On Tue, 07 Jan 2003 23:37:00 +0000, Ian Woollard
<[EMAIL PROTECTED]> wrote:
>Well, it spends 10 seconds between 95 and 99 degrees out of a 70 second
>orbital burn, then it rapidly returns to very nearly 90 degrees in the
>next 10 seconds. It seems to be moving perigee above the atmosphere and
>giving a nice circular 215x215km orbit.
The usual practice is to burn for orbital velocity (raising the
perigee, by definition), leaving the apogee fairly high and fairly
close to the launch point. When you've gotten to a respectable
altitude, you lower the nose below the horizon and keep burning. This
lower the apogee and pushes it forward, eventually lowering it to your
desired altitude and pushing it to halfway around the planet. When
you get there, you burn again to circularize the orbit, raising the
perigee the rest of the way up.
>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.
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.
-R
--
"You haven't been lost until you've been lost at Mach 3."
-- Paul Crickmore
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
