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