how flight control module knows when to enable recovey system in rocket ?


How flight control finds out when to enable recovery system to deploy parachute when it used in amateur rockets ?
Actually how it works ?

End of burnphase: |ACC|–> 0 , air resistance does the rest (that’s whay hobby rockets do not need a flight control module at all).

At the end of the burn phase the acceleration swaps from large and upwards to downwards due
to drag forces. This is not the time to enable recovery 'chute as the craft is at or near maximum

At the apex of flight the drag forces drop close to zero so the craft is in free-fall, acceleration is a minimum.
This is when to deploy the 'chute. However its not very well defined as the drag coefficient is very low in
a streamlined rocket so it could be travelling upwards fairly fast still, risking damaging the 'chute.

If you have independent measure of height, then this is useful information - when the height starts to fall, deploy.

If you have a measure of airspeed, then this is good information - low airspeed, deploy, the 'chute won't
get shreded. However if it's veered off sideways the airspeed may never drop low enough.

And of course in a finned rocket the craft will invert once it starts to descend, so this is another indication of
time to deploy 'chute, assuming your IMU can reliably keep attitude information through the burn-phase.