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Rocket Parachute Size Calculator

From FreeCAD Documentation

Rocket Parachute Size Calculator

Menu location
Rocket → Calculators → Parachute Size Calculator
Workbenches
Rocket Workbench
Default shortcut
None
Introduced in version
0.19
See also
None

Description[edit | edit source]

This calculator determines the parachute size required to achieve the desired descent rate based on the parachute parameters.

The desired descent rate depends on your needs. As a rule of thumb, the main parachute should slow the rocket to approximately 6.1m/s(20f/s) while the drone should allow the parachute to descend much faster 18.3m/s(60f/s) - 21.3m/s(70f/s). The editor has presets for these, with the value for the drogue being in the middle of the range. You can set a custom descent rate to suit your needs.

A key value for determining the descent rate is the coefficient of drag (CD). The exact value of this will depend on a variety of factors including the shape of the parachute. Presets are provided for parachutes cut from a flat piece of material (round, square, hexagonal) such as those provided by many kits, and for a true dome shape. Your parachute manufacturer may provide a better value for this coefficient.

Usage[edit | edit source]

  1. There are several ways to invoke the command:
    • Press the Parachute Size Calculator button.
    • Select the Rocket → Calculators → Parachute Size Calculator option from the menu.
  2. Enter the weight of your rocket and parameters for your parachute.

Calculation[edit | edit source]

Parachute size is determined by the mass of the rocket, desired descent rate, and drag characteristics of the parachute. The calculation is a two step process. First the area of the parachute is calculated:

A=2mgρvT2CD

where

m= the mass of the rocket
vT= desired terminal velocity
CD= drag coefficient of the parachute
ρ=1.22 air density, average at sea level in kg/m3 at 15C
g=9.807 acceleration due to gravity in m/s2

The diameter is then calculated from the surface area based on the shape.

For hexagonal parachutes:

D=2Asqrt3

For square parachutes, the diameter is the length of each side

D=A

For all other parachutes, the shape is assumed to be circular

D=4Aπ

Units[edit | edit source]

Calculations are done using metric units, but will display in your preferred units. Values can also be entered using any supported units by including the units with the value (eg 5 oz, or 23.2 g)

References[edit | edit source]

  1. http://www.rocketmime.com/rockets/descent.html
  2. https://descentratecalculator.onlinetesting.net/
  3. https://www.math.net/area-of-a-hexagon