The release of Starfinder motivated me to take another look at Spelljammer. Starfinder is set hundreds of years into Golarion’s future — or at least one possible future for Golarion. I was looking for something more compatible with existing campaign worlds. I am also a huge fan of cross-world campaigns, which I’ve been enabling via Plane Shift, Gate, Cubic Gates, and artifact-level magic portals between planes.
I was busy during most of the D&D 2e years, so I didn’t get time to really dig into Spelljammer when it was new. I was also put off by some of the fantastical physics of Spelljammer. It just didn’t feel right. What I really wanted was Traveller T20 with magic.
Going back to Spelljammer, I found so much to love… and so much to be irritated by. I love the settings, but the physics just don’t work in my campaigns. The result was my own set of house rules for Spelljammer, which are significantly different from the standard rules.
Gravity exists only where it is provided by physics or magic. In addition to the standard properties, each spelljamming helm creates gravity. Asteroids do not have significant gravity, by default. This can, of course, be rectified with magic. Spells can be used to create gravity, and those can be made permanent with the Permanency spell. The Rock of Bral, for example, is under a permanent magical effect which provides it with gravity. Magic devices which create gravitational fields are also widely available.
Air exists only where provided by physics or magic, and dissipates into space if not contained either by physics or by airlocks. Air generation is one of the functions of a standard spelljamming helm. Stand-alone air generators are also available for use on stationary objects such as asteroids, but the air will escape into the void of space if not contained by either gravity or physical restraints.
Each crystal sphere is a separate plane. Travel between planes may be accomplished via all the usual methods, or via the Phlogiston.
The Phlogiston is a river which flows between planes, much like the Styx flows between the various levels of Hell.
Crystal shells do not exist in most planes. In planes where an outer shell does exist, the Phlogiston passes through it automatically. (Unless overruled by a local magical effect, such as in the Astromundi Cluster.)