Turn 21 Theory

[Sylph]

I haven't looked into the code too far, but I've conducted some testing after noticing this same turn21 theory years ago.

I basically got a savegame after clearing the geoscape outside the ufo (tested on a large scout and a supply ship). Every turn, I saved the game, then ran my camping force into the ufo to see where the aliens were.

 

Most of the time, the aliens were hanging around near the door, even on turn 8 or so. They'd stay in this area (and presumably spread around the rest of the ufo too) until turn 21, at which point they'd start leaving the door.

 

I think worrying about whether AI has to path it's way out of the top/back of a ufo, and it taking 21 turns 'on average' for them to do this, would be a mistake. There's no doubt that the aliens will sit around, even right next to that door, until turn 21 when they start pouring out.

 

I'm sure the reason they trickle out one or two per turn has more to do with the fact that when an alien pops out and gets shot by the camping force, the rest of the aliens, knowing that there's a team camping their door, wait a turn before sending another alien to peek his head out.

 

 

Also, the turn 21 issue happens in alien bases too. Try invading an alien base and block the lift (remember aliens don't fire up those lifts, even with blaster launchers, so you're safe if there's no mind control.) At turn 21 there is a *very* noticeable increase in the amount of aliens that rush to the bottom of the lift, suggesting that they run for your soldiers at turn 21. In fact, most of the time if I sit on the starting lift I don't see any aliens until turn 21, and in the next 2-3 turns I get a good 5 at the bottom of each lift.

[NKF]

Just to throw one experience of mine into the lot:

 

Occasionally when using a Lightning, I like to stay inside the ship for a while and take potshots at any enemies nearby and only disembark when the area around the Lightning is safe. This has something to do with the strategy I used to use when I was playing TFTD with its ships that have hatches. Very effective.

 

One time though, I was playing a grenade-heavy squad and spent quite a lot of time arming all of their grenades. I might've passed turn 21 by then, but wasn't really paying any attention. Usually if I wait this long, the aliens approaching the Lightning will have abused the gaps in the walls to shoot through, but I was lucky they didn't. Finally when I had one soldier (the one "blocking" the official entrance) step out onto the ramp - there was a horde of floaters and reapers surrounding the ramp.

 

I notice this happening a lot in base missions where you plug a lift.

 

Perhaps the AI gets impatient after so many turns and just sets all of its units (the ones that aren't stuck for any particular reason) out to hunt the nearest X-Com soldier. This opposite seems to happen in TFTD though where the aliens just seem to stop moving, often putting you in the Last Alien Syndrome - which I must say isn't as common in UFO although it can and still does happen.

 

By the way, just to clarify, when we say turn 21, that's the turn counter you see after each side ends their turn, right? Not the actual turns that are twice that value.

 

- NKF

[kennyc]

yup, thats what i meant NKF, i can only assume thats what the others thought. i think its intentional game design to accomodate players who make use of reaction fire etc. even with 6 good reaction soldiers (60+each) you are likely to lose at least one in the gun battle that follows turn 21. i like it!

[Sylph]

To take advantage of reaction shooting to it's max I usually use the following strategy:

1. Use a squad consiting of 7 soldiers with guns, and 7 scouts with smoke grenades, flares, and stun launchers.

2. Move out scouts and smoke area around my craft

3. Explore landscape outside UFO and clear it

4. Camp to the side of the UFO door with all shooters (half sitting down so that the other half can fire over their shoulders). I take up a position so that an alien would have to turn around after leaving the UFO door in order to see them.

5. Stick some scouts in front of the UFO such that the aliens would not have to turn to see them

6. Wait

 

Once turn 21+ come around, the aliens start to leave the UFO. They shoot and kill the scout that stands outside, but after firing they get reaction-shot by the shooters. Sure, the scouting rookies die, but you have to expect some deaths on superhuman if you don't save/load in missions.

It's very rare that I lose one of my valuable soliders, and usually at about turn 25-30 aliens start to panic to such an extent that half the time aliens that remain 'stuck' in the ufo don't have any weapons when I come to find them.

 

You can get more success by smoking the UFO doors on turn 19, then sitting your firers out of visual range. The scouts hide around a corner from the ufo door, then each turn the scout pops out to have a look. If he sees an alien, he runs back behind cover, and the shooters all fire. The trouble is your soldiers don't get reactions training from this, which, in the long run, hurts more than losing and replacing a few rookies every mission.

[kennyc]

sounds like some good strategy there sylph, ive been experimenting with various set ups for battleships. ill take a screen and show you a configuration i found particularly useful for cleaning up aliens as they leave at turn 21.

[Sylph]

Battleships? I'd be interested in hearing how you deal with those. I leave them alone, because of the blaster launchers inside. I've never found a reliable way of defensing against blaster launchers until I'm using them, or psychic powers, myself...

