How to build a fiberglass rocket, part 13: parachutes

Rocket recovery

The recovery system in a high power rocket is extremely important. Needless to say, launching a large rocket extremely high into the air is pretty fun. But that thing is going to come down again, sooner or later, and the recovery system you’ve designed and built for your rocket will determine whether it comes down like a ballistic missile or floats down gently for a soft landing. It could potentially injure or even kill someone. More importantly, your valuable rocket could be completely destroyed.

If the rocket separates in the air, even with no parachute, that’s a good start. A rocket separated into two (connected) parts loses its aerodynamic design; it will fall, but awkwardly and more slowly than if it were in one piece. Of course, if it’s heavy, it’s still going to hit the ground pretty hard.

Drogue vs. main

Better to separate and have a parachute. A single chute can be sufficient – it depends on the rocket’s expected altitude and how heavy it is. Ideally, though, the rocket will be capable of “dual deploy,” which just means deploying two separate parachutes, a drogue and a main.

person running in street, pulling orange parachute behind in the air
a running start

The drogue chute is smaller and deploys at apogee. The idea here is that, at apogee, the rocket’s speed has ground to a halt. It’s no longer shooting upwards, but it hasn’t yet started falling back down very fast either. The rocket separates at this point and the smaller drogue chute deploys, slowing the rocket’s descent to some degree.

After the rocket continues its descent and is closer to the ground, the airframe separates again and the main parachute deploys. This larger chute slows the rocket’s descent considerably and allows for a softer, gentler landing.

This order is important. You wouldn’t want to release the main (larger) parachute at apogee because even a slight wind would carry it very, very far away by the time it landed. But if you only used a drogue chute during the flight, the rocket’s descent would not slow sufficiently and it’d have a rough landing.

person in street, holding open orange parachute above head
I am actually being pulled backwards here

A weighty problem

This brings us to my current predicament. The Darkstar Extreme is a fiberglass rocket, and it’s pretty heavy, at roughly 14 lbs before adding the recovery system or the motor. That’s about what two gallons of milk weigh. Imagine the force it would take to accelerate two gallons of milk vertically, thousands of feet into the air – and likewise, the size parachute you’d need to significantly slow the descent of something that heavy.

There are online calculators that can help determine the right parachute size (diameter) based on the weight of your rocket and how fast (or slow) you want it to land. There are a lot of trade-offs in rocketry, some of which may not be obvious. All else being equal, for example, the softer the landing, the better – but there is a trade-off here. You can slow the descent and get as soft a landing as you’d like by increasing the size of the parachute. But large parachutes take up a lot of space. It’s increasingly difficult to stuff a gigantic parachute into your rocket without increasing the size of the rocket (and its weight, which then requires an even bigger parachute, etc.).

I ended up buying a 2 ft diameter drogue chute and an 8 ft diameter main chute, both from Rocketman Parachutes. In the above pictures, I opened them up as soon as they arrived at my house and ran around in the street trying to catch the wind and fly them like kites – with mild success.

They’re also a very vibrant orange color, which should make the rocket a little easier to see during its descent in the sky, and easier to locate once it touches down.

I’m looking forward to launching the Darkstar Extreme in the near future and seeing these parachutes deploy! I should also be able to capture a lot of data with my flight computer, so I’ll be able to tell how fast the rocket was descending with the drogue, as well as with the main chute.

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