What Makes the Water Rocket Go?
Here’s another amusing example: water rockets. People often ask me why the rocket needs both water and compressed air. To a good approximation, the air provides the energy and the water provides the momentum. For any given volume, given a limit on max operating pressure, you can store vastly more energy in pressurized air than in pressurized water. That’s because virtually all of the water goes in while the pressure is zero … it just pours in as a liquid. Then it takes only a tiny bit more liquid to raise the pressure a great deal. In contrast, most of the air goes in after an appreciable pressure has been built up, so you’re doing a lot of work against this pressure. Meanwhile, it doesn’t do much good to have a big supply of energy if you don’t have a good way to transfer momentum. Air doesn’t weigh very much, so just throwing the on-board air out the back would not transfer a useful amount of momentum. Water is a thousand times denser, so throwing water out the back works a lot bet
