Air-flap release mechanism

If you don't want the rocket to crash after every launch, you have to add a recovery mechanism.

All recovery systems work according the same basic principle: change the CP / CG dramaticly, this to increase drag and lower the speed your rocket is coming down with.
Most off the recovery systems work with a parachute.

On this page you find a mechanism that triggers the release of the parachute by detecting the forward speed (= speed in the direction off the longest axis) off the rocket.
Close to apogee, this forward speed decreases and becomes almost zero if the flight path is vertical.

The detection mechanism is an airflap, a mechanism that is commonly used.
If the speed is high the rapid moving air forces the airflap in its locked position. But when the air-speed decreases below a threshold the mechanism triggers and releases the parachute.

There are many variations that can be found on the net but the construction i describe here has some distinct advantages.
It is very light, simple to construct, robust and adds lesser drag compared to most other air-flap trigger mechanisms.
Also it can easily be modified to other airspeed trigger levels to accommodate the design of your rocket.

The air-flap is made from PET material and is mounted on a steel wire.
This wire runs to and through the top part of the rocket (above the pressure vessel) so the air-flap can make a movement like a pendulum of a clock.
On the othe side of the air-fap, the wire is bended in the shape of a little hook, this hook can hold a rubberband that is wrapped around the bottle top in place.
The rubber band on its turn holds the heads of two tie wraps in retaining holes which keep the top on the rocket.
If the forward speed of the rocket decreases, the force of the rubberband on the hook is larger then the force of the air. The air-flap swings up and releases the rubberband from its hook.
The cable tie-heads unlock and release the top off the rocket with the parachute.
The top off the rocket is pushed off by means of an internal spring.

On the picture on the left you see the air-flap, pinched by a red clothing pin and a M10 nut for extra weight.
The clothing pin keeps the air flap down as long as it is on the launchpad. At launch it slips from the air flap, the acceleration force of the rocket is to large for the clothing to keep hold of the air-flap.
In the first phase of flight the acceleration forces, keeps the air-flap in a downwards position.

At the end of the air blowdown phase the accelleration force decrease but then the flap is held down by the force of the air caused by the forward speed of the rocket.
Around apogee the forward speed decreases and the flap is pulled up by the force of the rubberband working on the hook. The rubber band slips of the hook and releases the tie wraps. A spring pushes the top from the bottle.
On this, fairly large, rocket I use also a rubber band to pull the top away and release the chute. It is tied to the bottom of the rocket (the brown string you see running down on the left picture).
The chute is tied to the top part of the pressure vessel.
The biggest threat for this type of release mechanism is parabolic flight. If your rocket does not go straight enough up it will always have a forward speed.
If this forward speed is above the trigger speed level, then there is a big chance on a so called lawn dart. Your rocket will come down at about the same speed it went up and crashes dart-like to the ground.