The formulas provided below will calculate an estimated parachute size that will result in a descent rate of about 15 feet per second. If the landing area is hard, such as packed earth or salt, a slower descent rate may be desired to prevent damage to the rocket.

## Circular Parachute

$D = 39.6 \times \sqrt{W \times 0.454}$

Where:
$$D$$ is the diameter of the parachute in inches
$$W$$ is the weight of the rocket in pounds

### Example of a 5 pound rocket:

$39.6 \times \sqrt{ 5 \times 0.454} = 59.7$

Round up to a 60” circular parachute

## X-Form Parachute

$S = 51.5 \times \sqrt{W \times 0.454}$

Where:
$$S$$ is the size of the parachute in inches
$$W$$ is the weight of the rocket in pounds

### Example of a 5 pound rocket:

$51.5 \times \sqrt{ 5 \times 0.454} = 77.6$

Round up to a 78” X-form parachute

A smaller parachute will result in a faster descent. A larger one a slower descent. If the ground is soft, such as grass covered, you may want to use a smaller parachute for a faster descent. A faster descent will result in less drift. If the ground is hard, like frozen earth or salt, you may want a slower descent, and therefore a larger parachute.

Submitted by: Dean A. Roth