Timing Was Not on the Side of This Interesting Alternative Technology Hauler
On 2 June 2009 what appeared at first glance to be a standard Dornier 328JET commuter jetliner lifted off from Lockheed Martin’s Advanced Development Programs facility (Air Force Plant 42) in Palmdale, California. The aircraft, powered by a pair of underwing pod-mounted Pratt & Whitney PW306B turbofan engines, carried the civilian registration N807LM. Each engine nacelle under the high-mounted wings was decorated with the acronym ACCA, which stood for Advanced Composite Cargo Aircraft. Perhaps the tipoff to the jet’s origins was the Lockheed Skunkworks logo conspicuously appearing on the vertical stabilizer. But this aircraft, like many of the flying machines born at Plant 42, was unique.
Origins of the Carbon Comet
The US Air Force Research Laboratory (AFRL) wanted to see what benefits a composite fuselage would bring to a small airlifter airframe. But at the time, while there was tremendous potential for the substitution and/or incorporation of advanced composite materials used to reduce aircraft structural weights compared to conventional metal structures, the aerospace industry was hesitant to implement them in new aircraft. AFRL launched the Composites Affordability Initiative (CAI) to address the perceived risks and barriers. Government laboratories, including NASA, worked collaboratively with aerospace contractors to develop the required advanced materials and manufacturing technologies. This 10-year AFRL-led research and development effort ultimately led to the Advanced Composite Cargo Aircraft (ACCA) experiment.
Take Apart a Perfectly Good Jetliner…
The Lockheed Martin X-55 did indeed begin life as a Dornier 328JET. But aft of the port side forward entrance door the fuselage was replaced by a light-weight advanced composite structure. The forward fuselage, wings, and horizontal stabilizer were then bolted back onto the new composite fuselage. The weight savings alone was impressive. The new structure was comprised of about 300 structural parts as opposed to 3,000 structural parts in the standard aircraft. Fasteners were reduced from 40,000 to 4,000. The fuselage was formed using two cured pieces (upper and lower) and bonding them together with interior strengthening frames- much like a plastic model fuselage might be glued together.
There Will Always Be Bugs
Challenges presented themselves along the way- one of which was mating the aluminum portions of the airframe with the composite portions. In fact the program was delayed by the need to build a second fuselage “shell” when the lower fuselage skin did not bond correctly. The Air Force designated the ACCA as the X-55A in October of 2009. While the program never advanced far enough for the Air Force to name the jet, Lockheed Martin engineers dubbed the lightweight little airlifter the Carbon Comet.
Competition and the COD
The Carbon Comet was selected for the project instead of a design by Aurora Flight Science that was based on the Antonov An-72 Coaler- itself somewhat similar to the Boeing YC-14 airlifter. As an airlifter the fuselage of the X-55A was wider than the standard 328JET fuselage at about 9 feet in diameter and equipped with aft clamshell doors with a rear-loading ramp. With a fuselage length of 55 feet, the X-55A is roughly the same length as the Grumman C-2A Greyhound, which has been the standard for the US Navy Carrier Onboard Delivery (COD) mission since entering service in 1966.
The Last Word from Lockheed Martin
According to Lockheed Martin at the time, the ACCA was “the first of several steps intended to advance the readiness of multiple technologies needed for the next generation of theater airlift aircraft. The ACCA demonstrates how future aircraft designs and manufacturing processes can be tailored through the use of advanced composites and new structural concepts to significantly shorten design spans, reduce aircraft size and structural weight, accelerate manufacturing processes, reduce tooling costs and reduce assembly labor. Initial estimates suggest development cost could be reduced by 25 percent or more over today’s processes plus additional operating cost savings from less weight resulting in lower fuel burn. Historically, aircraft cost has been determined by the size and weight of the vehicle. With ACCA we are proving that while size does matter, it isn’t the be-all, end-all determination of aircraft cost”
Retirement and Some Interesting What-Ifs
The X-55A did not pan out. Part of the problem was that although Lockheed Martin, NASA, and plenty of other aerospace engineers worked on the materials they were difficult to work with on the scale required for an airframe the size of the Carbon Comet- or larger. But…if the X-55A had proved more practical, the aircraft might have itself been developed into a replacement for the aforementioned C-2A COD as a jet-powered COD aircraft. For that matter, Lockheed’s own S-3B Viking, which had recently been retired from service (or even the US-3A COD Viking) might have been rebuilt with a composite fuselage and become a jet-powered COD. Far-fetched? Perhaps, but stranger things have happened. Already. This year!
Today the Lockheed Martin X-55A Carbon Comet is on display at Joe Davies Heritage Airpark in Palmdale, California.
