Fabrication of the B-2 Stealth bomber's large contoured airframe from lightweight, strong, and stiff composite materials has led to improvements in composites manufacturing. Engineers at Northrop, Boeing, LTV, and the U.S. Air Force have developed a variety of automated production systems to boost quality while controlling costs.

With the recent development of the B-2 bomber, the aerospace industry has entered the realm of large-scale composite manufacturing. Carbon and glass fibers, epoxy-resin matrices, high-temperature polyimides, and other materials were used in creating the more than 10,000 composite components that went into the bomber's contoured airframe. A variety of innovative production techniques were developed to shape, cut, and fasten the strong lightweight composite structures.

"The B-2 bomber is the first real step in making primary aircraft structures from composites," said Mike Henderson, manager of the composites center at Northrop Corp.'s B-2 division (Pico Rivera, Calif.). "We addressed the same kind of manufacturing needs and issues that engineers will face in producing composite airframes for future large commercial airplanes." Citing industry predictions that composites will account for up to 60 percent of the material used to fabricate next-generation aircraft, Henderson said that the manufacturing improvements developed for the B-2 will be useful in future commercial and military aircraft construction programs. For example, Boeing plans to use the multimillion-dollar composites production facilities developed for the B-2 in Seattle to fabricate empennage structures for its soon-to-bebuilt 777 widebody transport plane.

The decision to commit to building a composite structure for the B-2 was made in October 1983, said Mark Wilson, chief of structures for the U.S. Air Force B-2 program. Composites offered designers weight savings, better flight performance, stealth capability, a smooth external surface (for aerodynamic and low-observability improvements), a greatly reduced fastener count, and the possibility of building aeroelastically tailored structures.

But this decision brought risk, recalled Dave Thomas, former deputy director of manufacturing for the Air Force B-2 program and now director of manufacturing for the F-15 program. "We recognized that the large composite structures we envisioned for the aircraft did not look particularly economical to build without some major manufacturing development work. For example, no automated equipment was available at that time to build or inspect the oversized structures we were planning." And without automation, the program planners knew, costs would skyrocket. An integrated approach to develop the required production systems was begun, involving research money and resources from the contractors and the Air Force B-2 program office.

New manufacturing systems developed for the B-2 Stealth bomber include automated composite tape lay-up machines, a robotic drilling and fastener-insertion system, portable force-feedback air drills, precision ultrasonic composite cutters, and flexible abrasive water-jet trimmers. Some have been commercialized, while research on others funded by the Air Force Manufacturing Technology Initiative (ManTech) is now available to the rest of the industry. Other systems, however, are still proprietary or classified.

Fatigue Tests

The B-2 bomber program reached a significant milestone in March when the third airframe out of the factory, which was constructed with no engines or avionics, completed 10,000 hours of simulated flying (equivalent to a service lifetime of 20 years or more) with only minor anomalies and no degradation of structural integrity, Wilson reported. "Getting through one life cycle provides confidence that the airplane will meet its structural design goals," he said. Conducted at Air Force Plant No. 42 in Palmdale, Calif., the "durability test article" is now entering its second lifetime fatigue cycle. In the tests, 180 computer-controlled hydraulic actuators simulate the structural loads of taxiing, taking off, cruising, and landing by bending, twisting, and vibrating the airframe.

To date, B-2 contractors have received $23 billion for their work on the Stealth bomber, and the Bush administration is seeking an additional $30 billion through fiscal 1997 for the remaining 60 aircraft for its 75-plane fleet. But whether this controversial and costly weapons program is fully funded or not, its legacy will be felt in future composite aircraft.

As prime contractor for the B-2 project, Northrop was paid $15.5 billion, of which $7.4 billion was awarded for directing the program's subcontractors. Northrop makes the forward center section (including the cockpit) and most of the parts on the plane's periphery such as the leading and trailing edges, wing tips, elevons, split rudders, landing gear doors, and fairings, as well as many internal secondary structures including crew station liners, ducting, and small parts such as access doors and covers. "There's a multitude of different composite materials in the plane-mostly epoxy-resin matrices, although we use high-temperature polyimides around the hot sections of the aircraft," Henderson said. "The reinforcement includes both woven and unidirectional carbon fibers, as well as aramid fibers, glass fibers, and combinations of these fibers."

Force-feedback Drill

"Anyone who has worked in composites assembly knows that there's a significant amount of variability in the feeds and speeds needed to drill a sandwich, or what we call a 'stackup,' of different materials such as a composite skin with a titanium and aluminum substructure," Henderson said. This difficulty is compounded by the fact that an out-of-round or an unacceptably rough hole in one of the B-2's components could mean many thousands of dollars in rework. The repair process normally consists of drilling out the hole to the next larger fastener size.

