Adapting AD&D Spelljammer rules to the OD&D Champions of Mystara campaign (and vice versa) is not an easy task. For this reason, and also because there isn't a common solution to this conversion problem, i've listed here all the possible and useful ways to convert Spelljammer to Mystara and vice versa. This guide takes into account all the differences and the similarities between the two game systems and proposes useful and easy ways to mix both systems (where possible) or suggests whichever to choose and why.

Note: when talking about Spelljammer throughout the text i am referring to a ship that functions with a spelljamming helm, not to the Great Spelljammer, the mighty Flying Fortress of the Spheres.

The peculiarity of the Spelljammer universe is its "geography". Each solar system in enclosed within a Crystal Sphere, that's to say an immense sphere made of a crystalline nearly unbreachable material where glittering stones are set to represent the stars and the constellations that can be seen from the planets' surface. Inside the sphere lies an entire solar system, comprising star(s), planetoids, comets, asteroids, planets and other strange space phenomena (sargassos, nebula, etc.).

Outside the Crystal Spheres lies the Rainbow Ocean, also called the Phlogiston or simply the Flow. Inside this immense ocean of colored ether-like substance, the Spheres bobs like corks floating on the water, and ships traveling through the Flow can reach other Spheres usign the currents that connects them. There are unidirectional and bidirectional currents, and sometimes the voyage is very long, but the Flow is an amazing place, and the dangers abound. To enter a Crystal Sphere from the Phlogiston, one must find a Portal (an opening that occurs randomly on the crystal sphere's surface) and sail through it before it closes. Another possible way to cross the boundary is to "force" a portal to occur, using magical spells or devices.

According to Champions of Mystara, Mystaraspace is not a Crystal Sphere, but it lies in a Galaxy just like those existing in real world. There is no mention of Crystal Spheres nor of the Flow, so this may lead to the conclusion that the D&D universe is the exact copy of the real one, except the presence of magic.

Now the question is simple: which of the two solutions is better to choose? It is possible to have Mystaraspace as a Crystal Sphere (a very handy choice) or to have the universe resemble our own, but in this latter case the Spelljammer universe must be considered unexsisting and must be totally rewritten and rearranged to suite the new celestial georaphy. There is another possibility, however: to merge both universes. This way, two possible scenarios exist:

1. Mystaraspace lies inside a galaxy where all the physical laws work as per D&D rules, whereas the Spelljammer universe lies in another galaxy or nebula, that can be connected to Mystara galaxy via a Wormhole (much like Deep Space Nine). The Spelljammer galaxy is totally made up of the Phlogiston, inside which the Crystal Spheres float around as per Spelljammer rules.
2. Mystara and Spelljammer are situated in two separate dimensions, namely the OD&D dimension (the Multiverse) and the AD&D dimension (the Flow or the Planescape dimension). In this case, Champions of Mystara rules apply to all the Prime Plane universe (there could be some areas where anomalies can be found, of course), and the Outer Planes are those described (well, hinted…) in the old Master and Immortal sets and in the Rules Cyclopaedia and Wrath of the Immortals set.

The main difference between Spelljammer set's background and the D&D space presented in Champions of Mystara is the way in which gravity functions. Spelljammer ships have a gravity plane, but both sides of a Spelljammer's gravity field pull toward that plane. This means that creatures can actually walk on the underside of their ship's hull. This also means that the crew must walk on the ceilings of the decks located on the other side of their ship's gravity plane. Also, there are no variations in gravitational strengths in the Spelljammer universe. The situation becomes even more complicated when two ships with different tonnage and different gravity plane inclination come into close contact: usually in this case, the bigger ship's plane becomes the predominating one, with disastrous effects for the unprepared crew.

According to D&D physics on the other hand, the gravity plane has two sides: a positive side that pulls objects down toward the gravity plane, and a negative side which repulses objects away from the gravity plane. Thus, everything "goes down" in the D&D universe. As per Spelljammer rules, in Mystaraspace gravitational objects coming within range of larger gravitational objects will reorient their own gravity planes to parallel that of the larger object - sometimes violently.

These two systems are obviously incompatible, so one must choose to adopt either one or the other. The real problem however is to adapt the vessel design once chosen. However, there is one possibility to use BOTH systems, by adding a premise to the whole nature of the Universe:

"Gravity functions differently in the various Crystal Spheres (or Galaxies or Nebulas if you don't want to use Crystal Spheres - see below)."

If we follow this theory then, we can state that in Mystaraspace the gravity works as per D&D rules, whereas in the Spelljammer spheres the AD&D system is in charge. As for the unexplored space, the DM is given complete freedom to decide which of the two systems work, or if there is another alternative gravity system.

