The Little Engine That Couldn't
The new Eclipse 500 lightjet will no doubt make a lot of customers happy
- By David Noland
- Air & Space magazine, November 2005
(Page 2 of 6)
Williams wasn’t the first to build a tiny jet engine. Back in the early 1950s, the French-built Turboméca Palas, with 330 pounds of thrust, inspired the creation of half a dozen oddball experimental Euro mini-jets. The Palas grew into the Marboré series (660 to 1,058 pounds of thrust), which powered a number of small military jets, such as the Morane-Saulnier 760 Paris four-seater and Cessna T-37 trainer. (The latter used the J-69, a version of the Marboré made by the U.S. company Teledyne CAE.) In the 1970s, the French firm Microturbo lowered the bar with the 220-pound-thrust TRS 18, which flew in the Italian Caproni A21J sailplane and in U.S. designer Jim Bede’s BD-5J airshow jet. Only 24 inches long, the TRS 18 is still the smallest jet engine ever to power a manned aircraft.
Those early mini-engines had a problem, though. Like all turbojets, they sucked up prodigious amounts of fuel. Worse, small aircraft are penalized by the pitiless exponential mathematics of scaling down: Reduce an airplane’s length by half, and internal volume for fuel shrinks eightfold. The BD-5J had an endurance of about an hour or so and a range of around 300 miles.
To be commercially viable, a small jet engine had to be fuel-efficient. That meant it had to be a turbofan. While Pratt & Whitney and Rolls-Royce began pushing ahead with turbofan technology in large engines in the 1960s, it was left to a young Purdue graduate and former Chrysler engineer named Sam Williams to create a small, fuel-efficient turbofan.
Williams left Chrysler in 1954 to start his own company. His first jet engine, prosaically named Jet No. 1, made its first run in 1957 at a meager 60 pounds of thrust. It weighed just 23 pounds; an old Williams publicity photo showed a smiling June Cleaver lookalike holding it in one hand. An improved version, the WR2, ran in 1962. Hewing closely to Frank Whittle’s 1930 turbojet configuration, the WR2 had a single-stage centrifugal compressor and a single-stage turbine. The reference book Jane’s All the World’s Aircraft described the engine as “simple in design, almost to the point of appearing crude.” In 1964, a more powerful version of the WR2 became the first Williams jet to fly, powering the Canadair CL-89 reconnaisance drone. The follow-on WR24 series, despite horrendous fuel consumption, was Williams’ first big commercial success, eventually powering more than 6,000 short-range Northrop target drones.
In 1967, Williams completed its breakthrough engine. The WR19, a turbofan based on the WR2 core, produced 430 pounds of thrust, weighed only 67 pounds, and was nearly twice as fuel-efficient as the WR2. It powered two short-lived 1970s contraptions: the Bell Jet Flying Belt, a Buzz Lightyear-style jet backpack; and the WASP II flying platform, a sort of aerial Segway Human Transporter.
The WR19 also caught the eye of military planners studying the concept of a long-range cruise missile. Williams’ timing was perfect; the WR19 was the only small engine with the fuel efficiency the cruise missile mission demanded. An up-rated version of the WR19, the 600-pound-thrust F107, eventually became the prime mover for the Navy Tomahawk and Air Force Air-Launched Cruise Missile, with production of more than 6,500 engines over 30 years. For creating the F107, Williams was awarded aviation’s highest honor, the Collier Trophy, in 1979.
Williams had begun tinkering with a small civilian turbofan based on his cruise missile technology as far back as 1971. But it would be a huge step to take a specialized Tomahawk powerplant, which only had to start once and run for three or four hours, and adapt the technology to produce a commercially viable engine.
Small size itself creates many design problems. Turbine blades can be made smaller, but air molecules can’t; as a result, skin friction and boundary layer effects are proportionally greater. (In engineering argot, a small engine is inherently less efficient because it operates at a low Reynolds number, an aerodynamic coefficient that relates component size to the air’s inertial and viscosity effects.) Compressor and turbine blade tip clearances are proportionally greater, resulting in greater tip losses. To maintain the most efficient turbine and compressor blade tip speeds, small engines must spin faster. Small turbine blades are also harder to cool. Oil passages become narrower, making lubrication tricky. Manufacturing tolerances shrink to watchmaker scale.