see
Pedantic Grognard
Officially, the depth of the air envelope around a planet is the same as the diameter of the planet. See "Air Envelopes of Objects" on p.18 of the Astral Adventurer's Guide.
If you ignore the canonical 5e air envelope for transition to/from 100 million miles/day speeds, you choose the 2e Spelljammer implicit design that that you can use spelljammers to get from any point on the surface of a planet to another super-rapidly compared to any other travel method.
For 2e, it always took 4 turns, or 40 minutes, to launch/land from an Earth-sized planet to reach where you could move at wildspace speeds, so New York to Tokyo (or Melbourne, or Pretoria, or anywhere else) travel in 80 minutes and a few seconds, if you could time your movement at 1,157.41 miles per second well enough.
If you use 1 mile from the surface as the transition point to wildspace speeds, and 5e ship flight speeds, that comes to a few seconds over 40 minutes for a 3 mph ship, 15 minutes NYC-to-Tokyo for a 8 mph ship like a Damselfly. So you're talking either faster than any modern travel method.
If you use 50-miles-from surface as the transtion and 5e ship flight speeds, it's under 34 hours for a 3 mph ship, under 13 hours for an 8 mph. Which is competitive with a modern NYC-to-Tokyo flight.
If you count the official 5e air envelope as part of the object for the 1 mile/1000-250 ft. distances, then it's faster to normally fly the great circle route from Tokyo to NYC (6,745 miles) at 3-to-8-mph than to fly the diameter of the Earth (7,917.5 miles) up at 3-to-8 mph, move at wildspace speeds, and fly the diameter of the Earth back down at 3-to-8 mph.
So the obvious choices are:
1) Use the 5e envelope as part of an object. Space travelers will mostly only visit small bodies (asteroids, small moons, and the like), because the time in atmosphere on large bodies is so long. Spelljammers will still be useful for same-planet travel (flight is an advantage), but will not utterly change travel/shipping times.
2) Do not use the 5e envelope as part of an object, and use 5e helm prices/availability. Spelljammers should rapidly replace all other methods of long distance shipping, because they move quantities of cargo comparable to conventional cargo ships, at capital costs similar to conventional cargo ships, at speeds faster than (or, with a 50-mile limit, merely comparable to) modern aircraft.
3) Do not use the 5e envelope as part of an object, and make helms legendary rarity (both in price and difficulty to make). Spelljamming helms are the exclusive province of powerful governments, high-level adventurers, and the like, since they are so rare and so powerful.
You'll notice all of those options have their disadvantages for official publication. The first one rules out Star Trek-style planet-of-the-week Spelljammer games. The second one blows up any existing game world it touches (the Realms, Eberron, etc.). The third one rules out Firefly-style random-nobodies-odd-jobs Spelljammer games.
My personal fourth option, which I developed when cogitating on these things a few years ago, is as follows:
Gravity Line Travel
A spelljammer that’s beyond the atmosphere (50 miles above the surface for Earth-like bodies) may exploit a gravity line to travel directly toward a celestial body (any object at least half a mile in diameter) in line-of-sight. While doing so, the spelljammer travels at a speed of about 4 million miles per hour (100 million miles/day). It automatically drops out of this speed when within a tenth of a mile of a celestial body (including its atmosphere) or another spelljammer.
One notable quirk of gravity line travel is that, when a helm returns to a celestial body, it arrives at the place it last departed that body, no matter how much time has passed or how many other bodies it may have visited since. Thus, if a spelljammer departs Toril by gravity line from a spot 50 miles above Waterdeep, next time it arrives at Toril by gravity line travel, it will arrive at that same spot above Waterdeep. The only exception is if the body has at least one active beacon arcane; in that case, the helmsman can choose to arrive at the nearest point above any active beacon.
(You can then make a beacon arcane legendary-rarity magic items that are expensive to operate, so putting several on the same planet is incredibly rare.)
If you ignore the canonical 5e air envelope for transition to/from 100 million miles/day speeds, you choose the 2e Spelljammer implicit design that that you can use spelljammers to get from any point on the surface of a planet to another super-rapidly compared to any other travel method.
For 2e, it always took 4 turns, or 40 minutes, to launch/land from an Earth-sized planet to reach where you could move at wildspace speeds, so New York to Tokyo (or Melbourne, or Pretoria, or anywhere else) travel in 80 minutes and a few seconds, if you could time your movement at 1,157.41 miles per second well enough.
If you use 1 mile from the surface as the transition point to wildspace speeds, and 5e ship flight speeds, that comes to a few seconds over 40 minutes for a 3 mph ship, 15 minutes NYC-to-Tokyo for a 8 mph ship like a Damselfly. So you're talking either faster than any modern travel method.
If you use 50-miles-from surface as the transtion and 5e ship flight speeds, it's under 34 hours for a 3 mph ship, under 13 hours for an 8 mph. Which is competitive with a modern NYC-to-Tokyo flight.
If you count the official 5e air envelope as part of the object for the 1 mile/1000-250 ft. distances, then it's faster to normally fly the great circle route from Tokyo to NYC (6,745 miles) at 3-to-8-mph than to fly the diameter of the Earth (7,917.5 miles) up at 3-to-8 mph, move at wildspace speeds, and fly the diameter of the Earth back down at 3-to-8 mph.
So the obvious choices are:
1) Use the 5e envelope as part of an object. Space travelers will mostly only visit small bodies (asteroids, small moons, and the like), because the time in atmosphere on large bodies is so long. Spelljammers will still be useful for same-planet travel (flight is an advantage), but will not utterly change travel/shipping times.
2) Do not use the 5e envelope as part of an object, and use 5e helm prices/availability. Spelljammers should rapidly replace all other methods of long distance shipping, because they move quantities of cargo comparable to conventional cargo ships, at capital costs similar to conventional cargo ships, at speeds faster than (or, with a 50-mile limit, merely comparable to) modern aircraft.
3) Do not use the 5e envelope as part of an object, and make helms legendary rarity (both in price and difficulty to make). Spelljamming helms are the exclusive province of powerful governments, high-level adventurers, and the like, since they are so rare and so powerful.
You'll notice all of those options have their disadvantages for official publication. The first one rules out Star Trek-style planet-of-the-week Spelljammer games. The second one blows up any existing game world it touches (the Realms, Eberron, etc.). The third one rules out Firefly-style random-nobodies-odd-jobs Spelljammer games.
My personal fourth option, which I developed when cogitating on these things a few years ago, is as follows:
Gravity Line Travel
A spelljammer that’s beyond the atmosphere (50 miles above the surface for Earth-like bodies) may exploit a gravity line to travel directly toward a celestial body (any object at least half a mile in diameter) in line-of-sight. While doing so, the spelljammer travels at a speed of about 4 million miles per hour (100 million miles/day). It automatically drops out of this speed when within a tenth of a mile of a celestial body (including its atmosphere) or another spelljammer.
One notable quirk of gravity line travel is that, when a helm returns to a celestial body, it arrives at the place it last departed that body, no matter how much time has passed or how many other bodies it may have visited since. Thus, if a spelljammer departs Toril by gravity line from a spot 50 miles above Waterdeep, next time it arrives at Toril by gravity line travel, it will arrive at that same spot above Waterdeep. The only exception is if the body has at least one active beacon arcane; in that case, the helmsman can choose to arrive at the nearest point above any active beacon.
(You can then make a beacon arcane legendary-rarity magic items that are expensive to operate, so putting several on the same planet is incredibly rare.)
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