Randy said:
> This is actually one of the flaws in orbitally dropped munitions (like
????
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?
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?
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.
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
> On Tue, 12 Mar 2002, Tomb wrote:
> I'm in orbit (ignore drag). I launch a Thor Javelin by applying thrust
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
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
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
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.
[quoted original message omitted]
Well, this isn't obviously true. Note that Skylab came down without the
benefit of thrusters (in fact that was why it came down). Also note that the
Space Shuttle requires tons of fuel and two solid rocket boosters to reach
orbit, but essentially it glides back to earth using only small thrusters to
pop it out of orbit.
When you go up, you are buying potential energy and when you come down you get
it back. Most of the time, you are worried about too much
energy on re-entry and have to take a gradual slope that allows you to
bleed off speed in the upper atmosphere before hitting the thick stuff. If you
hit it too shallow though, you can skip off, like a flat rock on a pond.
I can't remember if any Jupiter probes did this, but there is the idea of
aerobraking, where you skip in and out of the atmosphere a couple of times to
bleed off the interplanetary speeds and reduce you down to orbital speeds
without having to use a lot of thrusters.
--Binhan
> Randy said:
The projectile will have a velocity vector of the intial satellite, plus
whatever thrust you applied to the projectile. Since you are in orbit, the
planet's gravity will exert a continous and slowly increasing vector towards
the planet. The projectile will spiral inward towards the planet.
The best visualization of this is the giant funnels that people drop coins
into. The initial oribiting vector would be the rail but the track that
launches the coin is slanted in slightly, effectively providing thrust towards
the center. Gravity pulls the coin towards the center, but it still has a
forward velocity vector and so moves in a slowly closing spiral.
One reason you don't want Thor projectiles in geo-synchronous is that it
takes a long time for them to fall. You'd probably want them in LEO
(Low Earth Orbit) where they are only 100-200 miles up. They probably
need to be coated in thermal shielding (like the shuttle) to survive
re-entry since most metals can't go above a few thousand degrees temp.
Another issue is that re-entry will always be in the orbital direction
of the satellite. Which means targets on the far side of a mountain are going
to be pretty safe. This can be offset by having a bunch of satellites in
different directions (which you would need anyway to get decent coverage of
the planet using low orbit satellites)
--Binhan
> -----Original Message-----
[quoted original message omitted]
> From: KH.Ranitzsch@t-online.de (K.H.Ranitzsch)
> No object in a vacuum will spiral down to the planet. All orbits are
Or intentional deceleration.
2B^2
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[quoted original message omitted]
On 12-Mar-02 at 12:40, Randy W. Wolfmeyer (rwwolfme@artsci.wustl.edu)
wrote:
> On Tue, 12 Mar 2002, Tomb wrote:
And if you apply enough thrust the close edge of the ellipse is under the
surface of the planet and your missile impacts the ground. As a matter of fact
if you missile can pull 100Gs I bet the impact occurs far sooner than slowing
your orbit.