During March 1994, Boeing was selected for the direct lift STOVL concept after additional funding from Congress was awarded. Boeing cost shared the risk reduction program with DARPA which allowed Boeing to vie for a follow-on demonstration program. Later, in 1994, Northrop Grumman began critical technology validation with DARPA under a zero cost agreement.

The Joint Advanced Strike Technology (JAST) program was the US DoD central point for procuring future strike systems looking at affordable technologies and manufacturing processes. Following a six-month concept study it became evident that the most cost effective solution would be using one aircraft with three variants across the United States Navy,United States Marine Corps and the United States Air Force. The United States Navy required a Carrier Variant (CV) to work with its existing F/A-18E/F, the Marines a Short Take-Off Vertical Landing (STOVL) jet to replace its AV-8B and F/A-18C/D models, while the USAF needed a CTOL aircraft to replace its A-10 and F-16 and to complement its F-22.

In October 1994 the ASTOVL and JAST programs were merged as a result of budget legislation and the resulting program named Joint Strike Fighter (JSF). The UK MoD became a formal partner in December 1995 and committed $200 million to participate in a four year weapons system concept demonstration. Initial studies concluded that the JSF would be of a single engine design and in 1995, Pratt & Whitney, with their F119 engine, were awarded the contract to carry out preliminary design work on an engine for each of the JSF concepts.At the same time a partnership of General Electric, Allison and Rolls-Royce was also chosen to design an alternative engine in the form of the YF-120.

CALF stage 2 CTV studies were completed by the spring of 1996 and consolidated into the JSF program. On 16th November 1996, the US Secretary of Defence announced that McDonnell Douglas had been eliminated from the upcoming Weapons System Concept Demonstration phase and that Boeing, along with Lockheed Martin, had been chosen. Pratt & Whitney at the same time was chosen for the associated Engine Ground and Flight Demonstration Program.

Joint Strike Fighter competition

With Lockheed Martin and Boeing signing the development contract the JSF program began. Each company was tasked with producing two flight demonstrators, a CTOL/CV aircraft and a STOVL variant to fly against one another at a later date.

The winner of this competition would receive the contract to mass-produce the 5th generation fighter aircraft on a large scale making it one of the most important military procurement contracts in history. During 1996, the UK MoD launched the Future Carrier Borne Aircraft project to look at a replacement for the Sea Harrier fleet and later the Harrier GR-9 fleet. On the 17th January 2001 the UK signed a Memorandum of Understanding with the US DoD for full participation in the JSF program, which concluded that the JSF would be its choice for replacement of the Harrier fleet.

The battle of the X-planes had begun. During the competition stage, Lockheed Martin developed the X-35A (converted to X-35B at a later stage) and X-35C, Boeing designed and built the X-32A and X-32B. The X-32 was the first to get airborne, making its maiden flight on the 18th September 2000, with the X-35 taking to the air the following month on 24th October 2000.

The X-35A was used for early CTOL test flights, with all program objectives met and exceeded by 22nd November 2000. Shortly after, the “A” model was modified over a six month conversion process to allow for a 2nd engine to be added, this engine, a Rolls Royce/Allison was a shaft-driven lift-fan system, along with roll control jets along the wings. This was linked to the main Pratt & Whitney F119-611 turbofan engine to provide a separate source of thrust during the hover stage. For the STOVL selection process, the X-35A was now re-designated the X-35B. Aviation history was made on June 23rd 2001 when the first shaft-driven lift-fan propulsion system lifted an aircraft into the sky, for several minutes the X-35B hovered at approximately 20ft then made a vertical landing.

The X-35B made its final flight on August 6th 2001 when it took off from Edwards AFB, CA and flew to Palmdale, CA. During its 3.7 hour final flight, it had six aerial refuelling’s and made six touch and go landings. The X-35B flight test program was one of the shortest and most effective in history, lasting from 23rd June 2001 until 6th August 2001.

