Ok, so i'm currently writing something and a question popped into my head as one of the character's threw a firebomb at a bunch of enemies with full plate armor...and questions popped in my head.

how effective would it actually be? take the dark souls fire bombs or bombs from the game tenchu, how effective would it be against full plate armor, and how good is flame against armor in general, for both when the bomb hits the floor and when it breaks on say their breastplate.

do they have to be coated in oil and would it cause any harm to their skin if say their metal boots caught fire because of oil?

youtube.com/watch?v=zhZB3MI7OIM ( go to 2:33 to see bomb being used) heres a link to a dark souls bomb, its like a small grenade (medieval looking though) the tenchu bombs are like the thunder crash bombs in historical japan like 12-13th century i believe, en.m.wikipedia.org/wiki/Thunder_crash_bomb

I'm new to the site, so i'm not good with tags yet but no fantasy armors or resistance.

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    $\begingroup$ I am not familiar with dark souls or tenchu. How do their bombs work? Are they Molotov cocktails - hurled bottles of flammable liquid? $\endgroup$
    – Willk
    Feb 5, 2022 at 3:04
  • $\begingroup$ It vastly depends on what is it that you call a firebomb. For example, plate armor won't much more protective than paper tissue against a CBU-55 340 kg fuel-air cluster bomb. So, please clarify what a firebomb is supposed to be in your story. $\endgroup$
    – AlexP
    Feb 5, 2022 at 3:05
  • $\begingroup$ youtube.com/watch?v=zhZB3MI7OIM heres a link to a dark souls bomb, its like a small grenade (medieval looking though) the tenchu bombs are like the thunder crash bombs in historical japan like 12-13th century i believe, en.m.wikipedia.org/wiki/Thunder_crash_bomb $\endgroup$ Feb 5, 2022 at 3:35
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    $\begingroup$ You should add the description to the question. Comments are ephemeral. (And the YouTube link is to a cartoon. The effects of the bomb can be anything you like -- it can pop up parrot carrying a sign saying BOOM, it can level a city. It's a cartoon, anything can happen at any time.) $\endgroup$
    – AlexP
    Feb 5, 2022 at 3:54

2 Answers 2


The short answer: the person in the armor is more likely to survive the bomb then the unarmored one, but the results may differ due to the handcrafted nature of both armor and bombs.

Now, coming to the details. First thing, I need to note that the bomb you link to is not a fire bomb, but a primitive grenade. It depends not on the fire for the effect, but on the blast wave from the explosion and damage from casing fragments.

The most likely fuse mechanism is a timed fuse - i.e., there is something that burns with more or less predictable speed, until it reaches the explosive (most likely, black powder) at the core. Black powder starts burning too, and since it is enclosed, the resulting gases expand and break the cast iron shell.

So, answering your question about what will happen, when the grenade hits the armor is this - it bounces and rolls, and continues rolling, until the fuse reaches the core. Then it explodes. In theory, you may rig some chemical impact fuse, but it would be more complicated, expensive and less reliable.

Now coming to the effects of the shrapnel on the armor. I wasn't able to find any data on the kinetic energy of the black powder IEDs. The data on the black powder firearms is interesting as well. Wikipedia gives absurdely huge numbers like 1700 Joules to the projectiles of early arquebuses. This paper here (https://www.researchgate.net/publication/321442172_ANALYSIS_OF_BALLISTIC_CHARACTERISTICS_OF_16th_CENTURY_ARQUEBUSES_USED_IN_BATTLE_OF_PAVIA) gives much more believable 830J (and decreasing with distance) for 30g lead ball, and explores it's effect on the armor contemporary to it.

Now, your grenade doesn't fire lead balls, it fires cast iron fragments - lighter for the same size, having less initial energy, since they are not fired through the barrel, having less aerodynamic shape, so loosing the speed and energy more rapidly. I would estimate their energy at about 100-200 J, comparable to the muscle-powered weapons (I'm absolutely winging it with numbers by this point).

The average medieval armor is also not uniform thickens, it is thicker at the more expected directions of impact - from the top and from the front.

So, what happens when the grenade explodes near the guy in armor very much depends on how near it is and from which side. If he is unlucky enough to be directly on top, he is toast. The blast wave and the shrapnel shred him to pieces.

Inside the ~10m radius the fragments may or may not penetrate, depending on where they hit. Less so from the front (unless he is unlucky enough to get one in the eyeslit), more so from the back. Outside that radius the grenade would, most likely, have no effect on the armored targets.

There are also blast wave effects to calculate, on addition to the shrapnel damage, but I do not have enough data for it.


Two words: Molotov cocktail

An improvised weapon used to kill tanks. I think a tank is more plate armor than your guys are wearing.


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