[NKF]

The best way to get good at assaulting battleships is to just practice. Next time you find a battleship on the ground that you can easily capture, save the game and run a series of practice missions until it no longer seems daunting. Well, it will probably alway seem daunting, but there's no sense in avoiding them forever. They're generally easier than bases (having the same crew make-up), as there's more open space (making it easier to spot the BB carriers) and you don't necessarily have to venture inside the UFO to beat the missions.

 

The blaster bombs are difficult, but I've found that you can still effectively siege the battleship by having your troops spread out and cover the left, middle and right access to the lift. Space one soldier out to each side of the legs, and if you want a third (through each access), have them sit back in between the Battleship legs. This at least minimises the extent of the possible casualties you might suffer. Having a soldier or two hide in the blind spot of the lift can be handy for lifting off equipment or killing a distracted enemy at point blank range (such as the aforementioned alien with the BB) if they're right inside the door.

 

 

As for the BBs in general - you always have to rely on a bit of luck (the CE edition helps a lot here) and you must be more active in rooting out and killing the units carrying the BB's. Do that by agressively sweeping through the rest of the map for any alien with a BB that just might happen to have wandered in to the field. Use high explosives and rockets liberally to clear the terrain if that helps. Once found, focus all fire on the alien until it is down - or rush up to it and cross your fingers that it hasn't picked a target further away.

 

---

 

Regarding your UFO exit ambush, do you really have to sacrifice the rookie? I find that simply setting up a spaced out broken semi circle around the exit(s) to be more than sufficient. See, you are going to risk losing a soldier (or soldiers) either way you do it. However you don't want to just give the aliens a guaranteed chance at killing a soldier. Besides, it's much more efficient to have that rookie's firepower bolster your already impressive firing team - the more the merrier, after all.

 

The rookie (or tank) is only really needed to lure the aliens out if they get stuck and don't move as you'd expect them to. Even then, you only need to have the rookie act as bait briefly and then flee to safety (or get back in place in the firing team) before the aliens break through the door. Otherwise once you pass turn 21, the aliens will generally leave on their own and right into your ambush.

 

 

- NKF

[Sylph]

Easier than assaulting a base? Heh, I usually leave bases alone until I have psychic troopers, at which point it's not hard to clear all the blaster launching aliens on the first turn.

My usual tactic for base missions is to get psychic power, assault a base, steal a blaster launcher, then dust off immediattely. i then research blasters myself.

After getting blasters, I go back to the base, scout using hovertanks and mind control, kill the base controls with my own blaster launchers (usually on the first turn) then dust off, killing the base and getting me all the equipment intact.

As you can probably guess, I'm so scared of blaster launchers that I try my very hardest never to give the aliens a chance to use them!

Regarding your UFO exit ambush, do you really have to sacrifice the rookie? I find that simply setting up a spaced out broken semi circle around the exit(s) to be more than sufficient.

If you have troopers with the reactions to shoot the aliens, then my strategy will kill the alien before he kills my scout, just like yours.

If the reactions aren't in your favour, then my strategy works pretty much the same as the semi-circle, except that I gaurantee that it's only a rookie that dies, rather than risking the death of an experienced member of the team.

On superhuman difficulty, you can pretty much gaurantee that the aliens will be shooting at least once before you get a chance to have a reaction shot, so the rookie decoy is pretty necessary for me.

The main disadvantage to a firing team like mine would be alien grenades, but since when the alien leaves the UFO he only sees the rookie, the firing team isn't in danger to getting bombed. I assume that even with the spread out semi-circle a blaster bomb will still kill almost all of them...

[NKF]

Bases are mainly harder because of the tunnels. But I suppose the still have they surgical strike option for a quick win (snakeman base anyone?). A battleship mission, is easier because the aliens will mostly be concentrated inside the ship, and those outside don't have too many places where they can hide (for long).

 

If you have troopers with the reactions to shoot the aliens, then my strategy will kill the alien before he kills my scout, just like yours.

 

It's not the first alien I'm worried about. It's the aliens behind the one that just stepped out that bothers me. Your shooters will not be able to save the rookie from those ones.

 

 

 

If the reactions aren't in your favour, then my strategy works pretty much the same as the semi-circle, except that I gaurantee that it's only a rookie that dies, rather than risking the death of an experienced member of the team.

 

The semi-circle I like to use works with any level of reactions - it's about numbers. More troops, more fast snapshot weapons -- like the laser pistol and laser rifle -- equate to many more chances to get off attacks of opportunity than the alien has to fire back. Very good veterans can mix in some sure-hit snapshot weapons like the plasma rifle.

