Principles for building water rockets

Thrust

To make a rocket move we need a thrust of force. In the case of rockets that launch into space they use chemical reactions supplied by various explosive chemicals. Luckily for us we use water and air pressure to create thrust.

Thru experimentation I'll just tell you that the bottle 1/3 full of water is about right. Why add water in the first place why not air? The reason to add water at all comes down to the laws of motion explained by Newton. I'll just give a few equations. If you don't understand it it doesn't matter you can still make a good rocket.

F= Force; m = mass; a = accelleration

F = F_rocket = F_water ; Newtons 3^rd law

m_rocket < m_water; just the way it is

a_rocket = F / m_rocket ; Since the rocket mass is so small compared to the water mass it really goes fast

a_water = F / m_water ; Since the mass of the water is high it goes pretty slow compared to the rocket

Like I said if you don't follow don't worry. Basically the air acts like a spring between the rocket and the water. When the air pressurizes it pushes against the bottle and the water. Since water is pretty heavy, a lot of energy is transferred to the rocket which provides a good acceleration. Now you can use just air which works okay, but air pushing against air doesn't allow for as good of a pushing surface so the rocket won't travel as fast. It's like this push against a golf ball. Who goes faster you or the ball? Now push against a house. Who goes faster you or the house?

Center of Gravity and Center of Force

The center of gravity and center of force concepts are the very most important principles for creating a rocket that will fly straight as an arrow.

The center of gravity is the point where the rocket will turn and pivot. It can be found by balancing the rocket on an edge. Put the whole rocket on the table then nudge it slowly off the table. The point just before it begins to tip off the table is approximately the center of gravity.

The center of force is the middle of the surface that air pushes against air as it rushes by. The center of force will be approximately where the fins are located. This is where most of air will be pushing against the rocket when the rocket turns.

In the first scenario the center of gravity is behind the center of force. This is bad. Looking at the picture you will see why. Immediately as the wind pushes against the center of force it will flip the rocket completely backwards. That just isn't what we want.

Scenario two the center of gravity and the center of force are at about the same spot. This is an unstable situation making it turn easily. This is great for really big rockets that have a guidance system small changes allow the rocket to easily turn. However none of the water rockets I've build have complicated guidance systems. This could potentially be dangerous. There is no telling where this one will go. It could start chasing you or somebody else; not good. All though more than likely it just won't fly very well.

Scenario three the center of gravity is well in front of the center of force. This produces stable direction: the rocket will point the direction it's going. When the rocket turns a little the force of the wind immediately pushes the rocket back into alignment with the direction of travel.

There are two basic strategies to make your rocket's center of gravity in front of your center of force. The first is to move the center of gravity closer to the front of the rocket. The second is to move you center of force further to the back of the rocket.

To move the center of gravity forward you add weight to the front of the rocket. To move it back remove weight from the rocket. Basically make the front of the rocket heavier than the back of the rocket.

To move the center of force back move the fins further back on the rocket. Or to move the center of force forward move the fins up the rocket.

Alright there are the basic principles for building water rocket. Be safe, and have fun.