RE: Um, eh? (Orbital Mechanics for Dummies)

-----Original Message-----
From: Tomb [mailto:tomb@dreammechanics.com]
Sent: Tuesday, March 12, 2002 12:15
To: gzg-l@csua.berkeley.edu
Subject: Um, eh? (Orbital Mechanics for Dummies)

Randy said:
>This is actually one of the flaws in orbitally dropped munitions (like
>Thor from Renegade Legion).  It takes just as much work to drop
something
>from orbit as it takes to get it up there (aside from the fact that on
the
>way down you can use atmospheric drag to do some of the work).

???? 

[Bri] This has me puzzled also. 

Okay, I'm an idiot. Let's start by assuming I know very little. 

I'm in orbit (ignore drag). I launch a Thor Javelin by applying thrust
directly towards planetary centre of mass. What will happen to my
projectile? With thrust applied, it should move away from the
satellite.... and then what? 

[Bri] Assuming Thor Javelin is in geostationary orbit and that it is
being
fired toward the center of mass of the planet, it falls straight down
toward
the planetary center of mass (ignoring atmospheric concerns). If it is
not,
then things get more tricky. Also the planet is turning while the
projectile
is falling. As the projectile falls the forward inertia energy (that
kept
the projector in space) does not change, but the energy required to keed
a
geosyncrinous orbit at increasingly lower atltitudes increases. All of
which
is to say that the projectile still travels toward the center of the
planet
mass, but the surface area it is over changes with the time of the drop.
So
you still need to "lead" the target. Also the orbital platform will need
to
adjust for the force of the launch (even if you let gravity "draw" the
projectile out, the launch platform is now less massive and changes the
orbital balance).

PS - won't gravity play into the picture? I understood orbit was
achieved by balancing gravity with centripetal (or was that
centrifugal... never get those two straight) force?

[Bri] Yes. I thought that orbit was achieved by a balancing of gravity
against forward inertia (at a right angle to the axis of the pull of
gravity).

I'm thinking (ignoring atmospheric drag) that it can't take as much
energy to leave orbit to return to the surface as it does to get up
there... because gravity is applying a force to your object. But I may
be not accounting for the centripetal force.

[Bri] Correct. However to have a _sustained_orbit_ at a lower altitude
required _more_ energy than at a higher altitude. I believe this is due
to
the effect that gravity is stronger in close proximity. 

PS - Someone want to do something really interesting? After developing a
gravity based mechanic for FT, then come up with an algorithm to
calculate a method of entering orbit. Orbiting seems to be uber hard in
our movement systems....

[Bri] But then again, I am also probably off in my thinking. And I
invite