Who Says a Jet Can't Be Cheap?
Gerry Merrill says he can build you one for $150,000.
- By David Noland
- Air & Space magazine, March 2008
Taped to the wall of Gerry Merrill's office in Phoenix, Arizona, is a three-view engineering blueprint of a sleek, two-seat personal jet. Finely curved, it sports a V-tail and a single fat engine nacelle atop the rear fuselage. Merrill, a wiry 73-year-old, tells me that, compared to current propeller-driven light airplanes (which he dismisses as "archaic crap"), the airplane on his wall will fly twice as fast, get better fuel mileage, and be 30 times quieter. His proposed family of jet-powered light airplanes, he says, will revolutionize private aviation, if only he can raise the $120 million it would take to get them certified by the Federal Aviation Administration and into production.
The good news: "This is potentially a multi-billion-dollar business," he says. The bad: He can't break into the market.
Since the 1940s, jet engines have revolutionized military, airline, and corporate flying. But somehow, the jet revolution never trickled down to personal flying. A practical, affordable, jet-powered light airplane for private pilots—a Cessna Skyhawk, say, without the propeller—has for decades been only a dream. Five- or six-seat, $1.5 million to $3 million very light jets (VLJs) are just now on the market, but the vision of a single- or double-seat jet with the price tag of a propeller aircraft—one-eighth the cost of a VLJ—has never become more real than three-view drawings taped to the office walls of dreamers everywhere.
But Merrill stands apart from the rest; he probably knows more than anyone about small jet engines. A propulsion industry lifer who has worked on everything from torpedoes to funny-car dragsters, he's had a hand in the design of some 80 jet engines, ranging from the General Electric J79, used in U.S. fighters and bombers in the 1950s and 1960s, to the Teledyne CAE J402, used in today's air-to-surface and cruise missiles. But for more than 40 years, his abiding passion has been the idea of a small jet engine for light aircraft. He has spent most of that time bouncing back and forth between manufacturers, who weren't willing to bet the store on an engine that was not certified, and investors, who wouldn't put up the money for certification because the light-airplane market was too unpredictable.
Conventional wisdom holds that jet engines burn too much fuel at low altitudes to be suitable for light aircraft. Jets, it's said, are efficient only above 30,000 feet or so, where light airplanes fear to tread. "That's bullshit," says Merrill flatly. Sure, every jet flying today gulps prodigious amounts of fuel when going low and slow. But that's because they were all designed to run high and fast. The secret, Merrill says, is to simply optimize the configuration of an engine's turbines, compressors, and fan for, say, 250 mph at 15,000 feet instead of 500 mph at 40,000 feet. "It's not rocket science," he says. "My engine technology and materials are basically 1960s. The breakthrough is conceptual."
The results of optimizing a turbofan engine for low and slow are breathtaking—on paper, at least. According to Merrill's numbers, his theoretical 490-pound-thrust turbofan would, at 10,000 feet, push along his theoretical two-seat Cloudster airplane at 270 mph while burning 12 gallons of fuel per hour. That's 22 miles per gallon—about the same fuel efficiency that Cessna claims for its two-seat, propeller-driven 152, which flies not even half as fast. Climb to 23,000 feet, and the Cloudster will do 220 mph while burning seven gallons per hour. That's 31 miles per gallon. So much for the notion that jets at low altitude suck too much fuel, Merrill says.
Two primary factors account for the amazing low-and-slow fuel efficiency of Merrill's turbofans. All have extremely high bypass ratios and very low fan pressure ratios. (The bypass ratio is the amount of the air that passes through the fan—known as "cold thrust"—compared with the amount that passes through the core, or combustion chamber —"hot thrust." Typically, the higher an engine's bypass ratio, the better its fuel efficiency.) Merrill's engine for the Cloudster on his wall has a bypass ratio of 19, which is five or six times that of the current crop of VLJ engines, and double that of the most advanced jetliner turbofans. Merrill calculates a specific fuel consumption of around 0.30 pound per hour per pound of thrust—an extraordinarily low number, and one that is 50 percent better than the current VLJ mainstay engines, the Williams FJ33 and the Pratt & Whitney PW600 series.
But the real breakthrough in Merrill's engines is in their low fan pressure ratios. The ratio of pressures across the fan stage is a rough indicator of how much kinetic energy is wasted as air passes through the fan. "You've got to match the fan pressure ratio to the airspeed," says Merrill. "The lower the speed, the lower the FPR." Merrill is cagey about disclosing his engines' precise fan pressure ratio, but claims it is much lower than that on the VLJ engines.