Water to air ratio.
After following the tutorial on how to build you own water rocket, you’re now going to want to go ahead and test it. To do this you will need to attach a cork or bung onto an air pump needed to apply pressure to the rocket.
Next you will need to fill up the bottle roughly 1/3 of the way with water. We have tested the rocket with about 1/6 of water and the rocket was only able to reach about 1 meter high due to there not being enough propellant force to drive the rocket upwards. However, fill the bottle too much (2/3) and the bottle won’t expel all of the water as it will be too heavy, and you won’t be able to apply enough pressure.
The water in the rocket is what propels the rocket upwards due to Newtons third law of motion, that every action has an equal but opposite reaction. Therefore, as the water is pushed downwards the rocket is pushed upwards. However, you are not compressing the water, but the air inside it. The compressed air is what produces the force and the water acts as the mass, combining these two gives the acceleration of the rocket.
Now with this information you can figure out that not enough air means not enough force to drive the mass of water out. And too much air means wasted force as it will run out of mass to push out. That is why the golden ratio is about 1/3 water and 2/3 air.
Centre of mass and centre of pressure.
Now the relationship between the centre of mass and centre of pressure is that the further apart they are the more stable to rocket will be. On a bottle rocket the centre of pressure is roughly where the water will exit out of the rocket. Therefore, you need to apply different weights in the nose of the cone to increases the distance between the centre of mass and centre of pressure. However, there is that golden ratio again, between the rocket being too heavy so its max altitude will decrease, or it will be more unstable. Therefore, a double bottle rocket is more stable due to the large distance between the centre of pressure and centre of mass.
Nozzle diameters.
You can also play about with different diameter nozzles for the water to exit through. Due to Bernoulli’s Principle.
Due to the initial velocity and initial area inside the bottle being constant.
So as you decrease the area of the nozzle opening the water will exit at a higher velocity increasing thrust.
Due to the bove equation there is multiple ways to increase thrust, which are:
– Increase mass flow rate.
– Increase exit Velocity.
– Increase exit pressure.
– Decrease nozzle area if Pe>Pa and increase if Pe<Pa.