In the reception area at Burt Rutan's Mojave, California, skunk works, Scaled Composites, there sits on a corner table a small black tripod with a cup-shaped receptacle on top. Built by composites engineer Stan Stawski, who works at Scaled, it supported two tons before failing. It weighs less than four ounces.
Getting the most strength for the least weight is the real crux of aeronautical engineering. In airplanes, structure is a necessary evil. The same could be said of fuel, and the two evils come together in airplanes intended to fly extremely long distances.
Three factors control the range of an airplane: propulsive efficiency, aerodynamic efficiency and fuel fraction. Once a powerplant has been decided upon, propulsive efficiency is known. It comes with the engine, or the engine-propeller combination. Aerodynamic efficiency, expressed as lift-drag ratio, is principally (assuming a streamlined shape) a matter of maximizing wingspan and minimizing wetted area; that's the aerodynamicists' business. Fuel fraction is the structural engineers' contribution to the equation. It's the portion of the takeoff weight that is fuel. Most general aviation airplanes carry less than 20 percent of their total weight in fuel; long-haul airliners may carry 35 percent. To increase the fuel fraction from, say, 35 percent to 70 percent is not twice, but four times, or possibly eight times, as difficult as to increase it from 20 percent to 40 percent.
When tycoon-adventurer Steve Fossett approached him with a request for an airplane in which Fossett could make a solo unrefueled flight around the world, Burt Rutan didn't want to design another piston-engined globe-girdler; he'd already done that. A turbine would be more challenging. He knew that it would have to have a higher fuel fraction than anything he had ever designed before, including the two-person, two-engine Voyager of 1986. Turbines are less efficient than recips, and fully 82 percent of the airplane's takeoff weight would have to be fuel.
In 1999 Rutan and the two engineers he had put in charge of the project, Jon Karkow and Matthew Gionta, presented alternative proposals to Steve Fossett: one was a turboprop, one a turbofan using the 1,300-lbt Garrett F-109 engine. Fossett selected the turbofan, and preliminary planning for the project-then called Capricorn after the tropic whose length was the officially sanctioned minimum to qualify as a round-the-world flight (and they didn't want to call it Cancer)-got underway. Twin booms-incorporated, as on Voyager, to shift outboard the portion of the fuel that could not be contained within the wing, thereby reducing bending stresses in the spar-ended with inward-canted fins whose tips were joined by a horizontal stabilizer placed above the engine's exhaust.
Karkow, charged with preliminary design, had already considered various alternative arrangements, including a single fuselage (too big and heavy, no place to put the main landing gear); no fuselage at all (complications from control system and fuel weight asymmetries); and even a four-wheel taildragger (doubtful ground handling characteristics at high weights). In the end, the P-38 arrangement-the one Rutan had intuitively chosen in the first place-won out.
By spring of 2002, when Scaled and Fossett signed a formal contract for the construction of the airplane, the F-109 that Scaled had hoped to snag had slipped away, and the design team turned to the Williams FJ44 instead. The 2,300-lbt FJ44 is both considerably larger and slightly less efficient than the F-109, and, in compliance with the implacable Breguet Range Equation, the required takeoff weight rose from 18,000 to 22,000 pounds. The whole airplane had to get bigger.
