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# compare coefficients/aero models

## compare coefficients/aero models

(OP)

I have two (2) sets of drag coefficients, purportedly for the same thing. These drag coefficients are used in separate aero models. I know a lot about one aero model, and very little about the other. One thing that stands out to me is that one model uses a constant gravity model, whereas the other uses a varying gravity.

I was wondering if anyone had some thoughts on how I can objectively compare these drag coefficient sets, perhaps using the differing gravity models? Obviously, a simple plot of the two sets shows the differences, but I have this thought stuck in my head that somehow I can better correlate the coefficients, or perhaps the effects of the coefficients, to show how the aero models compare.

Any ideas?

Thanks!

### RE: compare coefficients/aero models

"varying gravity" ?

if one model has this feature, can you disable it (set it to be constant) and compare the two models ??

### RE: compare coefficients/aero models

rb,

Have you never tested vehicles up at the Rock?  g decreases with altitude...

### RE: compare coefficients/aero models

yes, Newton's law F = k*m*M/r^2 ...
the bigger r, the smaller F ...

and, courtesy of wiki "at an altitude of 400 kilometres (250 miles), equivalent to a typical orbit of the Space Shuttle, gravity is still nearly 90% as strong as at the Earth's surface" ... not a very strong effect ...

### RE: compare coefficients/aero models

Nope, not for most missions.  A fraction of a percent on the rock, but it is enough to make a difference, along with lower ambient air pressure, for rocket thrust calcs.

The 10% difference on Shuttle flights was a subject of a long ago conversation in a bar with one of the lads that wrote flight software.  Apparently that correction was included in the HP-41C software for the backup-backup reentry system.

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