Understanding Grenades

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This article aims to provide in-depth explanations on various aspects of how grenades behave and function in Enemy Unknown/UFO Defense and Terror From The Deep.

Blocked Throws

Sometimes you will get a message that you are unable to throw a grenade or that it is out of range even though there is no apparent obstruction to the target. This can cause some players immeasurable amounts of frustration. Some of the common causes for this include:

  • Low or hacked strength
  • Overhead obstructions

Strength controls how far you can throw a grenade. The higher the strength, the further you can throw. If a soldiers attempt to throw beyond his or her maximum range, you will get the message that it is out of range. Strength also controls how high the grenade is likely to be tossed when making long throws.

Overhead obstructions such as a ceiling, tree canopies or even the wing of the Skyranger provide the most common reason why an otherwise clear throw may get blocked. Even when it shouldn't, the game will sometimes think that the grenade will intersect with the ceiling immediately above and stop the throw. Very high or abnormally high (hacked) levels of strength can worsen this effect as it increases how high a grenade gets thrown for longer throws. Solution for this include manoeuvring your soldier away from the ceiling for a clearer throw, shorten the throw distance, or have a weaker soldier attempt the throw.

The entire game map is also by an invisible ceiling above the top-most layer. This invisible ceiling will sometimes prevent long range throws that would otherwise be quite legal. The only solution to this is to shorten the throw.

The Grenade Timer

All grenades are primed by setting their timer to a setting between 0 and 23. However, what do these numbers mean? It is easy and almost natural to assume that these numbers refer to the game turns. Yes, but not quite.

The confusion stems from the fact that each grenade timer delay does not correspond to the number of turns that you see during the alien movement phase. Each number in fact represents half a turn.

Throughout the battle, the game will have two counters that keep track of the number of turns taken in the battle. The main turn counter, the one that you see between move phases, is made up of two smaller turns. Your turn and the aliens turn. When the game reports that you are starting on turn 6 for example, each side will have taken 6 turns, making 12 turns in total. Grenades and their timer operate between these exchanges of turns between the player and the aliens.

As the turn order is always player first then the alien, even numbered turns represent the player and odd numbered turns represent the aliens. Observe the following delay settings:

0 = Explodes as soon as you end the turn, before the aliens move
1 = Explodes after the aliens end their turn
2 = Explodes at the end of your next turn
3 = Explodes at the end of the aliens' next turn
and so on.

Note that automatic doors are always closed just before you start your turn but after the aliens have ended their turn.

In general, 0 is the most common setting used as it provides a more immediate result. 1 is tactically advantageous if you are able to accurately predict where the aliens will be at the end of their turn. Higher settings are often used for demolition work when you want to give your sapper some time to retreat after placing the explosive.

In order for a grenade to then explode, it will need to fulfil both of the following conditions:

  1. The grenade counter has run out
  2. The grenade is on the ground when the turn ends

If either of these two conditions are not met, the grenade will not explode.

Except in very buggy circumstances, a grenade will never explode during a movement phase. Only between them, before either side gets control of their units.

Because of the strict detonation conditions, you can safely hold and carry a grenade even though it's well past its timer setting.

This allows you to arm grenades several turns in advance to avoid the 50% Time Unit arming cost when the grenade is needed the most. As long as the grenades are in your inventory, they will be safe to carry. (Unless the soldier carrying them is killed or rendered unconscious - then the grenades end up on the ground and may explode if the counter has also run out).

How The Timer is Implemented

Ever wondered why the delay settings for the timer begin at 0?

Rather than being assigned a number of turn and then counting down after each movement phase, a grenade is instead assigned a scheduled detonation time. This is a number that equals the current movement phase count plus the delay setting you selected.

If the current movement phase count is greater than this number, the first condition for the grenade to explode will be met.

For example, it's the 120th movement phase (that's 60 turns into the game). A soldier primes a grenade with a delay of 1. The grenade will then be set to detonate on turn 121.

When the turn ends and the aliens take over, the turn counter increments to 121. This is equal to the grenade detonation time. That's fine for now. As soon as the aliens end their turn, the movement phase counter jumps to 122. This is greater than 121, and thus the grenade's first detonation condition is met. If the grenade was on the ground, it will also explode.

If the grenade was held in the hand or inventory past this point, it will not explode. If it lands on the ground, regardless of which side is currently making the move, it will immediately explode as soon as the turn ends.

Experience Attribution

In order for the game to properly reward experience to soldiers who've harmed enemies with grenades, grenades carry an ownership flag to identify who to credit the experience to. Unfortunately this flag is only set when the grenade is thrown. It is never set when the grenade is picked up or dropped.

When the battle starts, all grenades will have their ownership flag default to 0. This is also the same reference to the very first unit on the map. (Unit 0 - in unitpos.dat terms). This is either the soldier that starts on the equipment pile, or a tank if one is on the ship.

For example, if soldier A throws a grenade to soldier B, who then picks it up, arms it, drops it by an enemy and then runs away, any experience earned from the attack will be awarded to soldier A, not soldier B.

This unusual behaviour is clearly a bug. To avoid it, always have the soldier throw the grenade.


Proximity grenade are even more unusual and award experience to the person that sets it off. The mine layer will therefore not earn any experience produced by using the mine. At least, without stepping on it.

Item Destruction

In Enemy Unknown/UFO Defense, if an armed grenade is destroyed by another explosion before it can go off, the grenade will no longer exist, thus negating the pending explosion. A stack of armed grenades will therefore only explode once.

In TFTD, grenades are given higher damage threshold that give them better protection against HE damage. Most grenades are indestructible to explosions of their own type while others like the Sonic Pulser have a negative damage threshold, making them effectively indestructible.

For more information on how explosives can destroy objects in general, refer to Object Destruction. Refer also to TFTD's item destruction table for TFTD.

Proximity Mines

Due to the odd behaviour of the 20 Armed Proximity Mine flags that govern whether a proximity mine is live or otherwise, a mine that gets destroyed will not clear their armed flag. This has the possibility of passing the armed status onto another object during the battle.

If this flag is not cleared, be extra wary as any item in the battlescape that is on the ground could explode on your approach, with the explosive force of the item's strength (where applicable). The most hilarious consequence of this I have ever seen is a dead X-com agent simulating a popper and blowing up next to an aquatoid.

In TFTD, the mine armor levels are increased a little, so most light explosions should not destroy the mine too easily.

Article TODOs

  • Section on strategies to briefly mention pre-arming, Dead Man's Trigger, grenade relay, etc. Passing the experience trick. Then linking to the right section.
  • Strength, weight, the throwing arc and an invisible ceiling.
  • Make a separate companion article called Understanding Proximity Mines
  • Cover a few other items mentioned in the Talk pages.
  • etc.