When building a two stage rocket, you will need a release mechanism. On the web you will find many ideas based on the principle of a crushing or an expanding sleeve mechanism. ( "original idea i believe was from Bruce Breggren) " A flexible tube or sleeve expands under pressure in a solid tube. The expansion forces hold the pieces together. After launch the acceleration forces and the pressure hold the two stages together. At the time the booster is burned out, the pressure drops to atmosferic and the acceleration stops. The expansion forces decrease and the tube looses grip. The second stage pushes itself from the tube and launches itself. Although this is an good idea it is not perfect, if your goal is: reaching maximum altitude, then it is better to wait before releasing the second stage. So it makes optimum use of the momentum of the booster. The mechanism shown here is a improved expanding sleeve mechanism. The combination with an airflap speed detection and release trigger enables the second stage to wait for the booster speed to decrease to a predetermined level and then release it self. Bottom assembled, in pre launch position. |
See both pictures and the drawing to find out how the construction works. The principle of holding the second stage is like a standard expanding tube mechanism. The top picture with both pieces apart. |
On presurization the inlet valve and the second stage valve let the air pass through to pressurize the second stage. The release valve is closed. The pressure makes the tube expand and holds down the second stage. After launch the pressure in the booster drops and the inlet valve blocks, the tube stays pressurized. Even if the acceleration stops and goes negative the pressurized tube holds the second stage in place. If the speed drops below a predetermined value this is detected by an airflap mechanism. A rubber band pushes a metal pin in the release valve. The valve opens and releases the pressure from the expanding tube. The tube looses grip and releases the second stage. For the inlet and release valve I used bicycle valves, the type that used an internal soft plastic part as a seal. Valves and tubes are glued with polyurethane two compound glue, the same as used for gluing the bottle parts together. |