[JGZinv]

A while ago I claimed that between me and Backslash we'd come up with
a hack of sorts to do gravity. Since we're not actively working on it, I figure
it's better to explain how it worked and should function, and perhaps the
general populace can refine the idea further. Rather than reiterate the idea,
I'll just repost my log with some edits.

Please note the dates, and if there's questions I can probably mentally chew on them
for a bit. It was a round-a-bout way that I came to the idea, but we've got lots of those.

Also, I think GravCube would be a great name for the end product. ^_^

11/23/08
(1:04:57 AM) JGZinv: hmm
(1:05:09 AM) Backslash: hm?
(1:05:34 AM) JGZinv: can your tractor beam have variable intensity ?
(1:05:51 AM) JGZinv: or double up the effect if two are used?
(1:06:19 AM) Backslash: yes
(1:06:44 AM) JGZinv: then I figured out gravity I think
(1:06:59 AM) JGZinv: simplcity is rather funny that way
(1:07:13 AM) JGZinv: got this off the chat ideas, and some HLP stuff
(1:07:32 AM) JGZinv: but set your invisible nav or waypoint
(1:07:53 AM) JGZinv: also spawn a cube...orb...whatever ship there
(1:08:02 AM) JGZinv: also invisible due to textures
(1:08:21 AM) JGZinv: have one tractor set to a 360 degree FOF
(1:08:28 AM) JGZinv: with long range
(1:08:37 AM) JGZinv: then have another set to shorter range
(1:08:38 AM) Backslash: and the beam itself invisible
(1:08:44 AM) JGZinv: that'd work
(1:08:49 AM) Backslash: yeah very nice
(1:08:52 AM) JGZinv: once you keep going closer
(1:09:01 AM) JGZinv: the effect doubles triples whatever
(1:09:10 AM) JGZinv: and sucks you in faster ^_^
(1:09:21 AM) JGZinv: walla.... Tach gravity ala mod
(1:09:40 AM) Backslash: heh.  well now lets see if we can make invisible beams
(1:09:51 AM) JGZinv: pure black texture?
(1:09:52 AM) Backslash: then I'll ask karajorma if he wants my tractor beam code
(1:09:57 AM) JGZinv: or was that green...
(1:10:05 AM) Backslash: not sure
(1:10:25 AM) JGZinv: either way... beam color is least of our worries probably
(1:10:44 AM) JGZinv: is you could set it up with several tractors
(1:11:08 AM) JGZinv: the jump between beam fields would be less noticable
(1:11:39 AM) Backslash: oooh, even better... beam damage attenuation.  I might be able to do that pretty easily based on some old code that phreak tried to get working
(1:11:40 AM) JGZinv: although this'll be interesting trying to escape the gravity well so to speak
(1:11:52 AM) JGZinv: attenuation?
(1:12:20 AM) Backslash: the beam 'damage' (in this case pull) gets weaker as it is further away
(1:12:24 AM) JGZinv: ah
(1:12:34 AM) JGZinv: yeah that'd do basically the same in reverse
(1:13:24 AM) JGZinv: only thing is I think to get best use out of this
(1:13:33 AM) JGZinv: you'd need some FRED modification
(1:14:03 AM) JGZinv: maybe some object edits so someone can drop in a "gravity well"
(1:14:21 AM) JGZinv: which is just a tractor'ed up invisa-cube
(1:14:44 AM) Backslash: hrm, wonder how many weapons on it we'd need to make it work well in multiplayer
(1:14:55 AM) JGZinv: worth testing though
(1:15:03 AM) JGZinv: depends on how FOF works too
(1:15:22 AM) JGZinv: if beams go through the "hull"
(1:15:29 AM) JGZinv: wouldn't be near as bad
(1:15:41 AM) Backslash: true
(1:16:09 AM) Backslash: hopefully we can force it to fire at more than one ship at once
(1:16:15 AM) JGZinv: hmm
(1:16:20 AM) JGZinv: multipart turret?
(1:16:29 AM) JGZinv: X like 8 turrets
(1:17:06 AM) JGZinv: figuring you have 6 sides to deal with
(1:17:12 AM) JGZinv: 48 turrets
(1:17:36 AM) JGZinv: hmm therein lies a problem too
(1:18:06 AM) JGZinv: if you have too many turrets all set to long range... it's going to be a instant yank more than a gradual build up
(1:18:22 AM) Backslash: true, getting only SOME turrets firing at the same ship at once
