Air-water rockets are a fun, safe and inexpensive way to explore the basics of rocket science as the laws of motion and the forces they experience in flight are similar to those of larger fuel rockets.
Isaac Newton first described these laws in 1687, and these very same laws are used today and helped send humans into space and the moon 280 years late
This includes:
- Momentum and Inertia – An object either remains at rest or continues to move at a constant velocity unless acted upon by a force.
- F = ma – The sum of the forces (F) on an object is equal to the mass (m) of that object multiplied by the acceleration (a) of the object.
- Exchange of force – When one body exerts a force on a second body, the second body simultaneously exerts a force of equal magnitude in the opposite direction. Classically described as “For every action, there is an equal and opposite reaction.”.
Some forces (F) a rocket will experience in flight are:
- the thrust of the engine
- weight due to gravity
- aerodynamic forces such as lift and drag.
Further, that these forces (F) will change dramatically during flight as propellant is pushed out from the nozzle to create thrust, accelerating (a) the rocket up. Two things occur as the rocket uses up its propellant, the mass (m) of the rocket decreases and its centre of gravity changes which affects the rockets stability. Then as acceleration slows as propellant is exhausted, aerodynamic forces, momentum and gravity take over as the rocket falls towards the ground. Consequently, there are many things to consider and experiment with to achieve the best results. Four Forces on a Rocket | Glenn Research Center | NASA
The physics of air-water rockets and fuel rockets are different but also similar. In a conventional fuel rocket, the propellant is pushed out through the nozzle by mixing explosive chemicals like liquid oxygen and hydrocarbons together as the energy source. In an air-water rocket, the propellant is water, and the energy source is compressed air.
How to build
1.25Litre PET Soda bottles are used as the pressure vessel of the rocket. Soda bottles have a typical pressure capacity of 14 Bar or 1400kpa (+200psi) and can be recycled. Our rocket launcher uses 4.5 Bar for +90 metres of lift into the air.
The Rocket Launcher uses a RC controlled 12V Air-Compress able to pressurise 4 rockets relatively quickly to 4.5 Bar.
Check valves ensure pressure is maintained in each rocket when others are lunched.
Air Tap Ball Valves are used to manually close off air to other stations to allow the pressurisation of launch of less than 4 rockets.
Compatible PET Soda Bottle Cap is drilled out to 20mm using a 20mm tapered drill.
Bicycle tyer inner tube used and cut into 20mm long pieces to make a pressure tight gasket.
A 19mm irrigation barb to snap-on hose connector adaptor is used as the latching system connecting to the Rocket Launcher.
A 12mm Brass Garden Hose snap-fit connector is used as the main Rocket Launcher release mechanism.
Also a 8mm x 260mm brass tube is soldered to the inside providing an air bypasses, through the water inside the rocket and also provides assistance in guiding the rocket straight when launched.
Stainless flexible cable is used and clamped to the snap-fit ring and then looped underneath to connect the servo via Teflon sleeves to ensure the cable moves freely.
When the servo is operated the snap-fit ring is pulled down to release the rocket under pressure.