In conventional metal airframe fabrication, the many rivet holes drilled in oversized wing and fuselage structures are produced by portable pneumatic positive-feed drills guided by prepositioned fixtures. But these simple devices were ill-equipped to put precision holes in multiple-material stackups without running at the slowest safe speed and without frequent stoppages to sharpen drill bits.

This producibility problem was solved by engineers at what was then Allen-Bradley's Rockwell Industrial Tools division and is now the Cooper Power Tools division of Cooper Industries (Lexington, S.C.). The engineers developed an 11-pound air drill with a two-speed transmission and an adaptive-control system that could sense the hardness of the material being cut and alter the spindle speed appropriately.

"The drill senses changes in the resistance of the material to drill penetration in about a nanosecond, which tells it when the bit has entered a new material," explained John Lawson, former project manager and now marketing manager at Cooper Power Tools. "About 0.05 second later, a custom microchip in the drill adjusts the cutter speed and feed accordingly." The sensory data for feedback control are provided by force transducers that monitor the torque and thrust of the spindle and by an encoder that measures the spindle position.

In a typical stackup, the drill might bore through the graphite skin at 3000 rpm, slowing to 600 rpm when it detects the hard titanium below. As it senses the softer aluminum, the drill would speed up again. The smart drill also retracts automatically when the bit dulls (when the strain gauges sense increased cutting force) and at preprogrammed times to allow the removal of cuttings.

The drill, Lawson said, can keep the hole-making process within preset limits for torque, feed rates, thrust, material type, and cutter configuration, which were determined by earlier production experience and stored in an IBM PC/AT computer.

The result of this development is an improvement in the quality of holes and reduction of costly rework. Average drilling time for a hole is reduced by one-half to two-thirds. Drill life is also extended; bits that formerly had to be resharpened after every 15 holes can now drill 60 holes before maintainance is required.

Ultrasonic Cutter

The standard system for cutting composite prepreg (preimpregated with resin) materials is the Gerber knife, a proven design developed from fabric cutters used in the garment industry. But that system is limited in accuracy to [+ or -] 0.030 inch, Henderson said, because its jigsaw blade-like knife, which reciprocates at 10,000 cycles per minute, leaves a large kerf when it shears through the tough, abrasive composite material. Tighter cut-part tolerances were required to produce "net ply lay-ups," a technique that reduces the amount of machining of the completed components by holding tighter tolerances upstream.

B-2 engineers turned to Design Technologies of the U.K. (now American GFM in Chesapeake, Va.) to obtain a more accurate prepreg cutter. The company had developed an ultrasonic cutter that could slice through the materials three times faster and with greater precision than traditional knives. The ultrasonic cutter, which employs a tungsten-carbide chisel vibrating at 20,000 Hz, produces cut-part profiling accuracies of [+ or -]0.005 inch, said Edward Mihalko, director of sales and marketing for American GFM. A vacuum that operates through many pinholes in the semirigid base keeps the prepreg fabric in place during cutting.

It's like the difference between an Exacto blade and a hacksaw," Henderson said. "The dramatic difference in kerf allows the ply to conform more accurately to the net configuration of the part." He also noted that the outlines of each successive ply are traced by computer-controlled ink-jet markers in the system, which eliminates a good deal of ply fixturing.

Northrop's ultrasonic knife, which can cut single plies at about 1600 linear inches per minute, has been redesigned by American GFM engineers to cut even faster, Mihalko said. Several of these improved machines have been sold to aerospace and automotive firms.

Robotic Drilling

The LTV Aerospace and Defense Co.'s Aircraft division (Dallas) has received about $2.2 billion for its subcontract to build about one-third (by weight) of the bomber's airframe. LTV makes the intermediate wing section, which lies roughly over the wheel wells and around the engines. The section is fabricated from advanced carbon-fiber-reinforced composite components, some with metallic substructures.

Because the assembly of aircraft components typically accounts for more than half the total labor cost of an aircraft, the automation of rivet hole drilling and insertion was a priority for LTV engineers. "The B-2 substructures are too big to manipulate simply and too thick to easily access both sides. These contoured composite parts-some with aluminum and titanium substructures-can be as large as 18 by 30 feet," said Jim Galloway, LTV manufacturing technology senior specialist and project manager. "So in the years after 1983, with $1.4 million in Air Force ManTech money and a $660,000 internal capital investment, we engineered the Robotics for Major Assembly (RMA) system-a computer-controlled five-axis robotic system that accurately drills high-quality holes and installs fasteners." So far, the RMA has installed more than 50,000 fasteners in a like-number of 3/16-, 1/4-, and 5/16-inch holes, while improving quality by more than 90 percent over manual methods and reducing production time and manual labor.

