# [amsat-bb] Re: info on satellite placement

Robert Bruninga bruninga at usna.edu
Tue Oct 31 09:09:51 PST 2006

```> Subject: RE: [amsat-bb] Re: info on satellite placement
>
> That is the most lucent simple explanation I could have
> imagined.  The missing link was the realization the rocket
> headed horizontal (or a srequired for a specific orbit).
> Obviously the further out, the longer it takes to do a
> revolution. I imagine at some point earths gravity will not
> be sufficient to hold an object of given mass in a circular
orbit?

Exactly,  If  you are at a given altitude and you are going to
slow, then your satellite will "fall" towards earth..  But as it
falls, it speeds up as it approaches its lowest point (on the
other side) and that makes it go higher to arrive back where you
are on this side... In otherwords, the circular orbit becomes an
elipse with a high side and a low side.

Now, If  you were at a given altitude and you were going to fast
for that altitude, then you would end up going higher on the
other side and slower... Again an elipse.... And because it is
higher and slower on the other side, it will fall back to where
you were in the first place... Again, an elipse.

The only problem with going too slow, or going to fast is
whether the elongation of your elipse just happens to run into
Earth.  That makes for a bad day..  Remember, the satellite is
orbiting around the CENTER of the Earth and its Gravity.  The
satellite has no idea how big the "earth" is...  So as long as
the low point of an eliptical orbit stays about 4500 miles above
the center of the earth, then it wont run into earth...  But if
the low point gets below that, then it runs into earth or the
atomsphere and dies...

> Or would it have to slow down below the speed limit in weaker
gravity?

Whatever speed at what ever altitude will always define an
eliptical orbit.  The only question is, is whether the Earth
gets in the way of that elipse...

Neat stuff!
Bob, Wb4APR

> -----Original Message-----
> From: amsat-bb-bounces at amsat.org
> [mailto:amsat-bb-bounces at amsat.org] On
> Behalf Of Robert Bruninga
> Sent: Tuesday, 31 October 2006 12:56 AM
> To: 'Simon'; Amsat-bb at amsat.org
> Subject: [amsat-bb] Re: info on satellite placement
>
> > I was wondering if any one can suggest where I might find
> > information on "how a satellite" gets placed into orbit.
> > I ... can only find that a rocket gets it there.
>
> Actually, the only thing the rocket does is get it going fast
> enough.  The only way to stay in orbit is to be going 17,500
MPH
> You could orbit the earth at tree-top level if it were not for
> air friction which would cause you to loose your speed and
hence
> fall to the ground.
>
> So the only thing the rocket does is 1) Get you going fast
> enough to stay in orbit, and conicidently, get you a hundred
or
> so miles above tha atmosphere so you can keep that speed up
long
> enough to do something useful.
>
> Notice how the rocket almost immediately starts turning
towards
> the horizon after launch, because it is horizontal speed that
is
> what defines an orbit.  It does travel upward at first to get
to
> less dense air friction as soon as possible, but then begins
to
> head horizontal until 17,500 MPH is achieved.
>
> > I hope to learn how and when its gets kicked off the rocket,
> > and what happens after that.
>
> Once the rocket gets to 17,500 MPH horizontally at the
altitude
> you want, that is when you separate the satellite and let it
> continue.  Usually the rocket uses some left over fule to
> slow-down and hence, fall back to earth.
>
> If you are not perfectly horizontal and not exactly at the
right
> speed, then your orbit will not be circular but will be an
> elipse.  AO-40 was launched into a very eliptical orbit to get
> out to 40,000 km at apogee for a very large footprint.
> Eliptical orbits are just fine, as long as they do not
intersect
> the earth or the earth's atmosphere.
>
> I said 17,500 MPH as a representative number.  It is different
> for each altitude orbit relative to the center of the earth.
> Remember that a 500 mile orbit (above the Earth's surface) 1s
> 4,500 miles above the center of the Earth.  But a 200 mile
orbit
> is 4,200 miles above the center, so the speed is not that much
> different.
>
> Hope that helps.
> Bob, WB4APR
>
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. Opinions expressed are those of
> the author.