(April 2011) TAKE A CLOSE LOOK at the inner workings of a modern turbofan engine and you’ll be glimpsing mankind’s ultimate triumph over that most fundamental of untamed ancient natural elements — fire. The science behind basic turbine engine design, after all, represents simplicity at its minimalist best: A single moving part compresses the air and mixes it with fuel to create a very hot fire, which in turn produces the thrust that propels an airplane forward. Not since cavemen first understood how to make fire by rubbing two sticks together have humans achieved something so monumentally significant from such straightforward engineering principles.
Since the widespread commercial adoption of turbine engines, starting with the Boeing 707 narrow-body jetliner in 1958, designers have sought to make improvements in every facet of their manufacture. Today’s modern turbofan engine is lighter, quieter, more reliable, more fuel-efficient and cleaner-burning than anything Sir Frank Whittle could have envisioned when he filed his first patent for the turbojet in 1930. In fact, designers of the latest turbofans are so adept at extracting the maximum performance and efficiency from their engines’ computer-perfected cores that you might be tempted to think they’ve reached limits of what’s physically possible.
But you’d be very wrong.
Engine manufacturers are broadening the technological boundaries of what they can achieve by turning to exotic superalloys and new types of material coatings capable of withstanding ever-higher temperatures, which translates into improved efficiency and higher thrust. Engineering teams are also sculpting new 3-D geometry for fan blades, compressors, turbines and even the engine nacelles themselves as they seek to squeeze every last ounce of fuel efficiency and performance from their designs. Some manufacturers are even crafting entirely new types of turbine engines that rely on geared turbofans, spinning blisks, rear-facing swept rotor blades or multicoupled internal stages, painstakingly assembled to increase turbofan bypass ratios and extract extra power from smaller engine cores.
Suddenly, the engineering that goes into modern turbine engine design doesn’t seem so basic after all.
“It’s not quite rocket science, but it has become incredibly sophisticated in the last decade as we focus our attention on improving every last detail of our designs,” said Walter Di Bartolomeo, vice president of engineering for Pratt & Whitney Canada.
The details include everything from optimizing the basic physics of the engine — such as compressing the incoming air to ever-smaller volumes — to improving how internal components are cooled. For example, Pratt & Whitney Canada’s newest engine, the PW800 family developed for large-cabin business jets, has achieved a 10 percent increase in fuel economy compared with current engines in the same thrust class by improving the airflow and adjusting the overall cooling scheme, while also incorporating the fourth generation of Pratt & Whitney’s Talon (Technologies for Advanced Low NOx), which not only addresses how much fuel is burned during the combustion process, but also how thoroughly it is burned.
Air Force Driving Switch to Biofuels
The Air Force last spring flew its first test flights in jets powered entirely by the biofuel blend. The flights took place at Eglin Air Force Base in Florida with an A-10 Thunderbolt II burning a combination fuel derived from camelina oil and conventional jet fuel.
On the commercial side, biofuel company Solazyme recently announced a partnership to develop aviation fuels with Qantas. All the world’s major turbine engine producers have begun testing biofuel blends. Researchers say it would take about 65,000 square miles (roughly the size of Wisconsin) to continuously produce the plant matter needed to meet all of aviation’s biofuel needs.
War of the Global Jets
In the 1990s, Gulfstream and Bombardier became locked in a battle to introduce the world’s first ultralong-range business jets, capable of flying more than 5,800 nautical miles nonstop. The Gulfstream V and Bombardier Global Express introduced in the late 1990s both derived their power from the Rolls-Royce BR710, a then all-new turbofan that burned less fuel per pound of thrust than any engine in its class.
Now the companies are back at it, scraping for every last mile of range in an escalating sales war.
“The practical goal is being able to fly 12,500 miles, plus some margin beyond that,” says Jim Kroeger, director of propulsion systems engineering and technology for Honeywell Aerospace. “That allows you to fly to any point on the globe nonstop. Once you can fly halfway around the world, range no longer becomes a limiting factor in where you can go.”
Converting from statute miles, the goal would be 10,800 nautical miles. Add in a couple hundred miles of safety margin and the magic number becomes about 11,000 nautical miles.
“There is this perpetual quest for more range, and as long as people want to fly farther, I don’t see us ever reaching a point where we’ll say we can’t go any farther,” Kroeger said. “How soon we get there is the big unknown.”
Gulfstream and Bombardier are taking an important step toward reaching that eventual goal with the new G650 and Global 7000 and 8000 business jets. Gulfstream’s G650 will be capable of flying 7,000 nautical miles when throttled back to its long-range cruise speed. Power comes from Rolls-Royce BR725 turbofan engines, the next generation in the BR700 series. The BR725 engine benefits from some of the core advances made with Rolls-Royce’s Advance2 technology initiative — a new family of turbofan engines Rolls-Royce is developing to power tomorrow’s business and regional jets.