Composite Structure Fabrication Technologies
The manufacture of fiber composite structures generally begins by combining the fiber with epoxy resin, or some other so-called "matrix" material. The resulting prefabricated sheets are called prepreg. Successive layers of these prepreg sheets are then placed in a mold that is shaped to the form of the part being fabricated.
The fiber directions in successive prepreg layers are diagonal to one another, in a fashion tailored to the load and stress field to which the part will be subjected. The stack of prefabricated sheets █ called a "layup" █ is then cured in an autoclave (essentially, a pressure cooker) under controlled temperature and pressure.
Initially, the task of making the layups in molds was done by hand. Later, beginning with simple, near two-dimensional parts, computer-controlled automated layup machines became available. Today, automated layup machines are capable of handling ever more complex parts.
Attachments between fiber composite structural elements have, for the most part, been made with bolts. In some cases, adhesive bonds have been used, in much the same manner as with metal parts. More recently, the layups for two or more parts have been joined in the curing process █ this is called cocuring.
These fiber composite fabrication processes permit the manufacture of parts in nearly final form ("near net shape"). However, some cutting, drilling, and other machining and finishing operations are usually required.
Much of this is done with conventional machine tools. But the tool shape and hardness, and the cutting speeds, must be adapted to the fiber composite material being worked. Laser cutting and water jet/hydroabrasive cutting are also used extensively in finishing operations for fiber composites.
For axially-symmetrical parts █ such as rocket motor cases █ filament winding is used (for example, in the Minuteman missile╠s upper stage). Filament winding has also been used to manufacture fiberglass/epoxy helicopter rotor blades. In addition, long parts of constant cross-section can be made by the pultrusion process: pulling the fibers and matrix material through a die. This is the analogue of the extrusion process for metals.
Most of the fiber composite structures produced to date have employed polymer matrix materials that cannot be subjected to severe temperature environments. This has been a strict limitation on the kinds of structures for which fiber composites can be used. But newly-developed composite materials do not have this limitation. These new materials include:
- Metal matrix composites
- Ceramic matrix composites
- Carbon/carbon composites
These new fiber composites can be used in higher-temperature applications such as rocket engines, hypersonic aircraft, and ballistic missiles.206 207
The PRC has been seeking to acquire or develop composite materials and structures technologies. One route has been through seeking co-production relationships for subassemblies of commercial aircraft and helicopters that have significant composite parts.208 There are also reports of indigenous development as well.
A wide range of composite materials and structures fabrication equipment is included in the Missile Control Technology List (MCTL), and is subject to export control regimes at some threshold of capability. These include:
- Composite filament winding
- Tape laying
- Fiber production equipment
The more advanced Western methods of composite structure fabrication for complex three-dimensional shapes are extremely sophisticated robotic machines █ some with as many as nine axes of motion. It is not believed that the PRC has been able to develop or acquire machines of this capability as yet.
Stealth and Composite Technologies
What is stealth? Simply put, stealth is the ability to conceal an attacker from a defender╠s detection and defensive systems and successfully accomplish the mission.209 Stealth does not make the attacker invisible, only more difficult to detect.210 To avoid detection, it is necessary to reduce or eliminate the attacker╠s "signature."
The "signature" is composed of five primary elements:
- Visual signature
- Infrared (heat) signature
- Acoustic (noise) signature
- Radio transmission signature
- Radar signature211
The first three signatures are relatively short range.212 The radar signature is the most important, because it can be detected at the longest range █ up to 400 miles away.213
In a stealth vehicle, attention is paid to all five signature sources.214 To reduce the infrared and acoustic signatures of an aircraft, the engines are buried inside the fuselage or wings. Special non-reflective paints and paint schemes reduce the visual signature. The radio transmission signature can be reduced or eliminated by secure communications or radio silence.
Defeating radar detection is relatively simple in principle.215 It involves designing the vehicle so that the incoming radar signal is reflected away from the defender╠s radar receiver, or absorbed by the vehicle itself using radar-absorbing materials.216 Radar stealth is accomplished in five ways:
- Designing the vehicle so that there are no surfaces pointing directly back to the source radar
- Using radar-absorbing materials on surfaces that could reflect back to the source radar
- Removing surface roughness by making the surface of the vehicle as smooth as possible
- Designing engine inlets to reduce reflection
- Burying engines and weapons inside the vehicle217
The F-117 and B-2 aircraft represent the cutting edge in manned stealth aircraft, because they combine all of the elements of design, materials, and manufacturing technology to achieve stealth, including radar and infrared invisibility.218
Why is stealth so important to the military? Stealth vehicles are difficult to counter by a defender.219 In military terms, stealth insures a greater probability of completing a mission and increased survivability of U. S. forces.220 Other benefits include:
- The ability to range over a greater area of enemy territory without being detected
- Reduced mission cost
- Increased effectiveness of other radar-jamming systems, such as chaff221
The PRC probably cannot build stealth aircraft or missiles with the same capabilities as the F-117 and B-2, now or in the near future. But the PRC is likely to try to acquire most of the key elements necessary to build them.
