"This program probably has one of the world’s most uninspiring viewgraph presentations,” says Barth Shenk, then the US Air Force program manager for the Advanced Composite Cargo Aircraft effort. “But looking at the original metal fuselage sitting next to the composite one now on the ACCA demonstrator, that’s when it sinks in. This is something fundamentally very different.”
The ACCA program was initiated in April 2007 to capitalize on the investment in composite materials made by government and industry over the last three decades through the Composites Affordability Initiative. Greater use of the fiber- and resin-based composite materials, which are both strong and light weight, is seen as the way to significantly reduce development time and costs for new aircraft.
The longer-term challenge to the aerospace industry is straightforward: advance composite technologies to build aircraft faster, using less labor, fewer parts, and simpler tools, as well as make them lighter and easier to maintain. The payoff: significantly lower costs to design, manufacture, and operate those aircraft. The ACCA program is a first step toward that goal.
Although composites are increasingly used in aircraft construction, ranging from panels and wing skins on the F-22 and F-35 to flaps on the C-130J, those parts are generally considered “black aluminum,” that is, parts fastened to frames, stringers, and ribs the same way metallic aircraft have been built for decades. “The government ’s Composites Affordability Initiative matured these technologies at the component-level,” notes Shenk, a civilian in the Ai Vehicles Directorate of the Air Force Research Laboratory, or AFRL, Wright-Patterson AFB, Ohio. “With ACCA, we had the opportunity demonstrate we could build a large, integrated airframe faster and cheaper than history said we could. We took on that challenge and succeeded in big way.”
The ACCA demonstrator, now officially designated X-55A, is the result. The X-55, a highly modified Dornier 328Jet commuter airliner, was flown the first time in June 2009, twenty-five months after the program began.
“Historically, airframe costs tend to scale with size, and airlifters tend to be among the larger aircraft,” Shenk observes. “Next-generation airlift platforms appear on paper to be prohibitively expensive to pursue, even though the age of the fleet, increased demand, and a renewed emphasis on fuel efficiency are compounding the need. So, airlifters stand to gain the biggest benefit from a program like ACCA. Army payloads aren’t getting any smaller, so any aircraft weight reduced is payload gained. Wit h the Joint Future Theater Lift tactical airlifter program in the initial study stages, the ACCA technologies are pretty timely.”
When the ACCA program began, AFRL selected two competitors, Aurora Flight Sciences and Lockheed Martin Aeronautics Company’s Advanced Development Programs, better known as the Skunk Works, for Phase I. This five-month effort funded preliminary design and risk reduction efforts. One contractor would be selected to proceed to Phase II—fabrication, assembly, and flight test of a demonstrator.
“Boeing and Northrop Grumman didn’t even bid,” says Mike Swanson, who was the Lockheed Martin ACCA program manager during Phases I and II. “We were skeptical that we could meet the AFRL requirements. But we accepted the risk.”
The requirements called for an aircraft with a fully pressurized fuselage and enough volume to accommodate standard-sized military 463L cargo pallets; structural robustness that could withstand common types of service-induced damage; and 463L pallet- and vehicle-compatible flooring and aft ramp loading. The aircraft had to be capable of landing on unprepared fields and to tolerate climate extremes.
“We looked at a clean sheet design, but that wasn’t going to get what the Air Force wanted,” notes Swanson. “A new design would’ve required more time than we had. We decided to take an existing airliner and transform it into an airlifter.
The 328Jet we selected is pressurized, has a digital flight control system, and is designed with a high mounted wing, which provides room for cargo and a ramp. We didn’t want to redo the systems; we just needed an aircraft to modify.” In October 2007, AFRL authorized the Skunk Works to proceed with Phase II.
“ACCA was very much like an old Skunk Works program— a small group of experts working very closely, with minimal oversight, to achieve a goal quickly,” notes Swanson. “The engineers were located across the hall from the shop floor. We could resolve issues in minutes.” At its peak, the ACCA program had only eighty Skunk Works employees assigned to it.
“There was a high degree of trust between the prime contractor and the customer,” Swanson adds. “That trust was the only way this project could work.” Shenk agreed. “Transparency was a key factor. It was nice doing business jointly in a collegial environment. We had weekly teleconferences, but we also had a lot of informal communication. That kept everything moving smoothly.”
The biggest technical challenge was developing a process to produce a flightworthy composite aircraft. “The weight savings that composites bring is great,” Shenk notes. “But their biggest advantage can be found in reduction of parts, tooling, and fasteners and in not having to build specialized facilities to fabricate and assemble aircraft.”
Four material technologies were key to the ACCA program. Large integrated sandwich structures brought an order of magnitude reduction in parts count—what would normally be two or more individual parts could be cast as one single assembly to reduce both assembly time and cost.
Highly tailored stiffening, or HiTS, is a process where the fibers are oriented in the composite matrix in the most efficient way for that component. Using HiTS reduced the amount of touch labor required to fabricate parts.
“Composite structures like to be large and continuous,” notes Swanson. “So, the idea is to make parts as large as possible. The use of joints between, say, barrel sections of a fuselage introduces complexity.” Traditional composite parts are cured in an autoclave with high heat and pressure.
