These are two staging mechanisms that are in general use. The diagrams show the release mechanisms - there should be extra support for the upper section (in the form of a straight side bottle section to rest on) or the mechanism will be damaged under the acceleration of launch.

Crushing Sleeve

The sustainer and the booster are linked according to the diagram. The pressures in the top (b) and bottom (a) bottles are the same as the bottles are pressurised from the bottom. The one way valve prevents water from returning to the bottom bottle.

On pressurisation, the flexible PVC tubing grips the PVC pipe (1), stopping it from escaping. When the rocket is launched, the pressure in the booster (a) falls and eventually, the PVC tubing (1) is released. If the weight of the top bottle under the acceleration from the booster is greater than the force from the pressure in the top bottle acting on the end of the nozzle (the outside diameter - 15mm in this case) then the sustainer will not be released. If the pressure is sufficient then it will break free.

Select Crushing Sleeve. The release pressure is put into the top text box and the diameter of the outside of the sustainer's nozzle section is put into the bottom box

To find out the release pressure, assemble the two bottles (with a little water in the top one to act as a seal) and take the pressure up, 5 psi at a time, releasing the pressure in the bottom section until the top bottle breaks free. I found that for the version on the right, 45 psi was needed. The diameter is 15mm in this case (the o/d of the 15mm pipe).

Expanding Tubing - Single Valve

The booster has a special top with an expanding rubber sleeve that swells under pressure, holding the sustainer in place. There is a non-return valve on the top of this that allows air to pass from the booster (b) to the sustainer (a) once there is a pressure difference between them. This has the effect of allowing a sufficient pressure differential to build up in order to inflate the tubing and, allows compressed air to travel into the upper chamber.

On launching, the pressure in the booster (b) falls and the tubing collapses, releasing the sustainer (a). Again, if the pressure in the sustainer is sufficient to force the two sections apart, it will, or it will wait until the acceleration has fallen enough for this to take place.

Select Expanding Tube and One Valve. Put the pressure at which the valve starts passing air in the upper box and the diameter of the inside of the neck in the bottom box.

Note that with this arrangement, the pressure in the sustainer is lower than that in the booster by the differential pressure (around 20 psi or so) and the diameter is the nozzle internal diameter (a bottle neck - 21mm or so).

There is nothing to stop you from putting a t-nozzle on top of the release mechanism to give you a smaller nozzle diameter. It will still have a high release mechanism diameter but the nozzle will be more effective. To find out the diameter of the t-nozzle required, elect to optimise the diameter and the value for Nozzle Diameter will be the t-nozzle diameter.

Expanding Tubing - Double Valve

Again, the booster has a special top with an expanding rubber sleeve that swells under pressure, holding the sustainer in place. There is a non-return valve on the top of this that allows air to pass from the booster to the sustainer once there is a sufficient pressure difference between them. This has the effect of allowing enough of a pressure differential to build up to inflate the tubing and, allows compressed air to travel into the sustainer (a).

Before air can get to the upper valve, it passes through the one way valve at the bottom. This just lets air in so that during pressurisation, the pressure in the release mechanism is the same as in the booster.

On launching, the pressure in the booster falls. Once the pressure difference between the booster (b) and the inside of the release mechanism (d) is great enough, air passes from the release mechanism (d) into the booster (b). Once the pressure in the release mechanism (d) has fallen further by the pressure difference required to inflate the expanding tubing, the sustainer is released. Again, if the pressure in the sustainer (a) is sufficient to force the two sections apart, it will, or it will wait until the acceleration has fallen enough for this to take place.

Select Expanding Tube and Two Valves. Put the pressure at which the top valve starts passing air in the upper box (20 psi in the example), the pressure at which the bottom valve starts passing in the middle box (50 psi in the example) and the diameter of the inside of the neck in the bottom box.

Note that with this arrangement, the pressure in the sustainer is lower than that in the booster by the differential pressure (around 20 psi or so), the sustainer is released once the pressure in the booster has fallen by the sum of the differential pressures of both valves, and the diameter is the nozzle internal diameter (a bottle neck - 21mm or so).

Again, there is nothing to stop you from putting a t-nozzle on top of the release mechanism to give you a smaller nozzle diameter. It will still have a high release mechanism diameter but the nozzle will be more effective. It is calculated in the same way as in the single valve description above.

Return to 2 Stage Optimisation

Copyright ©2000 Paul Grosse. All Rights Reserved