The Voidjammer setting is to be all about travel to other worlds in a fantasy setting. The setting will mix both Greek views of Ptolemaic astronomy and Aristotelian physics with Xi, the Chinese notion of spirit and flow. Space will still be a hostile place, but with more leeway for survival by plucky adventurers. But this survivability also includes plenty of dangerous denizens that can be far deadlier for the unwary.
But don't let this high-falutin talk about philosophy and weird science put you off. The setting will be dominated by the Rule of Cool, with archaic physics only used to make weirdly wonderful settings and all manner of high strangeness. Voidjammer will be chock-full of goofy-fun and themes from my favorite comics, cartoons and cheesy movies. But all will not be silly. The universe is deadly dangerous, especially when things seem the most relaxed.
So these are the basics is Space Travel in my campaign:
AETHERIC SPACE AND TRAVEL
In the Voidjammer 5th Edition setting, Space Travel is actually Planar Travel, specifically Aether Travel. It involves bodily entering the Deep Ethereal Plane plane (the Aether). The Deep Ethereal is a transitive plane that borders the Material, Shadowfell, Feywild, Elemental planes and the Realm of Dreams. It is a misty, fog-bound dimension that is sometimes described as a great ocean. Stars, planets, moons and other celestial bodies are all visible here, though clouds, storms and fog can mute colors and blur shapes, requiring skilled navigators.
The Deep Ethereal is most commonly accessed by spellcasters using spells such as Plane Shift. However, with the invention of Voidjammers, ships can bring large ships loaded with people and cargo into the plane. Though a spellcaster can go faster, Voidjammers can do so with greater safety, comfort and bring passengers and cargo.
For further details on the Etheric Plane, see page 48-49 in the Dungeon Masters Guide.
ENTERING THE AETHER
Those who traverse the Deep Ethereal typically do so in Aetheric Craft designed to shield its passengers from the damp and gloom of the realm. The realm is dark and cold, though this varies depending on where you are. Entering the Deep Ethereal is done by flying up to the edge of a planet or celestial bodies atmosphere (usually 1d4 miles), requiring travelers to must deal with both High Altitude and Extreme Cold (see page 110 in the DMG). After which a vessel with a functional Helm or Mana Sails automatically enters the Deep Ethereal.
AIR: The substance of the etheric (Aether) can be breathed by living creatures, but is not truly “air” and so it supplies no fuel for fire and is not as satisfying to breathe. Ships come equipped with an “Aetheric Converter” to transform Aether into air and water.
GRAVITY: There is no gravity here, with untethered objects must be tied down or otherwise attached to something, otherwise they will drift. Creatures wishing to move in the Aether must Swim, Fly or Climb (if hand/footholds are available) to get around. Ships with a functioning Gravity Pump have ships gravity, and occupants can move about normally.
NAVIGATION AND TRAVEL TIMES
As stars, planets and other celestial objects are visible in the Etheric, travel can be mapped by a skilled navigator (Arcana/Intelligence checks). This process is highly complex, requiring extensive knowledge of stars, constellations and knowledge of the Deep Ethereal, and so is decided by one or more Arcana checks as well as proficiency in Voidjamming.
Away from planets and other large objects, a ship moves at what is called cruising speed. The cruising speed of a ship depends on the helm propelling it. Once a course is set, a vessel with a minor helm allows a ship to travel between adjacent planets in 2d6+2 days, while a major helm moves between adjacent planets in 1d6+1 days. Travel between adjacent stars is measured in months rather than days. If a ship is near other ships, celestial bodies (asteroids, moons, planets), travel times are based on the ships base speed.
Aether Ships, Helms and other goodies.