The X-35C CV Naval variant was fitted with a new structure to absorb high impact landings. It also featured an increased fuel capacity and larger horizontal tails/control surfaces which provided greater low speed control for carrier approaches. Its maiden flight was on 16th December 2000 when it departed Palmdale and made a 27 minute flight to Edwards AFB. During this flight it climbed to an altitude of 10,000 feet and performed a number of rolls, turns, slide slips and general flight control trials along with landing gear tests.

In early 2001 a number of envelope-expansion flights began with air-to-air refuelling tanker qualifications, followed by its first supersonic flight leading to the first trans-continental flight by an X plane. Further testing during the program included Field Carrier Landing Practice (FCLP), which evaluated the performance and low speed handling qualities required during carrier approaches. Additionally, FCLP evaluated variable in flight engine speed tests and continued altitude testing. On 11th March 2001, the X-35C completed its test program with all objectives achieved.

• X-35A recorded 27 flights, 27.4 flight hours and max speed of Mach 1.05
• X-35B recorded 39 flights, 21.5 flight hours and max speed of Mach 1.05
• X-35C recorded 73 flights, 58.0 flight hours and max speed of Mach 1.22

Boeings X-32 ACTOL design was the first of the X planes to take to the skies and demonstrate overall flight characteristics along with software control. The X-32A was also used to evaluate the low speed handling required for the CV. The X-32A featured a non-moving intake along with a wide wing span and folding wing tips. It performed its first simulated carrier approach on 15th November 2000 and performed its first aerial refuelling 5 days later. The following day, 21st December 2000 the X-32A made its first supersonic flight. Only a few days later on 26th January 2001, the first flight with a loaded weapons bay took place, with all flight-testing being completed on3rd February 2001.

The design characteristics of the X-32B differed considerably to the X-32A; it was fitted with a direct lift system for STOVL operations and incorporated vectoring nozzles, a moving intake-cowl, shorter fuselage and a narrower wingspan. Certain parts had to be removed for STOVL demonstrations and supersonic testing could only be performed in the normal configuration. The X-32B made its maiden flight on 29th March 2001. On 11th May 2001, the X-32B arrived at Naval Air Station (NAS) Patuxent River in Maryland to continue flight testing and 78 flight tests were made during a four month period. By July, STOVL testing was concluded and demonstrations of the X-32B were over.

Eight months into the competition, Boeing re-designed the configuration for the production models due to shortfalls in payload and manoeuvrability performance, which was required by the USN. Changes proposed were the removal of the variable intake cowl for the X-32C, a re-designed wing with a replacement of the twin tails to the more conventional vertical and horizontal tails.However these changes were never introduced.

• X-32A recorded 66 flights and max speed of Mach 1.00+
• X-32B recorded 78 flights and max speed of Mach 1.00+

On 26th October 2001 the DoD announced that Lockheed Martin had been selected as the winner of the JSF X-plane competition.

F-35 production

Lockheed Martin as prime contractor teamed up with Northrop Grumman and BAE Systems to start production for what was, and still is, the largest aerospace defence program in history. Lockheed Martin is responsible for manufacturing the front fuselage, wings, flight control systems and also completes final assembly at Fort Worth, Texas. Northrop Grumman manufactures the mid-fuselage, weapons bay, arrestor gear, Active Electronically Scanned Array (AESA) radar, communications, navigation and target identification systems. BAE Systems manufacture the aft fuselage and tail as well as the crew escape, fuel, flight controls and electronic warfare systems. In addition to the key three manufactures a further 1,400 companies supply parts for the F-35 program.

During 2002 Canada, Denmark, Norway, Italy, Turkey, Australia and the Netherlands joined the international partnership and committed over $2.5 billion in funding towards the program. In 2003 production of the first major airframe components began and the first hardware delivery completed. During 2004, the STOVL variant was stripped of more than 2,700 pounds of excess weight during design changes.

AAssembly of the first F-35 commenced at Fort Worth in 2004, a year later, the assembly was complete. At the same time, the first F-35 engine test began.