Also note that if the two systems are allowed to coexist, the Spelljammers that will travel through Mystaraspace will have to be ready (i.e. properly arranged and refitted) to survive the hostile and alien physics laws that characterize this solar system. The same goes for Mystaran skyships that travel through the Spelljammer universe, of course.

In Mystaraspace, objects do not normally carry their own air envelope into space with them. On the contrary, the Spelljammer universe boasts this peculiarity: everything that leaves a planet (or asteroid) that is surrounded by an atmosphere, carries with it (or him) an air envelope whose extension depends on the size of the body. The air can be used to breathe in the void, but will exhaust in a certain number of days, becoming fouled and finally poisoning for those who breathe it.

This problem is easily solved. Simply, in Mystaraspace the D&D rule applies (no air envelope carried in space). So, the Spelljammers that have the misfortune to venture into this solar system will experience a rapid depletion of their air envelope, and unless they're equipped with air masks or air-tight compartments, the crew will suffocate. In game terms, the air becomes fouled in 1 turn for each day of "fresh air" remaining, and at this point the crew has only 1d6 turns to find out a solution before the air becomes "deadly". At this stage, a Saving Throw vs Death Ray is required each round to avoid dying.

Unlike Spelljammer's wildspace, the void between the stars in Mystaraspace is cold (temperatures ranging from -10° to -30°C -not as cold as the space surrounding Earth). This can be explained by an existing Gate to an icy dimension or plane.

On Mystara it is very difficult to bypass the Skyshield (atmosphere) without the use of magic or the ais of natural magical phenomena such as Tubular Breaches and Vortigern's Vortex. This inability to leave the planet strongly resembles the difficulty to breach a Crystal Sphere in the Spelljammer universe. However, since the subject of the topic in this case is the planet and not the solar system itself, the Spelljammer rules say that a ship requires a certain amount of time to takeoff from or to land onto a planet depending on the planet's size and the atmospherical conditions. When comparing this to the D&D rules, two possible hypothesis may be used to explain the difference:

1. Mystara (and maybe the other planets of the solar system) has a gravity well that exceeds that of the other planets of the universe, and due to its magical core prevents any huge body from escaping. In this case, the rules described in Champions of Mystara apply to both skyships and Spelljammers.
2. The inability to leave Mystara's atmosphere is ascribable solely to the weakness of the skyship's magics. In this case, a Spelljammer would be able to leave it as the AD&D rules describe, only because the engine is much more powerful than that of a common skyship, and Mystaran flying vessels that land on a planet of the Spelljammer universe will have therefore the same difficulty to leave its atmosphere as they had in Mystara.

Essentially, the reason why no Spelljammer exists in Mystaraspace is that the Mystarans have not yet discovered the existence and the use of spelljamming power and engines. They rely only on skyships built with an expensive and complicated procedure that are usually less fast and less maneuvrable than Spelljammers. On the other hand, in the Spelljammer universe the only possible way to travel between the stars is by means of spelljamming engines, and the enchanting techniques used in Mystara are now considered obsolete and unprofitable by the spacefaring nations.

Basically then, in Mystaraspace there isn't at the moment any spelljamming vessel only because this "technology" has yet to be discovered. Should the Mystarans or any other race that live in the other planets of the system (if any) stumble across a spelljamming ship and be able to study it, it is likely that they will also start to produce and use Spelljammers, beginning the race for space. If they have not such an opportunity, it is probable that the Mystarans will develop spelljamming engines in a century or so.

According to Spelljammer rules, the Ship's Rating is a reflection of the ship's speed and relative power, and it depends on the level of the individual piloting the ship (spelljamming officer) and on the power of the ship's helm. Also, the "tactical speed" (also called "impulse speed") varies according to the place where the vessel travels: in the void (wildspace) it's equal to 500 yards per SR point per ROUND; inside the atmosphere it's 240/80 feet per SR point per ROUND. The maximum SR is 10 (but it requires a 33^rd-36^th level mage or priest and a Major Helm to attain it). If you compare it to the maximum speed of normal skyships inside the atmosphere (360/120 feet per round -strong wind can increas or decrease this movement) or in the void (ten times the normal speed inside the Skyshield), it's easy to see that even a Spelljammer with a low SR is incredibly faster than a common flying vessel. This can be explained analyzing the type of engine used by the ship: simply put, a spelljamming helm is a much more powerful (and faster) means of transport than an enchanted skyship (think about the difference between a F-14 and a Sukhoi Su-2 or a Messerschmitt Bf 109).

However, a possible suggestion given in Champions of Mystara to balance the two systems is to convert each SR point to an Air Speed of 120/40 feet per round, the fastest speed allowed being 720/240 (at SR 6). Every two additional Ship's Rating points above the maximum only improve the Maneuvering Factor (see below) by one level, rather than increasing the vessel's speed. In the case, the two engines (Spelljamming Helm and Enchanted Hull) are considered to be more or less on the same "technological level", and this can be easier to handle for DMs.

