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# A Fighter With FlexibilityPUBLIC ACCESS

The Defense Department is Looking for Adaptability and Affordability in its Next-Generation Strike Fighter. Common Components and Modular Design Should Help the New Plane Meet the Special Needs of the Armed Forces.

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Associate Editor

Mechanical Engineering 120(01), 56-61 (Jan 01, 1998) (5 pages) doi:10.1115/1.1998-JAN-1

## Abstract

This article focuses on the US Defense Department planners who have opted to develop a single, affordable strike fighter flexible enough to match or surpass the capabilities of its varied predecessors. Efforts to build one plane for more than one armed service are risky—witness the failed attempt by General Dynamics and Grumman in the 1960s to develop the swing-wing F-111fighter/bomber for both the Air Force and the Navy. The prevailing belief today is that advanced integrated systems design and extensive simulation will help engineers avoid the pit-falls associated with combining the services’ diverse goals. To succeed, the department’s Joint Strike Fighter (JSF) Program will entail a degree of cooperation and frugality not seen previously among the American armed forces. At stake for the aerospace/defense industry is perhaps the largest US military procurement program ever. Although the aircraft from the JSF competition will be developed using some •of the most advanced engineering design techniques currently available, they will also have to satisfy today’s political and fiscal constraints.

## Article

The United States formidable fleet of fighter and ground-attack aircraft-including approximately 1,800 Air Force F-16s; 700 Navy carrier-borne F-14s, F / A-18s, and A-6s; and 300 Marine Corps A V- 8Bs-is aging and will need to be replaced early next century. In the past, separate aircraft were developed to meet the operational requirements of each U.S. military service. That expensive tailoring now flies in the face of post-Cold War realities: a diminished strategic threat and changing federal budgetary priorities.

Instead, Defense Department planners have opted to develop a single, affordable strike fighter flexible enough to match or surpass the capabilities of its varied predecessors. That one design, however, will also have to satisfy the often divergent demands of those three U.S. services and of Great Britain's Royal Navy, a partner in the craft's development.

Efforts to build one plane for more than one armed service are risky-witness the failed attempt by General Dynamics and Grumman in the 1960s to develop the swing-wing F-111 fighter/ bomber for both the Air Force and the Navy. The prevailing belief today is that advanced integrated systems design and extensive simulation will help engineers avoid the pitfalls associated with combining the services' diverse goals.

The Air Force, for example, needs an affordable aircraft that flies from conventional air fields to perform the critical air- to-ground strike job yet retain an effective air-to-air fighter capability. The Navy, on the other hand, is looking for a sturdy "first-day-of-war" carrier weapon that can penetrate 600 miles into hostile airspace without refueling. The Marines and Royal Navy want a plane capable of short takeoff and vertical landing (STOVL) that operates from amphibious assault ships and austere airfields.

These aims could prove incompatible. Clearly, however, the engineering challenge is compounded by the need to satisfy the desire of skeptical lawmakers to control costs. Therefore, the Defense Department has set relatively stringent (unit) price goals-$28 million for each Air Force plane,$35 million for an STOVL version, and $38 million for a carrier aircraft. To succeed, the department's Joint Strike Fighter (JSF) Program will entail a degree of cooperation and frugality not seen previously among the American armed forces. At stake for the aerospace/ defense industry is perhaps the largest U.S. military procurement program ever, according to Some industry observers. And if significant foreign sales emerge (beyond those to Great Britain), as seems likely, the number of aircraft manufactured-now planned for about 2,860--could double. In November 1996, the Pentagon announced that Boeing Co. and Lockheed Martin Corp. beat out McDonnell Douglas Corp. (which Boeing recently acquired) to reach the$2.2 billion concept-demonstration phase of the JSF contract competition. Seattle-based Boeing was allotted $661 million to build a prototype aircraft, while$718 million went to Lockheed Martin in Fort Worth, Tex., for its JSF effort. After a 1999 fly-off between the demonstrators-Boeing's will be called the X-32, Lockheed Martin's the X-35- one design will be selected. The first production aircraft is to be built in 2007, and the first operational aircraft should be deployed around 2010. Deliveries should continue through 2030.

