Calculating Drag for Model Rockets

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In summary, if you know the mass of the rocket, the magnitude of the force being applied it, and the force of drag as a function of velocity, you can calculate the rocket's velocity at any time using the equation given in the lastOneStandingTHIS thread.
  • #1
VantagePoint72
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Hi all,
For the past few years, building model rockets has been a hobby of mine. I've designed a few of my own, and I'd like to be able to do stability, height, etc. calculations before actually building the rocket. My question: If you know the mass of the rocket, the magnitude of the force being applied it, and the force of drag as a function of velocity, how do you calculate the rocket's velocity at any time. There's a similar question here: https://www.physicsforums.com/showthread.php?t=48326
but that problem simplifies things by making drag a constant times v, instead the actual physical case of a constant times v squared. I seem to recall that you aren't allowed to apply a non-linear function to the dependent variable in a differential equation, so you can't just change the v in the formula given in that thread to a v^2. Even if I'm wrong here, I'm still not entirely sure how to continue from where the thread left off as my integration skills are a little rusty. Could someone please help?

Thanks,
LastOneStanding
 
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  • #2
THIS is the equation you are looking for. Its pretty easy to apply if you know the values to plug in...

Going from that to speed/altitude, I'm pretty lazy when it comes to equation solving, so using f=ma (and a decreasing mass with time) and the drag equation, and throw it into an Excel spreadsheet using a numerical solving method. If you need help doing that, I can probably help you tonight.
 
  • #3
Ok, I was bored, so I did the spreadsheet/graph. Its attached. I didn't check it thoroughly, but the graph looks right. I had to clip the data to make it small enough to upload - stretch it down to about 500 and you'll get a flight profile for just about the entire ascent (even after engine shutoff).
 

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  • #4
Hey, thanks so much russ! This is a huge help, I really appreciate this! I knew that was the equation for calculating the force of drag, it was just using that to calculate the velocity at any time that I couldn't figure out.
 

FAQ: Calculating Drag for Model Rockets

What is drag and why is it important in model rocketry?

Drag is the force that opposes the motion of an object through a fluid, such as air. In the case of model rockets, the fluid is the air in the atmosphere. It is important to calculate drag because it affects the stability, flight trajectory, and maximum altitude of the rocket.

How do I calculate drag for my model rocket?

Drag can be calculated using the following equation: D = 1/2 * ρ * v^2 * Cd * A, where D is the drag force, ρ is the density of the air, v is the velocity of the rocket, Cd is the drag coefficient, and A is the frontal area of the rocket.

What factors affect the drag coefficient of a model rocket?

The drag coefficient of a model rocket can be affected by its shape, size, and surface texture. Generally, a more streamlined and smoother rocket will have a lower drag coefficient, resulting in less drag force.

How does altitude affect the calculation of drag for a model rocket?

As the altitude increases, the density of the air decreases. This means that at higher altitudes, there is less air to create drag on the rocket. Therefore, the drag force will be lower, resulting in a higher maximum altitude for the rocket.

Are there any tools or software available to help with calculating drag for model rockets?

Yes, there are various online calculators and software programs specifically designed for calculating drag for model rockets. These tools take into account all the necessary factors, such as rocket dimensions, altitude, and air density, to provide accurate drag force calculations.

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