Do Fireballs generate explosive force?

Question

My reading of the rules concerning Fireball and Delayed Blast Fireball in 5e work it out to be something along the lines of "the caster picks a point in space, and when the spell goes off (or after some delay/trigger in the case of Delayed Blast Fireball) suddenly there's magical fire in a 20 ft. radius around that point."

Notwithstanding the pitfalls of questioning the physical properties of magical effects (personally I like systems where everything makes a reasonable amount of sense), I wonder about how this would interact with small, confined spaces. Supposed a Delayed Blast Fireball was placed in a very small metal case, which was subsequently welded/sealed shut. What would happen when the spell is triggered?

I can think of three reasonable options at first:

1. We've created a grenade (albeit an extremely unstable one). This requires that force/pressure was generated by the Fireball, which creates implications for other applications.
2. The vessel contains the explosion, and since no force or heat was generated (only "magic fire"), the vessel is afterwards unharmed.
3. The vessel isn't blown apart (no force), but depending on its makeup might melt from the focused heat of the Fireball, leading perhaps to partial leakage.

Which scenario would apply? Or would something completely different happen?

Answer 1

By RAW, the container would take the fireball damage, and if the container is destroyed, the fireball would expand to fill it's normal area of damage (justification provided at bottom).

If the fireball failed to destroy the container, it would be contained. So, by RAW alone in that respect, you have successfully captured the fireball spell and prevented it from doing damage. This is extremely unlikely at best, and I'll demonstrate why below.

I'm going to assume the box you used is iron for my example.

Source: DMG pg. 246-247 - Objects

Using the table found on pages 246 and 247, we can see a resilient, small iron box has 19 AC and somewhere between 3 and 18 HP (3d6). So we'll assume maximum HP for this scenario. The important bit comes under the sub-section labelled Objects and Damage Types. Is your iron box immune to fire? If not, then it's going to take damage.

So let's apply the lowest Delayed Blast Fireball we can in this scenario.

Based damage = 12d6. You've put it in the box, so the minimum delay is going to be at least 1 turn, so add 1d6.

On top of that, the spell states it damages objects, and in order to halve the damage you need to make a saving throw.

The iron box does not have a save, so it takes 13d6 damage at a minimum.

This means you've dealt between 13 and 78 damage to the iron box. Now, unless you have incredibly unlucky rolls, it's been destroyed, and the fireball proceeds to fill the remaining area in a 20 foot radius.

As for why the fireball expands, it's because if the object is destroyed it did not impede the fireball. Consider that if all that was required to block a fireball was any object, all you would have to do to become virtually immune to most evocation spells is hold a paper thin table top in front of you wherever you go.

Answer 2

RAW, spells do what say they do and nothing more. Fireballs constrained by the space just fill the space, et cetera. There's no "blast" or whatever. It could potentially destroy the container it's in, but again, RAW, the area of effect of the spell was set when it was cast, there's no spill-over if the container is breached.

In your game, you can instead engage in the beloved pastime of pseudo-physics-arguing, but there's no right answer to the question in that case as it depends on how you're going to rule, so we can't give you an effective answer here for that case.

Answer 3

Allow me to briefly digress on "historical" fireballs. (Back in my day...!)

I had a very amusing afternoon, back in the 1E days, playing out the volume of a fireball. In the early editions, fireballs were specifically called out to conform to whatever space was available, and the spell said it fill about 33,000 cubic feet of volume, or about 33 10'x10'x10' "standard dungeon blocks".

Turns out, in those editions, semi-contained indoor fireballs are a really terrible idea. Alternately, semi-contained indoor fireballs are "all kinds of hilarious fun for your sadistic DM."

In older versions, the spell description also specifically calls out that "The explosion creates almost no pressure."

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Nostalgia aside, in 5E, the mechanic is described significantly differently. Rather than discussing how the ball of flame affects people, it simply states that anyone within 20' must make a save, and that the fire will go around corners.

From this description, it looks like option #3 is the closest.

As another answer suggests, the best solution would be to simply calculate the structure of the container, based on whatever thickness of material was used to attempt to contain the fireball.

If the fireball burns/melts through the container, then it simply expands to fill its original 20' radius, and proceeds as normal.

If the fireball does not break through the container, then it simply rages futilely within its walls for a brief moment, and then goes out.

The conundrum would now be how to "get the fireball into the box". Since there is no delay between reaching its target and exploding, you can't build the box around it. The 5E fireball lacks any statement on what happens if it runs into something before its planned termination point. However, on p.205 of the PHB, under A Clear Path to the Target, "If you place an area of effect at a point that you can't see and an obstruction, such a a wall [or a box, in our case], is between you and that point, the point of origin comes into being on the near side of that obstruction." (Thanks to SevenSidedDie for pointing this out.)

The delayed blast fireball eliminates the problem of getting the fireball into the box (if the rest of the party can assemble said box in under 1 minute, the concentration length of the delayed blast fireball). After that, everything would proceed the same as with a regular fireball, save that the box has far less chance vs. the increased damage of the delayed blast fireball.

Answer 4

This was addressed in 2e in an issue of the Dragon Magazine, in bygone eras.

I apologize for not having the issue information. It was a long time ago.

I do not remember the explanation for a sealed box. However, in confined spaces, such as halfways, the blocked areas forced the fire down the hallway until the same are was dispelled, and as long as the fire could move, it moved in that direction until the blocked area equivalent was dispersed. For simplification, once it moved down an escape path, at any breach, it tried to make a circle of damage.

The article explained it was not an explosive force but rather fire. So, it would set burnable things on fire that it touched, but did not blow down walls, doors, etc.

Now, heat does cause air to expand, so if the pressure created by the expanding air has enough energy, it could blow up a sealed box. But, it would more likely be like the fire coming up the elevator in Die Hard (even though that was a C4 explosion and should have destroyed the elevator walls).

Another way to consider it is a flamethrower. If the tank gets pierced, it can explode, because the tank is under pressure, but the flame coming out the front has no force, other than the fuel coming out under pressure...a flame thrower sets a pillbox on fire, but doesn't, on its own, cause an explosion.

A force explosion does not explode because of fire, it explodes because the chemical makeup expands quickly, and the most common chemical used to create pressure, which reacts to heat/fire.