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[iofhua]

I'm thinking of setting up a trap corridor that includes a minecart being bounced back and forth between two rollers. I have heard that invaders can get beat up pretty bad by a minecart in this type of setup - but I want to know if minecart tracks and rollers can be destroyed by building destroyers? I don't want a critical part of my defense destroyed by the first troll that comes along.

If trolls and other building destroyers can tear up minecart tracks, I will probably just use upright spears attached to a repeater.

[CaptainArchmage]

Minecart tracks are constructions, so they cannot be destroyed. However, the rollers are buildings and may be destroyed, though I have not seen that happen yet.

[iofhua]

What if I did something like this:

 outside        inside
    .                .
    .                .
    .                .
T~==========~T

Key:
. is spike trap
= is minecart track
T is roller
~ is a channel filled with 7/7 magma

The idea is that the enemy can't get over the magma to destroy the rollers. I'm hoping that the minecart will skip across the magma, and create magma mist in the process? Will this work?

The drawback is all the goblins and trolls and stuff coming into my fortress will be on fire... Which may cause additional problems.

[fricy]

I'm 90% sure that a minecart accelerated by rollers (and worse: exiting from a magma pit) won't be fast enough to generate magma mist, and won't have enough speed to seriously harm multiple invaders. Science is needed to make sure, but it's highly unlikely IMHO.
Also: I'm not sure if building destroyers will target the rollers, but I wouldn't be surprised if they did.

For safety you can experiment with impulse ramps, they are an exploit, but should be completely safe. Or build a multi-Z level track system to hide the sensitive parts and accelerate the carts in the kill zone.

[iofhua]

That's no good. I suppose I could learn to use impulse ramps, but I would like to keep things as simple as possible. Does anyone know exactly how fast a mine cart needs to go for it to skip on magma?

* It looks like rollers can push them up to 50,000 speed. If they need to go faster than that to skip I suppose I will have to get creative.

[Trif]

Rollers will work just fine. If I interpret the numbers here correctly, your cart needs to be faster than 3000 to skip on magma, and a one-tile roller on highest setting gets you to 50000. (Or is it 3000 to move through magma? I'm not sure.) A 6-tile long roller should get you to the highest possible speed, so you can definitely do some good damage without exploits.

Building destroyers will target rollers, but won't target tracks or track stops.

[fricy]

Rollers will work just fine. If I interpret the numbers here correctly, your cart needs to be faster than 3000 to skip on magma, (Or is it 3000 to move through magma? I'm not sure.)

No, that's not right, 3000 speed is 33tick/tile which is VERY VERY slow. And cartspeed in fluids is weird, I have some minecart speed log dumps somewhere (measured with dfhack), and I couldn't really understand how they work...

and a one-tile roller on highest setting gets you to 50000.  A 6-tile long roller should get you to the highest possible speed, so you can definitely do some good damage without exploits.

Doesn't work like that: rollers do not accelerate, but set the speed of the cart to roller speed. So a one tile tile roller is the same as a 6 tile roller in the end, it will never accelerate past 50k speed, which is 2ticks/tile...in fact it's 49990, 10 speed below the speed needed to skip a corner. I'm not sure if it's the speed necessary to skip the cart on fluids too, but I'm sure it's not less than 50k. My guess would be 100k, I know that's a "trigger number" for some minecart quirks..

[Trif]

and a one-tile roller on highest setting gets you to 50000.  A 6-tile long roller should get you to the highest possible speed, so you can definitely do some good damage without exploits.

Doesn't work like that: rollers do not accelerate, but set the speed of the cart to roller speed. So a one tile tile roller is the same as a 6 tile roller in the end, it will never accelerate past 50k speed, which is 2ticks/tile...in fact it's 49990, 10 speed below the speed needed to skip a corner. I'm not sure if it's the speed necessary to skip the cart on fluids too, but I'm sure it's not less than 50k. My guess would be 100k, I know that's a "trigger number" for some minecart quirks..

Oh crap, you're right about the rollers... Wow, I never noticed that.

Still, I'll have to disagree on your speed estimate for skips. I did a little testing in arena mode, and it turns out that pushing a cart once is too slow, but pushing a cart twice is enough to make it skip once on 7/7 magma. Which means that the speed should be between 20k and 40k.

[iofhua]

Alright I spent a couple hours designing this in notepad. Hopefully it will work. I have a region save with an embark that has both a volcano and river on it, so I just need to funnel magma and water to the right Z-levels to set this up. It's a bit complicated but hopefully I won't need to use impulse ramps to do this.

This design focuses mostly on just generating magma mist. If the enemy is standing on the one tile in front of the entry door at just the right time they could get smacked with the minecart, but the mist is what's going to do most of the damage I think. I think I also might use upright spear traps to create a "dodge-me" trap on the exit path where they could dodge into a pit that has an exit back outside, forcing them to walk back through the magma mist trap if they dodge into the pit.

A ramp down should add almost 5000 speed to the minecart. A roller and 4 ramps down should take it to just under 70,000 speed, which I'm hoping is enough to skip it across two magma tiles. I put a roller on every Z level to make sure that the cart stays at maximum possible speed.

