The Joint Strike Fighter: A plane for all reasons

March 7, 2002
The X-35 JSF will be a supersonic bomber, a stealthy fighter, and a short-take-off, vertically landing warplane flown by U.S. Navy, Air Force, and Marine pilots, as well as the British armed forces.

U.S. Marines will be able to land and take-off vertically with the JSF

The Pratt & Whitney engine for the JSF is tested in full afterburner.

The Air Force version of the JSF will have a refueling port located behind the canopy. The Navy and Marine versions, following tradition, will have a probe that extends to engage the refueling hose.

A lift fan is being installed in a JSF airframe. The main engine turns the counter-rotating blades in the fan by a shaft and clutch to generate 18,000 lb of vertical thrust.

The JSF's clean undercarriage and internally carried weapons reduce the plane's radar signature.

The JSF flies beside another aircraft as it undergoes flight testing.

The JSF tests its hover capability over a grated pit. Clam-shell doors aft of the cockpit on top of the fuselage are opened to let air into the lift fan and the rear nozzle is angled down for vertical thrust.

When it comes to developing and buying military aircraft, the armed services don't like to share. The Navy needs jets that land and take off from aircraft carriers, the Air Force wants all the speed and performance it can afford, and the Marines cite their need to support ground operations from forward airbases. Designing aircraft for specific arms of the service wasn't a problem during the Cold War when the enemy was well defined, defense funding was high, and planes like the F-16 and F-15 cost a paltry $30 million/copy or less.

Today, however, with a host of potential bad guys — none in the superpower category — and funding tighter than ever, the Armed Services have been forced to work together designing their next workhorse of an aircraft. They've come up with the X-35 Joint Strike Fighter and recently awarded production contracts to Lockheed Martin Aerospace Co., Burbank, Calif. To ensure the aircraft meets everyone needs, there are three variants, each supersonic: one for the Air Force, a beefed-up version with better lowspeed control for the Navy, and a STOVL (short take-off, vertical landing) version for the Marines.


The JSF variant designed for the Air Force, the CTOL version (conventional take-off and landing), is the simplest from an engineering standpoint. But it still has high benchmarks to meet. It has to match or exceed the performance of the formidable F-16 Falcon, be more stealthy, have a longer range on internally carried fuel (i.e., no external fuel tanks), and carry advanced avionics. The USAF plans to buy 2,000 JSFs at $28 million/copy to replace its F-16s and A-10s.

The JSF owes its military good looks to the design team's focus on stealth. Leading and trailing edges of the wing and tail, for example, have identical sweep angles, a stealth technique called planform alignment. The fuselage and canopy have sloping rather than vertical sides. The canopy and bay doors have saw-toothed seams, and the vertical tails are canted. A serpentine inlet duct hides the engine face, and the inlet itself has no boundary-layer diverter channel (the space between inlet and fuselage) to reflect radar energy.

The JSF's current night-vision capable canopy is shaped for stealth and uses a lightweight structure that protects against bird strikes and makes ejections quicker. The pilot will have exceptional visibility, especially forward toward the ground, thanks to the look-down angle over the canopy and the wide field of view over the aircraft's nose. Look-down capability is important because the jet will be tasked with air-to-ground missions. The cockpit will not have a heads-up display. Instead, targeting and navigation symbols are projected on the pilot's helmet visor. Stereo speakers in the helmet will give information, like missile warnings, with inherent directional information. If a missile is coming at the aircraft from the left side, for example, the warning will sound in his left earpiece.

Several sensors, including radar and two infrared systems, along with high-speed data links to other aircraft and military units, will give pilots an accurate and up-todate view of the battlefield. Northrop Grumman Electronic Systems is developing the electronically scanned array radar. It will have the same beam-steering capabilities developed for the APG-77 radar used in the F-22. The JSF radar will not be as powerful, however, because of limits to the size of transmit and receive modules. JSF radar will have a 90-mile range, two-thirds that of the F-22.

Northrop Grumman builds both IR sensor systems, including both the distributed aperture system (DAS) for navigation, missile warning and infrared search and track, and an electro-optical targeting system for long-range detection and precision targeting. EOTS is based on the Sniper XL pod developed for the F-16 and is behind a radar opaque window under the JSF's nose rather than turret mounted. Embedded behind the window, it can be used without degrading the plane's stealthiness. EOTS incorporates a midwave third-generation forward-looking infrared system, dual mode laser, CCD TV, and laser tracker and marking. DAS, on the other hand, places six infrared cameras around the JSF so that it gives 360° coverage.

The JSF carries an integrated core processor (ICP) based on an off-the-shelf processor. It integrates and coordinates all sensor displays, and executes pilot commands. The JSF ICP costs less, but is an order of magnitude more powerful than the F-22's processor, which was custom designed a decade ago. And while the F-22 uses about 2.5 million lines of software, the JSF will use twice as many. That's because the JSF's three different versions have more advanced electronic systems and they must operate in both air-to-air and air-to-ground modes. Special care is being taken to design the software as modular or "layered" to permit modification and growth.

The JSF two parallel weapon bays located in front of the landing gear will each have two hardpoints for carrying bombs and missiles. It will typically fly with an internal payload of two guided, 1,000-lb weapons and a pair of airto-air missiles. It can carry much more externally, but at the cost of its stealthiness. Military planners envision the aircraft operating strictly with internal weapons during initial phases of an air campaign, letting it break down air defenses and hit heavily defended targets while cloaked from radar. Later missions would use externally carried bombs and missiles.