To meet its goals, the Defense Department's JSF Office has forged a set of requirements that seemingly all participants can accept. Designing a common aircraft like the JSF is an exercise in defining what aspects to share among the services without reductions in performance and what aspects to vary with minimal cost increases. The JSF Office specified a single-engine, single-seat supersonic combat aircraft, with a top speed of Mach 1.5, that features stealth capabilities similar to that of the new F-22 air-superiority fighter plus turning performance and agility on a par with the F-16. The joint aircraft will use a new generation of far-seeing sensors and precision weapons to find and destroy difficult targets.

The JSF is also being designed to provide significantly higher sortie rates as well as reduced maintenance and logistics requirements.

One of the more-crucial compromises in the plane's design concerned its planned weapons load, which was eventually defined to take part in a specific combat scenario. At the start of hostilities, the aircraft would carry a modest internal (and therefor concealed) payload: a pair of guided bombs and two AIM- 120 air-to-air missiles for self-defense. Once enemy air defenses are weakened by attacks and radar invisibility becomes less important, external weapons would be fitted to wing pylons as well.

The U.S. Navy version of Boeing's joint strike fighter, a single-seat, multiservice airplane, will have a reinforced structure so that it can withstand the rigors of catapult takeoffs and arrested landings on aircraft carriers at sea.

A wind-tunnel model of the shaft-driven lift-fan exhaust nozzle for the short-takeoff and-vertical-Ianding variant of Lockheed Martin's JSF design is tested, in a horizontal mounting, at NASA's Lewis Powered lift Facility in Cleveland.

## Common Threads, Uncommon Threats

"All of the services want to fly at supersonic speeds, shoot air-to-air missiles, and drop bombs on a target, but they land and take off totally differently and have vastly different operational suitability requirements," said David Wheaton, Lockheed Martin vice president and program manager of the company's JSF team. "The challenge was to come up with a configuration that has a high degree of commonality and afford ability, but at the same time does everything we wanted to do in up-and-away flight and still be able to take off and land in all those modes."

A key concept behind the effort to build one plane rather than three was to minimize the differences between variants. Parts are identical where possible; otherwise, they are designed to fit together in the same way so the same workers can build different models on a single production line.

"We identified commonality as providing the highest payoff in terms of affordability, so we concentrated on it," said Fred May, Boeing's deputy JSF program manager. "For example, each of the service variants has the same outer mold line [outer geometry], which allows the use of common tooling. This way, the airplanes will be pretty much the same on the outside, and the insides can be tailored to meet specific needs, the same as we tailor the insides of our airliners for specific customers."

According to aviation-industry observers, the JSF competition pits Lockheed Mar tin 's somewhat traditional, relatively conservative design against Boeing's more revolutionary one. It can also be seen as a contest between lo n g-time fighter maker Lockheed Martin and Boeing, which has not built a successful operational fighter in half a century. (The bet is that the latter company can transfer its vast commercial-aircraft experience to military planes, plus it has the added significant fighter expertise from McDonnell Douglas.)

"We took what we think is a low-risk approach regarding the aerodynamic configuration and the integrated flight control and propulsion system," Wheaton said. Lockheed Martin 's X-35 features a conventional wing/ body/ tail layout, which Wheaton said offers "a great deal of flexibility to meet future changes." Furthermore, the STOVL variant will rely on a shaft- driven lift-fan propulsion system in which a vertically oriented fan located aft of the cockpit takes power off the engine via a shaft/clutch/ gearing arrangement to send air thrusting downward during vertical flight.