(1:18:41 AM) Backslash: heck either way this'd look cool with visible beams.  laser light show
(1:18:48 AM) JGZinv: yeah... 2 long range... 2 med long range...2 med... 2 short
(1:19:07 AM) JGZinv: with stepped area of effect length
(1:20:16 AM) Backslash: if we're really really lucky, the fire-beam sexp won't care which ship is targeted
(1:20:39 AM) Backslash: in which case we'd only need 1 turret per range increment
(1:20:43 AM) JGZinv: I'd think it'd go after whatever it can get
(1:20:56 AM) JGZinv: but that's me... not a coder talking
(1:21:21 AM) Backslash: we're probably both right and wrong in various ways... I may be a coder but I dunno as much about FRED or weapons, yet :-P
(1:21:43 AM) JGZinv: well it's an idea
(1:21:52 AM) JGZinv: unless you want me to go post it at HLP
(1:21:58 AM) Backslash: I like the idea
(1:22:44 AM) Backslash: for simplicity you could leave out the tractor beam idea and just ask if a beam could be made to fire at multiple ships at once
(1:23:19 AM) JGZinv: how bout an always on beam instead?
(1:23:33 AM) JGZinv: I think Nuke said that was already doable
(1:23:44 AM) JGZinv: back when we talked about lightsabers in FS2
(1:24:00 AM) Backslash: well if the refire rate is very low, probably, yeah.  unless he had a different method
(1:24:10 AM) JGZinv: basically that...
(1:24:29 AM) JGZinv: super high refire... so there wasn't a noticable shut off
(1:24:36 AM) JGZinv: no energy cunsumption
(1:24:46 AM) Backslash: heh now all I need is the knowledge of how to make invisible models.  also how to add turrets to ships.
(1:25:00 AM) JGZinv: we covered invis models too
(1:25:26 AM) JGZinv: adding turrets... I'd like to know as well but it's either in PCS2, Max, or Truespace
(1:25:34 AM) JGZinv: take your pick
(1:25:45 AM) Backslash: I prefer the first to
(1:25:47 AM) Backslash: *two
(1:28:12 AM) JGZinv: set it up your way far as you can
(1:28:18 AM) JGZinv: then ask a person to turret it
(1:28:48 AM) Backslash: true
(1:31:37 AM) JGZinv: That's because you haven't finished the texture. Unless you apply a texture to it and have one of the same name in your data/maps directory, the game will read it as invisible. If you want to just see the ship. Make a small (maybe 16X16?) square of a single color, name it 'HappyPants' or whatever you want, apply it to the object in truecrap and then save it as a .pcx in your maps folder. When you go in game, your modded ship should be just the model painted in a single color. I personally use red or gray but other colors might work for you.
(1:32:15 AM) JGZinv: so just don't apply a texture
(1:32:19 AM) Backslash: hahaha
(1:34:24 AM) JGZinv: btw main reason I said to stick a waypoint or whatever with it
(1:34:30 AM) JGZinv: was that if you wanted to...
(1:34:38 AM) JGZinv: we could make it work like a powerup
(1:34:58 AM) JGZinv: based on the distance from the waypoint... it could change the beam intensity
(1:35:07 AM) Backslash: hm!
(1:35:10 AM) JGZinv: but that seems like the harder way now
(1:36:21 AM) JGZinv: on player ship distance 45
 - change beam intensity 80%
(1:36:32 AM) JGZinv: or whatever the SEXP equivalent
(1:36:38 AM) Backslash: true.  at least we have options to try!
(1:36:50 AM) JGZinv: yeah 2 ways... that should work
(1:36:59 AM) JGZinv: worst case on the beam lock
(1:37:13 AM) JGZinv: is you pile a bunch of these things together
(1:37:22 AM) JGZinv: so they are each their own ship
(1:37:33 AM) JGZinv: so they can lock on to more than one thing
(1:41:17 AM) Backslash: will take a lot of fiddling I can tell, but still, it's something to try!
(1:41:23 AM) Backslash: I'm madly table editing right now
(1:41:55 AM) JGZinv: for this or something else?
(1:42:12 AM) JGZinv: this sound right for the post?
(1:42:13 AM) JGZinv: Basically we need to figure out:

A. Can a beam from an AI controlled ship target multiple other ships at once?

B. If not, would a 360 degree FOF with an always on type refire rate essentially do the same thing?

C. If A is good to go, how can we go about implementing it?
(1:43:55 AM) JGZinv: eh scratch C
(1:45:02 AM) JGZinv: meh posted
(1:46:17 AM) Backslash: yeah, table editing for testing this.  stand by
(1:46:23 AM) JGZinv: k
(1:46:34 AM) Backslash: gonna start with just automatically firing at everything in sight
(1:46:43 AM) JGZinv: oh yay
(1:47:00 AM) Backslash: see what that looks like, for one thing.... light show :-D
(1:47:08 AM) JGZinv: rave in space
(1:47:15 AM) JGZinv: next week on JAD
(1:47:33 AM) Backslash: yeah really
(1:47:57 AM) JGZinv: btw beams can target multiple points on one target
(1:48:05 AM) JGZinv: so eh... sorta there
(1:48:15 AM) Backslash: yeah I've seen it do that
(1:48:39 AM) Backslash: oh crap... hopefully that won't mess too much with the direction of the pull
(1:48:55 AM) JGZinv: pull inward....
(1:53:34 AM) JGZinv: seemed to fit some of our questions
(1:53:36 AM) JGZinv: http://www.hard-light.net/forums/index.php/topic,51903.msg1066706.html#msg1066706
(1:55:31 AM) Backslash: well I've got half the battle done... the turrets are invisible but they don't fire yet


11/23/09
(10:14:53 PM) Backslash: yar!  it works but only sort of.  So far I've only got it to shoot at the player, but that's a start.  Looks like crap... contrary to what I had expected, the beam doesn't 'track' the target so it fire lots of separate beams as the player moves.  but eh, all that should be invisible eventually, right?
(10:31:40 PM) JGZinv: yo
(10:31:54 PM) JGZinv: wanted to yak at ya anyway
(10:31:59 PM) Backslash: ah good
(10:32:02 PM) JGZinv: did you see the topic?
(10:32:05 PM) Backslash: yep
(10:32:08 PM) JGZinv: ok
(10:32:24 PM) JGZinv: did you try the other way with the 360 FOF always on?
(10:33:02 PM) Backslash: yeah... it mostly doesn't work, or at least, it works but looks horrible.
(10:33:11 PM) Backslash: hopefully a few code tweaks will help
(10:33:14 PM) JGZinv: looks can always be changed
(10:33:21 PM) JGZinv: we just want it to work
(10:33:30 PM) Backslash: it causes ships hit by beams to shudder and rotate as well as get pulled in
(10:33:40 PM) JGZinv: sounds good to me....
(10:33:46 PM) JGZinv: y4 it a problem?
(10:34:03 PM) JGZinv: that seems like the beam thwack in the beam table is having the effect
(10:34:21 PM) JGZinv: set thwack to 0.... won't do that
(10:34:30 PM) Backslash: well the shudder and rotate isn't 'regular'... goes in all sorts of directions
(10:35:05 PM) Backslash: and uh... I'll look into that, but afai remember that's how I was applying the pull in the first place
(10:35:13 PM) JGZinv: ahhh
(10:35:39 PM) Backslash: but yeah, it mostly works.  my first time I rofled because I immediately got pulled into the center turret and died because of such a fast collision
(10:36:18 PM) JGZinv: well that's the idea if you're really close... but I get what you're sayin
(10:36:31 PM) JGZinv: were you able to break out of it?
(10:36:45 PM) Backslash: not the first few times, no
(10:36:51 PM) JGZinv: or is an impossible task to break the gravity so to speak
(10:36:54 PM) Backslash: I'm trying to tweak the strength
(10:37:14 PM) Backslash: recently tweaked it too much the opposite way so there wasn't enough gravity
(10:37:26 PM) JGZinv: wonder how this would affect caps
(10:37:41 PM) JGZinv: like in the wing commander movie
(10:37:47 PM) Backslash: hmm good question
(10:37:48 PM) JGZinv: with the kilrathii crusier
(10:38:16 PM) JGZinv: make a black hole graphic... a big "grav cube" set up like this
(10:38:23 PM) Backslash: I suspect it will not affect them as much as we wish it would, since they have such higher mass
(10:38:30 PM) JGZinv: when it gets close enough it looses health
(10:38:46 PM) JGZinv: yeah but really it should work that way
(10:38:54 PM) JGZinv: technically for big ships
(10:39:00 PM) JGZinv: appearance wise
(10:39:24 PM) JGZinv: they have the engines to push through an arena of low gravity like most of space
(10:39:30 PM) Backslash: true
(10:39:33 PM) Backslash: didn't think of it that way
(10:39:36 PM) JGZinv: high gravity we could do in cutscenes
(10:39:45 PM) JGZinv: with really high values
(10:40:06 PM) JGZinv: what you need is almost an immunity to beams flag
(10:40:20 PM) JGZinv: so fighters or big ships could plow right through it
(10:40:35 PM) JGZinv: but only effect a group you didn't want protected
(10:41:15 PM) Backslash: the visual effect is currently not what we want (not that it matters) but does look pretty cool, almost Borg scanning like
(10:41:24 PM) Backslash: yeah
(10:41:30 PM) Backslash: all jumped in at once
(10:42:13 PM) JGZinv: maybe someone can use it as is...
(10:42:44 PM) Backslash: well as is still needs code changes to have the tractor/gravity part actually work
(10:42:59 PM) Backslash: but yeah, I'll see if karajorma would like me to commit that to his branch
(10:43:17 PM) JGZinv: ^_^

