Choosing the right parachute for the needs of your vehicle is extremely important for a successful recovery. This page will outline the two primary types of parachutes and some helpful methods for determining the correct parachute size for your needs.
The first thing most people think when they hear the word "parachute" is a main chute. This large parachute is responsible for slowing the vehicle down to a managable speed before it hits the ground. It is deployed at a certain altitude after apogee and before the vehicle hits the ground.
Typically, it is best practice to aim for roughly 15 ft/s or less as a descent rate under the main chute. This should be the foundation upon which you build your parachute selection. There are three main factors that go into the descent rate of the vehicle:
Usually, vehicle mass is the most inflexible factor, so parachute diameter and CD are the ones to consider when making a selection. If you are constrained by size, i.e. parachute compartment length/diameter, then it may be possible to find a parachute with a high CD that allows for a smaller size while getting the same performance descent rate wise.
The Drag Equation
When trying to find a parachute that hits that 15 ft/s sweet spot, the drag equation is quite helpful. It relates parachute area and CD to drag force. During recovery, the vehicle is at terminal velocity, meaning the drag force is equal to the weight force. Therefore, the equation is as follows:
D = W = (1/2) * CdArv2
Which can be rewritten to find velocity:
v = sqrt( 2W / CdrA )
Where v = Velocity (m/s), W = Vehicle Weight (N), Cd = Parachute Drag Coefficient, r = Air Density (Typically 1.229 kg/m3), and A = Parachute Surface Area (m2).
Note that these values are in SI Units, and it may be necessary to convert values to be used in the equation. Be very careful to keep track of units so you don't get a false result.
One final note about descent rates: While a descent rate slower than 15 ft/s is safer for the vehicle, it may not be desirable due to drift. Depending on the altitude at which the main parachute is deployed (typically you won't even be allowed to deploy at less than 500 ft) and the wind speed on launch day, the vehicle can drift quite far from the launch site. If the descent rate is too slow, then it may drift an unmanagable distance before landing. This is why 15 ft/s is the sweet spot for descent rate.
Large rockets like the ones ARA typically builds often utilize dual deployment, which means that two separation events occur during the flight. One at apogee and one at a specified altitude after apogee. The latter event deploys the main chute as discussed above. However, the apogee separation event deploys the drogue parachute.
A drogue parachute is a small parachute intended to slow the descent of a vehicle from apogee just enough that the velocity at main deployment does not cause too much of a shock to the main parachute. At large vehicle sizes, the terminal velocity without a drogue would usually be enough to tear the main parachute apart when it deploys. Additionally, it is impractical to deploy the main chute at apogee because the vehicle would almost certainly drift too far away from the launch site to be recoverable.
Just like with main parachutes, there is a sweet spot for the descent rate under drogue that you should aim for. Usually, you want this speed to be around 75 ft/s or less, depending on the strength of the recovery system components, and a couple other factors that depend on mission constraints.
Other than the target descent rate being different, the other factors to take into account when choosing a drogue parachute are the same as for choosing a main parachute. Read above to learn how to determine the right parachute specifications.