 

It also helps that the shooters are made up of a mix of veterans and rookies. The veterans get off the opening shots (hopefully - and crippling the alien would be a bonus), the alien shoots at the nearest rookie (cross your finger that it misses), then the other rookies join in now that the alien's initiative has dropped. As a bonus, any rookies that meet the condition to enter a reaction fire queue, but don't get to fire because their buddies manage to kill the alien first will still reap the experience as if they had reacted.

 

I assume that even with the spread out semi-circle a blaster bomb will still kill almost all of them...

 

That's true for any explosive. On the bright side, the broken semi-circle cannot work on Battleships or small scouts! It works with any other UFO though if placement allows. It doesn't work on the Battleship on account of its struts, and the small scout... well, I suppose it would be pointless. In any case, it's free from the wrath of blaster bombs.

 

But that still leaves the alien grenade. For them, my solution is to vary the positions of the soldiers so that their placement is pretty much random - hence why I called it a broken semi circle. Some fly, some kneel up close, some get postioned well back. Doesn't particularly matter where the are as long as they are within visual of the door landing, and out of the immediate view of any alien leaving the ship. With such erratic distances from each other, the damage from an alien grenade is adequately minimised. Later on, you can even just have everyone fly at different elevations to nerf the alien grenade for good.

 

That leaves the small launcher as the only area effect weapon that'll bother you. Then again, flying suits do resist stun, so all in all it works out well.

 

I just feel that other means should be tried before deliberately sacrificing a rookie. I mean, you'll start to run out of rookies before long! (in the mission, I mean)

 

- NKF

[Sylph]

Well, remember that turning on the spot doesn't promote reactions fire. The aliens can leave their UFO then turn to face your men and still get the first shot.

 

In my strategy, if the alien's reactions are poor, then my rookie will be able to kill the alien just like they can in yours. It's also worth noting that I normall have 7 or 8 veteran soldiers looking right at that door, so it's not common for them all to fire.

 

The two strategies are functionally identical, particularly if you have been putting the rookies slightly closer than the veterans in your broken semi circle. I'd just imagine it's harder to get the rookies closer without blocking LOs if your shooters are spread out in a semi circle.

[Knan]

Always fun when this backfires... a veteran reacts first, hits a rookie in the back of the head, then in turn gets shot by the alien.

[NKF]

Well, you can't really avoid the alien turning - but see with the rookie, anyone will have a fair shot at the first alien but only the rookie has to deal with the second alien (or more - depending on how crowded they are back there). Without the rookie anyone could get killed on either encounter, but there are more soldiers who could potentially retaliate or open fire on the second alien. The difference is 1 vs. many or many vs. 1 for the second encounter. Or another way, losses could go from 0 - 1 and an almost guaranteed 1 against 0 - 1 to 0 - 1.

 

Line of fire shouldn't be a problem if the UFO door's not almost flush against the edge of the map. Get close, but not too close. You've got roughly 20 tiles to work with from the landing just outside the door, so there's plenty of room to space out 8 - 10 soldiers. If done right you'll never have to worry about friendly fire from the opposite side of the field or getting shot in the back. Getting everyone to kneel (even when in the air) helps, but that doesn't discount the really freak mis-fires that result in the soldier having to turn to shoot the soldier beside them.

 

- NKF

[Sylph]

As an example of my spacing, just to make it more clear:

 

	 
		----------=------
 XX
 XX
 XX
		   x

 

The second alien can kill the scout from time to time, although often they leave the ufo to do it just like the first did. I suppose it boils down to whether you feel you can sacrifice a scout in order to gaurantee the safety of your veterans.

I've gotten used to losing 5+ scouts every mission, and my score is still very high at the end of each month, so I'm ok with it. If you were going for perfect mission scores, or perhaps couldn't afford the cash on new recuits, then yes, the broken semi-circle would be better.

[Sylph]

A possible solution to this question crossed me today -

 

From the 'smoke' entry in ufopaedia:

 

"It takes 15-20 turns for the cloud from a smoke grenade to clear"

 

I'm wondering whether, upon mission start, the smoke from a crashed ufo is preventing the alien AI from leaving their craft, and once the smoke has dissipated, the alien AI starts to work properly?

[Zombie]

Nice thought, but this happens whether the UFO was forced down and crashed or it landed on it's own (therefore without smoke). It's gotta be something else.

 

- Zombie

[NKF]

I recall we had a discussion recently on psi and the grace period you get before the attacks start, I believe it was mentioned (by Seb76 if I'm not mistaken) that the 20 turns is hard coded.

 

Perhaps it influences more than just the psi grace period?

 

- NKF

[Zombie]

That seems like more than coincidence, so if Psi attacks are coded to begin 20 turns in, most likely the aliens will start to come out of their craft after 20 turns too. It's possible.