12/6/08
(1:28:42 AM) Backslash: well it doesn't work with anything except my build which is buggy
(1:29:10 AM) Backslash: but that's a matter of time.
(1:29:12 AM) JGZinv: the build or the cube?
(1:29:15 AM) JGZinv: is buggy
(1:29:32 AM) Backslash: the build

12/9/08
(1:55:17 AM) JGZinv: not sure quite what you need done with it
(1:55:45 AM) JGZinv: besides the whole works-on-my-build-only bit
(1:56:03 AM) Backslash: yeah it's mostly just porting the code changes I've done from the old unstable HEAD build to the latest 3.6.10 branch
(1:56:38 AM) JGZinv: ah... so partly the code freeze too I'd imagine
(1:57:36 AM) Backslash: yeah
(1:57:53 AM) JGZinv: blah
(1:58:11 AM) Backslash: once my changes are in place, all the cube is is a gun that fires beams at you :-P

12/16/08
(1:53:22 AM) JGZinv: kara said he was interested in our gravity
(1:53:29 AM) JGZinv: though I've not told him how we did it
(1:54:38 AM) JGZinv: I think he's thinking we came up with some uber complicated method
(1:54:49 AM) JGZinv: as he want's to stick it in the galactica
(1:54:56 AM) JGZinv: for carrier landings
(1:55:06 AM) Backslash: well technically it is more complicated than it should be :-P
(1:55:18 AM) JGZinv: unless he does staggered grav cubes
(1:55:28 AM) JGZinv: I don't think it'd work for that application off hand
(1:55:35 AM) Backslash: I agree
(1:56:00 AM) JGZinv: if you built them into the hull and defined them only to activate on a certain event like landing
(1:56:16 AM) JGZinv: they maybe... it'd work out... but it'd be a pain either way
(1:56:17 AM) Backslash: I'm hoping that once he sees how we did it, he can make a new sexp that does the same thing as what the beam does without the bad sideeffects.
(1:56:33 AM) Backslash: cause we're kind of bunging the whole chicken at the problem when an egg might do

[JGZinv]

I realize we didn't provide any files with this right off, but not one question or comment
after the years people have desired gravity?

[General Battuta]

I think you should probably remove the chat transcript and summarize it as a post. It's a bit hard to follow.

[Nuke]

gravity is easy, define a point in space as center of gravity or a position along an axis as plane of gravity, assign some acceleration to that point, then make it follow the inverse square law. apply resulting acceleration to the player. ive done it with scripting but it breaks collision detection. i have a theory to fix it (write the velocity computed in the previous frame to the old velocity variable in an objects physics info), but i havent got around to coding it. the need for terrain kinda slowed down progress on that, so im mostly working on my terrain generator, gloriously dubbed dirtgen.

[Aardwolf]

TL; DR.

I think you should probably remove the chat transcript and summarize it as a post. It's a bit hard to follow.

Yes, summarize pl0x.

[JGZinv]

*sigh* Reading HLP's IRC room is more complicated than this, but I guess I must.


Main Theory Purpose:

There's two methods for making this idea work.
Both rely on Backslash's tractor beam code which was made in a non Head
build at the time during the code freeze in November. Supposedly Bax sent
the code to karajorma to be included in the main build before we got to the
recent releases.

Originally it was intended for FringeSpace, since the result would mirror
Tachyon's "gravroids" but I'm putting it out here for other people to try
and mature or use.


Explanation of how it should work in practice:

Once you've got the setup described below, when in game you should have several "rings of
influence" around the GravCube. If you want to imagine Saturn's rings... that works.

