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Ejection Charge Calculator

Thrust To Weight Calculator

Rocket Workbench

Description

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 ${\displaystyle 6.1m/s(20f/s)}$ while the drone should allow the parachute to descend much faster ${\displaystyle 18.3m/s(60f/s)}$ - ${\displaystyle 21.3m/s(70f/s)}$. 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 (${\displaystyle C_D}$). 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

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

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:

${\displaystyle A=\frac{2mg}{\rho v_T^2{C}_D}}$

where

${\displaystyle m=}$ the mass of the rocket

${\displaystyle v_T=}$ desired terminal velocity

${\displaystyle C_D=}$ drag coefficient of the parachute

${\displaystyle \rho =1.22}$ air density, average at sea level in ${\displaystyle kg/m^3}$ at ${\displaystyle 15C}$

${\displaystyle g=9.807}$ acceleration due to gravity in ${\displaystyle m/s^2}$

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

For hexagonal parachutes:

${\displaystyle D=\sqrt{\frac{2A}{sqrt3}}}$

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

${\displaystyle D=\sqrt{A}}$

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

${\displaystyle D=\sqrt{\frac{4A}{\pi }}}$

Units

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

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

Ejection Charge Calculator

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Rocket Workbench