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

On Tue, 12 Mar 2002, Tomb wrote:

> 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?

It will also move it into an elliptical orbit, but not as efficiently as
thrusting backwards.  You want to cut as much of your velocity that is
tangential to the surface as possible, this is what keeps you in orbit.
Now if your orbit is pretty low to begin with, that downward thrust
might be enough to push you into a higher drag region, and then air
resistance will do most of the work, but will also be more likely to
burn
up the projectile.

>
> 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?

You're correct, except the centripetal force is gravity (the force
pulling
you towards center).  Centrifugal force is a psuedo force that you
only feel if you take the orbiting satellite as stationary in your
reference frame.  Basically orbit is achieved if you're moving fast
enough
tangentially to the surface that the force of gravity pulling you down
only does so enough to pull your motion into a circle (or ellipse).

>
> 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.
>

There are two parts to getting into orbit from the ground, getting to
the
appropriate altitude (fighting gravity to get there), this is
perpendicular to the surface, and then getting the tangential velocity
to
stay in orbit at that altitude.  Now it's been a long time since I
actually worked any of the numbers, but I think the bigger part is
getting the orbital velocity.  It's not too hard to send a sounding
rocket
up to low orbit altitude, getting it to stay there is hard.

> 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....
>

Actually I've used the gravity mechanic for vector movement that others
have mentioned.  Determine range bands for the gravitational force, and
then apply them to your motion as an additional thrust.  Orbit isn't too
hard to achieve, you just have to a rough idea how much velocity you
have
to have at a certain altitude.

P.S. If I'm screwing any of this up (I'm sure there are others on here
that know this stuff as well if not better than me) let me know.  It's
been a while since I studied orbital mechanics in any classes.