Do siege weapons have a maximum rate of fire?

Question

Siege weapons typically take a series of multiple actions to make an attack. Ballistae, for example, require the input of a load action, aim action, and fire action to make a single attack roll - regardless of whether they are free-standing (DMG 255), mounted on the deck of a water ship (Ghosts of Saltmarch refers to the DMG), or mounted to a spelljamming vessel (Astral Adventurer's Guide in various places, cf. Giff Bombard).

Since the actions of the crew reset on their initiatives (or on their side's initiative in Spelljammer), a ballista with a crew of one can only fire every third round, but one with a crew of three can fire once a round. But is there anything, RAW, preventing one from assigning them a crew of six so as to achieve a fire rate of two ballistae per round? Or a crew of nine for three shots per round?

I realize this is going to pretty quickly collide with the DM enforcing verisimilitude, but I am interested in whether there is any RAW restriction. Similar questions about handing a weapon to another player in combat, or passing a necklace of fireballs down a line are limited by the free object interactions involved in passing off the shared items or weapons. In this case, the ballista is stationary, so I think the RAW limit would be the movement rates of the crew and how many could effectively move through the space of the ballista and still give way to allow a new crew-member to access it.

Answer 1

There is no limit on rate of fire in the core rules

There is no limitation on the number of times a siege weapon can be fired in the core rules. In fact, those rules are so slim we can look at their entirety here (Siege Equipment section, page 255 DMG):

SIEGE EQUIPMENT
Siege weapons are designed to assail castles and other walled fortifications. They see much use in campaigns that feature war. Most siege weapons don't move around a battlefield on their own; they require creatures to move them, as well as to load, aim, and fire them.

That’s it. The rest of the section consists of a list of example weapons; here is the Ballista:

BALLISTA
Large object
Armor Class: 15
Hit Points: 50
Damage Immunities: poison, psychic
A ballista is a massive crossbow that fires heavy bolts. Before it can be fired, it must be loaded and aimed. It takes one action to load the weapon, one action to aim it, and one action to fire it.
Bolt. Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3dl0) piercing damage.

The actions needed to load, aim and fire the weapon are not its own actions. They are the operators' actions. While each operator may have only one action per turn, with more operators you can have more actions to operate the weapon. Indeed it is common practice to have a large enough crew so that they can fire the weapon each round by pooling their actions. There’s nothing in there that limits how many times it could be fired in a round with a large enough crew.

Reasonably, the crew needs to be next to the ballista to manipulate it, so there may be a ceiling by the movement of the crew and how many crew members could be near enough the weapon to move there, take their action, and move away again to make room for others. With a normal speed of 30 and medium size, this might be up to six sets of crew in combat, for a total of six attacks per round, or sixty per minute.¹

As PlayPatrice points out in the comments, even the ten shots per minute you would get from one attack per round outpaces the rates of fire with more modern day artillery weapons. Historically ballistae could be fired about two times per minute, or once every 5 rounds, so the DM might - as the question suggests - want to apply common sense to impose a limit over what the mechanical rules would allow.

Restrictions for vessel-mounted siege weapons

There are limits to rate of fire per round for siege weapons that are part of a vessel, due to special vessel combat rules.

Ghosts of Saltmarsh has rules for siege weapons that are mounted on ships. While the weapons are the ones from the DMG, when they are part of a vessel, firing them becomes one of the vessels action's instead of using individual actions by crew members for individual weapons (and in fact, crew are not assigned to specific weapons). The vessel’s actions are limited by round. For example, here is the text from a Galley (p. 187):

ACTIONS
On its turn, the galley can take 3 actions, choosing from the options below. It can take only 2 actions if it has fewer than forty crew and only 1 action if it has fewer than twenty. It can't take these actions if it has fewer than three crew.
Fire Ballistas. The galley can fire its ballistas (DMG, ch. 8). […]

And here is the Ballista's weapon feature of the Galley:

WEAPONS: BALLISTAS (4)
Armor Class 15
Hit Points 50 each
Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3dl0) piercing damage.

