These are my experiences with setting up Turrets for defensive purposes in "creative" (create without limits, safely) mode.
The Fundamental Problem
If you set up one turret and watch for a while, it becomes clear that one is not enough. The turret aims at the closest opponent, but others can get by while it is engaged. One turret does not have enough firepower to handle the typical groups of multiple astronauts, for example. The obvious answer is to set up multiple turrets so that they can dispatch each opponent more quickly, and thus handle multiple attackers more effectively.
Here is an extreme example of setting up multiple turrets:
These turrets will not actively try to fire at one another, but because of the way they are placed, as soon as they start firing at a target, there is no way to avoid some of them hitting each other:
As soon as a turret is hit by another turret, it starts to fire back. The grid of 16 turrets was reduced to six after killing just one enemy mob. After a couple more mobs there were just two turrets left.
A Wide Formation
The obvious way to avoid them killing each other is to place the turrets far apart.
I placed four of them at the corners of a square, about 30 blocks apart from one another. This was still too close together to avoid them hitting each other, and some mobs got through without being shot. After a few opponents had been killed, two turrets were also gone, and at least one Brown_Mob was shot, probably by a turret:
Having brown mobs explode close to the entrance to the base is not particularly desirable. Inasmuch as the brown mob is very low to the ground, it seemed to avoid being hit most of the time. There were no other brown mob explosions in my turret tests, so I suspect they were not getting shot.
The next thing I tried was to stack the turrets, so that they fire above or below one another when engaging the same opponent. This is done in two steps, like so:
First a structure is built, then the turrets are placed, then the un-needed blocks are drilled out.
While this arrangement works a bit better than the widely-spaced square, they still manage to shoot each other (somehow) and I was soon down to only two turrets.
It is also possible to get two turrets to occupy the same space. This example started like the first photo above. On the right, the block holding up the upper turret was drilled out, causing it to fall down to the same spot occupied by the lower turret:
This double-turret does not work at all. When two turrets occupy the same space, neither can fire without hitting the other, when then starts firing in defense, and soon one of them is killed.
Since the two double stacks were hitting each other, the next thing I tried was a triple stack:
- A triple turret takes on a green mob
This arrangement actually worked fairly well, inasmuch as the turrets never managed to hit one another. I left this arrangement running for about 12 hours, during which time they managed to kill a truly impressive number of astronauts:
- Disks litter the battlefield in all directions around the triple-turret
However, during that 12 hours turrets were occasionally killed by an astronaut which they had engaged. I started with 3 turrets, and during the 12 hour period there were 5 times that a turret was killed and had to be replaced. It was usually the center one, though I'm not sure there was a reason for that. 3+5=8, so I needed a total of 8 turrets to defend the area for 12 hours.
Supernova544 discovered the reason the middle turret gets killed (read the blog article User_blog:Supernova544/1_entity_turrents). To summarize, the top and middle ones are still too close together. The top one is aiming at a downwards angle and its shots hit the middle one. The angle isn't as bad lower down, so the middle and bottom turret are okay (as seen in the Scooter8pie image below).
I got equally impressive results from 3 turrets placed on a more normal way, at different places and different heights:
- Another impressive debris field of disks
These turrets had to be replaced occasionally, but were fairly efficient as the field of disks shows. Placing them above ground level is a particularly good way to keep them from shooting each other when they are trying to shoot astronauts.
I also noticed that turrets seem to ignore the small green mob: