Simple Atmospheric Model for Space?

Jan Philips on sci.space.history asked "How low can you orbit?".
I have a space simulator that could be used to play around
with this kind of problem. I had not really worried about
orbital decay yet. I tried some tests to see if I could match
a graph in "Space Planners Guide" but my decay times are order of
5 times shorter for orbits under 250 miles. I also get much
shorter times for the Vostok and Mercury examples given by
Chris Jones.

I am using the atmospheric model at:
http://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html
but I don't think this model is really supposed to be used at orbital
altitudes.

I found some real space atmospheric models like:
http://nssdc.gsfc.nasa.gov/space/mod...mos_index.html
ftp://nssdcftp.gsfc.nasa.gov/models/...ic/met/met.for
http://nssdc.gsfc.nasa.gov/space/model/atmos/msis.html

But these need far more detail to run than I want to deal with.
For example the last URL above says:
The model expects as input year, day of year, Universal Time,
altitude, geodetic latitude and longitude, local apparent solar time,
solar F10.7 flux (for previous day and three-month average), and
magnetic Ap index (daily or Ap history for the last 59 hours).

I have a graph on page 16.18 of "The Standard Handbook for Aeronautical
and Astronautical Engineers" but I want something I can code up.

Does anyone know a simple atmospheric model that just takes an
altitude as an input but would be closer to reality than what I have
now?

My simulator is at http://spacetethers.com/spacetethers.html

See sample inputs:
"62 Decay of Circular Orbits"
"63 Decay of Eccentric 160 km Orbits"

Thanks,

-- Vince

In article ,
(Vincent Cate) wrote:

I have a graph on page 16.18 of "The Standard Handbook for Aeronautical
and Astronautical Engineers" but I want something I can code up.

If you can't follow the references back to the original data, then why
not simply scan in the graph and recapture it? (On the Mac there is an
old-but-good app called "Data Thief" designed for exactly that task, but
it's pretty easy to hack out something yourself if you can't use that.)

HTH,
- Joe

,------------------------------------------------------------------.
| Joseph J. Strout Check out the Mac Web Directory: |
| http://www.macwebdir.com |
`------------------------------------------------------------------'

In article ,
Vincent Cate wrote:
I found some real space atmospheric models like...
But these need far more detail to run than I want to deal with.
For example the last URL above says:
The model expects as input year, day of year, Universal Time,
altitude, geodetic latitude and longitude, local apparent solar time,
solar F10.7 flux (for previous day and three-month average), and
magnetic Ap index (daily or Ap history for the last 59 hours).

Unfortunately, there are *reasons* for this. The upper atmosphere is
extremely variable -- with time of day and *most especially* with solar
activity. No simple, constant model can describe it well.

And yes, this means that orbital lifetimes are also highly variable.
Predictions of Skylab's orbital life disagreed by years, largely because
they made different assumptions about solar activity.

The discussion in section 8.6.2 of Vallado's "Fundamentals of Astrodynamics
and Applications", 2nd ed, is the best readily-accessible one that I'm
aware of.

Does anyone know a simple atmospheric model that just takes an
altitude as an input but would be closer to reality than what I have
now?

Vallado gives a piecewise-exponential model that varies scale height with
altitude (using a small table, it's not a single formula), and says it's
suitable for general studies. (Ah, and he says he swiped it from Wertz's
"Spacecraft Attitude Determination and Control", so that's two books to
look for.)

In his appendix B, he spends ten pages giving a more precise model.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
In article ,
Vincent Cate wrote:

I found some real space atmospheric models like...
But these need far more detail to run than I want to deal with.
For example the last URL above says:

The model expects as input year, day of year, Universal Time,
altitude, geodetic latitude and longitude, local apparent solar time,
solar F10.7 flux (for previous day and three-month average), and
magnetic Ap index (daily or Ap history for the last 59 hours).


Unfortunately, there are *reasons* for this. The upper atmosphere is
extremely variable -- with time of day and *most especially* with solar
activity. No simple, constant model can describe it well.


I just finished reading Zelazny's "Flare". After an 80 year Maunder
Minimum, humanity has grown complacent. Accountants have cut radiation
shielding out of budgets for Lunar settlements, space ships, satellites,
etc. There's a lot of stuff in LEO.

Then big sun spots occur. Lots of people and electronics get fried. The
upper atmosphere swells up and a lot of LEO stuff goes crashing down. It
seemed to a fairly plausible hard sf book. Which surprised me as I had
always thought of Zelazny as a writer of fantasy.

Hop
http://clowder.net/hop/index.html

(Henry Spencer) wrote in message ...
And yes, this means that orbital lifetimes are also highly variable.
Predictions of Skylab's orbital life disagreed by years, largely because
they made different assumptions about solar activity.

If I can get within a factor of 2 half the time I will be much better
off than I am now. :-) I am thinking of my simulator as something for
learning and checking out design ideas, not trying to predict exactly
when a particular satellite will reenter.

The discussion in section 8.6.2 of Vallado's "Fundamentals of Astrodynamics
and Applications", 2nd ed, is the best readily-accessible one that I'm
aware of.
[...]
Vallado gives a piecewise-exponential model that varies scale height with
altitude (using a small table, it's not a single formula), and says it's
suitable for general studies. (Ah, and he says he swiped it from Wertz's
"Spacecraft Attitude Determination and Control", so that's two books to
look for.)

Thanks. That is what I am looking for. Strangely, while Vallado is
$178.60 used on abebooks.com it is only $54.75 new on Amazon.
Wertz is $99.69 on abebooks and $152 new or $104 used on Amazon.
I went with Vallado new. :-)

Thanks again,

-- Vince

In article ,
Hop David wrote:
I just finished reading Zelazny's "Flare"...
...a fairly plausible hard sf book. Which surprised me as I had
always thought of Zelazny as a writer of fantasy.

Zelazny is much better known for his fantasy, but his output included a
sprinkling of quite decent hard SF.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Vincent Cate wrote:
The discussion in section 8.6.2 of Vallado's "Fundamentals of Astrodynamics
and Applications", 2nd ed, is the best readily-accessible one...
(Ah, and he says he swiped it from Wertz's
"Spacecraft Attitude Determination and Control"...

Thanks. That is what I am looking for. Strangely, while Vallado is
$178.60 used on abebooks.com it is only $54.75 new on Amazon.
Wertz is $99.69 on abebooks and $152 new or $104 used on Amazon.
I went with Vallado new. :-)

It's a good book to have. It goes into depth on a lot of things that
other orbital-dynamics books just skim over.

The one disturbing note is that I've gotten one or two hints that he (or
his editor) is not as careful as he should be about proofreading stuff
taken from other people's papers. Notably, his equation 9-57 (mean lunar
perturbation of perigee height) is simply wrong, as one can see by
consulting the original paper or the discussion in Wolverton's "Flight
Performance Handbook for Orbital Operations".
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

(Henry Spencer) wrote in message ...
The discussion in section 8.6.2 of Vallado's "Fundamentals of Astrodynamics
and Applications", 2nd ed, is the best readily-accessible one that I'm
aware of.

This did the trick. My simulator now matches orbital decay graphs
in the books I have (3 sources) to within 25% or so.

If anyone wants to play with it, it is a Java applet at:

http://spacetethers.com/spacetethers.html

See samples 63 to 66. It is easy to try out your own tests.

-- Vince