Producing a certain thrust by accelerating a large amount of air to a relatively low speed takes less energy and thus less fuel than producing the same thrust by accelerating a smaller amount of air to a higher speed. However, efficiency drops when the plane's speed exceeds that of the exhaust gases, meaning that planes with high-bypass turbofans are limited to about Mach 0.85-0.9 while fighters with turbojets or low-bypass turbofans can fly supersonic. Since fighters need to be streamlined, they would also become too fat if their hull contained a high-bypass turbofan with the same thrust as one or two low-bypass turbofans with afterburner.