The use of a new generation of composite materials on ACCA that could be cured outside of an autoclave and at considerably lower temperatures reduced tool ing costs and allowed very large single-piece structures to be fabricated.
Finally, the use of Pi joints, which look like the Greek letter (π), allowed parts to be bonded, almost like a plastic model kit, rather than having to spend time drilling holes and installing fasteners—and adding complexity—to connect parts together. “We took a leap-frog approach with the use of bonded parts,” Swanson notes. The use of Pi joints eliminated the need for ninety-five percent of traditional metal fasteners on ACCA.
The demonstrator, the second 328Jet Dornier built for development testing in the 1990s, arrived in Palmdale, California, in November 2007 and was essentially beheaded. The flight station, a small section of fuselage, and the passenger door were separated from the rest of the fuselage. The horizontal tail, rudder, the engines, and the nearly seventyfoot- long wing were also removed to be reinstalled later.
The 328Jet’s original metal fuselage with ribs, stringers, and formers was placed off to the side in a hangar. The composite fuselage that replaced it in late 2008 is wider and taller with no stringers or ribs and with smooth sidewalls. The eight structural support frames that are there are located in high stress areas—four to bear the main landing gear, three to support the tail, and one to strengthen the area around the cargo ramp.
Using the large sixty-foot-long by twenty-foot-wide oven in the Skunk Works’ Advanced Prototyping Shop, the fuselage was cast in two halves, upper and lower. The lower fuselage half is the largest single piece of the aircraft at fifty-five feet long and ten feet wide and was cast much like a fiberglass boat hull. A twelve-inch wide seam running the length of the fuselage is the visible symbol of where the upper and lower fuselage halves were bonded together. The interior of the aircraft was left unpainted to show the composite materials.
“We made sensible design decisions, not just using composite parts for composites’ sake,” notes Shenk. One major part of the X-55 that did have to be made of composites was the cargo ramp, which not only had to support the weight of pallets or vehicles being loaded but also had to hold aircraft pressurization. Cost and complexity were reduced by not installing hydraulics to operate the ramp. Instead, the ramp is operated by a simple hand-crank mechanism.
Creating the replacement vertical tail involved engineers and technicians at four Lockheed Martin facilities. The vertical was designed in Marietta, Georgia; laid up at Michoud, Louisiana; assembled in Fort Worth, Texas; and completed and installed in Palmdale. Installation into the aircraft required only fifteen minutes, and it fit perfectly.
The final numbers for this aircraft’s transformation are startling: The original metal fuselage and vertical tail contained about 28,000 fasteners. The X-55 has only about 4,000 fasteners and nearly all of them are in the area where the metal f light station joins to the composite fuselage. The original fuselage and tail had 3,000 metal parts. Those same structures on the X-55 are made up of only 300 composite parts, with sixty-two of those in the tail and twenty-nine in the cargo ramp.
Keeping the flight station intact reduced risk considerably. The original digital flight display system was transferred to the new aircraft configuration to eliminate the need to develop specialized software. Many of the original wiring harnesses were reused, but some new ones were required, including those necessary for flight test instrumentation.
“We could save fifteen to twenty percent of structural weight in an aircraft if the manufacturing processes were optimized for production,” says Brian Shoemaker, the Lockheed Martin ACCA build team manager. “We did have some teething pains on this aircraft. Some parts didn’t fit the first time, a few had to be repaired, and some didn’t initially bond well. But across all the new composite structure, we encountered almost no delaminations and only a couple of voids [air pockets in the material]. The fact the aircraft received an airworthiness certificate by the FAA is proof the process works.”
The real proof came when the aircraft took to the air.
First flight of the ACCA came on 2 June 2009. Rob Rowe and Joe Biviano, Lockheed Martin test pilots, f lew the aircraft from Air Force Plant 42 at Palmdale, testing basic aircraft functionality and control. The flight lasted eightyseven minutes and marked the final milestone for ACCA Phase II. Two other test f lights in July and August opened the aircraft’s performance envelope.
Last September, AFRL awarded Lockheed Martin the ACCA Phase III contract. This nineteen-month program will include further flight envelope expansion, training of government pilots, construction of a fuselage fatigue test article, and additional materials testing. Full documentation of the program will also be completed in this phase.
Longer term, AFRL is working with industry and NASA to develop the next phases of X-55 a s a testbed for advanced technology programs, including further work in composite materials, energy efficiency, ISR technology, and advanced aerodynamics.
Another potential test project is to design and build a new composite wing for the X-55 to replace the existing metal one. “The Joint Future Theater Lift aircraft will likely have short takeoff and landing capability,” says Shenk. “We could incorporate high lift devices into the X-55 to provide a STOL baseline for JFTL. We have a rugged test asset with volume, payload capacity, and endurance that opens the door for a number of experiments.”
“Most of the existing processes in the aerospace industry are set up to build metallic aircraft,” concludes Swanson. “With this program, we hope to fundamentally change how aircraft are going to be built in the future.”