Drogue Chute Sizing
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In some cases a drogue is not needed. When an airframe will fall horizontally then drag may slow the rocket enough that a drogue is not needed. This can often be the case when an airframe is split into two approximately equal parts. For example the author has a 5 pound rocket that falls at about 45 feet/second in a flat spin when the fin canister and payload sections are separated at apogee. In this case a drogue is not needed. A small one (12" or 18") may be used to produce a more stable descent, so that the fin canister and payload sections don't bang into each other.
Some rocket designs don't allow the airframe to be used to slow a rocket's descent. In these cases a drogue is needed. For example, rockets whose airframe cannot be split, or those that deploy the drogue and main parachutes from the same location.
A drogue's purpose is to slow a rocket's descent, but still allow it to descend rapidly enough to avoid significant drift. Descent needs to be quick, but not too quick that the rocket and/or main parachute will be torn apart when the main parachute is deployed. The author aims for a descent rate of about 50 feet/second.
It's useful to know how fast a rocket will descend without a parachute under different conditions. For example, a 4" diameter, 5 pound rocket will come in nose first at about 253 feet/second (172 miles/hour). The equation for calculating terminal velocity is shown below.
| Vt | = terminal velocity in feet/second |
| W | = weight in pounds |
| Cd | = drag coefficient - assume 0.75 |
| rho | = atmospheric density (about 0.0024 at sea level) |
| A | = surface area in square feet |
If a 4" diameter, 5 pound rocket comes in nose first then:
Vt = sqrt( (2 * 5) / (0.75 * 0.0024 * 0.087)) = 253 feet/sec
If the same rocket has a 60" long airframe and falls horizontally, then:
Vt = sqrt( (2 * 5) / (0.75 * 0.0024 * 2.6)) = 46 feet/second
If a rocket will fall at a reasonable rate without a drogue, then one is not needed unless you want a small (12"-18") one to add stability without significantly affecting the descent rate. If a drogue is needed then the terminal velocity equation can be rearranged to calculate the desired size of a circular parachute. The simplified equation is shown below, and assumes a desired descent rate of 50 feet/second.
| D | = drogue diameter in inches |
| W | = weight in pounds without propellant |
Example of a 5 pound rocket: D = 24 * sqrt( 0.14 * 5 ) = 20 (or 24) inches
Submitted by Dean Roth
