Actually, it IS Rocket Science!
I have been designing, building and launching water bottle rockets over the past three years. While this has been a fun hobby, some of my rockets have soared smoothly while others have spiraled out of control. Obtaining predictable altitudes has also been inconsistent. This year, I decided to approach my passion for rocketry in a scientific experiment, and I set out to determine the optimal amount of propellant to use in a rocket to achieve maximum altitude. My hypothesis was that adding more water to the rocket would make it fly higher due to the increased mass expelled from the rocket, but past a certain point, adding more water would begin to weigh down the rocket because there would be less room for compressed air. I designed my experiment to measure the altitude of rocket launches while increasing the amount of fuel used by 100 ml for each set of four rocket flights launched. I purchased an altimeter and fins. Other materials used were my bicycle pump, launch pad, goggles, headphones, and a remote trigger to activate the launch. I kept the PSI constant at 60. I recorded the data in my log. I measured the altitude of four flights of no fuel (water), 100 ml of fuel, 200 ml of fuel, 300 ml of fuel, 400 ml of fuel, and 500 ml of fuel. The flights consistently flew higher for each next water level. At 500 ml of water, they were flying about twice as high as the ones at 100 and 200 ml. When I graphed all the data, I was surprised to see that there was no visible curve in the data trend at all. The altitude was steadily increasing but there was no sign of a peak altitude, let alone a gradual decrease afterward. To see how long it would take to reach this point, I increased the water level to 750 ml, and flew four flights at that. The flights were in fact lower than the flights at 500 ml, so I had already passed the peak altitude. Backtracking, I flew flights at 600 ml and it fit in nicely with the graph; higher than the ones at 750 ml, but not as high as 500 ml. Unfortunately, I was only able to get three flights of 600 ml because on the fourth flight, the rocket landed perfectly on the top of a very high tree. The rocket was recovered, but the altimeter was nowhere to be seen. In conclusion, for a rocket with a pressure chamber volume of 7 liters, the optimal amount of water appears to be 500 ml, making the water-to-air ratio 1:14 for optimal flights. This was only tested in one type of rocket and only at 60 PSI, so if I were to repeat this experiment, I would expand it by testing different pressures too. In the future, I would also like to test different densities of water by adding salt or soap to change the density.