Crystal Shells[]
All wildspace is bounded by crystal shells or crystal spheres. Inside the crystal sphere is the vacuum of wildspace, the planets, and stars. Outside the crystal sphere is the rainbow ocean of phlogiston and more crystal spheres. The size of a crystal sphere is determined by the size of the planetary system inside. Usually a sphere has a radius at least twice as big as the orbital radius of the outermost celestial body in the system (i.e., the distance from the shell to the outermost body of the system is the same as the distance from that outermost body to the center point of the system).
Because of their great size, the outside of a crystal sphere appears perfectly flat. The curvature is so gradual that it is. completely undetectable to anyone who is close enough to see the crystal sphere through the obscuring phlogiston. The spheres consist of an unbreakable, dark, ceramic material of unknown origin. Some legends state (and various theologians agree) that the smooth-surfaced shells were created and positioned by the gods themselves to protect their worlds from the ravages of the phlogiston, which is held to be the prime matter of the universe. Less charitable philosophers maintain that such shells were placed by an even higher authority to keep gods and men in and confine their activities.
Whatever their origin, the crystal shells are uniform throughout space. All appear as great, dark, featureless spheres of unidentifiable matter. The crystal spheres are definitely solid. They have no gravity along either their interior or exterior sides (an exception to the rule that all large objects have gravity). No magic has been found that can damage or alter the surface of a shell, except for those spells which cause portals to open. Even this, however, is believed to be nothing more than an artificial triggering of a natural phenomenon, since portals also occur naturally and seemingly at random. They are apparently immune to the effects of wishes and even the wills of the outer planar powers (though this does not rule out the possibility that the outer planar powers created them perhaps they were wise enough to prevent even their own tampering).
The Phlogiston[]
The phlogiston is a turbulent, unstable, multicolored, fluorescent gas (or gas-like medium) which fills the regions between the crystal spheres. Very little is known for certain about the phlogiston or this region. Every planetary system known is encased within a crystal sphere. Essentially, the crystal sphere keeps the wildspace in and the phlogiston out (this, of course, is a tremendous simplification, but it is easy to grasp). Like the phlogiston, crystal spheres are a great mystery; their origins and substance are unknown. Within wildspace are the celestial bodies: planets, suns, moons, asteroids, and a host of other items collectively lumped together under the heading "planetoids." Most celestial bodies have an atmosphere of some sort although it is dangerous to assume that this is an inviolable rule.
Gravity[]
The reason everything drags its own atmosphere around through space is gravity. This is also the reason why people can stand on a space sailing ship without falling off its deck and can stand on a spherical planet without falling off the bottom side. Every body in space has its own gravity. Gravity is an accommodating force in that its direction seems to be "that which is most convenient." In an object the size of a planet, gravity is directed toward a point at the center of the planet so that people can stand anywhere on the surface and dropped objects fall perpendicular to the surface. In smaller objects, like space craft, gravity is not a central point but rather a plane which cuts horizontally through the object, usually at the lowest deck. Significantly, this gravity plane is two-directional; it attracts from both top and bottom.
A sailor can actually stand on the bottom of the ship's hull and move around as easily as if he was walking on deck. In this case, "down" is actually "up," back toward the plane of gravity that cuts through the ship at the lowest deck. One of the stranger side effects of all this is that an object falling off the side of a spelljammer can oscillate back and forth across the plane of gravity, falling first in one direction until it crosses the plane, then reversing direction and falling back across the plane again, and so on until something causes it to stop. To a person standing on the deck, the object appears to fall down, then up, then down, then up. This trick is commonly used to amuse passengers new to space travel. More than one groundling has gotten in trouble for standing at the ship's rail and tossing an endless stream of apples overboard just to watch them bob.
Air[]
All objects drag air with them whenever they leave an air envelope. A typical human, for example, will drag enough fresh air with him/her to breathe for 2-20 minutes. After that time runs out the air will turn foul for a like period of time and then become deadly and unbreathable.
The amount of air a spelljamming ship carries with it is based on its size. The larger the vessel, the more air is dragged into its pocket. The amount of air is equal to its keel length x beam length /2 + its HP (K x B/2+HP). For example, a man-o-war has a keel of 200 ft, a beam of 20 ft, and its HP is 550. 200 x 20/2+550 is 2550, which means its minimum crew size of 10 people can survive 255 days. When a vessel reaches its air limit the air becomes fouled; it smells bad and is stale and humid. All attacks and proficiency checks in a fouled atmosphere are at disadvantage. Air remains fouled for the same amount of time it remained fresh, once that time wears out it becomes deadly; each turn everyone aboard must save versus poison or pass out. If unconscious each turn they must save versus poison or die.
While important, air is relatively easy to replenish. Entering a larger air envelope like that of a planet or asteroid is one of the most popular and cheapest methods. Green plants will refresh air, some vessels make great use of these for just this purpose. Many asteroid colonies keep at least half their surface area reserved for plants for this reason as well.
Propulsion[]
Spelljammer ships are powered by engines when travelling through Wildspace. It is dependent on the ship but most engines are magical in nature, with "jets" at the rear of the ship. The engines provide enough force to move these ships throughout Wildspace.
A few engines are non-magical but through many years of learning it has been accomplished. They work in a similar fashion to the magical engines.