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[RocheLimit]

I found this method out in my old DF40.24 fort and just confirmed that it still works as late as DF43.05.

Basically, you fill 1 minecart with water and 1 minecart with magma, then dump them on a track stop.  The trackstop is either not set to dump (if you are fine with using DF Hack to change the dump direction) or set to dump but disabled by a lever.  Be sure that all tiles around the 1x1 dump zone are not drops, as dwarfs will toss dumped items down rather than place them in the dump zone.

When ready, either set the dump direction or pull the lever; the two minecarts will dump 2 units of magma and 2 units of water onto the same tile, instantly creating a tile of obsidian.

While each tile does require quite a bit of setup, and if you go the lever route it isn't truly 'instant', the process cannot be stopped from operating, can block a hall with an indestructible tile stronger than any bridge, and will kill anything on the affected tile.  It's also very useful for small construction projects and touching up existing structures.

Slightly detailed instructions

Spoiler (click to show/hide)

Gif of the walls being formed

Spoiler (click to show/hide)

[Fleeting Frames]

Mhm. I prefer to set them on a track stop that dumps on a track stop that dumps where you want the wall to be, though - no need to link to a lever or use a dump zone, hauling-route works fine(also use this double stop to transport dozen minecarts full of 10 logs each).

Though, got to say, hadn't considered it in military applications. Would make pretty good procedural buildingdestroyer trap without requiring a cave-in, as track stops are considered Traps by game.

[Bumber]

TIL: You can attach levers to track stops.

This could come in handy. Safer item dumps, for instance.

[Skullsploder]

This. I already have a very safe trash compactor design but this could be good for automating drop chutes, from the trade depot to the magma forge for example.

[Fleeting Frames]

@Bumber:
I've tried using it in minecart-waterwheel power generators, but for some reason they remained on after water stopped moving ^^;;

[Dunamisdeos]

If you were to set this up high in the air and have both dump carts set to dump into open air, could you create an obsidian bombardment system?

Magma and Water meet in mid-air, obsidian blocks falls to the ground and hits a goblin on the head, etc

[Fleeting Frames]

Not sure if it has been done, but it's been discussed at least. Though at the time they used floating screw pumps and magma sea underneath to swallow the obsidian.

[Dunamisdeos]

If it worked you could set up a checkerboard of sorts and rip up anything underneath. Even if it didn't get hit, it'd get hit with the dustsplosion(s).

[Skullsploder]

If it worked you could set up a checkerboard of sorts and rip up anything underneath. Even if it didn't get hit, it'd get hit with the dustsplosion(s).

Why not have a pair of pipes sharing an output tile, one drawing from a magma source and the other from a water source? The tiles immediately before the output would have to be bridges or something, and it would probably be best to have the tunnels "airlocked" so that as soon as some predecided amount of liquid is inside the second to last tile, that tile is blocked off from the rest. Then when it's time to drop some obsidian, you open the floodgate on the final tile.of each tunnel and.the fluids meet.

Code: [Select]

oooo oooo
MF+F F+FW
oooo oooo

Unless dumping into the magma sea it would also be an extremely quick way of making arbitrarily tall single-tile obsidian columns

[PatrikLundell]

Most likely any obsidian formed would bring the bridges with it as it falls, making the construction single use (repeatable with reconstruction, but so is cast obsidian supported by a lever controlled support). You can try to construct it, of course, using two three tile long raised bridges at the sides, a hole in the center, and a single (or double) tile flow control bridge at each side. My prediction is that it will at best work a few times before one or both feeder bridges as crushed, with a high probability for both bridges to be brought down every time.
If you do test it, please report what the results were.


WBBBW
Mb bw
WBBBW

W = Wall
B = raised bridge
M = Magma
b = (lowered) control bridge
w = Water

It can be extended into an airlock design as well. I wouldn't recommend open space at the sides of the drop hole, as the fluids will flow diagonally at times, which means they will eventually do so at the same time, fusing with the wall.

[Skullsploder]

Most likely any obsidian formed would bring the bridges with it as it falls, making the construction single use (repeatable with reconstruction, but so is cast obsidian supported by a lever controlled support). You can try to construct it, of course, using two three tile long raised bridges at the sides, a hole in the center, and a single (or double) tile flow control bridge at each side. My prediction is that it will at best work a few times before one or both feeder bridges as crushed, with a high probability for both bridges to be brought down every time.
If you do test it, please report what the results were.


WBBBW
Mb bw
WBBBW

W = Wall
B = raised bridge
M = Magma
b = (lowered) control bridge
w = Water

It can be extended into an airlock design as well. I wouldn't recommend open space at the sides of the drop hole, as the fluids will flow diagonally at times, which means they will eventually do so at the same time, fusing with the wall.

My experience with obsidian is limited to large, single level obsidian farms where water is dropped onto magma via retracting bridges, equally and all at once across the surface, so I don't know much about finicky stuff like this. I was under impression that the water and magma had to share a tile to form magma? If that is the case, then the feeder bridges should never be taken down since obsidian can never form on tip of them. You're right about the open spaces though, a floodgate or raised bridge in each should do the trick.

[PatrikLundell]

It's true water and magma have to share the same tile to form obsidian, but I'm not 100% convinced fluids can't occasionally flow over empty empty space tiles to leave 1/7 fluid on the other side. I also don't know anything about how newly formed obsidian tiles interact with adjacent non supporting structures (such as bridges). I know, however, that trying to drop a single tile block of rock through the middle of a 3*3 retracted drawbridge smashes it (haven't tried raining drawbridges, but I don't expect any difference in the behavior).

Another thing to consider is that if there's an excess of one fluid that ends up on top of a block down at the ground, the weirdo cave-in logic will throw it up to land on what probably is a random side of the obsidian formation hole, with what I guess is about 50% probability of being on the wrong side, and if you then send another batch of fluid down towards the hole you're going to get obsidian in the wrong tile (and if that tile isn't separated from from supporting rock it will fuse with the rock to form a layer both above and below the bridge, forcing the bridge to be deconstructed, while if it isn't the bridge will probably be destroyed).