What effect does reducing a Spelljamming Vessel to 0 hp have?

Question

For sea-going vessels, Ghosts of Saltmarsh says that (186):

A ship component is destroyed and becomes unusable when it drops to 0 hit points.

So, for example, if the 'movement component' was reduced to 0 hp, the ship would be unable to move. Further (186):

A ship is wrecked if its hull is destroyed.

Of infernal war machines, Baldur's Gate: Descent into Avernus says that (216):

When an infernal war machine drops to 0 hit points, it ceases to function and is damaged beyond repair, and any souls trapped in the vehicle's furnace are released to the afterlife

However, I have been unable to find any similar statement of consequences for spelljamming vessels while looking through the Astral Adventurer's Guide for Spelljammer: Adventure in Space. I can see that Spelljamming vessels have hp and damage thresholds, and I can see that they can be damaged by PC's, by ship-to-ship weapons, and by Crashing. Various means are given to repair them. However, I am not finding any reference to what the general consequences are of a vessel reaching 0 hp.

Is such a ship unable to move? Does it stop moving immediately, or continue in motion on its last speed and course? Does it lose structural integrity and start to come apart? Can it be repaired from a state of 0hp, or is it permanently destroyed? What happens if it is at 0hp and continues to take damage?

There are some clues in a few places in the text, but it is unclear whether these are narrative flourishes or rules (and if they are rules, whether they are general to all spelljamming ships). For example, the Turtle Ship says:

If the ship takes damage while on or under the water, interior hatches can be sealed to prevent the ship from flooding completely.

Would taking damage once the Turtle Ship was at 0hp mean that the interior hatches were overwhelmed, such that it would flood and then sink?

The Living Ship says:

This ship’s main distinctive feature is the fully grown treant on the aft deck. The treant has a speed of 0 because its roots are woven into the deck; it and the ship are inseparable. If the ship is reduced to 0 hit points, the treant dies of shock.

Here is a specific consequence of a ship being reduced to 0hp, but it seems to be restricted to this one type of ship.

In the Light of Xaryxis adventure,

at the end of chapter 4 and beginning of chapter 5,

the players might be stranded when their ship is "disabled or destroyed". "Disabled" clearly refers to their spelljamming helm being rendered inoperational so that they cannot control their ship. But how could their ship be "destroyed"?

Another ship is commanded to "demolish what's left" of it

Does this refer to doing damage after a ship has reached 0hp?

Answer 1

The rules for spelljamming ships do not need to specify what happens because the normal rules for objects apply (ships are objects, probably composite objects)

The DMG has the following rules on objects:

[...] the only hard and fast rule is this: given enough time and the right tools, characters can destroy any destructible object. Use common sense when determining a character’s success at damaging an object.

[...]

For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

[...]

Hit Points. An object’s hit points measure how much damage it can take before losing its structural integrity. Resilient objects have more hit points than fragile ones.

[...]

So, we have two options:

1. Spelljamming ships are objects (because they have been given one set of hit points for the whole ship)
2. Spelljamming ships are composite objects whose hit points equal the total value of the hit points of the individual objects they are comprised of.

If option 1 is true, then once the ship reaches 0 hp it is destroyed. If option 2 is true then once the ship reaches 0 hp all of the objects it was composed of are destroyed (and thus the ship is destroyed by virtue of there being no objects for it to be made of).

Either way, the ship is destroyed.

In addition, some ships have special rules on what happens when they are destroyed, for example, the Tyrant Ship states:

Reducing a tyrant ship to 0 hit points not only destroys it but also destroys the ship’s spelljamming helm and eyestalk cannons.

But which option is correct?

Either option is viable, but for simplicity's sake while running the game, this DM would go for option 1, unless my players had a particularly cunning plan that required option 2 to be true. In that case I'd probably use the nearest equivalent sailing ship component parts to simulate individual parts of a spelljamming ship.

Disabled or destroyed?

The full quote provides context to your question.

If the characters and their allies are defeated, the reigar returns to the esthetic and commands it to demolish what’s left of the Second Wind. Enough of the ship’s air envelope remains to keep any surviving characters and crew members alive for 8 hours.

The demolish command is only given when the characters and their allies are defeated. The ship is destroyed at that point, not before.

There is another option you are not considering with your interpretation.

You have said disabled can only mean the spelljamming helm is destroyed. Disabled can also mean "render defenceless and incapable of escape" in the situation outlined.

If the attacking ships have destroyed/disabled all of the ships weapons, and is faster than the target ship so the target has no hope of escape, it has been disabled and the characters have been defeated.

But what about lateral movement post destruction?

Spelljamming ships can travel at a speed of

100 million miles per 24 hours

Or 4.1 million miles per hour. It is reasonable to wonder what happens if it is destroyed travelling at that speed.

Thankfully we never have to think about that as the rules state:

A spelljamming ship automatically slows to its flying speed (discussed later in this chapter) when it comes close to something big enough to have its own air envelope and gravity plane (see “Air Envelopes” and “Gravity Planes” below), such as another ship, a kindori (see Boo’s Astral Menagerie), an asteroid, or a planet.

So all ship to ship combat occurs at a more reasonable terrestrial set of speeds noted in the spelljamming ships stat blocks.

But I want to know?

Ok, let's pretend, somehow, a spelljamming ship didn't decelerate automatically. We are now in a situation explicitly not covered by the rules.

The answer is pretty simple. DMs are allowed to use their judgment to apply reasonable outcomes to situations not covered by the rules.

In this case, everything, the ship, it's crew, etc are converted into highly exotic forms of matter the moment they impact anything.

As to what happens to the things they impact...it depends. Is it a planet or celestial body? Then nothing. Otherwise? Also highly exotic forms of matter.

Answer 2

You are thinking in Newtonian Mechanics. In Newtonian Mechanics, things not subject to any force move linearly, indefinitely long. There is no difference between you moving in a direction, or the world moving in the opposite direction.

That is not what we ordinarily experience. That was thousands of years of development of... the philosophy, well more clearly science, but science was a branch of philosophy at the time.

What we ordinarily experience is that things stay, and if you left them as they are they stop. Yes, it is obvious who stays and who is moving. We could call it "Aristotelian mechanics". That is how things of the everyday, for an uneducated layman, appear when working.

D&D5 game designers, thr Spelljammer designers, did not think so deeply behind the things. They want to create a way to integrate the various D&D worlds, so to go with that they also tried to create some space technology or something like it.

It is very unlikely that they knew about this important part of the history of the science, but somehow they have still hit on these important problems as Aristotle did thousands of years ago.

Reading the books, their decision is very obvious.

They left the whole question open.

This part is very consequent in the rule books. Note, they were mostly various art students in the USA, so nothing reductionism, nothing science, forget this all.

How to approach this

As the DMG points out, the DM is the master of rules and should make a ruling.

How should a DM rule?

If your team does not have a special affinity to such a mixed-system game, do not try to calculate Keplerian orbits of dead ships. Be simple. Be Aristotelian. Do it like you would DM a game about a sea transport.
If the ship dies, it stops.

This works in two ways:

1. It is simpler
2. It is easy to implement in the game, even if you feel that it is just a "game-ism."