Even acquisition of these elements will be insufficient to permit the PRC to build effectively stealthy aircraft or missiles. System integration of stealth is a major additional task facing the PRC.
The PRC╠s Acquisition of Stealth Design Technology
The PRC╠s efforts to solve the stealth design problem received a major boost when the PRC was able to import both high performance computers, and software packages known generically as "finite element" software. This software is used to assess aerodynamic forces and stresses on three-dimensional structures.
"Finite element" software also has the capacity to solve complex sets of Maxwell╠s equations. These equations relate to electromagnetic radiation (that is, radar) around a structure.
With high performance computers and "finite element" software, the PRC now has the capability to design aircraft which are aerodynamically feasible and then evaluate their stealth capabilities, too.
The Department of Defense has sought tighter export controls on "finite element" software.222 This software is distinctly dual-use, with civilian applications including automobiles, off-shore oil drilling platforms, and the design of nuclear reactor plants. One of the main concerns of the Defense Department, however, is its use in stealth applications. The software is also critical for anti-submarine warfare.223
The PRC╠s Acquisition of Composite Materials Technology
Building composite structures for aircraft is, in some ways, similar to building a fiberglass boat: the rigid fiberglass is technically a composite material, made up of layers of fiberglass fabric and epoxy resin. In composite structures for aircraft, the fabric is woven from ceramic, polymer, or carbon/carbon materials, instead of fiberglass.224
Large rolls of the fabric are run through machines that apply a coating of uncured resin to the fabric (known as prepreging the fabric). This material bonds together, forming the composite structure.
In stealth aircraft structures, radar-absorbing layers and coatings are integrated into the composite structure.
Some PRC joint ventures are adding to the PRC╠s ability to produce composite airframes:
- British Petroleum America proposed to sell to the PRC proprietary technology for resins and reinforcing materials, as well as the technology and training to operate a facility.225 The company also planned to sell the methodology for translating manufacturing requirements into optimized semi-finished materials. BP America specifically sold the PRC technical data for hot-melt prepreg formulations,226 and for an acrylonitrile plant.227 The prepreg technical data was sold to the AVIC China Helicopter Corporation.228
- Hexcel was willing to supply the PRC with high-temperature curing resins and the production equipment and training to apply the resin to fabric materials.229 Specifically, Hexcel planned to give the PRC the technology for 250 F and 350 F epoxies.230 The company planned to transfer to the joint venture a solution-impregnation coating tower for fabrics, and hot-melt impregnating equipment for tapes.231 The joint venture was supported by exports of carbon epoxy prepreg to the Chengdu Aircraft Industry Corporation232 and the Xian Aircraft Company.233 In addition, Hexcel was going to transfer Boeing Aircraft Company╠s specifications for advanced composites,234 graphite,235 Kevlar,236 and conductive fabrics.237
Kevlar is used to make high-strength smooth surfaces on stealth aircraft. The graphite and conductive fabrics are used for radar-absorbent surfaces of stealth aircraft. In addition to their uses for stealth technology, the growing importance of composite structures in all aircraft construction provides an incentive to the PRC to acquire this technology even for non-stealth aircraft █ military and civilian.
The PRC╠s Acquisition of Composite Structures Manufacturing Technology
Obtaining the design capability and the materials-production capability were still not sufficient for the PRC to build aircraft with composite structures. The missing element of the Chinese puzzle was the ability to manufacture aircraft parts with consistent performance time after time.
The answer to this question was found in a joint venture with the Sikorsky Aircraft Company.238
The Sikorsky Aircraft Company joint venture with the PRC proposed to build the composite tail section of the civil S-92 helicopter.239 Sikorsky would teach the PRC to design and fabricate the tail section using proprietary technology to meet Federal Aviation Agency standards of quality and performance.
The project included teaching the PRC to fabricate aircraft components using carbon fiber materials (which are also used in stealth aircraft).240 In addition to showing the PRC how to use the materials, Sikorsky also taught the PRC about:
- Bag molding
- Mold releases
- Die manufacturing241
The key requirements the PRC expected to obtain from the venture were precision tooling, repeatability, and a high production rate.242
The PRC acquisition of composite technology is an interesting case study. It indicates a broad-based set of joint-venture initiatives directed toward providing for the PRC a state-of-the-art composite materials/aerospace structure capability.
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