Recovery Systems

What goes up, needs to fall back down at least as long as its not floating around in space. After the rocket lifts off successfully, Recovery ensures that the rocket reaches the ground in a safe and sound manner. This is essential to protect the rocket as well as the objects on the ground.

One thing that i’ve learnt is, irrespective of the trust you have in your recovery system, its always required to clear the viscinity of the launch site off all people, animals, objects, everything. Do not proceed if you haven’t ensured this.

For our rockets, if the body diameter is less than 15 mm and total weight doesn’t exceed 20 g (with propellant loaded), a recovery system isn’t included as the free fall of the rocket itself would suffice, as after the propellant burns out, the weight of the rocket is usually reduced to 4-5 grams. This method of recovery is known as tumble recovery.

Now, beyond these specified limits, a time tested recovery method is through parachutes. Though a few across the world have successfully tried propulsive recovery for hobby rockets, we’ve not deweled into it as we’d be continuing with parachutes for the forseeable future.

Though the recovery method is parachutes, there are stark differences in the means of ejection. Broadly, its classified into non electronic and electronic means. Non electronic is through pressurizing the body tube hence forcing the nose cone to eject, which in turn drags the parachute out. For this, the nose cone usually consists of a press fit connection to the interstage / body tube. The pressurization of the body is usually caused by the explosion of a small charge from a specifically designed rocket motor that directs the hot gas upwards. This somewhat reminds me of the hot-staging process in Saturn - V during stage separation. For this to work, propellant is rammed into the motor in sequential stages such as:

  1. the primary charge (flight propellant)
  2. the delay charge (coasting phase)
  3. the ejection charge (usually timed at apogee or just after it)

The quantity and the ratio between each charge determines the timing for parachute ejection. Here, the delay charge (charge that burns whithout producing any thrust) is essential as it allows the rocket to decelerate and reach apogee. If the parachute is ejected while the propellant is still burning, it may be ripped off from the launch vehicle.

Coming to the electronic means, in simple terms consists of just sub systems, the trigger and the actuation, the latter being ususally servo based and works in a manner that when it receives a signal, it unlatches a hook or a latch mechanism releasing the nose cone and hence, the parachute. Also, in our designs, the nose cone is spring loaded to ensure sufficient ejection force is present. Speaking of the trigger, the means are limitless. Some of these are:

  1. electronic timer based trigger.
  2. gyroscope & accelerometer sensor based trigger that works on the basis of rocket acceleration.
  3. barometer based trigger that triggers at a particular air pressure level.