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.
Burt Rutan, Sir Richard Branson and Steve Fossett stand by the Virgin Atlantic GlobalFlyer.
Freelance aerodynamicist John Roncz, who had developed airfoils and propellers for Voyager, and his former assistant, Mark Mangelsdorf, who now worked for Scaled, created the thick low-drag airfoils for the project and resized it for the larger engine. With the booms, 42 inches in diameter, now 30 feet apart, the horizontal tail had become too long and whippy; it was discarded in favor of two separate empennages of conventional proportions, one on each boom.
The preliminary design was frozen by fall of 2002. Detail design and execution was now parceled out to separate teams. Rutan himself was regularly consulted along the way, but he was mainly absorbed in working on Tier 1, his suborbital rocket plane. Joe Ruddy handled the all-carbon wing and fuselage structures, including the 500-pound, 114-foot-long main spar that holds everything together. To meet the rigorous weight requirement, all inessential structure was pared away. The design limit load factor is just 2Gs at maximum weight. The complete structure, with a wingspan greater than that of a 727, weighs a mere 1,727 pounds. Bob Morgan designed the extremely efficient tricycle landing gear for a single cycle per flight, retracting under compressed air pressure and free-dropping for landing. Clint Nichols did the propulsion design, and Shawn Keller the electrical systems.
Chuck Coleman designed the all-important fuel system. Largely automated (unlike that of Voyager, which included a maze of spigots and plastic hoses in the cabin and at one point had to be re-plumbed in flight), it incorporates 13 tanks with a dozen filling points; the 13th tank is the central “header” from which the engine draws fuel. Each wing has four integral fuel tanks outboard of the boom; the inboard portion of the wing between the fuselage and the boom, which has negative dihedral in order to allow fuel to drain into the boom, is divided, along with the boom itself, into separate tanks ahead of and behind the spar.
The four tanks in the booms and inboard wing panels hold about 3,600 pounds each. During the first two and a half days of a flight expected to take more than three, the system is automatic. Every 10 minutes, jet pumps in the booms send fuel to the header tank, alternating between the fore and aft boom tanks so as to maintain the CG location. After 60 hours or more, the pilot throws eight switches to open electric valves in the outboard wing tanks and allows an additional 3,600 pounds of fuel to drain into the booms.
The record flight, which is supposed to take place either in April or else after October this year, is expected to begin somewhere within the continental United States, possibly in Ohio, land of the Wright brothers. A long runway is needed; in fact, rocket assist was considered for the F-109 engined version, and then (at least provisionally) discarded for the more powerful FJ44. Laden with 18,000 pounds of fuel, indicating a constant 100 knots, the airplane will climb gradually to an eventual cruising altitude of 45,000 feet. There it will maintain a speed of .4 Mach-not the most efficient speed aerodynamically as the airplane gets lighter, but one required to keep the engine happy at that altitude. With an expected tailwind of 60 knots or so-much more if Fossett can locate and hitch a ride on the jetstream-the gangly jet should make it around the planet in something between 60 and 80 hours before alighting, with twin drag chutes billowing, at the airport from which it departed.
The cockpit, less than eight feet long and pressurized to provide a 10,000-foot cabin at 45,000 feet, is relatively spacious for a single pilot, and is equipped with a state-of-the-art glass panel. A tiny bubble canopy provides the only view out for takeoff and landing; there are also a couple of small round windows in the cabin sides. Flying above weather in an almost entirely automatic airplane, the intrepid pilot will risk extreme boredom.
On January 8th, with fanfare and folderol more appropriate to a new ride at Disneyland, the airplane, now dubbed the Virgin Atlantic GlobalFlyer, was presented to the press. That this was no ordinary roll-out was apparent from the in-hangar fog-and-light show with which it began and the exceptionally tasty breakfast buffet-several cuts above the usual for this kind of event-with which it ended. The airplane’s many noses had been adorned with paintings of a flying woman, presumably a virgin although she didn’t look much like one, whose red wrap appeared to be unwrapping apace. Fossett and sponsor Sir Richard Branson of Virgin Atlantic posed for the cameras in silver jumpsuits, together with a less gaudily attired Rutan. Swarms of reporters thrust eager microphones at them. Branson, who resembles a latter-day Buffalo Bill Cody, and who is listed as the backup pilot for the world flight should Fossett be unavailable, modestly conceded that some delay might be involved if his services were needed, since he does not yet have a pilot’s license.
As I was nibbling my sixth serving of porcini strudel I ran into Dick Rutan, somewhat grayer and more Michaelangelesque of build than when I last saw him. Few of the reporters present, most of whom were from the broadcast media, had any idea who this man was, despite his brother Burt’s having lost no opportunity to remind them that the GlobalFlyer would not be the first airplane to circumnavigate the globe unrefueled, but the second. It struck me that being unrecognized and largely ignored ennobled Dick. He seemed to harbor little resentment at the Fossett/Branson threat to steal his circumglobal thunder, but the frantic self-aggrandizement, the patent craving for publicity and admiration, the wholesale reliance upon wealth to achieve what others had had to do with labor, patience and courage-all this seemed mean and trivial by comparison to what he and Jeana Yaeger and a multitude of unpaid volunteers had done with Voyager.
If the GlobalFlyer‘s trip succeeds-and there is no reason, other than the unavoidable perversity of Destiny, why it should not-then while Fossett and Branson bathe in the brief spotlight of media attention it will behoove color che sanno-those who know-to remember, and remind their friends, that most of the credit for the accomplishment justly belongs to the engineers and craftsmen who made it possible: the women and men for whom building a practical jet airplane with a fuel fraction of 82 percent was not an unimaginable goal.