Jet engines, like people, come in many shapes and sizes. There are sleek and narrow engines called turbojets that generate enormous thrust. Used almost exclusively by the military, they can fly a fighter jet faster than twice the speed of sound, but they also burn a lot of fuel. Engines from another big group, called high-bypass turbofans, are bulkier, slower, but much more fuel efficient. Everybody knows these engines. They are the heavy lifters of commercial aviation that hang from the wings of most passenger and cargo aircraft. They use a large fan at the front of the engine that pulls the plane forward like a powerful propeller.