marcus.1178

Atmosphere+drag model / ISP

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Hello there,

I plan to program some simple software to optimize the start/orbiting of rockets in some optimal way. At the moment it works somehow, but I have some questions:

1) How can I accurately calculate the drag? Can I just use the formula of http://wiki.kerbalspaceprogram.com/wiki/Atmosphere#Drag  with an appropriate constant or does the drag also depend on the Mach number (as in reality).

2) Does the specific impulse ISP for a specific rocket engine depend only on the (static) pressure?

3) Which functions of the API can be used to determine the current thrust/drag?

Thanks for your help.

Best wishes

Marcus

 

 

 

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24 minutes ago, marcus.1178 said:

1) How can I accurately calculate the drag? Can I just use the formula of http://wiki.kerbalspaceprogram.com/wiki/Atmosphere#Drag  with an appropriate constant or does the drag also depend on the Mach number (as in reality).

There's a bunch of modifiers and multiples in physics.cfg. It is modified by the Mach Number, (DRAG_MULTIPLIER), a couple flat modifiers/multipliers, and then there's also some facing curves... mostly only the tip and tail need to be considered.
The A and Cd values can be teased out for each part from the PartDatabase.cfg. There's a post that talks about calculating volume of parts for buoyancy that talks about how the drag cubes are formatted. And THEN there's also another curve to convert the Cd from the file to another Cd value... again in Physics.cfg.

There may be another variable in there somewhere, given the change log for 1.2 included some discussion of making some better drag distinctions for pointy and not-so-pointy ships. 

 

24 minutes ago, marcus.1178 said:

2) Does the specific impulse ISP for a specific rocket engine depend only on the (static) pressure?

Just the pressure, as per the usual equations.

 

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Thank you very much purpletarget.

In the physics.cfg there are tables for DRAG_TIP, ... and so on.

They have 4 columns: what does the different column mean? 

It says that x is "Mach" and y is "multiplier". What about the third and fourth value?

Are they used for inter/extrapolation?

 

 

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