Register to reply

Calculating drag

by LastOneStanding
Tags: drag
Share this thread:
LastOneStanding
#1
Oct21-04, 10:36 AM
P: 718
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: http://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
Phys.Org News Partner Physics news on Phys.org
Physicists discuss quantum pigeonhole principle
First in-situ images of void collapse in explosives
The first supercomputer simulations of 'spin?orbit' forces between neutrons and protons in an atomic nucleus
russ_watters
#2
Oct21-04, 11:44 AM
Mentor
P: 22,237
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.
russ_watters
#3
Oct21-04, 01:16 PM
Mentor
P: 22,237
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).
Attached Files
File Type: zip profile.zip (38.3 KB, 34 views)

LastOneStanding
#4
Oct21-04, 06:26 PM
P: 718
Calculating drag

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.


Register to reply

Related Discussions
Calculating air drag Introductory Physics Homework 1
Help with Drag. Introductory Physics Homework 3
Calculating viscous drag in water Mechanical Engineering 2
Calculating air drag for a massive rocket General Physics 6
Calculating drag Engineering, Comp Sci, & Technology Homework 5