Spelljammer introduced into the AD&D universe a comprehensive system of fantasy astrophysics, including the Ptolemaic concept of crystal spheres. Crystal spheres may contain multiple worlds and are navigable using ships equipped with "spelljamming helms". Ships powered by spelljamming helms are capable of flying into not only the sky but into space. With their own fields of gravity and atmosphere, the ships have open decks and tend not to resemble the spaceships of science fiction, but instead look more like galleons, animals, birds, fish or even more wildly fantastic shapes.
The Spelljammer setting is designed to allow the usual sword and sorcery adventures of Dungeons & Dragons to take place within the framework of outer space tropes. Flying ships travel through the vast expanses of interplanetary space, visiting moons and planets and other stellar objects.
Like the Planescape setting, Spelljammer unifies most of the other AD&D settings and provides a canonical method for allowing characters from one setting (say, Dragonlance) to travel to another (say, the Forgotten Realms). However, unlike Planescape it keeps all of the action on the Prime Material Plane and uses the crystal spheres, and the "phlogiston" between them, to form natural barriers between otherwise incompatible settings. Though the cosmology is derived largely from the Ptolemaic system of astronomy, many of the ideas owe much to the works of Jules Verne and his contemporaries, and to related games and fiction with a steampunk or planetary romance flavor. A strong Age of Sail flavor is also present.
One criticism of the setting was the unrealistic cost of even the weakest type of helm at 100,000 gold. This gave players an opportunity to acquire vast amounts of wealth in a very short time which would unbalance a campaign. Additionally this made it difficult to form an in game economy.
This was changed in the 2005 HackMaster supplement "HackJammer", which included an overhaul of the original helms with the weakest helm being valued at 10,000 gold.
All bodies of any size carry with them an envelope of air whenever they leave the surface of a planet or other stellar object. Unlike real-world astrophysics, this air envelope is not dispersed by the vacuum of space. These bubbles of air provide breathable atmosphere for varying lengths of time, but 3 months is considered "standard".
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.
Note that unlike the Ptolemaic system, the crystal spheres are not nested within each other.
As a living thing (although it does not consume any matter, it does absorb heat and light through its dorsal side and uses them to produce air and food for its inhabitants), the Spelljammer has a complex life cycle and means of procreation. Normally the ship has no captain and wanders the cosmos seemingly aimlessly. When the Spelljammer has a captain, obtained through another complex process, it will create Smalljammers (miniature versions of the Spelljammer) that go forth as its spawn. Apparently there can only be one Spelljammer at any one time. One Smalljammer will mature into a full Spelljammer ship if it is ever destroyed.
For more details on the Spelljammer ship, please consult the Legend of the Spelljammer boxed set, which was released by TSR in 1991.
The core Spelljammer product line consisted of four boxed sets: Spelljammer: AD&D Adventures in Space (ISBN 0-88038-762-9) introduced the setting and provided the basic rules for spelljamming travel. Legend of Spelljammer (ISBN 1-56076-083-4) expanded on the setting, in particular the Spelljammer itself. The War Captain's Companion (ISBN 1-56076-343-4) provided more detailed ship-to-ship combat rules, and The Astromundi Cluster (ISBN 1-56076-632-8) provided a roleplaying campaign.
The line was expanded by a number of accessories. Lost Ships (ISBN 0-88038-831-5) expanded the number of ships, while Practical Planetology (ISBN 1-56076-134-2) assisted DMs who wished to create their own spelljamming setting. The Rock of Bral (ISBN 1-56076-345-0) provided a home base for adventuring parties, and Realmspace (ISBN 1-56076-052-4), Krynnspace (ISBN 1-56076-560-7) and Greyspace (ISBN 1-56076-348-5) gave information about the crystal spheres housing TSR's three main campaign worlds. TSR also published a DM's screen (ISBN 1-56076-053-2) and two Monstrous Compendiums (ISBN 0-88038-871-4 and ISBN 1-56076-071-0).
A series of five connected adventures was released in the modules Wildspace (ISBN 0-88038-819-6), Skull & Crossbows (ISBN 0-88038-845-5), Crystal Spheres (ISBN 0-88038-878-1), Under the Dark Fist (ISBN 1-56076-131-8) and Goblins' Return (ISBN 1-56076-149-0). Also published was a longer campaign, Heart of the Enemy (ISBN 1-56076-342-6) and Space Lairs (ISBN 1-56076-609-3) contained short standalone adventures.
When TSR started to produce a line of handbooks in 1992, they added the Complete Spacefarer's Handbook (ISBN 1-56076-347-7) to cover spelljamming travel.
A full product list is given in the external links.
In 2002 a team of freelance game modification developers created "The Arcane Space Tileset" for Neverwinter Nights. This tileset included Spelljamming ships, Space and Atmospheric terrains, along with Monsters and NPC's, all set within the Spelljammer Campaign setting.
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