 sorties conducted.[254] The F-35A achieved IOC with the USAF on 2 August 2016, and the F-35C with the USN on 28 February 2019.[2][3] The F-35A conducted its first Red Flag exercise in 2017; system maturity had improved and the aircraft scored a kill ratio of 15:1 against an F-16 aggressor squadron in a high-threat environment.[255]

The F-35's operating cost is higher than those of some older fighters. In fiscal year 2018, the F-35A's cost per flight hour (CPFH) was $44,000, a number that was reduced to $35,000 in 2019.[256] For comparison, in 2015 the CPFH of the A-10 was $17,716; the F-15C, $41,921; and the F-16C, $22,514.[257] Lockheed Martin hopes to reduce it to $25,000 by 2025 through performance-based logistics and other measures.[258]

The F-35I Adir (accompanied by a 253 Squadron F-16I Sufa) on its debut flight in Israel, December 2016

The Israeli Air Force declared the F-35 operationally capable on 6 December 2017.[259] According to Kuwaiti newspaper Al Jarida, in July 2018, a test mission of at least three IAF F-35s flew to Iran's capital Tehran and back from Tel Aviv. While publicly unconfirmed, regional leaders acted on the report; Iran's supreme leader Ali Khamenei reportedly fired the air force chief and commander of Iran's Revolutionary Guard Corps over the mission.[260][261]

On 9 December 2010, a media report stated that the "USMC will base 216 F-35Bs on the East Coast and 184 of them on the West Coast...Cherry Point will get 128 jets to form eight squadrons; Beaufort will have three squadrons and a pilot training center using 88 aircraft; Miramar will form six operational squadrons with 96 jets and 88 F-35s will go to Yuma for five operational squadrons with an additional test and evaluation unit".[262] In 2011, the USMC and USN signed an agreement that the USMC will purchase 340 F-35B and 80 F-35C fighters. The five squadrons of USMC F-35Cs would be assigned to Navy carriers while F-35Bs would be used ashore.[263][264]

In February 2014, the USAF announced that the first Air National Guard unit to fly the F-35 will be the 158th Fighter Wing of the Vermont Air National Guard based at the Burlington Air Guard Station. The 158th currently flies aging F-16s; the F-35A is expected to arrive in 2020.[265] On 11 March 2014, the first F-35A assigned to Luke Air Force Base arrived; 144 are to be stationed there over the course of the next decade.[266][267] On 8 January 2015, RAF Lakenheath in the UK was chosen as the first base in Europe to station two American F-35 squadrons, following an announcement by the Pentagon. 48 F-35s, making up 2 squadrons, will add to the 48th Fighter Wing's already existing F-15C and F-15E Strike Eagle jets.[268]

The USMC plans to disperse its F-35Bs among forward deployed bases to enhance survivability while remaining close to a battlespace, similar to RAF Harrier deployment in the Cold War, which relied on the use of off-base locations that offered short runways, shelter, and concealment. Known as distributed STOVL operations (DSO), F-35Bs would operate from temporary bases in allied territory within the range of hostile ballistic and cruise missiles and be moved between temporary locations inside the enemy's 24- to 48-hour targeting cycle; this strategy accounts for the F-35B's short range, the shortest of the three variants, with mobile forward arming and refueling points (M-Farps) accommodating KC-130 and MV-22 Osprey aircraft to rearm and refuel the jets, as well as littoral areas for sea links of mobile distribution sites. M-Farps can be based on small airfields, multi-lane roads, or damaged main bases, while F-35Bs return to rear-area USAF bases or friendly ships for scheduled maintenance. Helicopter-portable metal planking is needed to protect unprepared roads from the F-35B's engine exhaust; the USMC are studying lighter heat-resistant alternatives.[269]

F-35B ZM148 of No. 617 Squadron landing on HMS Queen Elizabeth, 2019

The United Kingdom's Royal Air Force and Royal Navy both operate the F-35B, known simply as the Lightning in British service;[270] it has replaced the Harrier GR9, which was retired in 2010, and Tornado GR4, which was retired in 2019. The F-35 is to be Britain's primary strike aircraft for the next three decades. One of the Royal Navy's requirements for the F-35B was a Shipborne Rolling and Vertical Landing (SRVL) mode to increase maximum landing weight by using wing lift during landing.[271][272] In July 2013, Chief of the Air Staff, Air Chief Marshal Sir Stephen Dalton announced that No. 617 (The Dambusters) Squadron would be the RAF's first operational F-35 squadron.[273][274] The second operational squadron will be the Fleet Air Arm's 809 Naval Air Squadron in April 2023.[275]

No. 17 (Reserve) Test and Evaluation Squadron (TES) stood-up on 12 April 2013 as the Operational Evaluation Unit for the Lightning, becoming the first British squadron to operate the type.[276] By June 2013, the RAF had received three F-35s of the 48 on order, all initially based at Eglin Air Force Base.[277] In June 2015, the F-35B undertook its first launches from a ski-jump at NAS Patuxent River.[278] When operated at sea, British F-35B shall use ships fitted with ski-jumps, as will the Italian Navy. British F-35Bs are not intended to receive the Brimstone 2 missile.[279] On 5 July 2017, it was announced the second UK-based RAF squadron would be No. 207 Squadron,[280] which reformed on 1 August 2019 as the Lightning Operational Conversion Unit.[281] No. 617 Squadron reformed on 18 April 2018 during a ceremony in Washington, D.C., US, becoming the first RAF front-line squadron to operate the type;[282] receiving its first four F-35Bs on 6 June, flying from MCAS Beaufort to RAF Marham.[283] Both No. 617 Squadron and its F-35s were declared combat ready on 10 January 2019.[284]

In April 2019, No. 617 Squadron deployed to RAF Akrotiri, Cyprus, the type's first overseas deployment.[285] In October 2019, the Dambusters and No. 17 TES F-35s were embarked on HMS Queen Elizabeth for the first time.[286] No. 617 Squadron departed RAF Marham on 22 January 2020 for their first Exercise Red Flag with the Lightning.[287]

On 22 May 2018, Israeli Air Force chief Amikam Norkin said that the service had employed their F-35Is in two attacks on two battle fronts, marking the first combat operation of an F-35 by any country.[288][12] Norkin said it had been flown "all over the Middle East", and showed photos of an F-35I flying over Beirut in daylight.[289]

The first U.S. combat employment began in July 2018 with USMC F-35Bs from the amphibious assault ship USS Essex, with the first combat strike on 27 September 2018 against a Taliban target in Afghanistan.[290] This was followed by a USAF deployment to Al Dhafra Air Base, UAE on 15 April 2019.[291] On 27 April 2019, USAF F-35As were first used in combat in an airstrike on an Islamic State tunnel network in northern Iraq.[292]

On 25 June 2019, the first combat use of an RAF F-35B was reportedly undertaken as armed reconnaissance flights searching for Islamic State targets in Iraq and Syria.[293]

In July 2019, Israel reportedly expanded its strikes against Iranian missile shipments; IAF F-35Is allegedly struck Iranian targets in Iraq twice.[294]

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• Keijsper, Gerald. Lockheed F-35 Joint Strike Fighter. London: Pen & Sword Aviation, 2007. ISBN 978-1-84415-631-3.
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