For Lockheed Martin, 2006 was to be a very important year; the first F-35 was completed on schedule and moved from the factory floor to ground testing. On 7th July 2006, JSF AA-1 was unveiled to the world and received its official name, Lightning II, after the P-38 Lightning and the English Electric Lightning interceptor. On 15th December 2006 aircraft AA-1 an F-35A variant was flown by chief pilot Jon Beesley over the skies of Fort Worth to make its maiden 35 minute flight.

Characteristics

Three variants are being produced: F-35A CTOL, F-35B STOVL and F-35C CV. Although similar in resemblance to the X-35 with same basic shape, many characteristics have changed during the system design and development phase. The demonstrators used off-the-shelf components, where as the F-35 uses newly designed components with state of the art technology to reduce productiontime and costs. The major changes between the X-35 and the F-35 are;

Cockpit

The cockpit has been designed for the needs of a 21st century fighter pilot with priority given to situational awareness. At the initial design stage the design team were well aware that monitoring an aircraft’s systems could divert a pilot’s attention from information that is critical to the mission success, so a fresh approach was considered.

A new eight by twenty inch LCD display thatis controlled by finger-on-glass technology replaced many of the hard-switches that have traditionally graced a cockpit. The LCD touch-screen now incorporates all radio, mission systems computers, identification and navigation controls. The screen can be divided into several smaller screens allowing the pilot to decide on the amount of information displayed and its relevance at any given moment.

The multi-function display (MFD) is colour coded; green indicates good conditions, yellow indicates potential problems requiring the pilots attention and red indicates a serious problem requiring immediate attention. These colours are also representative of exterior objects and phenomena detected by the on-board sensors with red representing and adversary or threat, yellow for unknown and friendly forces are shown in green and are displayed in shape form. Shades of grey are used to outline maps as well as various subsystems including fuel, flight controls and weapons. This all helps the pilot’s comprehension and situational awareness. Voice recognition is also used by the pilot for normal flight tasks such as altering radio frequencies and changing navigation coordinates. Voice activation is not as fast as finger activation so split second decisions are still undertaken with the finger.

All three variants share identical cockpit design with only one functional difference; the conventional and carrier aircraft have a button to drop the landing hook which is used for carrier and emergency landings. On the STOVL variant, this button is used to convert into and out of the STOVL propulsion mode. Detents in the STOVL controller indicate the desired touchdown sink rate of the aircraft.

The engine throttle (pilot’s left) and side stick (pilot’s right) are active controllers. These are designed to give the pilot feedback as a function of the flight envelope and flight mode. An active controller can be programmed to allow an increase or decrease of controller force while the pilot experiences “g” forces. The active throttle utilises internal motors allowing the throttle to be moved back automatically during certain flight modes. In STOVL mode, this allows the option to input soft stop detents and afterburner detents at will. A unique feature of the active throttle is the inability to cut the engine through the throttle. Instead, a single toggle switch is used to cut the engine and these features were first introduced on the X-35.

The F-35 is also the first fighter to incorporate a virtual head-up display projecting the information to the pilot’s helmet visor called the helmet mounted display (HMD). The helmet mounted display provides heads-up display (HUD) to the pilot in any direction the pilots head turns and also allows the pilot to target threats with the head rather than the aircraft motion. HMD incorporates night vision giving the pilot situational awareness throughout 360 degrees by utilising the aircrafts array of infrared sensors known as distributed aperture system. Removing the HUD eliminates weight as well as simplifying cockpit design.

Canopy

To reduce weight and improve radar signature characteristics, the canopy has a forward opening design with integrated bow frame whereas the X-35 had a side opening two piece frame.

Radar

All variants feature the AN/APG-81 AESA radar, which enable the pilot to engage targets at long range and provide high situation awareness, the X-35 had no radar fitted.

Horizontal tail

The size has been increased slightly and aspect ratio reduced on the F-35A/B, the F-35C has a larger tail surface area.

