Randall Clague wrote:
My atmospheric model is simple exponential drop off with a
scale height of 6 odd kilometers- I tried a model that was supposedly
NASAs and 'more accurate', but it seemed to give density that went up by
a factor of 4 as my vehicle ascended, so I doubted that model, either
that or the atmosphere is very weird.
The atmosphere is more than a bit weird - the density function resists
simple modeling - but a simple exponential dropoff [
1.225*EXP(-Altitude/6000) ] is only accurate over a narrow range.
6000 meters is too low in any case, and gives you artificially low
density all the way up. I've seen a NASA-affiliated site which
recommends 8400 meters as the scale height. That's accurate to +/- 3
db up to 32 km, but by 120 km its error peaks at over 15 db.
Actually I used 1.2 / 2^(alt/6000) which is a bit nearer, so I wasn't quite
as far out as you're assuming.
Mark Spiegl wrote a module for RASP that uses lookup tables and
interpolation, and it's accurate to 3 db up to 430 km. But best of
all, there is a freeware DOS program written by NASA Glenn (I think)
that will give you ICAO 1976 standard atmosphere values - all of them
- from -5000 meters to 1,000,000 meters, at down to 10 meter
intervals. The output format isn't terribly friendly, but I've
converted it to Excel, and plan to incorporate it into my sims RSN.
The DOS program is 33k zipped, and I'll be happy to send you a copy.
10m intervals? So I'll know the atmosphere to 0.01% and the Cd factor to
10%, a real measure with a micrometer, cut with an axe kind of deal
-R
--
"You haven't been lost until you've been lost at Mach 3."
-- Paul Crickmore
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