Raptor Rolls into Production

If all goes as scheduled, the F-22 Raptor Advanced Tactical Fighter (ATF) will have taken 23 years to go from a formally identified Air Force requirement in 1981 to operational hardware in 2004. The project is still on schedule and the first of America’s nextgeneration air-superiority fighters soared smoothly into production last month.

“The next century’s guarantor of air dominance for U.S. military forces is coming of age, and, ahead of schedule,” says F-22 Program Office Director Brig. Gen. Michael Mushala. “To accomplish so many program goals in record time really is a testament to the men and women who make the Raptor America’s most advanced fighter.”

But there have been changes. The final number of planned production aircraft, for example, has shrunk from 750 to 339. And the prototype that earned Lockheed Martin and Boeing the ATF contract has been redesigned to meet new requirements and reduce production costs. Now that the first operational aircraft are being assembled, details are finalized and more is known about the design and manufacture of the F-22 Raptor.

Fighter or flying supercomputer
The F-22 will be the first aircraft to rely on “integrated avionics.” The term means different things to different people. Pilots experience integrated avionics as a single source of coordinated information. To software engineers, integrated computer systems share data about the situation, the mission, and aircraft systems. And hardware designers see integration as common processors with enough connectivity and bandwidth to support the required processing.

Put simply, the plane won’t carry separate boxes full of electronics for radios, navigation devices such as an automatic direction finder and GPS, or even a radar and electronic warfare suite. Instead, two Common Integrated Processors (CIPs), each a bit larger than a breadbox and built by Hughes, will process and share all signals and data for sensors and mission avionics. CIPs will even control flight surfaces such as the ailerons and vertical tails.

Each CIP has 66 module slots, similar to expansion slots for printed circuit boards in PCs. Currently, 19 of the slots in CIP 1 and 22 in CIP 2 are vacant, leaving room for future improvements. And each module currently uses only 75% of its computing capacity, leaving room for growth. There’s also space, power, and cooling for a third CIP.

So instead of devoting separate boxes to each avionic system, the F-22 uses CIPs programmed to automatically emulate or configure the hardware to provide electronic functions necessary for a mission. For example, if a module acting as a UHF radio fails, the CIP detects the failure, loads the UHF radio program into a compatible module, and brings the UHF radio back on line. This lets the plane accommodate combat damage and makes it easy to upgrade avionics.

Each CIP operates at 10.5 billion instructions/ sec and carries 300 Mbytes of memory. Software engineers had to write 1.7 million lines of code for the computers, 90% of it in Ada, the common computer language for the DoD. (The only exceptions to the Ada requirement were for special processing or maintenance needs.)

All this gives the Raptor the computational power of two Cray supercomputers and makes the F-22 extremely capable. But the densely packed electronics also generate considerable heat. So air and polyalphaolefin-based liquid systems cool the CIPs and other electronics, as well as the racks holding them.

This cooling boosts the MTBF for the CIP modules to 25,000 hr. Cooling also lets the Boeing-built power supplies, each about 6 3 6.5 3 0.5 in. and weighing less than 2 lb, put out 400 W.

Keeping it flying
With only 339 planes in the pipeline, it is important each be 100% mission ready for most of its operational life. So the plane carries extensive diagnostic and built-in test capability to reduce maintenance downtime. Virtually every piece of hardware checks itself and reports failures or if its operation degrades. There can be more than 15,000 fault reports on the engine and airframe, and another 15,000 for the avionics. Most faults don’t trigger pilot warnings or affect the aircraft’s mission capability.

Technicians will access the F-22 fault system through a laptop computer called a Portable Maintenance Aide. It reads and records faults, along with the levels of consumables such as fuel and oil. It also uploads new operational flight programs and software tailored for specific missions or newly added hardware.

To protect against fire, the plane carries infrared and ultraviolet sensors, along with a Halon 1301 fire suppression system. Halon is the only ozone-depleting chemical used on the F-22, and efforts are underway to find a replacement. The alternatives under consideration might take up more room than Halon, but the designers are ready. There’s space onboard to carry fire-fighting agents that occupy 2.5 times the volume of Halon currently carried.

