How did aircraft engines like the 707 become more fuel efficient and powerful into the 777?
In: Technology
It’s all about bypass.
707 engines are, like military fighter jet engines, non-bypass. All the air that’s compressed by the front blades feeds combustion. This is great for thrust-to-weight ratio, but not efficient at less than max power. Alas, an airliner spends almost all it’s time at less than max, and fuel economy is really important to an airline.
777 engines are high bypass. Most of the air that’s compressed by the front blades pushes out the back just like the exhaust. Bypass engines don’t grow in power output as fast, the 777 engine might be 20x the fan cross-sectional area of the 707, but it only makes 5x as much thrust at max. What it does make is lots of thrust at the most economical cruise settings, where airliners spend most of their time flying.
All-bypass, what are generally called propeller airplanes, turn the front fan in a way that’s unconnected to combustion. They are even more economical, but not easy to scale up to 707 passenger capacity.
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