As I read it, using the Fire Ballistas action will fire all four Ballistas, once. So, the four ballistas mounted on a Galley could each be at most fired three times per round, using all three of the Galley’s actions.

This still is not due to an inherent limitation on the siege weapon’s rate of fire per round, but due to a limitation on how many actions are available each round to fire it.

——

¹ Maybe you can construct better rotation patterns with enough space around the weapon — I’ll skip this optimization problem. Often the weapons will be on battlements or cluttered ship decks with limited space to spare.

Answer 2

If the ballista is a creature, it may not take all three actions in one turn even if it is fully crewed.

"Such weapons are slow to load and fire ..." [Spelljammer Adventures in Space]

I have always understood this to mean that the actions must be consecutive. They aren't the actions of the crew occurring simultaneously, but rather the actions of the ballista/crew combo. First turn; wind up and load the weapon, next turn; aim it, next turn; fire it --- rinse repeat. Loading and aiming would interfere with one another and aiming and firing certainly can't occur at the same time, the latter would disrupt the former. A fully crewed Roman ballista was lucky to get off two shots a minute (1 shot per five rounds).

"... a ballista with a crew of one can only fire every third round, but one with a crew of three can fire once a round." [original question] This has no basis in RAW. I always understood the rules to mean that the ballista could not be operated at all by less than three crew. The ballista can still only fire one round per three turns and a mangonel once every 5 turns.

"Such weapons are slow to load and fire, Player characters are almost always better off using their own weapons and spells in ship to ship combat, reserving ship board weapons for targets that are too far away to be damaged by other means." [Spelljammer Adventures in Space, (from the beginning of 'ship to ship combat')]

Sorry, I have the digital copies of Adventures in space and Ghosts of Saltmarsh and cannot provide page numbers. DMG pg 255 is pretty sparse on clarification anyways.

Answer 3

Yes

It’s on p.6 of the PHB:

1. The DM describes the environment
2. The players describe what the want to do
3. The DM narrates the results of the player’s actions

Is there a RAW hard limit? No.

However, the DM is, thankfully, not a computer; they come with human intelligence and are empowered and explicitly authorised to apply it. A modern manually operated artillery piece has a rate of fire of about 2 rounds per minute.

Can a ballista fire faster than that? Well, for a DM adjudicating heroic fantasy where mortals contend with gods? Sure. For a gritty, down-to-Earth campaign where PCs have human limitations? Surely not.

Answer 4

The Ballistae is a Creature - Not a weapon.

Like any other creature - gets 1 attack action per round unless otherwise enhanced through specific magical / nonmagical means. It has AC, Hit points and damage immunities, and an independent attack bonus and damage amount for its ranged attack.

The original posters question assumes that the Ballistae (or other siege weapons) are weapons, rather than "Large Object" creatures as described in their stat block.

Let's look at the creature stat block:

BALLISTA,

Large object

Armor Class: 15

Hit Points: 50

Damage Immunities: poison, psychic

A ballista is a massive crossbow that fires heavy bolts. Before it can be fired, it must be loaded and aimed. It takes one action to load the weapon, one action to aim it, and one action to fire it. Bolt. Ranged Weapon Attack: +6 to hit, range 120/480 ft., one target. Hit: 16 (3dl0) piercing damage.

This is a creature stat block. not a weapon. A creature can have only 1 action, 1 move action, 1 reaction, and 1 bonus action each round. It has its own attack bonus (+6) that is independent of the person who fires the weapon. It has the special requirement that 3 crewmembers must spend actions on it in order for it to make it's attack action. It cannot - by the rules of action economy - make more than 1 attack per round.

Though - as a creature - with a 6 man crew. You could cast haste on the catapult and get 2 attacks per round.

This is a creature with the "Large Object" type - not a weapon "Wielded" by the crew. This arguably also means the 'Creature' can recieve the "help" action from any crewmembers who did not spend an action on one of the three requisits.