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Meikle641
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### Floating Rocks

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Post by Meikle641 » Fri Apr 09, 2010 6:27 am

I've been trying to figure out how to make a special material (rock or crystal, whatever) that floats in the air. Assuming you had enough you could have flying castles or weird airships (like in Escaflowne), or something.
The problem I have is figuring out how much x pounds of it can support, and at what altitudes each "plateau" of weight would float at.
Or am I going about this the wrong way? I'd rather not Magical Tea Party this for my campaigns (one is ongoing) if I don't have to.
Thanks.
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Post by Username17 » Fri Apr 09, 2010 6:34 am

I've done a lot of thinking about floating rocks. Rocks don't float the way balloons float. If they were simply buoyant, they would come crashing down when you put weight on them. So what they need is a rather powerful force that pulls them towards the specific height they normally float at. Even then, they need a fair amount of friction on that spring, otherwise they will be constantly bouncing up and down.
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Post by Lokathor » Fri Apr 09, 2010 6:59 am

Make its repulsive force be inversely proportional to the square of the distance between them. Like opposite-gravity, and fiddle with the constants so it doesn't balance with gravity on the surface. Then, each big hunk of the stuff will eventually balance at an altitude where gravity and lift cancel each other out. Though as frank says, it'll probably bounce up and down around that height for a fair bit whenever anything major bumps it, until air resistance slows it down, if you want to be technical.
>_>
Like Pandroa's floating mountain things. They're held up "magnetically" because the core of the planet holds up the unobtanium rocks.
Last edited by Lokathor on Fri Apr 09, 2010 7:00 am, edited 1 time in total.
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Meikle641
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Post by Meikle641 » Fri Apr 09, 2010 7:07 am

Well, my original ideas involved them being natural cognizance crystals that generate power daily, which somehow causes lift. Probably ripped from the ground by sun or heat hitting them, causing enough to make them fly?
Thought was that the altitude was fixed on how much the rock gains each day. Hypothetically you could just keep them at the right height by having fire on it for x time or something, to keep equilibrium.
Rather complicated, though.
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Post by Lokathor » Fri Apr 09, 2010 7:21 am

Heat activated? Well that's a possibility, but unless the flight duration is extremely long even after the heat is gone it seems like they'd come drifting (or crashing) down every single night. Which might be acceptable to you, but it's just not what you implied at first.
Though, "heat activated flying crystals" is basically just as magical tea party as "lighter than air crystals" or "anti-gravitic crystals". Once you get into "that couldn't really exist land" you can just pick any ol plausible explanation and go with it. Just watch what you say to make sure that things stay consistent, and that the players can use stuff the same way the non-players can, and you basically avoid the perils of magical tea party.
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Post by TavishArtair » Fri Apr 09, 2010 7:28 am

It seems the most sensible kind of floating rock has an inherent tendency to push away from the earth which increases on either a geometric or exponential principle the more of it you get together (and thus cause their repulsion fields to overlap and combine, the bottom of which presumably touches some point on or near the earth). Thus causing fairly small amounts to not really get very high (though occasionally drift around someone's head if you give them a poke, see ioun stones), and for large amounts to be capable of floating in the goddamn sky. And occasionally someone does something clever like make a shield or armor of the stuff and leap off tall precipices and use the property of it to coast their way down and not just go splat on the ground.
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Post by Meikle641 » Fri Apr 09, 2010 7:32 am

Well, as long as I can say x amount of rock can make whatever bullshit float at whatever, and price it I'm happy. Handwaving the exact details would be fine with me, as long as I can have aided flight using this sort of stuff.
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Post by Username17 » Fri Apr 09, 2010 7:34 am

Magnetic levitation is pretty limited, in that it has to have a source which is much closer than the core, or gravity will be altogether negated and the damn thing will just fly off into space. A much closer magnet causes the opposite problem, where the repulsion sphere is a much smaller, and therefore steeper sphere than the gravitational attraction sphere, causing the rock to naturally slide down - coming crashing to Earth in a rounded but still very rapid arc.
I assumed at first that you wanted a stable floating mountain that you could like walk on and shit without having it oscillate noticeably while you walk on it, spin, or be in imminent danger of falling down. But with you talking about temperature based heights, I'm not sure any more. Do you actually want giant stone pistons that fly around based on wind patterns?
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Post by Meikle641 » Fri Apr 09, 2010 7:45 am

...No, not really. I'm not really good at this sort of thing, but that was one rationalization of how it could work.
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Post by Vebyast » Fri Apr 09, 2010 8:15 am

Speaking as a guy that knows some physics and some controls theory (and who also is up way too late, so double-check my numbers and read the wiki links), a few points. Also note that I've been typing this for a while, so it's probably going to be out of date by the time it's posted.

- You don't actually need that much force to maintain a target altitude. Once you can resist gravity, controlling where you go is peanuts. As an illustration, let's say that your rock increases its upward force by 1%. After ten minutes, your rock is going fifty meters per second, and it's ascended roughly 18 kilometers. Keeping your rock up is stupidly difficult, absolutely; a 10 meter cube of granite masses about 3 million kilograms. However, once you have that granite cube in the air, you can basically poke it with your finger and it'll start moving. A medium creature with fly 60' (perfect) could probably get that 10m cube going ten or fifteen meters per second with no real difficulty.
- Rocks going at speeds we can attain are immune to air resistance, for all intents and purposes. Air resistance is directly proportional to the square of your speed through the fluid. Even in the case of that 10m cube of rock, if it's only going a few meters per second, air resistance is only going to be a few hundred newtons. That's a lot when you're only dealing with a few dozen kilograms, but rocks are heavy. Those thousand-some newtons will take literally days to slow your rock down to a stop. Also, if you've ever seen the bowling ball experiment for conservation of energy, remember that your rock isn't going to oscillate slowly, or over a small area. It's going to oscillating violently with an amplitude almost equal to the original displacement from its target.
- "Proportional to the square of the inverse" is possibly the second worst possible controller. Not only does it not converge without a damper, but even with one it doesn't converge cleanly at all. If you're really concerned about doing the physics right, check out something like the PID controller. Simple to understand, simple to implement, and it works well enough that everybody uses it.
- If you go with anything dependent on buoyancy, you're going to have to take weather into account, end of story. A change of a few kPa are enough to give your rock that .01g acceleration I mentioned earlier, and then you fall out of the sky.
- Magnetic levitation doesn't work like that. There is no configuration of static "permanent" magnets which can cause another permanent magnet to be stable. If you want your rocks to not follow the field lines north and dive-bomb Santa, you need either a superconductor, a feedback servo system (AKA computer), or a diamagnetic material. All of those would be rather complex, and all would be far too easily exploitable by the party.

TL;DR: making rocks float is hard. Making them float stably is almost impossible. Making them float stably at a controllable altitude requires sufficiently advanced technology. If you want your rocks to conform to real-world kinematics, they're either computerized vacuum airships (which aren't really all that great) or they're magic.

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Post by Lokathor » Fri Apr 09, 2010 8:17 am

Basically your options go like this:
[*]Something pulls them from above, but the pull weakens as they get too high for some reason, and they hover at the right height.
[*]Something pushes them from below, but the push weakens as they get too high for some reason, and they hover at the right height.
[*]They're lighter than some (but not all) air, so air gets under them and they float up until they're on top of the right air layer, and they sit there.
[*]They for some reason project their own lift, like with giant rockets or giant fans or giant "they push up when you heat them or expose them to sunlight" power. As with the other three, the lifting power of this method needs to drop off eventually for sort some reason, and then they just hover at whatever height.
Depending on the specifics of any method, the rocks could be sitting at a stable height (or not), or could be floating all the time (or not), or they could be uselessly spinning in place like asteroids (or not), or they could even need external power to function (or not).
First you have to clearly decide on the results you want, and then you can pick the method to get the results you want.
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Post by Rejakor » Fri Apr 09, 2010 4:11 pm

I think the best floating rocks i've ever seen in fiction or game were the vimana in Wen Spencer's Endless Blue. The name is stolen from a hindu description of flying machines/an architectural feature of hindu temples. But that's fine. First of all, they were big. Like, floating mountain big, but more bullet shaped. These floating rocks being conglomerative and absorptive is a great idea if you don't want a sky filled with little pieces of floating rock (which is a cool image, but might not be what you're going for). Second, they had dirt and whole ecosystems on them (which is cool). Why didn't all that stuff cause it to unbalance and fall off? Well. Third, they seemed to hold their current inclination unless hit with a considerable kinetic blow, and even then slowly returned to their previous inclination, which hints at some kind of self-balancing mechanism (and lets you build castles and crap on them). Fourth, and possibly most importantly, they were moving - orbiting the planet, basically. Which is a lot cooler than just sitting there floating in space. Hell, in the book the rain that hit the vimana fell off a certain point on them and was dangerous because of the height it fell from (although you'd assume it would diffuse into spray from that height). Fifth, they maintained a certain height, as if being pulled from above and pushed from below by great forces that equalled out at a certain height. So with a strong kinetic impact, they could be removed from that height, but they'd quickly return to it.
All this crap is mostly pointless, but does end up with interesting and interactable floating rocks. If I was going to have floating rocks, i'd have rocks that kept their height by push and pull, moved at speed (possibly due to the same), attracted and absorbed lesser rocks in their flight path, and held their inclination(current rotational profile..words) with an amount of force enough so that a direct trebuchet hit would shudder it but not spin it.
But it all really depends on what kind of floating rocks you want.
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Post by Vebyast » Fri Apr 09, 2010 4:25 pm