"Stacked composite and metal structures require specialized drilling techniques," Galloway noted. "Our goal was to do the drilling process in one shot without any reaming, and to do the countersinking immediately afterwards." The LTV team had to experiment to optimize the feeds and speeds of the drill head and drill geometry to limit the amount of thrust exerted on the somewhat fragile tacked-together part. They also had to control the size of the chips when the bit hit the metal substructure so as not to enlarge the hole in the composite layer. They used a straight-fluted polycrystalline-diamond drill, which provided better chip evacuation.

Cooling is provided by gaseous nitrogen pumped at high velocity and pressure (750 psi) through a hollow spindle and drill bit to emerge from coolant holes on each flute. The jet of nitrogen gas also flushes out the cuttings. This novel arrangement eliminates the need for a liquid-coolant recovery system since the gas merely dissipates. LTV has applied for a patent for the unusual design, Galloway said.

The tool head is positioned by a six-axis electric servo-driven vertical gantry robot constructed by LTV personnel. The robot has an accuracy of about [+ or -]0.15 inch, but requiring greater accuracies, the team developed a vision system-based adaptive offset feature that incrementally offsets the hole locations every 60 inches along the large contoured components. This system eliminates the effect of assembly irregularities such as tolerance buildup, which would invalidate preprogrammed hole positions.

The ranging and positioning system is based on an International Robomation Intelligence (Carlsbad, Calif.) vision system, an infrared laser diode array built at LTV, and a Sony camera. It locates tack fasteners, whose positions are used to guide the robot. A depth-sensing tool holder from Craft industries Inc. (Troy, Mich.) maintains the countersink depth. Sealant is applied after drilling is completed.

At first, the RMA's automatic fastener-insertion system used the drill head spindle to push in the two-piece titanium flush fasteners, but residual interference in the hole would sometimes cause the composite surface to flex. In addition, the automated device had some problems in choosing the correct fastener length. So as part of a recent Air Force Industrial Modernization contract, LTV engineers are developing an adaptive grip length determination system that would read the hole depth off the drill and select the correct fastener. A small air hammer that fits on the head will complete the insertions. Improvements in the fastener-handling system are also being studied. Despite all the automation, a technician working on the other side of the components secures the fasteners with nuts.

Water-Jet Trimming

"Hand-routing composite parts is a messy difficult operation," said Daren Davis, manfacturing technology senior specialist at LTV and project manager. "With these large cumbersome parts, manual labor would produce unacceptable results in terms of quality, cost, productivity, and safety. We wanted to take the human error out, to cut faster with no heat effects, and to hold high tolerances, so we decided to trim large composite panels with abrasive water-jet technology."

Begun in 1985, this factory automation project was funded inhouse with about $1.5 million, Davis said. A gantry robot was purchased from Cimcorp Inc. (Shoreview, Minn.), which provided a working envelope of 18 by 40 by 5.5 feet, and a positioning accuracy of [+ or -]0.013 inch. The water-'jet cutting equipment came from Flow Systems Inc. (Kent, Wash.). A vision system similar to the one used in the RMA finds the location of three indexing targets, which provides position-correction data to the Cad-based part program.

When ready to cut, a 60-horsepower dual-intensifier pump directs deionized water containing 80-mesh garnet abrasive out the nozzle at a pressure of about 40,000 psi. The Mach 2-velocity stream can cut up to four-inch-thick graphite epoxy, six-inch-thick aluminum, and two-inch-thick titanium. The typical feed rate for cutting quarter-inch-thick composite is about 12 to 15 inches per minute. Internal hole finish is about 250 microinches.

After the water slurry cuts through the material, it is caught by a compact stream catcher that replaces conventional water tank traps and support fixtures. The catcher consists of a stainless-steel canister filled with stainless-steel burnishing balls. The canister is fixed to the lower end of the robot's end effector so that it can travel behind the part as the water-jet head moves along its front surface. As the stream enters the canister, it fluidizes the bed, dissipating the jet's energy into heat and wear. The waste material is vacuumed away.

The water-jet robot is also designed to trim horizontal stiffener blades. In this case, the problem was to stop the high-speed stream from cutting adjacent blades only four to six inches away. This was accomplished by a similar, though smaller, stream catcher that interposes itself between blades.

Outboard Wings

With a $4 billion share of the B-2 contract, Boeing produces more than half the structure, fabricating the 65foot-long outboard wing sections-the largest structural aircraft parts ever made from composite materials-as well as the aft center section (a 50-foot-long structure that contains the weapons and electronics bays). Boeing also makes smaller components such as the weapons bay doors.

In general, the components are autoclave-molded graphite-epoxy composites made from a high-modulus temperature-resistant Fiberite 934 matrix from ICI Americas (Wilmington, Del.) reinforced with Thornel T300 (yield strength over 3.2 Gpa) carbon fibers from Union Carbide Corp.'s Speciality Polymers and Composites division (Danbury, Conn.).