The outer most rings have a low beam intensity, so they have a light pulling
effect. Middle rings have a bit stronger intensity setting. So when you transition
from the outer to the inner ring zone, your ship will be pulled in faster. Since it's a
tractor beam, you'll have to fight to get loose of it.



This goes for the zone closest to the Cube as well, at which point the intensity
should be very high, making it extremely difficult to escape the "gravity well."


Method 1:

Place an invisible cube "ship" there, "invisible" meaning don't apply a
texture or by other means. Make sure this ship cannot move or fly off.
It should only be capable of firing turrets at targets. It should also be
invulnerable unless you want a player to destroy it (such as if the model
was a roid or device).

The reason for using a cube is that you can have
a very good field of fire, for very little poly count. Using a sphere for
example, would require a more complex model and is unnecessary.

The cube should be pre setup with tractor beam turrets on each side.
The number of turrets will depend on how many "attenuation rings" is
needed for a smooth effect, and if multipart turrets could fill in for some
of this (I've not tried turreting a ship so don't know).

Each turret does not need to be a complex model, again it's an invisible object.

Each turret should have either a 180 degree FOF or a 360 degree FOF.
Not sure which would be more useful.

Every side should have tractor beam turrets setup so that at least one
has a long range, at least one medium range, and at least one short
range. Again more "step downs" or attenuation may be required so it is
a smooth effect. Note that for a 6 sided cube, you're looking at 18 minimum
turrets (3 per side).

The beam texture used for every turret, should be using a "invisible texture,"
if I remember right this was a pure green color. This way no beam effects are
seen by the player.

No beam whack.. or other table settings which would cause a significant jerk.
You want a smooth and gradual build up of the pulling effect.


Method 2:

Essentially this is the same as Method 1, however you set the beams to always
be firing (instant refire, no wait), have infinite ammo/energy, and continue with
the same "stepped" intensity as before.

Bax found this method to work, but the ships when caught would shudder and
rotate. We thought that it might be related to the beam thwack, or that some
minor code tweaks might be needed.


Optional Method Add-On:

Set an invisible nav or way point on your
location you want as the center of gravity.
At the same location as the "GravCube" described above.

Since the game now has a point of reference, you can set it up so that the
turret intensity could be changed based on distance from the Cube, to a
incoming ship. This would make having several tractor turrets unnecessary.

Simple example: on player ship distance 45 - change beam strength 80%...
or whatever the SEXP  equivalent would be.


Thoughts/Notes:

This should work for caps well, because they have in game, larger engines and
can push through gravity wells with an easier time.

FRED would need some kind of edit to make the most use of this device, mainly
something that would show the range of the device based off of the figures
listed in the ship/weapons table. IE it'd have to read the range of the beam
and turn that into some kind of graphic (a bubble?) which would show the area
of influence the Cube would have.

You might be able to take this same idea and use it for special purposes such as
carrier landing, or something, terrain.... but you'd want to set it up so that the Cube's
were able to be turned on and off by a sequence of events. Not left on all the
time as it would affect several other things in the local area. You can also force
the beams down a certain path if you don't use a Cube, or it's properly aimed /
if you don't use method #2.

You can do the same thing with roids, ships, objects, whatever. Long as it can play host to
some turrets. The beams don't have to be invisible either... it depends on your use for it.

If you have problems with firing at more than one ship at once, I theorized that with
Method 1, you could add more turrets per side. Like 8 turrets, figuring you have 6
sides to deal with would be 48 turrets. 2 long range... 2 med long range...2 med...
2 short. But if you have too many turrets all set to long range, you'll end up with a
"yank" and not a smooth pulling motion.

This is why I think the Method 2 is the best bet and less resource intensive.

[Herra Tohtori]

Is it possible to remove the zones and just make the force/acceleration continuous based on the distance? For a concentrated beam weapon you could use slowly dissipating linear curve, but for point sources of gravity (ie. normal objects of significant mass) you could us the more normal a = G*M/r^2 for all objects around the planet, sun or some more exotic gravitationally significant target.

One thing that might be interesting to try is to apply this to subspace vortices, but with negative mass... based on the size of the ship (required diameter of the outgoing or incoming ship) it would cause a gravitational disturbance that would temporarily push ships in the immediate proximity a bit away, which might be pretty cool.

Of course, toying with this idea you could give outgoing vortices positive mass, which would suck nearby ships towards them (sort of like the classic black hole / white hole hypothesis of the ends of an Einstein-Rosen bridge).


