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Air Resistance approximation re-entry

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#1foxcode  Members   -  Reputation: 147

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Posted 01 January 2014 - 01:46 PM

Hi guys, sorry if this is the wrong place to post, but nearest I could find.

I have been thinking about the air resistance and subsequent heat generated when a spacecraft re-enters the atmosphere. In the past I have done A level physics and maths, but we always neglected air resistance.

How would you go about approximating the drag on a lander as it comes down through the atmosphere? I do not need an exact calculation, I imagine a good approximation would depend on the velocity and surface area of the lander, as well as the density of the air.

I am also interested about the heat generated from friction, though this is less important more of a curiosity. I am familiar with the very basics of thermodynamics, but I do not remember dealing with surface areas, which I assume will be very important regarding the rate of heat transfer.

I realise this is kind of a vague question, just really looking for some avenues to explore.

#2jjd  Crossbones+   -  Reputation: 1926

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Posted 01 January 2014 - 02:12 PM

This should give you a pretty good idea of what is involved (or at least, what terms to search for).

-Josh

--www.physicaluncertainty.com
--irc.freenode.net#gdnet

#3foxcode  Members   -  Reputation: 147

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Posted 01 January 2014 - 03:11 PM

Thanks jjd, haven't  read all of that article yet, but after a short browse it looks very useful

#4Mizmoon  Members   -  Reputation: 120

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Posted 06 January 2014 - 11:40 AM

Here is the basic formula for air drag:

Fd = 0.5 * Cd * Rho * Velocity^2 * A

Where:

*Fd is the drag force in newtons, applied to the opposite direction of the velocity.

*Cd is the coefficient of drag - this is determined in wind tunnel test. It is a constant which you can tweak to your preference, but there are some examples of the Cd of different primitives to be found here: http://en.wikipedia.org/wiki/File:14ilf1l.svg

*Rho is the density of air. This can be tricky since it changes with temperature, and temperature drops at higher altitudes. Air humidity also affects density. I don't know the exact equation for this, but i suggest you use a constant air humidity, and then sample data from this calculator (http://www.denysschen.com/catalogue/density.aspx) and then make a function out of these points. You might have to find samples of air temerature at different altitudes aswell, but that should not be hard.

*Velocity relative to the airflow.

*A is the reference area in square meters.

Edited by Mizmoon, 06 January 2014 - 12:01 PM.

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