How a water rocket works

  • A rocket transforms the potential energy stored in pressurized gas into kinetic energy.
  • The exhaust gas and fuel (in our case water) exits from the rocket’s nozzle and are expelled at high velocity, endowing them with significant momentum.
  • The rocket propels itself forward by expelling a jet of exhaust, which transfers momentum to the rocket, causing it to move in the direction opposite to the exhaust with an equal amount of force.

 

What makes a water rocket fly?

  1. As the compressed air expands, it exerts pressure on the water.
  2. This pressure forces the water downward through the nozzle.
  3. The water jets out of the nozzle, creating thrust to overcome the rocket’s weight and air resistance.
  4. This thrust propels the rocket upward into the sky.
  5. The fins and nose cone stabilize the rocket, preventing it from tumbling.
  6. The water rocket is propelled till gravity over comes the thrust bringing the water rocket down.

What happens if there is too much water to air ratio?

The rocket exhausts its air pressure before expelling all the water. Without the expulsion of water, the rocket loses thrust faster because of the added weight. The rocket along with the remaining water falls back down to the ground.

What happens if there isn’t enough water?

The rocket exhausts its water supply before running out of air pressure. Without water flowing through the nozzle the rocket generates less thrust. The remaining air pressure escapes with minimum thrust added. The empty bottle then glides momentarily before descending.

Rocket thrust equation

Thrust (F) = Mass flow rate (ṁ) * Velocity (V) + (Pressure (Pe) – Pressure (Po)) * Area (A)

Rocket thrust is calculated by multiplying the mass flow rate by the velocity and adding the product of the pressure difference and the area.

Bernoulli’s principle

Bernoulli’s principle states that as the speed of a fluid increases, its pressure or potential energy decreases. This principle is demonstrated in water rockets: when the rocket is released, the pressure inside the body decreases as the speed of the expelled water increases, propelling the rocket upward.

Bernoulli’s equation

(0.5 x Density * Fluid flow speed²) + Pressure + (Density * Gravity * elevation) = Constant

Typical rocket layout