Just in case anyone is wondering why more efficient engines are bigger: the energy is quadratic in speed (mv^2/2) while the momentum is linear (mv). For a given

Just in case anyone is wondering why more efficient engines are bigger: the energy is quadratic in speed (mv^2/2) while the momentum is linear (mv). For a given amount of energy (which comes from burning fuel) you can choose to push the airplane forward by pushing air back in 2 ways: 1. less mass, more speed, 2. more mass, less speed. It turns out 2 is better, for example you can push 4 times as much mass for half the speed, which results in twice the momentum of the air pushed backwards. Now the amount of air you can push is the amount of air you can get, and that's proportional with the front area of the engine. So, you always want to have as large an engine as possible. Bonus: the larger the engine, the slower the air moves through it, and so the less noise it produces. When you read that engines have become both more efficient and more quiet over the years, the second part was just a nice side-effect of the first.