Water rockets

Here is where I learnt about water rockets. Water rockets run under the principles of Newtons 3rd Law of motion – “for every action there is an equal and opposite reaction.” The reaction of the pressure building (potential energy) turns into a reaction of thrust (kinetic energy). As seen in the diagram, the thrust propels the rocket whilst fighting its weight and air friction.

Thrust 

“The thrust T is the resultant of the forces due to the pressures exerted on the inner and outer walls by the combustion gases and the surrounding atmosphere, taking the boundary between the inner and outer surfaces as the cross section of the exit of the nozzle”

Thrust (T) is calculated in this equation: 

The Net force is the total force, deciding what’s happening to the rocket. It is positive as the Thrust force is larger than the weight. This can be worked out with (Thrust – Weight). 

Other factors that I looked at were : Solid/Liquid propellant differences and Model and real rocket comparisons.

How it works

With air being pumped in it will compress and expand. That will then push the water out the bottom, causing the thrust force. A good thing to note was what happens with measuring how much water to have.

With too much water:

The rocket runs out of pressured air before all of the water is pushed out. Since the water isn’t being pushed out there will be a lost of thrust. The rocket and all the extra water weight fall to the ground.

And with not enough:

The rocket runs out of water before it runs out of the pressured air. Since there is no water left the rocket loses thrust. All of the extra air pressure is wasted and the bottle coasts for a little bit then drops to the ground.

To find out the highest the rocket we make goes, I will be doing four tests with different water pressures at the same psi and record the best time. 

Designing of the rocket

Due to the fins and model of the nose the rocket will lean, causing Lift along with change in direction of thrust. It’s good to notice the weight will still be directly down. Leaning also involves itself in the flight path. All rockets will turn but good design and calculations can alter that. 

After all the rockets pressure or water is released, depending on if there is too much water or not, it will ether drop sooner or coast for a bit. 

There are a lot of calculations with equations when launching, which will be tested when simulating. Here are some equations I need to note when calculating launch 

Forces ;

  • Vertical

  • Horizontal

Acceleration ;

  • Vertical

  • Horisontal

 

In-flight, the drag equation is 

Drag = coefficient x density x (Velocity squared / 2) x reference area

When calculating the weight (W), each component has individual weights that you add together. eg. 

W = Wa + Wb + Wc + Wd

Wi = Mi + g

Mi = competent mass

g = gravitational acceleration.

 

Determining Center of gravity and pressure

Each component has (some) weight and are located (some) distance from a reference.

The centre of gravity multiplied by the wight is equal to the weight multiplied distance of all the components away from the reference. 

The equation looks like this: cg W = DaWa+DbWb+DcWc+Dd+Wd…

Similar to centre of gravity. Each component has (Some) area located (Some) distance from a reference. The centre of pressure multiplied by the Area is equal to the area times distance for each component from the reference. 

This equation is : cp A = DaAa+DbAb+DcAc+DdAd…

These are shown in the diagram below 

The center of gravity is the point on witch the rocket will turn. And the center of pressure is the point where wind pressure balances out. This would mean that you would want the cg higher than the cp for a stable flight path, done buy adding weight higher up.