Since the 1940s, the U.S. Air Force’s X-planes have typically been designed for speed, maneuverability, and weapons delivery. Often built under black budgets, the existence of many X-planes remained secret until after their retirement.
Breaking that mold is the USAF’s newest X-plane, the Advanced Composite Cargo Aircraft (ACCA). Designed to demonstrate the use of structural composite materials in a short takeoff and landing (STOL) cargo aircraft, the ACCA will be the Air Force Research Laboratory’s (AFRL) first flying X-plane since the Grumman-built forward-swept-wing X-29 of the 1980s.
The program will also be the USAF’s first serious attempt to demonstrate new cargo-carrier capabilities since the Advanced Medium STOL Transport competition of the 1970s, when Boeing’s YC-14 and McDonnell Douglas’ YC-15 squared off. Neither plane went into production but the YC-15 lived on as the baseline for the McDonnell Douglas C-17 Globemaster III.
Lockheed Martin will build the ACCA, which will be based on a Fairchild Dornier 328J regional jet that will have its aluminum mid/aft fuselage and empennage replaced by sections built of advanced composites. (Aurora Flight Sciences also competed for the ACCA program, but its design based on an Antonov An-72 was on the losing end of the October 2007 downselect.)
“We usually don’t build demonstrators for structural technologies,” said Barth Shenk, AFRL’s ACCA Program Manager. “But we’ve invested a lot of money in composite materials for manufacturing and wanted to see if we could develop a capstone program that would put together these technologies under a highly constrained time and cost bogie.
“We’re running a lean program office with five people. We want to keep this light, lean, and fast all the way through the program.”
If all goes well, ground checks will begin as soon as late summer. Contract award to first flight is expected to take only one year—with wheels up planned for late 2008.
Lockheed’s integration of advanced composites on the ACCA flight demonstrator will enable a reduction of 80 to 90% in parts count and lessened corrosion and fatigue issues compared to conventional aircraft manufacturing approaches. A lighter aircraft will also lead to less fuel consumption. It is also expected that the company will make extensive use of rapid prototyping techniques to speed design and production.
At press time, Lockheed was “well into the fabrication of the fuselage and components,” according to Mike Swanson, ACCA Program Manager at Lockheed Martin Advanced Development Programs (Skunk Works) group. Unlike most other large composite structures for airframes that have to be cured in huge, expensive autoclaves—such as those being built for Boeing’s 787, for example—Lockheed is taking advantage of new techniques developed under the U.S DOD’s Composites Affordability Initiative to cure the material at much lower temperatures and pressures.
The Skunk Works’ experience with composite materials comes from several earlier programs, including: the DarkStar UAV, which had a composite center fuselage; and, most recently, the Polecat UAV, a tail-less vehicle with a 90-ft wingspan (which remained a secret until Lockheed released a photo at Farnborough 2006). The composite materials for both UAVs were cured in low-temperature ovens and built without the use of autoclaves.
“We’re trying to build the fuselage as a large, single-piece component with upper and lower sections,” said Swanson. “The way of thinking about how you develop an airframe structure is fundamentally different than what’s been done in the past.”
Extending from the frame just behind the entrance door of the 328J to the rear of the airplane, the composite fuselage being constructed for the ACCA will be 55 ft long and 9 ft in diameter. The cockpit and everything forward of the entrance door will remain in its original state, with fasteners used to bring the forward metal section together with the mid and rear composite sections.
Lockheed chose the 328J for the ACCA program based on AFRL criteria for relatively high speed (hence turbofans instead of turboprops), STOL performance, and tolerance against foreign object debris from operation on rough fields (hence the high wing and engines). The ACCA will be a one-off demonstration aircraft with no intention for it to go into production.
Like most X-planes, though, many of the technologies and manufacturing techniques that will be proven during the program will likely make their way to other military aircraft besides STOL transports, including long-range strike aircraft and unmanned systems.
“This is more about the technology itself than any particular program,” said Swanson. “Even though we’re using an air mobility aircraft, these technologies could find their way to any number of future military programs.”