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Polymer composites make up about 26 percent of the structural weight in the U.S. Air Force's next-generation fighter and will help the F-22 avoid detection in hostile environments.

Graphite-based thermosets epoxy and bismaleimide each account for more than 12 percent, and thermoplastic polyetheretherketone-based APC-2 on unidirectional tape takes care of 1 percent for structures where toughness is beneficial, according to Dave Trawinski, a member of the F-22 team.

By comparison, polymer composites make up about 30 percent of the Air Force's B-2 bomber and 18 percent of the Navy's upgraded F/A-18E/F.

Lockheed Martin Corp. and Boeing Co. units are building nine flyable F-22s and two airframes for static and fatigue testing in the engineering and manufacturing development phase. The program goal: replace the F-15 with delivery starting in 2004 and extending into 2012. Both are in the same weight class, about 40,000 pounds.

The dogfighter F-22 is a separate program from the ground-attack Joint Strike Fighter. On Nov. 16, the Pentagon selected Lockheed Martin and Boeing to compete for JSF development.

Air Force research on developing an F-15 replacement got serious beginning in 1986 with the awarding of demonstration-validation contracts. In 1991, the Lockheed-led team won what then was called the Advanced Tactical Fighter competition.

A decade has brought changes:

A target of having initial operational capability by the mid-1990s for the single-seat F-22 was moved to 2004.

Projected inventory of 750 aircraft was reduced to a probable order for 442. That is four wings, a training wing and spare aircraft, considered a bare minimum to cover two regional conflicts.

Design reviews made significant changes to advance technologies in engines, avionics, flight controls and stealth characteristics involving electromagnetic and radio-frequency interference. And some minor design changes continue.

The contractor team and the Air Force's Ohio-based F-22 Systems Program Office sought to incorporate as much as 38 percent of polymer composites in the structure. Thermoplastics, however, proved too expensive in some applications, and the team fell back to using aluminum.

The Air Force drove the F-22 team toward more material commonality than exists on earlier programs.

``We have one database of knowledge of materials rather than splinter materials,'' said Trawinski, F-22 materials and producibility manager at the Lockheed Martin Aeronautical Systems unit.

To make the material choices, Trawinski worked closely with Al Fenstermaker, manager of F-22 parts, materials and process technology for Boeing's military airplane division, and Harry Miller, F-22 materials and processes engineering chief at Lockheed Martin Tactical Aircraft Systems.

Boeing manufactures the wing sections with composite skins and internal spars and the aft fuselage with composite upper skins and keel panels in Seattle. Lockheed Martin Tactical Aircraft Systems, formerly a General Dynamics Corp. unit, produces the midfuselage in Fort Worth, Texas.

Lockheed Martin Aeronautical Systems fabricates the forward fuselage and empennage, or aircraft tail, and performs final assembly in Marietta, Ga.

Cytec Industries Inc. and Fiberite Inc. are key polymer suppliers, and Hexcel Corp. provides the carbon fiber.

Dow-United Technologies Composite Products Inc. makes extensive use of an exclusive process that the company calls advanced resin transfer molding.

RTM has found its way into more than 150 parts, including kick ribs, spars and substructure in larger assemblies for the forward fuselage, empennage, wings and edges.

``Cytec Engineered Materials provides well over 70 percent of the composites on the F-22,'' said Patricia Harrison, business manager for the Cytec Industries unit.

Cytec's proprietary 5250-4 BMI resin is used on tapes and woven fabrics of fiberglass, Hexcel's IM7 carbon fiber or Dow-Corning's silicon carbide fiber, mostly for structural purposes.

``Cytec provides 5250-4 as an RTM material, the bulk of which is manufactured into wing sine wave spars by Dow-UT,'' Harrison said.

In addition, Cytec BMI adhesives, designated Metlbond 2550, are used in various weights on glass and mat and as a barrier designed for sandwich structure. Cytec manufactures polymer composites in Anaheim, Calif., and adhesives in Havre de Grace, Md.

Fiberite supplies the Marietta and Fort Worth sites with toughened epoxy 977-3 on Hexcel IM7 fiber in both woven fabrics and unidirectional prepregs, according to Bill Pelot, senior account manager for Fiberite in Marietta. Fiberite makes the epoxy and prepreg in Greenville, Texas.

``We also supply thermoplastic prepreg APC-2 on IM7 unidirectional tapes on specific areas that get a lot of abuse,'' Pelot said.

APC stands for aromatic polymer composites, a class of advanced structural materials based on continuous carbon fibers and thermoplastic polymer resin matrix. APC-2 is a particular grade that uses the semicrystalline polymer PEEK.

Joint venture Dow-UT does about 90 percent of the RTM work on the F-22, according to a January 1996 Defense Department report.

The Air Force and Boeing have recognized the venture's success with the advanced process using carbon fiber to reinforce 3M Co.'s PR500 toughened epoxy and Cytec's BMI in making the parts.

In Wallingford, Conn., Dow-UT makes the wing spars and more than 100 other parts for each F-22 under a $30.2 million contract.

``Dow-UT's ARTM will save approximately $250,000, on average, per aircraft,'' said Boeing engineer Geary Long, compared with hand lay-up of composites and based on an anticipated buy of 442 aircraft.

Dow-UT's ``advanced'' RTM process involves the application of resin powders to uncut fabric to stiffen the material and maintain fiber orientation during component manufacturing.

A small, minority-owned Philadelphia firm is another qualified RTM supplier. Workers at C.A. Spalding Co. learned the skills through a mentor-protégé program with Lockheed Martin Aeronautical Systems and in sessions at Dow-UT's 180,000-square-foot advanced research, development and production facility.

Air Force contracts funded five F-22 mentor-protégé programs involving ceramic, sensor and control, engine part, and RTM technologies. The Air Force paid $3.8 million for the efforts to assess Spalding's capabilities, train workers on RTM technical aspects, transfer composites technology and fabricate and test prototype parts.

``Only a handful of companies have RTM capability, so this makes us very competitive in both defense and commercial markets,'' Javier Kuehnle, Spalding chairman and chief executive officer, said in a news release.

The Spalding program began in November 1993, and although it ended last September, Lockheed Martin's commitment to its protégé will continue, according to Tom Burbage, F-22 team general manager.

The DeLand, Fla.-based Intellitec division of Technical Products Group and the aerospace subsidiary of Kaman Corp. in Bloomfield, Conn., also supply RTM.

Thermoforming is on target.

``We have been pleased by the thermoplastic forming work at Superform,'' Trawinski said.

Superform USA Inc. thermoforms internal equipment shelves, cockpit glareshield, nose access center and side panels and avionics bay, side bay and main landing gear doors using Amoco Corp.'s polarylsulfone-based Radel 8320 thermoplastic graphite tape incorporating Hexcel IM8 carbon fiber.

``We use our patented process called diaphragm forming,'' said A.J. Barnes, vice president of technology for the Riverside, Calif., subsidiary of British Aluminium.

Soon, for these applications, Superform will make the transition to Fiberite APC-2 tape using Hexcel IM7 fiber because of the difficulty in procuring Radel. Amoco discontinued production of Radel.

In a computer software venture, NovaLogic Inc. of Calabasas, Calif., sells the F-22 Lightning II. The $50 game simulates five training and 31 combat missions.

A reality check for the F-22 team is scheduled in late May, when the first unit rolls out of Lockheed Martin's Georgia plant and lifts off into the skies over Marietta.