Drag at Orbital Altitudes

1) Was the International Space Station/Space Station Freedom meant to
orbit at 300-400km originally (c1990), or was it slated for higher
altitudes?

2) How much would drag be reduced (percentage wise) if the station
operated at 600km vs 400km? (For the sake of argument, say the station
could get there in the first place.)

(Mike Miller) wrote in message . com...
2) How much would drag be reduced (percentage wise) if the station
operated at 600km vs 400km? (For the sake of argument, say the station
could get there in the first place.)

The density at 400 km averages about 3.725E-12 Kg/m^3
and at 600 km averages about 1.454E-13 Kg/m^3.

drag = Cd * dragArea * 0.5 * density * v*v;

So the density alone gets you a factor of 25. The velocity is
slower higher up to, so you win some from that too.

My simulator (Java applet) at http://spacetethers.com/spacetethers.html
can simulate drag on sats. Check out samples 63 to 66.

You can also get the density at some altitude in meters
with an input like:

AirTest 400000

-- Vince

(Mike Miller) wrote in message . com...


2) How much would drag be reduced (percentage wise) if the station
operated at 600km vs 400km? (For the sake of argument, say the station
could get there in the first place.)

Hi Mike,

The atmospheric density is approximately 25 times less at 600-km
versus 400-km. 9.89x10^-14 kg/m^3 at 600-km and 2.62x10^-12 kg/m^3 at
400-km (based on MSIS atmospheric model with a solar flux [F10.7]
number of 120, taken from 2nd edition of SMAD). Since atmospheric
drag is directly proportional to density, the drag force should
decrease by about a factor of 25. (this ignores the small change in
velocity from being in a higher circular orbit)

Patrick Marsden

(Vincent Cate) wrote in message om...

The density at 400 km averages about 3.725E-12 Kg/m^3
and at 600 km averages about 1.454E-13 Kg/m^3.

drag = Cd * dragArea * 0.5 * density * v*v;

So the density alone gets you a factor of 25. The velocity is
slower higher up to, so you win some from that too.

Excellent. Thanks for the replies.

Next question: how much worse is the radiation environment at 600km vs 400km?

Mike Miller, Materials Engineer

Mike,
If your serious about this stuff, this is an excellent reference (in my
humble opinion).

Title Space mission analysis and design / ed.
Author Wertz, James R
Larson, Wiley J
Edition 3rd ed.
Publisher Torrance : Microcosm, 1999
ISBN 0-7923-5901-1

KR,

Rogier

(Mike Miller)
Next question: how much worse is the radiation environment at 600km vs 400km?

There is an online version of the AP-8/AE-8 model for the Van Allen belt
radiation at:
http://nssdc.gsfc.nasa.gov/space/model/models/trap.html

But as of 11/22/03 the online version would not run for me, so I downloaded:
http://nssdcftp.gsfc.nasa.gov/models.../fortran_code/

Documentation related to these is at:
http://nssdc.gsfc.nasa.gov/space/mod...s/radbelt.html
http://nssdc.gsfc.nasa.gov/space/mod...etos/aeap.html

Key terms a
L-value: equatorial distance to a geomagnetic field line measured in
earth radii
B/Bo: magnetic field strength normalized to the equatorial val
Bo: magnetic field strength at the magnetic equator (minimum value)

So:
L-value = 1 + altitude/EarthRadius
EarthRadius: 6378000 meters

L-value for 400 km is about 1.06272
L-value for 600 km is about 1.09407

So running the Fortran code (online version is similar but less options):

1) I select: AP8MIN

2) I want to look at range of energy values so I put in:
6, then 0.1,2,8,32,64,128

3) For L-values I enter: 0 then 1.03,1.33,0.03
To get the range: 1.03,1.06,1.09,1.12,1.15,1.18,1.21,1.24,1.27,1.30

4) For B/Bo I enter: 10 then 1,2,3,4,5,6,10,15,20,50
Though I only use "1" for the table below:

5) For output I enter "1" for "E * L TABLE - integral"
Then I pick the B/Bo value of 1 (equatorial).
And I get the table:

integral flux [PROTONS /cm*cm*sec]
L \ E/MeV 0.10 2.00 8.00 32.00 64.00 128.00
------\--------------------------------------------------------------
1.03 I 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00
1.06 I 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00
1.09 I 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00
1.12 I 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00 0.000E+00
1.15 I 4.004E+02 3.563E+02 3.198E+02 1.473E+02 6.786E+01 2.048E+01
1.18 I 2.023E+03 1.931E+03 1.829E+03 1.492E+03 1.179E+03 7.216E+02
1.21 I 4.728E+03 4.629E+03 4.445E+03 3.748E+03 3.093E+03 1.911E+03
1.24 I 9.318E+03 9.038E+03 8.342E+03 6.131E+03 4.848E+03 2.853E+03
1.27 I 1.836E+04 1.765E+04 1.566E+04 1.003E+04 7.599E+03 4.260E+03
1.30 I 3.660E+04 3.483E+04 2.969E+04 1.654E+04 1.200E+04 6.403E+03

So this model predicts that at 600 km (L-value 1.09) you are still not
getting significant radiation. But you don't want to get much above
800 km (L-value 1.12). Using AP8MAX gives a similar result.

If you are not at the magnetic equator (can't orbit there as the Earth
rotates) the above is not going to be exactly right. But if you are
over the Earth's equator, I don't think it is far off. However, if your
orbit passes through the South Atlantic Anomaly you will get some
radiation.

-- Vince