This will be the most complicated thing I've ever made in DF, assuming I can pull it off.

Spoiler (click to show/hide)

Magma Mist Trap

*** Collapsed View ***

   █.█                            . is a floor
   █.█                            ╤ is a roller (set to highest speed)
   █.█                           ▼▲ are ramps
╔╤═~═~═▼═▼═▼═▼╗               ╗╝═╚╔ are minecart tracks
║  █D█        ║                   D is a door (set to forbid passage)
║  █<█        ║                   < is a stairs up
║  ███        ║                   █ is a wall
╚▲═╤▲═╤▲═╤▲═╤═╝                   ~ is a channel with 7/7 magma
              
             
*** 2 Z-Level Below Trap ***
                   
 ☼                              ☼ is a gear assembly
 │                              │ is a horizontal axle
 │                  
 │                    
 │            
 ☼─☼──☼──☼──☼
 │                
 │               
(to waterwheel)    
                  
                        
*** 1 Z-Level Below Trap ***
                          
  (to magma source)
   ~                       
   ~                             ~ is 7/7 magma
  █~█                            █ is a wall
  █~███                          o is a vertical axle
 o█~~~█                          
  █████                                  
                                    
                                    
   o  o  o  o
                       
                          
*** Z-Level of Magma Mist Trap ***

   (entrance)                 
    .                 
   █.█                          ~ is a channel with 7/7 magma beneath
   █.█                          < is a stairs up
   █.█             
╔╤═~═~═▲          
║  █D█         
║  █<█        
║  ███          
╚▲█o  o  o  o
                  
               
*** 1 Z-Level Above Trap ***
              
       ▼═▲               
   █████████████                           > is a stairs down
   █>............. (exit path)                           
   █████████████          
 ▼═╤▲█o  o  o
            
              
*** 2 Z-Levels Above Trap ***
                                         
         ▼═▲             
                                  
                               
                               
    ▼═╤▲█o  o   
               
                   
*** 3 Z-Levels Above Trap ***
                         
           ▼═▲             
              
                    
                         
       ▼═╤▲█o  
                    
                      
*** 4 Z-Levels Above Trap ***                  
                                            
             ▼╗             
              ║                
              ║                  
           ███D█                
          ▼═╤═╝█   
          █.████                      δ is a lever (controls door)
           .δ                      
           .
          (maintenance access)

[Trif]

It's a good design! The only thing I'd change is to put some more space between the down ramps and the magma channel. Minecarts can just fly over 1-tile gaps at pretty much any speed, so you'll probably need to extend the channel to get magma mist.

[BoredVirulence]

I can't comment on skipping magma, but I can admit that rollers generally suck for minecart traps. I made a circular hallway with several max powered rollers. The problem wasn't lethality, but that the minecarts never had the momentum to continue to another roller after hitting something. So the trap had to be reset after any injury.

If you can power the minecart enough, it will be a good design, but I'm skeptical that you can do it with rollers alone.

[Lielac]

I can't comment on skipping magma, but I can admit that rollers generally suck for minecart traps. I made a circular hallway with several max powered rollers. The problem wasn't lethality, but that the minecarts never had the momentum to continue to another roller after hitting something. So the trap had to be reset after any injury.

If you can power the minecart enough, it will be a good design, but I'm skeptical that you can do it with rollers alone.

If you completely carpet the hallway in rollers it might work.

[fricy]

Just an FYI: with ~59000 speed I was able to skip 8 tiles of magma, but no magma mist was generated, the cart landed and lost all speed on tile 9. Now I'm pretty sure the minimum speed to skip over magma is 50k.

@Trif: You can push carts twice in arena mode? wow, I was not aware. Still 40k speed sounds too slow. hmm maybe I'll test it some other time..
Unfortunately I can't log the speed of carts in flight, only when they touch down. So no more science for today, I don't have the patience any more to count ticks.

[Lielac]

@BoredVirulence: I only put a rooster in, but my proof of concept seems to work. I'm going to put an aged grizzly bear in next.

Edit: The grizzly bear died even faster, and the minecart kept going just fine.

Edit2: In this small circle, minecarts come around fast enough to hit a dorf in the ass if said dorf is putting it back on the tracks. ... Oops.

[iofhua]

Well I can confirm that one roller set to highest speed is more than enough to skip a steel minecart across 3 tiles of 7/7 magma. In fact a setup like this:

0R~~~R0

0 is wall
R is roller
~ is magma

launched the minecart back and forth across the magma so quickly that the poor dwarf who dropped the minecart on the roller (it was on at the time) got pummeled into squishy red goo faster than I could blink. While I'm ecstatic that it's so easy to skip minecarts across magma, it doesn't generate any magma mist at all. That's what I wanted more than anything.

According to the Wiki, skipping minecarts should generate mist, and should do so independent of the minecart's weight. I'm guessing this is wrong or a bug - is this working as intended? I would love to build a magma mist generator with a minecart and some rollers.