If it is feasible to use one continuous equation to define the acceleration instead of zones, it would be a huge step towards things like true atmospheric flight as well as (obviously) simulation of orbital mechanics, should someone be mad enough to try it....

...and, as a combination of the two, there's also the Adama Maneuver.

[JGZinv]

Is it possible to remove the zones and just make the force/acceleration continuous based on the distance?

JGZinv -

Since the game now has a point of reference, you can set it up so that the
turret intensity could be changed based on distance from the Cube, to a
incoming ship. This would make having several tractor turrets unnecessary.

Basically we're on the same track, different method though. As to the rest of all that, no clue.
This is why I tossed this out here so it can be refined be people who at least know the system.
All I claimed was that it works to a large degree... it's been talked about forever and ever.


There is a problem however with the design based on distance, unless there's a tractor designated per ship,
multiple ships entering the same field would experience an incorrect amount of pull, when they are at different distances.
IE the closest ship would trigger high gravity, but ships at the edge would suddenly shift from low to high even though
they are at the edge of the field.

[Bob-san]

Is it possible to remove the zones and just make the force/acceleration continuous based on the distance?

JGZinv -

Since the game now has a point of reference, you can set it up so that the
turret intensity could be changed based on distance from the Cube, to a
incoming ship. This would make having several tractor turrets unnecessary.

Basically we're on the same track, different method though. As to the rest of all that, no clue.
This is why I tossed this out here so it can be refined be people who at least know the system.
All I claimed was that it works to a large degree... it's been talked about forever and ever.


There is a problem however with the design based on distance, unless there's a tractor designated per ship,
multiple ships entering the same field would experience an incorrect amount of pull, when they are at different distances.
IE the closest ship would trigger high gravity, but ships at the edge would suddenly shift from low to high even though
they are at the edge of the field.

Why not triple-mount a tractor-gun? Set three different distances on it; 1500m, 1000m, 500m, or something. At the same strength or whatever would draw them in at various strengths.

[JGZinv]

I'm not trying to be rude here, but isn't that exactly what I just described
and the basis of this whole device?

The cube should be pre setup with tractor beam turrets on each side.
The number of turrets will depend on how many "attenuation rings" is
needed for a smooth effect, and if multipart turrets could fill in for some
of this (I've not tried turreting a ship so don't know).

Every side should have tractor beam turrets setup so that at least one
has a long range, at least one medium range, and at least one short
range. Again more "step downs" or attenuation may be required so it is
a smooth effect. Note that for a 6 sided cube, you're looking at 18 minimum
turrets (3 per side).

Method 2:

Essentially this is the same as Method 1, however you set the beams to always
be firing (instant refire, no wait), have infinite ammo/energy, and continue with
the same "stepped" intensity as before.

If you have problems with firing at more than one ship at once, I theorized that with
Method 1, you could add more turrets per side. Like 8 turrets, figuring you have 6
sides to deal with would be 48 turrets. 2 long range... 2 med long range...2 med...
2 short. But if you have too many turrets all set to long range, you'll end up with a
"yank" and not a smooth pulling motion.

[Nuke]

my atmospheric flight mod does this
altitude is in meters (above one planitary radius), down is a normal indicating the direction of gravity in relation to the ship. pp is a table of physics properties for the gravity emitting object, it contains the surface gravity and planetary radius. and omass is the mass of the object affected by gravity. i believe this is correct, but im not entirely sure.

Code: [Select]

--determine the force acceleration of gravity on an object at a specific altitude
get_grav_force_and_accel = function(alt, down, pp, omass)
--compute gravity stength at altitude
local grav_accel = (pp.surf_grav_accel * (pp.radius^2))/((pp.radius+(alt*0.001))^2)
--convert acceleration into force
local grav_vec = (down * grav_accel) * omass
return grav_vec, grav_accel
end
since the gravitational constant is rather huge and i was kind of concerned that floating point math would make direct computation of the force of gravity between objects rather difficult. instead i give the acceleration at 1 planetary radius and apply the inverse square law to that. this gives a realistic gravitational falloff (which in freespace units often spans the entire useable mission area). ive orbited planets with this code and as far as i can tell it works.

[JGZinv]

Link showing how tach uses grav roids.
Mainly the trap on the monster roids level illustrating
the slingshot effect you get.

http://www.youtube.com/watch?v=Zub9N7mabhQ

Video quality is horrifying compared to what I uploaded.

[Nuke]

fly on mars
http://www.youtube.com/watch?v=MMmhsxv69uI