Report on the design of Physoc Air Pressure Rocket
The basic principle behind this type of rocket is reasonably simple. A bottle about a quarter full of water has air pumped into it, then you release the rocket and the water shots out the back giving you thrust via Newton’s Third Law.
A primitive rocket (a 500ml cola bottle say) of this type is very easy to build, but in order to allow safe launch of a larger rocket, we need to build a more complicated system.
This launching mechanism basically consists of the connector and nozzle from a normal common or garden hose. This is attached to a hose pipe in the normal way. At the end of the hose pipe is a bicycle tyre valve. This is attached to the pipe by using a jubilee clip to squash the hose pipe onto it. The valve will also have some araladite on it also. This is very tight and will not blow out at the pressure we will put inside the bottle. A bicycle pump can be attached to this and this is how we pressurise the bottle.
Next we use looped pieces of a strong string (possible bale twine or garden string) that go up through another jubilee clip around the hose connector and down outside it. The jubilee clip is then tightened. We use three bits of string in a equilateral triangle shape. This ensures that when we pull on the string to pull the hose connector down, the force down is equally distributed around the connector. This is important as if only one side where pulled down the rocket would launch crooked.
This is then mounted on a plank of wood and a bamboo cane put along side it. This plank of wood is then attached to the base. This is shown in the third diagram. The base is there to make sure it doesn’t fall over and also to make sure the plank with the launching mechanism is pointing straight up.
The rocket is a 2l cola bottle with the bottle top glued into the hose nozzle. I used Araladite (excuse spelling), this is a really strong super glue that can be bought in any car shop. On the side of the rocket there is something like a toilet roll tube that will go along the bamboo cane making sure it goes up straight at the start anyway.
To help the stability of the rocket after launch we use a rounded nose cone at the top, this makes it more aerodynamic, so the air flowing pass it will be close to the ideal of laminar flows. This means we don’t get turbulence which cause exert a horizontal force on the rocket and send it sideways. Along with this the rocket has a cylindical plastic sheet (the middle of a cola bottle) attached at the bottom using long sticks (I used kebab sticks). This is supposed to improve stability further, I believe it works in two ways. One it lowers the centre of mass moving it closer to the centre of pressure which is a good thing for stability and two it must change the air flow along the side of rocket.
Altogether the rocket should be stable.
That’s how the rocket will be made and launched.
The most important thing is that the rocket launches straight up. The base stops the launcher falling over. The hose connector is pulled down by three strings making sure the rocket is released evenly. The bamboo makes sure that the rocket is still pointing up after the first few seconds when most of the water is being forced out. And the rocket is designed that it stays vertical.
Using the strings the rocket can be launched from a safe distance, so none of the physoc committee get hurt. The students watching will be kept a good distance back (e.g if it were launched on the rugby pitch, students would be kept on the paths either side. This can be done with the help of the security guards), so when the rocket comes down (this technically should be back on top of the launcher but with a small wind it could be up to 10m radius around it I’d estimate. I will know more when I test it this weekend).
So in conclusion, I cannot see how anyone can be injured during the launches (2 most likely but 4 max). Of course any suggestions are most welcome and I will be happy to talk further will the college safety officials.
Peter Elliott
PRO, DU Physoc
SSTP