Like the F-16, the Air Force JSF will sport an internal gun and a refueling receptacle on top of the fuselage behind the canopy. The gun, being developed by Boeing and Mauser-Werke of Germany, is an advanced 27-mm gasoperated revolver gun firing electrically primed ammunition at 1,800 rounds/min. Unlike the F-16, however, the aircraft carries an extensive internal suite of electronic countermeasures.

Powering all JSFs is a single JSF119-611 jet engine from Pratt & Whitney, an updated version of the afterburning turbofan engine used in the F-22 Raptor. The low-bypassratio engine has been updated with a new fan and lowpressure turbine, taking the engine from the 35,000-lb class to the 40,000-lb class. It has also recently been renamed the F135 engine. To ensure the military doesn't rely on a single engine supplier, and to maintain the nation's jet-engine industrial base, Congress insisted on an "Alternate Engine." GE, Allison, and Rolls-Royce are refining their F-120 engine to meet this requirement.

The engine isn't the only component borrowed from other planes to save design time and money. The main landing gear comes from the Navy's A-6 Intruder while the nose-wheel gear is off an F-15E Eagle. The auxiliary power system was originally designed for the F-22 and the engine-driven hydraulic pumps are YF-23 vintage. The B-2 Spirit bomber contributed its accessory drive and the ejection seat is from the AV-8B Harrier. The JSF cockpit's two large color displays were original to the C-130J Hercules, and the F-16 Falcon donated several subsystems and controls.

Carrier operations account for most of the differences between the Navy and other JSF variants. Carrier landings, for example, are so severe, they're often referred to as "controlled crashes." The JSF, in a low speed approach to a carrier landing, will descend at about 11 fps, and will withstand sink rates up to almost 18 fps. By comparison, the typical sink rate for an Air Force JSF will be about two ft/sec.

To help handle better at low speeds, the aircraft will have larger wing and tail-control surfaces. The increased wingspan also boosts the strike-fighter's range and weapon or fuel load. Even without external fuel tanks, the JSF has almost twice the range of the F/A-18C. Larger leading-edge flaps and wingtips provide the extra wing area, while the wingtips fold so the aircraft takes up less space on the carrier's crowded flight and hangar decks. The Navy's JSF will also have two extra control surfaces — ailerons outboard of the flaperons on the wings — for additional lowspeed control and flying precise glide slopes. The Navy JSF currently flies landing approaches at about 130 to 135 knots, about 25 knots slower than the Air Force version.

The airframe is strengthened and a tailhook added so the plane can withstand catapult launches and arrested landings. The landing gear has a longer stroke and higher load capacity. And the plane will have a refueling probe on the right side of the forward fuselage.

The Navy plans on buying 300 JSFs at an estimated $38 million/plane to replace some of the Navy's F/A-18 Hornets. There is also talk of building an "electronic" JSF to take over the chores of the EA-6B Prowler, the Navy's aging, electronic countermeasure platform.

Marines will be flying the STOVL JSF, letting them stay close to the front and operate from less than pristine airfields. The major difference between it and other JSF versions is its engine configuration. Lockheed Martin's STOVL concept gets its vertical thrust from a lift fan powered by the second low-pressure turbine on the jet engine combined with vectored jet exhaust. According to Lockheed, using a lift fan rather than just engine exhaust yields three advantages: thrust is independent of engine, thereby letting engineers size the engine for conventional flight and simplifying the transition from conventional flight to hovering or vertical landings; the additional thrust generated by the lift fan far exceeds its additional weight; and lower exhaust temperatures and pressures create a safer, less-hostile ground environment for men and machinery during hover than that produced under direct-lift aircraft such as the Harrier. For example, the lift fan reduces the risk of hot exhaust being ingested back into the engine and robbing the plane of vertical thrust or causing compressor stalls.

The lift fan, designed and built by Allison, consists of counterrotating blades putting out up to 20,000 lb of thrust, enough to support half the JSF's weight. The fan inlet is hidden beneath clamshell doors just aft of the cockpit. Its output nozzle is directly below. Moving inlet guide vanes modulate airflow and therefore thrust. Auxiliary engine-air inlets just behind the lift-fan intake supply extra air to the engine when the JSF hovers.

Engine exhaust roaring through a three-bearing swivel nozzle supplies the rest of the vertical thrust, another 18,000 lb, enough to let the JSF hover. The nozzle can be positioned at 0, 40, and 105, and sweep 12 left or right. Two roll ducts located on the wings just inboard of the wing fold are supplied with relatively cool fan air that bypasses the engine's core to provide roll control. Each roll duct can put out 1,500 lb of thrust. The engine's swiveling exhaust nozzle controls yaw, while adjusting the thrust put out by the lift fan and engine exhaust controls pitch.

The Marine's JSF will be the first supersonic STOVL aircraft. It will carry less fuel and have no internal gun because the space is taken up with STOVL hardware. Still, a gun pod will be available. Plans are for the U.S. to purchase 600 of the STOVL JSFs priced at an estimated $35 million. The British Royal Navy and Air Force also intend to buy 150 of these planes to replace Harriers. Other nations, including the Netherlands, Denmark, and Singapore, are also considering purchasing the JSF.

The Marine's version of the JSF will use a novel lift-fan design for vertical lift.

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