To further protect against fire (especially in combat) the aircraft’s eight fuel cells fill with nitrogen as fuel is depleted. The JP-8 fuel, a naphthalene derivative with a relatively high flash point, is not particularly flammable. Its fumes are, however. Putting inert nitrogen in the fuel cells eliminates fumes and the fire risk. An On-Board Inert Gas Generation System (OBIGGS) supplies nitrogen by pulling it out of ambient air.

The plane will be the first fighter to make extensive use of high-tech composites and new alloys, and it will be in service for more than 25 years. Designers took special care to test those materials, using more than 13,000 coupons, or samples, of six different types of composites. Chief among them was bismaleimide (BMI), a composite with high strength and temperature resistance used for the aircraft skin.

Composites comprise 24% of the F-22 structural materials by weight, while titanium alloys account for 42%. Airmet 100, a steel alloy, will see its first application as part of the jet’s main landing gear. As a heattreated specialty steel, it offers a high level of corrosion protection. These metals also went through rigorous, with metal parts undergoing 5,300 static and durability and damage tolerance tests (DADT) on metal parts.

Manufacturing processes
The F-22 is the result of several exotic manufacturing processes. These include hot isostatic pressing (HIP), resin-transfer molding (RTM), automated fiber placement (AFP), and electron-beam (EB) welding

In HIP casting, metallic castings are heated in a static pressure environment (more than 10,000 psi) to collapse, or “heal,” voids or gas pockets. On the F-22, structural titanium components are HIPed to eliminate voids left after casting. HIP also strengthens six large structures on the F-22: the rudder actuator housing (one for each rudder), the canopy deck, the wing side-of-body (SOB) forward and aft fittings (four total, two for each wing), the aileron strongback (one for each aileron, two total), and the inlet canted frame (one each for the left and right inlets).

The F-22 is the first aircraft to take advantage of resin transfer molding for composite parts. RTM economically and repeatedly fabricates intricately shaped details to tight dimensional tolerances. In the past, technicians making large composite parts had to pressurize hundreds of fabric layers containing an embedded resin and curing, or “baking,” them in an autoclave. This was time consuming and labor intensive. With RTM, “preforms” are assembled under vacuum from stacks of fabric placed in metal tooling that matches the shape of the part. The tool is injected with pressurized, hot resin. After cooling, out pops the finished BMI or epoxy part.

RTM is used to make more than 400 parts for the F-22, from inlet lip edges to load-bearing sine-wave spars in the wings. The economics are compelling. At Boeing, for example, RTM reduces the cost of wing spars by 20% and cuts in half the number of reinforcement parts needed to install spars in the wings.

AFP technology lets composites replace titanium for the pivot shaft on the horizontal stabilizers. AFP involves computer-controlled positioning of reinforcing fibers. This makes possible the complex geometry of the pivot shaft: a 10-in.-diameter cylinder at one end, a rectangular spar approximately 4 in. wide at the other, and an offset between. It resembles an oversized hockey stick. The shaft is composed of more than 400 layers of composite tapes with widths ranging from 1⁄8 to 1⁄2 in. The shaft cures in stages to prevent internal cracking and wrinkles, which could create voids. The process takes up to 60 days, but saves 90 lb per aircraft compared to titanium. Engineers are investigating use of thicker tapes, which should greatly reduce production time.

Automated electron beam welding lets Boeing and supplier Aerojet build lighter weight titanium assemblies for the aft fuselage. EB welding takes place in a vacuum chamber and uses a stream of electrons to weld titanium parts together. This prevents exposure to oxygen, which can create brittle surfaces. EB welding can also join thick titanium parts (i.e., more than an inch) considerably better than other methods. This reduces the number of fasteners in some fuselage components by up to 75%. Besides the economic and weight savings, fewer fasteners means fewer openings for possible fuel leaks.