Air, Gravity & Fire: Spelljammer 5e Conversion


There are overlooked game mechanics for a normal D&D adventure that become more important when you transport that adventure into space. As DMs and players we take for granted that certain physics of a fantasy world operate much the same as they do in our world. Characters need to breathe. If they fall from a high place they will take damage. And, fire burns. You may think that these same physics in a Spelljammer campaign would mirror what would happen in our outer space. You would be wrong.

Before we begin any discussion about physics in space we need to go over some basic definitions for the universe of Spelljammer.

Crystal Spheres

Crystal Spheres are exactly what their name entails; they are gigantic spheres made out of magic crystal. These spheres are also sometimes referred to as Crystal Shells. A Crystal Sphere contains all of the planets, moons and suns of each fantasy campaign world.


Between all of the celestial bodies in each Crystal Sphere is a void known as Wildspace. Wildspace is the equivalent of space in the real world. There is no air or gravity within Wildspace.

The Phlogiston

The Phlogiston is an endless ocean filled with a multicolored flammable element that is neither air, earth, fire or water. The Crystal Spheres actually float along within the Phlogiston.  Currents also flow through the Phlogiston that act as express lanes for Spelljammer ships traveling between the spheres. Like Wildspace there is no air or gravity within the Phlogiston.


In the original Spelljammer rules air was actually a fairly complicated mechanic. Any object that was at least the size of a person carried along a pocket of breathable air when it went into space. This pocket was enough to last for 2-20 turns (20 to 200 minutes in 2nd Edition AD&D rules). The larger the object was the more air it dragged along with into space. The air pocket surrounding a ship was based off of the tonnage of the ship. A ton equated to 100 cubic yards (76.5 cubic meters).

As a ship traveled through space the air pocket surrounding it would gradually become stale and eventually it would turn toxic. Landing on a celestial body or coming within the air pocket of another ship triggered a math equation to determine how the mingling of air affected the quality of each air pocket.

This mechanic is my absolute least favorite in the original Spelljammer rules so I’m dumping it for something simpler. I don’t really care about how objects like planets or asteroids get air. That isn’t a question that a game with magic needs to answer in my opinion. The questions I am concerned with are how a ship gets air and how a character gets air if they are off the ship. Rather than invent another complicated mechanic I am going to invoke a few MacGuffins.

A Spelljammer ship already contains a simple, yet powerful magic device that allows it to travel through space. It isn’t too much of a stretch of the imagination to include the production of air into the duties of the Spelljammer Helm. For individuals operating outside of the air pocket provided by the Helm I invented a simple magic item that is easily obtainable by anyone traveling to space.

Since there is no breathable air in Wildspace or the Phlogiston suffocation may seem like a certainty for groundlings journeying into space. Space-faring races do not worry about such things though. Any celestial body larger that is more than 1 mile (1.6 km) in diameter generates its own breathable atmosphere. This atmosphere extends from the surface to the edge of Wildspace.
A functioning Spelljammer Helm also generates an atmosphere surrounding the ship with a pocket of breathable air. This pocket extends thirty feet (9.1 meters) out from all points of the ship’s structure and is enough for the maximum crew of each ship to breathe normally on deck. Should the Spelljammer Helm be disabled or destroyed the air pocket will slowly begin to dissipate, but enough breathable air will remain for the crew to survive for seven days.
Adventurers unlucky enough to find themselves outside of the air pocket of a Spelljammer ship or bold enough to explore smaller celestial bodies will want to equip themselves with a Pearl of Air. These pearls are mined from the gas giant planets by space dwarves and sold at most space ports. A Pearl of Air provides its holder with enough breathable air to survive outside of an air pocket for 24 hours.


Gravity is a simpler mechanic in the original Spelljammer rules than air and I will be carrying over most of those gravity rules. Simply put any object with an axis that is 25 ft. (7.6 m) or longer generates a plane of gravity strong enough to attract other objects. I am going to make one change here. Only Spelljammer ships and celestial bodies generate gravity in this conversion. This gravity is all equal to Earth gravity.  The gravitational plane of an object extends to the same distance as the air envelope or atmosphere does.

The gravitational plane of a Spelljammer ship runs along the long axis (bow to stern). Because this plane bisects the ship from top to bottom it is possible to have ship decks with reverse gravity. If you look at the ship deck plans included in the boxed set these decks are usually cargo decks. This reverse gravity also means that a person can fall off of the deck of a Spelljammer ship they will not fall endlessly through the void of space. Instead they will be pulled back towards the bottom of the ship so long as they remain within the gravity field. The original Spelljammer rules also allowed for combat in a weightless condition, but with penalties to rolls for non-natives of space. I am going to convert this rule over to all weightless combat rolls being made with Disadvantage.

There is no gravity in Wildspace or the Phlogiston. However all Spelljammer ships and celestial bodies larger than 25 feet (7.6 meters) in diameter generate a gravitational pull equal to that of Earth’s gravity. For a celestial body the force of gravity acts much like it does in our world. Any creature or smaller object on the surface of the body is pulled towards the center. The gravity field of a celestial body extends from its center to the edge of Wildspace.
On a Spelljammer ship this force manifests as a gravity plane along the ship’s long axis. Creatures or smaller objects above this plane are pulled down by this force; creatures or smaller objects that are below are pulled up. The gravity plane of a ship or object extends the same distance as the air envelope.
Creatures or objects that fall off of the deck of a Spelljammer ship will fall through the gravity plane and then be pulled back up towards the bottom of the ship. This makes it possible for a creature or object to bob up and down next to a ship.
The lack of gravity in Wildspace and the Phlogiston affects the travel of normal ranged weapons. Missiles fired beyond a gravity field have an extended range. The long range of the weapon listed in the Player’s Handbook is the normal range in space and the new long range is four times that number. Melee combat is also possible outside of the gravity field, but all attacks are made with Disadvantage.


Fire is a fairly straightforward mechanic in Spelljammer and doesn’t need any conversion.

Within the air envelope of a Spelljammer ship or a celestial body normal and magical fire functions exactly as it would on the ground. However, the lack of air in Wildspace has an affect on fires. Magical fire like Fireball can be cast normally outside of an air pocket, but this fire has no chance of catching anything ablaze. Normal fire will not function at all outside of an air envelope in Wildspace.
Fire in the Phlogiston is extremely deadly. The Phlogiston permeates the air envelope of a Spelljammer ship making an explosive mixture. Spelljammer crews know to extinguish all flames before exiting a Crystal Sphere, however, accidents do happen. The following table shows some examples of fire damage in the Phlogiston.
Type of FlameFireball DamageArea of Effect
Lit Candle1d64 inches/ 10.2 cm diameter
Lit Lantern3d61 foot/ 0.3 m diameter
Cooking Fire3d63 feet/ 0.9 m diameter
FireballSpell damage X 3Spell area of effect X 3
Casting any spell that creates magical fire in the Phlogiston causes the spell to immediately trigger with the caster as the center of the area of effect.
Fire on the deck of a Spelljammer ship is also very dangerous. Shipboard fires will expand at a rate of 1 foot (0.3 m) per round. Initially a fire can be put out by one crew member in one round. Every round the fire is allowed to expand increases the number of crew members and rounds needed to extinguish the flames by one.

Okay, then. There are some basic physics to expand the world of Spelljammer a little bit more. We are getting very close to having a full picture of how a Spelljammer campaign will look. Next time I will be going over some of the peculiarities of magic in space.


Art credit- Jim Holloway

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