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RE: Longevity-sat proposal



Folks:

We need to all back away a minute and remember our
high school physics.  A body in motion will maintain
the vectors of its motion until and unless it is acted
on by an outside force.   Suppose I draw an satellite in
orbit about the earth with no motor, no fancy devices,
and spinning.  


--->  will be the satellite orientation when things are
perfect.    X will be the earth and I will make an ascii
depiction of an elliptical orbit.  This same kind of
drawing can be done for a circular one.




                             ----> 
           ---->
                                          ---->

---->                                         X  ---->

                                          ---->
           ---->
                             ---->

This is a poor attempt to draw an elliptical orbit but it
should be illustrative of the point I am trying to make.

The orientation of the vector is the axis of rotation on
a spin stabilized satellite.  Unless you act on this by
some external force that is large in the short term
(such as magnetorqueing), over short time periods, this will
not change.  In order to get the antennas to always point at
the earth, you must act on the satellite by some force.
In AO-40, the goal was to start in the mode above. This
is approximately the mode we are in now.  We made a design
error and have all of the magnets for the magnetorquers
aligned so they are supplying the mystery force but for
a few orbits this is a good approximation.

Over longer periods of time, all sorts of things perturb
this.  Nonspherical and nonuniform distribution of the mass
of the earth,  the moon, Jupiter,  solar forces, and even
mystery forces caused by too many magnets lining up the right
(wrong?) way . . . 


What we were going to do was have a bunch of wheels inside
suspended magnetically.   When you slow one of these wheels
down or speed them up, the rest of the satellite will do
the "equal but opposite reaction" game and change its
angular momentum by "the same amount" you changed the
wheel but in the "opposite direction".


<<< In order to get the satellite antennas that are fixed
on the body of the spacecraft to always be earth pointing,
this changing of the speed of something must be constantly
happening or the satellite will revert to the spun stabilized
picture I drew so poorly above>>>

By anyone's definition, this is not a cheapsat.

In order for a gravity gradient boom to work, there has to
be a fairly large gradient.  They worked extremely well on
the UOSAT's when they were almost passive and even better
when they provided the bias and magnetorquers were used to
close the control loop and keep the satellite earth pointing.
But nothing like enough gravity gradient will exist at 10000 km
without a huge long boom and I do mean huge.  By anyone's
definition, a huge long extensible gravity gradient boom is
not a cheapsat.  UOSAT's are in low earth orbit and the
amount of boom needed to get near gravity gradient lock
was achievable.

Finally,  you could throw up your hands and say okay, I will
give up gain antennas on the satellite so I can just throw it
off and not have to control it.  If you believe that another
20 dB needed on the ground will ever be called cheapsat,
go figure out exactly what it would take to make your 435
Mhz antenna 20 dB better.  You better get federal government
funding.

Next, no launcher that currently operates that we can ever
hope to get a ride on delivers to 10000 km circular. (Does there
exist one at all?) That means to get there, we would have to provide
our own propulsion.  Since we are 1 for 3 on getting it right and we
spent years working on all of that, by anyone's definition this is
not a cheapsat. You most certainly cannot do this without sensor's
and computers.

In short, there simply does not exist a model for HEO that will
ever fit into the cheapsat in my opinion.  The only orbit that
begins to fit cheapsat kinds of ideas is a LEO or any other
orbit where we are just dropped off and start having fun.

Bob
N4HY











-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Greg D.
Sent: Friday, November 07, 2003 3:38 AM
To: amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] Longevity-sat proposal


Actually, I thought the idea of a high circular orbit was rather good.  
Something perhaps a little higher, so that the bird would drift slowly 
across the sky.  Is there a problem with this?

The boom won't work, but in a circular orbit a slow roll in the opposite 
direction from the orbit should keep the antennas aimed toward Earth.

Greg  KO6TH


>From: Cathryn Mataga <cathrynm@junglevision.com>
>CC: amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] Longevity-sat proposal
>Date: Thu, 06 Nov 2003 00:51:07 -0800
>
>
>Woops, Okay, I see.  I've been informed by smarter people
>than me, that this is not going to work. Sorry for the noise.
>
>
>Cathryn Mataga wrote:
>
>>3.  Orbit 10km high, roughly circular over the equator.
>>     (The idea here is to hopefully get an orbit where we could
>>     talk to the satellite using a dish and a linear actuator.)
>>     Would be a daytime satellite anyway.  Good
>>     for field day, things like this, I suppose.  Don't know how hard
>>     it is to get to this place.
>>4.  Satellite points toward earth using Gravity boom. Spin stabilized.
>cribe amsat-bb" to Majordomo@amsat.org
>>
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