As for the so called "Spelljamming Speed" (the "warp speed"), both Spelljammers and Voidships (Skyships equipped to travel into theVoid) have the same rating: 100 million miles per day (average of 4 million miles per hour). The Spelljamming speed can be attained only when the vessel is in "open space" (far enough from any bodies of 10 Sjr Tons or more). A vessel traveling at spelljamming speed slows to impulse speed when coming within a distance of 10' x radius of the ship's own gravitational field from another gravitational field, or whenever it makes a turn.

This refers to the tactical (impulse speed) maneuvrability of the craft. This is a factor of sails, oars, fins, and other controlling devices. In general, the hull design of a ship determines its MC (or MF for D&D): the bigger and bulkier the ship, the lower the MC. In AD&D the MC value ranges from A (referring to the most maneuvrable ship) to F (which indicates that the vessel is below minimum maneuvrable standards). In D&D the MC value ranges from 5 (referring to a small aerodynamic craft) to 1/5 (a bulky unmaneuvrable ship). Each MC value simply refers to the number of moves allowed to the ship in one round: A(5) = 5 moves/round; B(3) = 3 moves/r; C(1) = 1 move/r; D(1/2) = 1 move every 2 rounds; E(1/3) = 1 move every 3 rounds; F(1/5) = 1 move every 5 rounds.

It is also possible to convert the Spelljammers' MC into a D&D MF based on its hull length and shape instead of using the simple and handy correlation given above.

In the Spelljammer rules, Tonnage is a measure of volume rather than weight, whereas in the D&D rules it reflects actual weight and the numbers given are in line with seagoing vessels of real-world Earth. To translate a Spelljammer's tonnage for use with the D&D skyship rules, either recalculate the vessel's tonnage based on the skyship design (using the rules given in Champions of Mystara, Designer's Manual, page 13) or simply double the Spelljammer's tonnage (or divide by two the Skyship's tonnage to convert from D&D to AD&D).

Alternatively, the D&D Tonnage and AD&D Tonnage can be used both simultaneously, the first measuring the weight of the ship according to D&D rules (see ref. above), and the second measuring the space (volume) inside the ship as per Spelljammer system. This way, the cargo can be considered using its volume or its weight or both (more precise but much more complicated to figure out).

The Hull Points in the D&D system are calculated based on the material and on the total surface of the hull (see Champions of Mystara, Designer's Manual, page 13), whereas according to Spelljammer rules, the Hull Points are equal to the Tonnage of the ship. There is obviously a large gap between these two methods (and usually AD&D ships tend to have much less HP than D&D vessels), so one of the two must be chosen. Either recalculate the Hull Points based on the skyship's design (using D&D rules) or find out the AD&D Tonnage (volume) of the ship and use it as the new Hull Points.

The AR (or AC) reflects the difficulty that an attacker has in causing significant damage to the ship because of its construction. In both systems the AC depends on the material used to build the hull (modified by specific enchantments), so either use the D&D table in Champions of Mystara or the AD&D rules in War Captain's Companion.

Something completely alien to AD&D rules, the Cargo Capacity in D&D is the encumbrance (measured in coin weights or tons) of cargo and passengers the ship can carry in addition to its crew and gear. Cargo Capacity is limited by both the avaliable space and how much weight the ship's lifting enchantments can lift. The AD&D rules rely only on the space available on the ship to calculate what can be transported, ignoring the weight of the items. This problem can be resolved in two ways:

1. The Spelljammers are (technically speaking) so advanced and powerful that they have not a weight limit for the loaded items, but only a space (volume) limit. Simply put: anything can be loaded in the spelljammer as far as its cargo hold and decks are not full, with no influence on the ship's speed. The D&D rules apply only to Skyships because of their inferior building technology and magic.
2. The Cargo Capacity must be calculated for the Spelljammers too, using the rules provided in Champions of Mystara and referring to the areas of the ship labeled as "Cargo Hold".

In D&D, this is the maximum weight the vessel can carry and still fly at full speed; determined by the strength and amount of technology or magic used to provide the ship with lift. Since the Spelljammer system doesn't use it, it must be either calculated for Spelljammers usign D&D rules, or simply assume that the Spelljammers don't have a Lift Capacity, and thus that they can travel at full speed without considering its cargo (the use of Lift Capacity is strictly connected to the Cargo Capacity).

Make any necessary changes in range, damage, etc. based on the AD&D Conversion Guide given in the Rules Cyclopedia, or adopt the AD&D statistics only for the siege and huge weapons given in War Captain's Companion.

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