# Simple model for atmospheric friction

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I'm looking for a very simple model to simulate atmospheric friction when landing on a planet. What I want to do is give a space ship some friction parameter S (for example if it has wings or a very large surface the friction parameter will be made bigger). I also give the planet some friction parameter P that represents how much friction it's atmosphere gives (e.g. this will be 0 for the moon because it has no atmosphere at all). Then, in function of S, P, and the velocity of the ship, I want to apply some force on the ship (in the opposite direction as it's velocity) created by the friction. What sort of formula would be simple but somewhat realistic? AFAIK, when something falls on the earth it'll get a constant speed after a while because friction compensates gravity. I want something like that. To calculate the amount of friction, should I use the speed, the speed squared, the speed to the third power, or something else? EDIT: also, if the ship has wings I'd probably also have to simulate lift and how it flies like an airplane, but for now I don't want to implement that yet, just simple friction that reduces your velocity. [Edited by - Lode on October 10, 2005 6:19:58 PM]

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for slow moving the force is usually proportional to v for fast it's more like v^2 so something like this

f = c*A*V (slow) or
f = c*A*V^2 (fast)

where c is just a constant and depends on the fluid you're in(in your case air) and it could vary with height(ie get larger as it comes closer to the surface)

and A is very important paramater, it's basically the area exposed to the wind(porjected area, in the direction of relative velocity wind-spaceship) for the space ship you can basically assume the wind has a velocity of zero, since the ship is going so much faster then air. for example the projected area of a sphere is always the circle with that radius. you could calculate this area for each triangle in your spaceship. like so

Aproj = A*(N DOT Vel) where N is the normal of the triangle and A is the scaler area of that triangle. for each triangle the force could be accumulated and added to that triangle. or for simplicity, you could approximate the entire projected area and simply exert a force on the center of mass(however this would eliminate any cool torque effects)

Tim

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http://farside.ph.utexas.edu/teaching/329/lectures/node70.html

This expands on the difference between the low speed V and high speed V^2 drag.
It also gives you a formula which takes into account the density of the fluid. This would be important for entering the atmosphere, as the air density would increase as you get closer to the surface.

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Yep - as Wombat says, keep the formula the same, but calc air density from the planet + distance.
The APress games physics book has a lot of nice stuff on drag and aircraft physics in general.

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