I second the vimana flavor. Those were pretty cool. Of course, they were made even cooler by the fact that the planet was inside out, and they were orbiting the sun rather than the planet, but that's secondary.
Also, I seem to remember that their self-righting mechanism was stronger than their return-to-course mechanism. The one they hit with a space-diving battlecruiser maintained its orientation after rolling a few times, but I think they mentioned that its course was permanently changed.

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Rejakor
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Post by Rejakor » Fri Apr 09, 2010 6:00 pm

Yes but I believe the HEIGHT was unchanged. So they stay at the same height, but can be 'steered'. Which is sweet.
Possibly have them slowly accelerate up to a 'top speed' in a certain direction (which we will label 'forwards') for the rock, which could change if it absorbed other smaller rocks, but you can change where the forwards will take it with sufficient magic/muscle+ropes.
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PhaedrusXY
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Post by PhaedrusXY » Fri Apr 09, 2010 6:31 pm

Have them be the tops of mountains/buttes that actually mostly exist in another plane (the Ethereal?). The "floating" parts are just what happens to exist on the Prime. They could even move relative to the Prime if their primary plane moves relative to the Prime, but their motion would be pretty much immutable. Trying to stop one would be as difficult as trying to move a "normal" mountain.
This would let you have "floating" castles and stuff, but not rocks that you could craft into flying suits of armor or use as ships. So I don't know if that gets you what you want out of it. I kind of like the idea, though and might just use it for my own games.
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Meikle641
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Post by Meikle641 » Fri Apr 09, 2010 7:24 pm

Yeah, I think Rejakor has the gist of it. I'm sure I can cobble something together for smaller shit, though.
Relatively stable substance that stays roughly the same height. The attraction of smaller rocks is appealing, too.
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Post by Rejakor » Sat Apr 10, 2010 10:04 am

Ooh, I like that idea Phaedrus. That's cool. I might use that in future.
Although I don't want to discourage flying pirate roks. Because that's kickass awesome. And heroes flying around the sky with floating rock armour and floating rock weapons, some kind of magic air bladders to provide jet propulsion, hell, festooned with lead weights and strange air-windmill propellors?
SKY DWARVES.
*apoplexy*
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Post by Meikle641 » Fri Apr 01, 2011 3:02 am

Idly, I was wondering if anybody would be willing to whip me up some mechanics and fluff for this. I'd be willing to pay, assuming someone could give a fair price.
I wouldn't be asking this had DSP not ditched my project, along with some others, for Pathfinder. Been over two years, assholes.
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Post by fectin » Fri Apr 01, 2011 4:18 am

Actually, if you reject Newton, the old Greek descriptions of motion could really make floating rocks work well.
Essentially, (IIRC) things seek their natural state. That's usually "at rest and down" but doesn't have to be. If you had a type of rock with a natural state that was e.g. 5000 feet up and at rest (assume the ground is also at rest, otherwise it gets wierd), you have basically the behavior that you want.
Otherwise either go for rocks that treat the density of air as much higher or for rocks that are (inexplicably) drawn to some sort of gravetic halo around actual centers of mass.
Underall, I guess it comes down to how much detail you need. Unless you actually need a lot of detail, I'd just say "fuck it, every cubic foot of rock supports X weight".
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Post by erik » Fri Apr 01, 2011 12:27 pm

Quasi-related. In my Hollow Earth campaign (currently on hiatus), I 'splained away why there was gravity on the inside of the planet by granting a repulsive force from the mini-"sun" in the center of the planet that just happens to peter out before reaching the outer layer of the planet, but pushes shell surface walkers with roughly the same force that gravity pulls when on the outside of the Earth's shell.
Doesn't really help floating rocks, but it reminded me of a similar quandary of trying to work around physics.
In this particular case of floating rocks I'd say don't sweat the physics terribly and just "magic" it. How do immovable rods work? Do whatever they do for a stationary mountain. Have a large portion of the mountain be of a material that has the properties of an immovable rod. Erosion is a problem that inhabitants will have to cope with.
Or ramp up the crazy magic a bit more and have shamans/engineers/cloud giants/astral beings who harvest and process clouds of magic aether and forge the clouds into mountains, some workable material, ship hulls, whatever. So long as it is consistent within its setting it doesn't matter exactly how it is done.
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