Multi-Stages

The separation system of the stages described below is modeled on That of Bruce Berggren: http://www.geocities.com/wrgarage/index.html. The 2nd floor nozzle is a rigid tube of 8 mm in diameter and about 6 cm long. It is fixed on a plug either by gluing or threading this tube to put a nut on each side of the plug. This nozzle is fitted in a flexible plastic tube (Vinyl) 8 mm in diameter interior. This vinyl tube is fixed on the upper plug of the first floor, while the other end is closed by a valve that lets the air pass in the first direction towards 2nd floor.

When the two floors are in place and that we press the first floor, the valve lets the air pass to fill the second floor. The pressure is the same outside the vinyl tube inside. The friction forces between the two tubes are sufficient to keep both floors together. During the phase Propulsion of the first floor, the pressure dimuned in the tank of the down, so outside the vinyl tube while inside, the pressure is always that of departure since the valve does not leave Take the air up and down.

The Vinyl tube will then start to "swell" under the effect of the pressure difference and thus the friction forces will decrease. When the force generated by the second stage pressure on the valve will be greater than the friction forces of the tubes, then the second floors will be released.

The diameter of 6 mm for the nozzle makes it possible to have a thrust relatively low, but longer compared to the first floor. As the speed of the rocket is already important, the effectiveness of Ailerons is good, which allows for good stability, throughout the propulsion phase of this second floor, despite a low center of gravity.

The important points are :

On the one hand the adjustment of the friction force of the two tubes, it must be big enough so that the separation is not premature, but not too strong so that the second stage is released in time. This adjustment is essentially done by the length of the tubes, the greater the length and the greater the friction.
On the other hand the stability of the second stage under load (with water), so be sure to put the fins as low as possible.

The nozzle of the 2nd stage

The release system of the first stage

Note the 2nd stage fins very tilted downwards for a better stability. The parachute on the 1st stage is just above the hand of the person holding the rocket. It is protected by a nose cone which is ejected when the 2nd stage goes away.