Re: [amsat-bb] AO-40 Orbital Stability

["Stacey E. Mills, M.D." <w4sm@xxxxxxxxxx>]

At 07:39 AM 2000-12-25 -0500, MCGWIER ROBERT wrote:
>Stacey:
>
>Your pictures bear out WHY it is important to have
>a burn of the 400 N motor.  The instability (perigee
>bouncing around) is a real problem when your
>perigee is 360 km.

Perigees in highly elliptical orbits ALWAYS bounce around.   That, by 
itself  is not a sign of  orbital decay or "instability".   If you look at 
the 10 year chart that I posted, it's less "busy" and you can see the 
scales on the left better.   As mentioned in my earlier e-mail, the 
perigee's fluctuate from about 270 km to about 700 km.  Remember, however, 
that in a highly elliptical orbit, the time at perigee is minimal.  If you 
look at the total energy of the orbit, as reflected by mean motion, there 
is no net loss even after 40 years.   In fact, there is a slight increase 
in energy (decreased mean motion).  In other words, gravitational forces 
pump more energy into the orbit than the perigee drag removes.

>The graphs are so busy that
>I can't make out the units.  What are the maxima
>and minima of the perigee heights AND what
>was the apogee of the spacecraft.  It was not
>clear that your graphs assumed the current
>conditions.

The graphs do not plot apogees, but they can be derived from the graph 
since eccentricity is plotted.  The orbit does not significantly 
circularize.  To do so requires a LARGE amount of perigee drag.  These 
graphs ARE based on the latest NORAD elements.

I realize that everyone is "perigee shy" after AO-13, but the height at 
perigee is not  the only factor, it's the nature of the perigee fluctuation 
that is critical.  For AO-13, it was a combination of high inclination, 
along with disadvantageous  RAAN and ArgP that conspired to have the moon 
and sun pull the perigee into the earth.  Drag was really not a factor 
until near the end of its life when the orbit began to slowly circularize 
and the drag heating eventually destroyed the S/C.  In its current low 
inclination orbit AO-40 does not suffer from these problems and it will be 
around for far longer than any of us.....even if we don't change the 
current orbit.




--
  _______________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
   Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
  _______________________________________________________________________

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