[LINK] Orbital mechanics, was: Finally up to date on the Shuttle

Robin Whittle rw@firstpr.com.au
Sun, 16 Feb 2003 18:56:46 +1100


Here's one way of looking at descending from orbit:  Lets consider
firing some super powerful rockets in an instant to completely stop all
orbital velocity.  Now, as we know from Bugs Bunny, you start picking up
velocity as gravity accelerates you downwards.  Previously your orbital
velocity at the orbital radius had exactly countered the acceleration of
gravity, but with that gone, you start accelerating at 10
metres/sec/sec, or less if you are a long way from the surface.

Maybe, depending on the orbital height, you will soon be falling with a
greater velocity than you were in orbit with.

If you were orbiting at 100 metres (ignore the atmosphere for a moment,
maybe think of this on the Moon) and you suddenly stopped the circular
motion, you would accelerated downwards until you hit the deck.

If you were in a 10,000 km radius orbit, then you would ultimately reach
a much higher velocity than from 100 metres.

But how this works with the Shuttle I am not sure.   Maybe it slows down
with the rocket burn and then speeds up as it gets lower.  That would
make sense, although since the horizontal distances are much greater
than the vertical, this returns me to a counter-intuitive perplex state.

When in doubt, Google!  (Google has 28 instances of this phrase
already.)

   http://www.colorado.edu/physics/2000/applets/satellites.html

A Java applet for the Earth and Moon.  Instructions - click your
trackball / mouse button while moving it and launch your satellite. 
WARNING - this is ADDICTIVE!    I got one to loop the Moon three times
in between orbits of the Earth before eventually hard-landing on the
Moon!   You can also leave the cursor still - near the moon's approach,
and click and release the button so the satellite has no velocity to
start with.

   
   http://www.medphys.ucl.ac.uk/~martins/orbit/orbit.html

6 Mbyte Windows orbital simulation program with a 17 Megabyte textures
file which is needed for operation (I used the Avsim site).  Unzip the
textures into \textures\ in the directory of the main program.  It has a
steep learning curve and I did not attempt it.  But it apparently does
quite realistic simulations of orbits, launches, dockings etc. anywhere
in the solar system.  Maybe someone can try it out and report on how the
velocities change after the rocket burn for re-entry.


Still, after quite a bit of Googling, I couldn't find an answer to our
question of how vertical and angular velocities, and the combination of
the two, change in a near Earth orbit re-entry.

One interesting tidbit of information is that the International Space
Station drops about 180 metres a day (2mm a second), due I assume to
atmospheric drag.  

   http://www.hq.nasa.gov/osf/station/viewing/issvis.html

So if they don't boost it with rockets every few months, the drag would
get worse and it would re-enter and burn up.   This makes me think that
the "weightless" condition in the ISS is not perfect.  An object would
naturally stay in orbit if it wasn't for the drag, so an object
free-floating in the ISS is in orbit around the Earth, but not subject
to atmospheric drag.  So sooner or later the ISS would be dragged
backwards, or is it moving faster forwards . . . or ????   I still don't
entirely grok it . . .   Anyway, an object floating in the ISS would not
float there forever before hitting a wall.

 - Robin