4. Parachute Deployment
During the majority of the flight, the parachute has to remain snug and safely in the capsule and is only to be deployed when desired. This function is placed with the Parachute Deployment Device or PDD.
First, you should wonder if a parachute deployment system is required for your missions. For a CanSat-like mission, the parachute can often be stored on top of the can during flight and has to be deployed when the cansat is ejected from the rocket. In this case, the mass and complexity benefit of not having a parachute system weighs up to the uncontrolled parachute deployment.
There are two main categories in PDD's: pushing and pulling systems.
The simplest pushing system would be a spring system which is released upon command. These systems are under tension during the flight and can be released using a servo, wirecutter, boltcutter or wire melter. Even though these systems are straightforward, the fact that they are continuously under tension is a risk. An alternative is a cold or hot gas deployment system where the parachute is ejected using either a stored cold gas such as CO2 or a gas generator. These systems do not apply a force on the parachute during the stored flight but are inherently more dangerous as they contain high-pressure systems or pyrotechnics.
The pulling systems are generally more complex than the pushing systems. An example of a pulling system is the slug gun deployment device which ejects a mass, and the momentum of the mass pulls the parachute out of the canister. This system is quite similar to the hot/cold gas ejection system, but only ejects a small mass instead of the entire parachute. This can be beneficial when the parachute becomes too big for a hot/cold gas ejection system. As an alternative to a slug, you can also use a smaller parachute to extract a larger parachute. Another pulling system is the tractor rocket. In this case, a small solid rocket motor is used to extract the package from the rocket. The advantage of this system is that the kick-back onto the rocket is much less and that there is a constant pull on the system instead of a sudden jerking. However, as the system contains a rocket motor, it is more dangerous than the other systems.
When deploying a parachute system, you have to determine when this has to be done in the flight. During most rocket flights there is an onboard knowledge of where the rocket is which can be used for the actuation. The most simple deployment logic is a timer, where the parachute deploys x seconds after liftoff detection or after separation. Other inputs can be static pressure (altitude) or vertical velocity (apogee detection). Of course, you can opt for even more complex sensors and might attempt to measure Mach number or dynamic pressure to ensure the parachute deployment occurs inside the parachute envelope.
You should always keep in mind that any onboard system has to be thoroughly tested, and the more complex a system is, the more complex the verification and validation efforts will be.