Boeing's X-32 is substantially different- and, some say, marginally more capable-with its deep, blended-delta-wing arrangement and an STOVL propulsion system. similar to the Harrier jump jet. For vertical landings, most engine exhaust is ducted through rotating lift nozzles positioned near the aircraft's center of gravity. To withstand the catapult takeoffs and arrested landings of Navy carrier operations, the naval versions of both aircraft will feature a beefed-up airframe structure, stronger landing gear, and an arresting hook. Outstanding low speed flight-handling characteristics are also crucial to accomplish this task. For example, the Boeing Navy plane will be fitted with a moveable vortex fence in-board on the wing, which provides some nose pitching moment, according to May. Similarly, the wing and tail of the Lockheed Martin Navy version will be enlarged to per! Tut slower landing approaches.

Common to both JSF designs is the core of Pratt & Whitney's F119 turbofan engine, two of which currently power the F-22. The company's Large Military Engine Division in West Palm Beach, Fla., received a \$900 million contract for this phase of the JSF program.

Here again, commonality is key. " All the JSF variants will share a common engine core and control system, manufacturing processes, as well as much the same support system and diagnostics system," said Robert A. Cea, Pratt & Whitney's JSF program manager. Tests on upgraded versions of the Fl19 engine are planned for early this year, STOVL tests are scheduled for the summer, and deliveries of flight- test power plants are expected in late 1999. (To avoid placing all of its eggs in one basket, the U.S. government is funding an alternative JSF engine, the YF120, to be built by the team of General Electric Aircraft Engines in Even-dale, Ohio; Allison Advanced Development Co. in Indianapolis; and Rolls Royce Military Aero Engines Ltd. in Bristol, England.)

Pratt & Whitney's 36,000-pound- thrust baseline F119 engine is to be beefed up to meet the greater power demands of the two new designs. Boeing's version will be in the 40,000-pound-thrust class, while Lockheed Martin's engine will develop about 38,000 pounds of jet thrust.

"To meet the greater thrust needs for both versions, our engineers scaled up the sizes of the fans and, rather than a single-stage low-pressure turbine, the JSF power plants will have twin-stage low-pressure turbines because you're driving a bigger airflow," Cea said. There is no difference between the Air Force and Navy power plants (the F119 is fully ready for marine use), he noted, and the design criteria chosen for both JSF engines is the STOVL variant "because it has the highest duty cycle."

These increases in power should not pose a problem for the F11 9, because the most demanding operating conditions the engines will see-sustained STOVL operations at sea level is where the turbofan is strongest. Said Cea: "The F1 19 core was designed for supercruise operations- sustained supersonic flight without afterburners. This means the engine core is derated at sea-level static, so it's kind of loafing at those condition s. It therefore has a lot of room to grow."

To support his assertion, Cea cited a recent successful ground test of a developmental engine called Caesar in which the advanced turbofan operated "at the hotter JSF temperatures." The engine is fitted with the company's new Supervane and Superblade technology concepts, which improve the effectiveness of aerodynamic cooling by more effectively directing airflow through the internal spaces of turbine blades and compressor vanes. "The idea is to get the most cooling for the least amount of air," he said. The JSF power plants will be the first to feature these technologies.

The Marine Corps/Royal Navy versions of the competing JSF designs rely on different propulsion systems to achieve short takeoffs and vertical landings. The Lockheed Martin STOVL variant (top) uses a shaft-driven lift fan with reaction-control ports at the wing roots, while the Boeing design (bottom) has center-mounted lift nozzles similar to those used on Harrier jump jets plus a ventral jet screen unit.

## Vertical Landings

The strike-fighter competitors, as mentioned previously, differ in their approach to STOVL propulsion systems. Lockheed Martin 's setup relies on a lift-fan system in which the fan is connected to the main engine by a shaft. "When the clutch engages," Cea said, " the shaft draws horsepower from the main engine and drives the lift fan, which sends cool ambient air p asses through a thrust-vectoring nozzle on the plane's underside." The rush of cool air from the lift fan bathes the forward ventral area in front of the inlets, which stops the ingestion of hot exhaust rebounding 'from the g round, a condition that causes the engine to run hot. This feature is important for successful vertical landings. (When the lift fan is not in use, access doors above and below are closed.) Allison is developing the X-35's lift- fan and clutch systems.

