While spelljammer helms have long been a staple of cross-sphere interactions upon the Prime, it's only in recent centuries that their usefulness has been acknowledged across the planes. Though they have no great speed advantage on most planes given the nature of intraplanar travel, and but for a few planar paths, a full ship is somewhat inconvenient to take from one plane to another, a helmed ship provides a far greater level of safety and ease of transport than the more common caravan.
In a broad description, a helm is any device that converts some form of ambient energy into motive force for an attached vessel. By far the most common variety on the planes, though, are the spelljammer helms, enchanted devices in the form of chairs (most commonly thrones) that work via the innate magical energies of one sitting in them. Those using such a device feel as though they become one with the ship, and are able to fly it as though moving their own bodies, though they find themselves magically drained after usage. Rarely, other varieties can be found or purchased, most often either furnace helms — helms powered by the energies released upon the destruction of an enchanted device — or lifejammers — helms powered by the drained life energy of one sealed within.
Helms were and are created almost solely by the arcane; possibly accounting for helms' previous lack of presence on the planes, as for most of their known existence, they have eschewed planar travel whenever possible. No others possess the secret of their creation, though others have managed, through experimentation, to find ways of adapting or altering an existing helm towards other uses. Still, as helms rise in popularity in the general multiverse, so too do the arcane (or "mercane", as many on the planes have nicknamed them), trading in spelljamming thaumatology wherever will have them.
- Spelljammer: Adventures in Space - Concordance of Arcane Space, pgs.8,33-39
- Spelljammer: Adventures in Space - Lorebook of the Void, pgs.6,8,10,12