 

In those Psi tests I ran a while ago, the attacks didn't always start at turn 20 though. It depended on the Psionic stats of the target unit. The lower the stats, the faster the attacks started. 20 must be a "base number" of turns which is modified up or down by stats.

 

- Zombie

[Bomb Bloke]

Twenty turns? Usually they start just one or two turns in, assuming you're lucky enough to get any "grace period" at all.

[JellyfishGreen]

After reading the Psi pages on the wiki I get the impression that if you get spotted, the psi attacks can begin; if you somehow manage to break or avoid contact, your location is revealed anyways after turn 20, and thus the psi attacks can begin.

 

I couldn't find any collection of this "Turn 20/Turn 21" info on the wiki though, and it seems a very important topic if anyone can stick it in there.

 

... First post of the year. Yes, I'm still alive, and so's the toddler. But busy. And lost in Dwarf Fortress.

[Zombie]

Nice to see you again JFG, how old is the little one now? 2? 3?

 

I seem to recall the turn 20 thing is found on a talk page on the Wiki somewhere... can't recall the exact place though. In any event, you are right, this info should belong on a main article page. I'll just dump a recent post of mine at the Xcomufo.com forums over here as I'm too lazy to rewrite the points from memory:

 

Here's what happens: at the beginning of a mission (turn 0), the aliens know the stats and locations of all your troops. (I just tested this again to be sure: edited 10 Sectoid Leaders and 10 Commanders to be on a Large Scout, then edited my soldiers to have 10 Bravery and 0 for Psi Strength and Psi Skill. On the aliens first turn, half my troops were under control by them). The aliens have an intelligence stat which allows them to remember the stats and locations of your troops, and the higher the number the longer they remember. For aliens, that number is between 2 and 8 (which translates to a 2-8 round memory). After that time period is up and the alien doesn't see any of your units, it's memory is wiped clean and is basically blind and has to search for more targets.

 

This changes when one of the aforementioned conditions happens. If you kill all the aliens except for two (or one, it doesn't matter) then the aliens are granted the omnipotent power to see the stats and locations of your troops again (hence why I said "re-reveal" in my previous post). Or, if it hits turn 20 and there are still aliens left on the map, they get a free look around. In either case, the aliens use their intelligence stat yet again to remember what was shown to them. After this time period is up, the aliens will be totally blind because there are no more "free-looks" coded into the executable. So if you can last this long and remain hidden, the remaining aliens are going to have to search for you which may take a while.

 

That's what I believe happens. It needs to be tested though, so if I get some time tonight or this weekend I'll do a little more editing.

I don't recall what needs to be tested though. I think I wanted to edit the intelligence stat of the aliens to determine behavior after turn 20. But the pre-turn-20 stuff is pretty sound. Seb76 found that coded into the executable.

 

- Zombie

[Bomb Bloke]

Indeed, long time no see, JFG.

 

Anyway, this is also relevant to the alien intelligence/memory issue.

 

Basically if you see an alien, that counts as them seeing you.

 

But yeah, I can't seem to find the contents of this thread in the wiki anywhere.

[JellyfishGreen]

Added! under Alien Inventory Behavior which is the closest we have to an AI page, and mentioned already on the Psionics page.

[Timbukto]

Do you think there may be some relation as far as being targeted by blaster bombs as well? That by turn 21, they know your location and thus can fire at will if any blaster bombers still exist by that turn? I also am very curious to know how Aliens can know your location. Blaster bombs are obviously similar to PSI in that the firing unit does not need to see your units, and potentially, line of sight does not need to be maintained by the alien for a blaster bomb firing as well?

[NKF]

With firearms, the primary concern is with visual confirmation - they'll never (actively) shoot someone beyond their 20 tile range. That's plenty of room even for the blaster and its mighty blast radius.

 

Of course, the AI might treat the blaster as a special case. Might be a good time to play with the good old rooted alien (energy recharge set to 0) out in the middle of a field game to test this.

 

- NKF

[Triv]

This 21-turn theory doesn't seem to work too well for me. :\

 

My biggest problem during crashed-ufo missions is flushing out the aliens within said ufo. My last mission, I cleared everything outside the ufo. Camped out a bit outside the doors and took down 2 or 3 who decided to come outside and investigate. Then there were 3 left inside. 2 of which would literally NOT move. I think they were soldiers but I'm not sure. Either way, one of my men had to die no matter what because they would not stop camping the second doorway in the ufo. Smoke grenade didn't help, either. The last alien, a leader, was upstairs.. well, leading, I would assume, so that's acceptable (and predictable) for him to stay up there throughout the mission.

 

You know, a simple ability to open a door while not standing directly in the doorway would solve all these ufo close-quarter combat gripes.