"The main engine is fitted with a three-bearing rotating or swiveling nozzle," Cea added, that serves as the other major support for vertical operations. Supplied by Rolls Royce, the vectoring exhaust system can swivel 110 degrees downward from the horizontal. It also provides some yaw control.

The lift- fan airflow balances off the main-engine thrust in the rear. Two small roll- control (reaction- control) nozzles positioned to each side of the fuselage Gust outboard of the wing roots) port engine fan air to provide the aircraft 's third and fourth STOVL supports.

According to Wheaton of Lockheed Martin, his company has taken a low- risk approach to STOVL operations; by contrast, with Boeing's directed-engine-thrust arrangement "in which the engine has to do all the work, the engine tends to get big and heavy and technically challenging." The shaft-driven lift fan "reduces the strain on the core engine," he said.

The Rolls Royce-supplied STOVL propulsion system on Boeing's X- 32 has two centrally located lift nozzles (the lift module) providing most of the downward thrust when the plane flies in hover mode. For vertical flight, the primary (aft) vectoring exhaust nozzle is closed, diverting the £low through the vectorable lift nozzles, according to Cea. The Boeing aircraft also features a j et-screen system in which an air duct ports cool bypass air from the engine and diverts it through a rectangular opening positioned ventrally forward of the lift module.

While the jet screen provides some vertical thrust during STOVL operations, its main purpose is to protect the engine from hot-gas ingestion by screening the engine inlet with cooler air. This arrangement of exhaust ports, which provides three-point STOVL stability, is considered simpler than the X-35's, but is thought to supply less vertical thrust. The X-32 also has a puffer-j et reaction control system with several small yaw- and pitch-control nozzles at the rear.

The Navy's version of lock heed Martin's JSF, here attacking an enemy aircraft using an AIM-120 air-to-air missile, features a larger wing and tail to permit slower landing approaches.

## Airframe Design

Boeing engineers have designed the X-32 to be assembled from large integrated modules. The plane's common forebody contains the cockpit and most of the electronics, while the tailored lower-fuselage section accommodates its engine and weapons bays. All variants get the common aft- body/tail section. These modules are then attached to a one-piece blended delta wing. The tough bonded thermoplastic wing will be lightweight and sufficiently compact to fit the tight spaces on small aircraft carriers without folding. The large wing structure will also be able to hold about 18,000 pounds of aviation fuel, which should be sufficient for the Navy's needs.

The airplane maker has released very little information on the plane's surprisingly short but stealthy high-speed engine inlet. Designers of most stealthy aircraft bury the engine and its highly radar-reflecting face deep within the airframe behind a serpentine-shaped duct. The X-32's engine, however, is located relatively far forward, leaving little room for stealth components and for the technology needed to slow incoming high-velocity air down to speeds usable by the turbofan. The airplane's high-speed inlet/forebody compression system, shaped like a hippopotamus's mouth, is said to accomplish both of these tasks in a very restricted space.

With its clipped-delta wing, horizontal aft stabilizer, and outward-canted vertical tails, Lockheed Mar tin's X-35 resembles the F-22. Wheaton said the similarity is accidental-the prime design driver was the integrated flight control and propulsion system. After that portion of the overall design was determined, company engineers "wrapped an airframe that did the other things we needed around it."

A key to meeting the tough JSF cost goals, he said, will be the use of large integrated parts. The aircraft's wing box, for example, carries through the fuselage, which eliminates the side body joint between these traditionally independent components. This design reduces much of the structure, weight, and assembly typically associated with this interface, Wheaton said.

The JSF will be able to locate and kill mobile targets such as Scud missile launchers at night and in bad weather. Rather than using separate antennas for its electronic sensors, optical sensors, and communications systems, JSF will use multipurpose "shared apertures." One set of antennas, built into the fighters' composite wing and body skin, will track enemy radars and communicate with allied forces. Advanced infrared sensors will give the pilot 360-degree night vision. The forebody will be designed around a phase d-array radar antenna that is being developed by Hughes and Northrop Grumman to minimize reflections. JSF is also being designed with "off-board" sensors in mind, using jam-resistant data links to gather information from other surveillance systems.

The future single-sea t aircraft will incorporate a high degree of automation. A helmet-mounted display, replacing today's heads-up display, will provide the pilot with a precision synthetic view of the terrain and operational situation.

The JSF will also have an "open " flexible avionics suite, allowing easy installation of follow-on systems. Computers are to be based on commercial processors. There will also be a significant reduction in cables through the use of a few powerful data buses.

Another change is a new, integrated approach to subsystem design. In the past, engineers tended to optimize individual components to reduce weight and /o r increase performance, an approach that often led to total system inefficiencies. The JSF/ Integrated Subsystems Technology (J/IST) Demonstration Program will bring advanced subsystems technology to maturity, enabling optimization at the total vehicle level.

Lockheed Martin is currently working on electromagnetic flight-control actuators that would be lighter and cheaper than hydraulic actuators. Boeing's j/IST assignment is to develop is an integrated power/ thermal-energy management package in which new subsystems such as hybrid starter/generator units and combined cooler! power generators replace conventional air-conditioning and auxiliary power units.

Engineers at lockheed Martin are using virtual-product-development technology to create virtual-reality simulations of the JSF assembly line, which is planned for the company's Fort Worth, Tex., manufacturing facility. These next-generation design tools should help the aerospace firm meet the Defense Department's stringent cost goals.

Lockheed Martin engineers used the CATIA CAD/ CAM system to design the Marine Corps/Royal Navy STOVL variant of the JSF. The large lift-fan unit (behind the cockpit) and the vectoring exhaust nozzle (aft) provide most of the vertical thrust.

## Single Assembly Lines

The JSF will feature not only common mold lines across the fuselage and wing box but also common components such as the canopy, radar, the ejection system, subsystems, most of the avionics, and structural geometries. This high level of commonality makes it possible to manufacture both prototype aircraft on single (separate) production lines with simple holding fixtures that can accommodate cousin parts and assemblies for each variant. Boeing's X-32 will be built mostly in Seattle, while Lockheed Martin's X-35 will be constructed in Palmdale, Calif.

Both companies are using integrated product teams to develop major components of their planes. Each team operates like a small company with total responsibility for a specific part of the aircraft.

In addition, both firms plan to incorporate a good deal of composite structures in their designs. Large metal components (made of titanium and aluminum) will be produced using high-speed machining as well as unitized welding and casting techniques. The idea is to build fewer parts lighter and cheaper by avoiding the large numbers of fasteners used on current airplanes. Said Boeing's May, "The biggest expense in building aircraft today is drilling and filling holes."

Included on the production lines will be a substantial amount of manufacturing automation and jigless assembly techniques to minimize tooling. This determinant assembly approach uses key features designed into the part and precision locating techniques to quickly align _various components with little setup effort. Whenever practical, digital component definitions from each company's CATIA design database-from Dassault Systemes in Suresnes, France, and IBM in Armonk, N.Y. will be transferred directly to computer-numerical-control machines for part fabrication.

Both airplane manufacturers are making extensive use of advanced simulation software and virtual- reality technology to hash out the detailed operational and ergonomic designs of their JSF aircraft. Said Wheaton, "We're using sophisticated computer models that propagate the effect of any design changes to all three variants automatically." Both companies will also use product data- management software, intranets, and World Wide Web-based technology to create a single data environment that will ensure all relevant information will be readily available to the entire engineering team, no matter where team members are located.

Although the aircraft from the JSF competition will be developed using some •of the most advanced engineering design techniques currently available, they will also have to satisfy today's political and fiscal constraints. As a result, the winner will have to be a jack-of- all-trades; hopefully, it will be master of all, rather than master of none.

Copyright © 1998 by ASME
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