Vertical tail

With a more up-right design, greater aspect ratio and higher sweep along with increased rudder area it has improved supersonic stability and flight control.

Wings

The wing area has increased from 450 square feet to 460 square feet with the wingspan increasing to thirty-five feet on the F-35A/B. The F-35C has a wing area of 668 square feet and wingspan of forty-three feet. These changes improved take-off and up-and-away performance.

Length

This has increased by 7 inches with most changes being made in the forward fuselage to allow for equipment to be installed.

Volume

The overall volume has been increased to accommodate internal equipment, a weapons bay and additional fuel.

Environmental Control Systems

The integrated power package (IPP) provides conditioned air and liquid cooling for the aircraft systems. Integration with the IPP and engine provide a unique feature for the cooling system, engine fan-duct heat exchangers act as a cooling source to the hot air side of the IPP. The Environmental Control Systems (ECS) use both fuel and air as a cooling source and the IPP also provides emergency power as well as engine start capability. The systems in use with the X-35 were taken from F/A-18 ECS pack.

Nose landing gear

This has been redesigned and tailored for STOVL operations; the demonstrator aircraft used the nose wheel from an F-15 and modified main landing gear from an A-6.

Electro Optical Targeting System

EOTS for short provides targeting information and laser designation capability for the HMD,this is housed between the radome and nose landing gear.

Armament

A GAU-22A four-barrel 25mm canon mounted internally on the F-35A, externally on a pod for the F-35B/C.

Weapons bay

The X-35 had no weapons bay but the F-35 has a large bay capable of carrying two satellite- guided JDAMS and two AIM-120 missiles. The F-35B has a slightly smaller bay than that of F-35A/C.

Store stations

Seven external store stations have been added to the production model. The F35A/C can carry up to 18,000 pounds of ordnance and the F-35B can carry up to 15,000 pounds. Ordnance that can be carried includes pods, fuel tanks, launchers and various types of air-to-air and air-ground weapons.

Engine

The production engine is a Pratt & Whitney F135 that has an output of more than 40,000 pounds of thrust, 2,000 pounds more than the F119. Also under testing was the F136 engine*, designed by General Electrical Rolls-Royce as one of the program requirements was that the aircraft be designed for engine interchangeability.
*The F-136 had its funding cut at the end of December 2011 leaving the Pratt & Whitney F-135 as the sole engine for all three variants.

F-35 Fleet to date

Since the first flight of an F-35 in 2006 more than 7,400 flights have been flown and 11,600 cumulative flight hours being logged. The F-35 fleet as of 13th December 2013 consists of:

• 67 Low Rate Initial Production (LRIP) aircraft;
  • 34 Aircraft based at Eglin AFB, FL – 18x F-35A, 14x F-35B and 2x F-35C.
  • 10 Based at Edwards AFB, CA – 2x F-35A, 2 x F-35B, 1x F-35C on loan for SDD and an additional 5x F-35A for operational testing.
  • 16 F-35B based at MCAS Yuma, AZ
  • 4 F-35A based at Nellis AFB, NV foroperational testing
  • 3 Forth Worth, TX based F-35s are expected to depart to Eglin AFB shortly

 

• 20 System Development & Demonstration Aircraft (SDD) aircraft
  • 6 Static aircraft in addition to AA-1
  • 4 F-35A at Edwards AFB, CA
  • 9 Based at Patuxtent River, MD – 5x F-35B and 4x F-35C

In the second part of the Joint Strike Fighter series, we look at the F-35 program from 2006 to 2014, the SDD fleet and the many Operational Test and Evaluation developments over the last eight years.

AeroResource would like to thank; Laurie Quincy, F-35 International Communications Manager Lockheed Martin Fort Worth, Joe Stout, Director of Communications LM Fort Worth, Benjamin Boling, Production Manager and host around LM Fort Worth and especially John Haire, Public Affairs Director Edwards AFB, who’s extensive help behind the scenes over the last 2 years with F-35 flight operations has made this article possible.