The Phlogiston, Wildspace, and Crystal Spheres

Crystal spheres

A crystal sphere  is a gigantic spherical shell which contains an entire campaign settings' cosmology.  Each sphere varies in size but typically they are twice the diameter of the orbit of the planet that is farthest from the sun or planet at the center of the sphere (the system's primary). The surface of the sphere is called the "sphere wall" and separates the void of "wildspace" (within the sphere) from the "phlogiston" (that surrounds and flows outside the sphere). The sphere wall has no gravity and appears to be impossible to damage by any normal or magical means. Openings in the sphere wall called "portals" allow spelljamming ships or wildspace creatures to pass through and enter or exit from a crystal sphere. Portals can spontaneously open and close anywhere on the sphere wall. Magical spells (or magical items that reproduce their effects) can allow a portal to be located. Other magic can open a new portal or collapse an existing one. Ships or creatures passing through a portal when it closes may be cut in two. (the crystal spheres are not nested within each other but all float like islands on the Phlogiston)

Wildspace

Wildspace is similar to the outer space of science fiction, with planets, asteroids and stars, but with different physics. Gravity is either none or the same as that of Earth, and is directed towards the center of planet-sized bodies; on large objects in space like spacecraft and enormous creatures gravity is directed towards a flat plane running through the object's long axis, allowing characters to stand on the decks of ships.^[1]

The Phlogiston

 is essentially a big ocean of a unique element that is neither air, fire, water, or earth The phlogiston (also known as "the Flow") is a bright, extremely combustible gas-like medium that exists between the Crystal Spheres. A signature property of the substance is that it does not exist within the boundaries of a crystal sphere, to the degree that it cannot be brought into a crystal sphere by any known means up to and including the direct will of deities. Every crystal sphere floats in the phlogiston, very slowly bobbing up and down over time. Travel between Crystal Spheres is facilitated by the formation of "Flow rivers" — sections of the phlogiston which have a current and greatly reduce travel time. Travel through the "slow flow" (i.e. off the Flow rivers) is possible, but very dangerous.