Acoustical Apogee Detection
This method uses the fact that there is a lot of noise produced by air flowing past the rocket during ascend. It can be noticed in many on board videos e.g. on youtube. Close to apogee the noise level drops almost to zero. This change in noise can be used to trigger the deployment mechanism.
Drawback: If the rocket's trajectory has a significant horizontal component the noise level will never vanish completely. As a consequence the parachute will not deploy.
AAD seems to be unexplored. That's why we go for it.
Rather than launching a rocket only with acoustical apogee detection or implementing a backup recovery system we decided to design a small experimental payload. This will be launched with a standard rocket with deployment timer.
The first attempt is a simple detection circuit using an electret microphone and a two stage amplifier followed by a low-pass filter. The result is a DC voltage which is dependant on the noise level recorded by the microphone. The output voltage is limited to 1.4 V in order to protect our data logger to which the signal is fed. We hope to find a correlation between vertical speed and noise level.