"In deciding to go with a composite airframe, we also decided to automate as much of the processing as we could to reduce cost and improve quality," said Dan Arnold, manager of the parts, materials, and process group in the Military Airplane division of the Boeing Defense & Space Group. "There really was no other way to go about it and keep it economically feasible." To date, Boeing has spent about $400 million to build its composites-manufacturing facility in Seattle. Besides B-2 components and parts for the 777 transport, the plant also builds composite replacement wings for the Navy's A-6 Intruder and parts of the YF-22 Advanced Tactical Fighter prototype.

"We had five Air Force ManTech programs on our segment of the B-2 project, which comprised a research and development investment of about $4 million to $5 million," Arnold said.

Many of the B-2 structures that were to be built by Boeing (as well as by LTV) were so large that an efficient automated method by which prepreg composite tape could be applied and conformed to contoured tooling had to be developed. With ManTech funding and B-2 contractor oversight, engineers from Cincinnati Milacron Inc.'s Industrial Robot division (Cincinnati) developed a giant six-axis gantry robot with a tape-laying head that could spool out six-inch-wide zero-degree tape on huge tooling fixtures.

Even more crucial to the project was the team's evolutionary development of the robot control software, which allows tape layup of more severely contoured surfaces than was previously possible, said the Air Force's Dave Thomas. This "natural path" software, he explained, moves the tape head in such a way that it can hold tight tolerances in the critical structural areas, while allowing the natural path-the way the tape wants to lay up-to "wash out" in noncritical areas. The precise manner in which software commands the tape head to spool out the material avoids puckering and misaligned layup, he said.

If the hundreds of composite plies in the B-2 wing skin were to be layed up by hand, it would take some 60 days of manufacturing time, Arnold estimated. With the robotic tape-laying system, it takes less than 40 days. This also helps avoid problems with the aging of prepreg materials through exposure to factory floor conditions.

The Boeing facility has six of these systems, Arnold said. Both the tape-laying robot and the software are currently marketed by Milacron and by Ingersoll Milling Machine Co. (Rockford, Ill.).

A derivative of the tape-laying system was developed to build 60-footlong 6- to 8-inch-wide U-shaped composite channels that serve as integral blade stiffeners (stringers) for the wing panels. Each unique stringer is set in position on the huge wing skins, where it is held in place with tools. The combination is then bagged and cocured in large autoclaves, which effectively bonds them together into a single structure. This automated channel laminator reduced hand lay-up by 69 percent, while improving quality and reducing scrap.

"Rigid graphite-epoxy tooling is used to create the smooth outer mold line on these parts," Arnold explained, "but in an effort to get away from rigid inner tooling, we used a silicon elastomer to shape the inner mold line. The elastomer provides a fluid-like pressure on the inner surface of the lay-up, which allows some forgiveness in handling the substantial ply drop-off on these parts." From inboard to outboard on some of the larger structures, the lay-up thickness can go from 0.6 to 0.180 inch, he said.

A recent ManTech effort has defined new elastomers for inner surface tooling that lasts for 20 rather than only seven autoclave-molding cycles. A switch to the new materials could save from $10 million to $20 million over the life of the project, said Lt. Col. John Campbell, chief of the Air Force Manufacturing Systems division.

The need for many big constantcross-section components such as Land T-stiffeners and brackets led Boeing engineers to develop (with ManTech funds) a large composites pultrusion machine, Arnold said. Materials are pulled through two hot dies to consolidate and partially cure them. They then undergo secondary curing in autoclaves. Arnold noted that the scheme required quite a bit of materials development work so that the [+ or -] 45 degree fabric would not stretch and distort as it is pulled. We had to laminate the plies with zero-degree tape so that we could pull the combination through the dies," he explained.

Giant Autoclaves

The Boeing plant includes two of the largest-volume autoclaves ever built for composites work. These 90-foot-long 25-foot-diameter chambers are necessary to handle the large parts being built there. One autoclave, which can reach a temperature of 850 [degrees] F and a pressure of 250 psi, is for epoxy materials. The second chamber achieves higher temperatures and pressures and is used to process thermoplastics and polyimides. Two additional smaller autoclaves process smaller parts.

Due to the oversized and highly contoured nature of the parts used in the B-2, conventional nondestructive ultrasonic inspection systems with their water immersion tanks would have been impractical, Arnold said. Instead, a sophisticated system that employs water-jet coupling to convey the ultrasonic signals to the parts was developed with ManTech funding. The ultrasonic heads are moved about by a robot arm. Both through-transmission and pulse-echo transducers operating at 1, 2.25, and 5 Mhz are used, depending on part thickness and configuration. The system is designed to find half-inch flaws. Inspection data are stored on magnetic disks for later analysis.