Choosing the parachute Size of a rocket
Parachutes are a necessity when preparing to launch a rocket for obvious reasons. The size of the parachute matters too! You don’t want to lose a rocket from it drifting too far away from its intended landing spot or breaking from falling too fast.
Let’s take a closer look into why parachutes are important and what happens when you choose the wrong size.
Why is a parachute so important anyway?
Firstly, not all parachutes have the same purpose. Some parachutes are used to prevent a rocket from drifting too far, while other parachutes help by slowing down the speed a rocket descends. No matter the specific purpose, the ultimate goal of the parachute system is to prevent the rocket from being destroyed when it is descending to the ground after a launch.
Without a Parachute
The rocket will suffer major damage, preventing any possibility of being launched again.
With a Parachute
The rocket will land at a safe speed, allowing it to be recovered and launched again in the future.
Parachute Separation Phases
Real-life demonstrations by the UF Rocket Team on the importance and usage of parachutes
What are some important factors to consider during the descent of a rocket?
Impact velocity is the speed that the rocket hits the ground at.
Drift radius is how far the object drifts away from the launch pad and is calculated by multiplying the descent time by the wind speed.
Duel deploy is when a rocket uses two parachutes whilst descending. A drogue parachute to reduce drag and drift radius, and a main parachute with even more drag to minimize impact velocity. With both of these parachutes used together, a rocket is able to safely descend with a low impact velocity and drift radius. The more time a rocket spends in the air during its descent, the further away from its landing spot it will drift from when there is wind blowing.
What is a Drogue Parachute?
The drogue parachute is a smaller parachute that is deployed at apogee (max altitude) and allows for the rocket to descend without drifting too far away from its intended landing spot. The drogue parachute also provides enough drag for the main parachute to deploy without ripping apart from the descent velocity. Without the drogue parachute the rocket drift radius will be too large if the main parachute is deployed at apogee, and the main parachute will tear apart due to a large descent velocity if deployed at an altitude of 500 feet.
What is the Main Parachute?
The main parachute is the larger parachute that provides enough drag for the rocket to land without taking any catastrophic damage. The main parachute is typically deployed at 500 feet above the ground and is larger than the drogue parachute, so if it were deployed at apogee, the rocket will drift very far away from it's landing point if the wind is strong.
Importance of Duel Deploy
- Minimize Drift Radius
- Minimize Impact Velocity
- Minimize Deployment Velocity of Main parachute
- Minimize Time Spent in Air
What to Think About
In order to meet all these goals, we use a drogue parachute and a main parachute. The drogue parachute is in charge of reducing the drift radius and reducing terminal velocity. The main parachute is big parachute deployed at around 500 feet to minimize the impact velocity. By using this combination of chutes we can allow the rocket to descent at a fast enough speed to minimize drift, and then safely recover the rocket with a main parachute.
How to choose a parachute Size
Parachutes help increase drag which decreases the terminal velocity of a falling rocket. Drag can be increased by changing the size and shape of the parachute
To determine the size of your parachute, you need to know a few equations first.
Equation of Drag
D = 1/2 • ρ • v² • S • Cd
D = Drag
ρ = Air Density
S = Size of Parachute
Cd = Shape of Parachute
Equation of Weight
W= m • g
W = Weight
m = Mass of Object
g = Gravity
Equation of Parachute Size
Now that you know the equations for drag and weight we must set the drag equal to the weight. This will help us find the size of the parachute that will allow the rocket to descend at a speed without being damaged.
D = W
1/2 • ρ • v² • S • Cd = m • g
S = m • g • 2 / (ρ • v² • Cd)
How are the Parachutes Deployed?
The drogue parachute is deployed at apogee because acceleration is equal to 0 and velocity is minimized. If the drogue were to be deployed anytime after apogee, there would be a risk of the drogue tearing apart due to a large descent velocity.
A barometric altimeter measures the air density to approximate the altitude. When a specific altitude is reached (500 ft), an ejection charge is ignited and deploys the main parachute.
Want to calculate the descent speed of your own rocket?
This page allows you to predict the speed at which your rockets will descend. (Note that it is only an estimate, and values will vary with wind, different air pressures, etc.)
Rocket Landing Simulation Under Different Circumstances