Spelljammer WTF Spelljamming? (5e) Orbital Mechanics


My campaign is 5e homebrew world, and has been running for nearly 2 years.
The PCs have just reached level 11, and have built the [dominant civilization's] second spelljammer in history (history only goes back 500 years)
They need to go blow up a moon, because of course, they do. More campaigns should get a chance to blow up a moon.

1. not very much of the world is actually explored. just circumnavigating east/west PCs will see 2 other habitable continents.
the world has very little mage tech, it is a cross between early medieval and the self-centeredness of the roman empire.

2. They need to travel 800 miles south to pick up a MASSIVE magic implosion bomb. (they recovered it and gave it to an NPC back around level 6)
they can target the destination from space, as it is on the edge of an 80-mile diameter crater.

3. 5e spell jammers have 2 speeds Fly speed (4 mph) and wildspace speed (away from gravity sources - 100 million miles a day / 400,000 mph )
so it takes 15 hours to get to the edge of space (62 miles on earth) and then 45 min to get to the normal moon (310,000 miles)
the trip to the bad moon is less than a week.

4. they don't actually know this - they just got the ship to fly last session, staying within 100' of the surface.
information from the world's first SJ ship includes a set of blueprints, a sending transcript about space whales, and then no further magical communications.

I will allow geo-synchronous orbit because that makes it easier to return to their homeland. What if they want to speed up or go counterspinwise?
what if they travel at Spelljammer speed away for 1 minute then return to the planet?
quick trips to the moon, and back would be funnier they could stay in the frame of reference and get 4 skill checks a day to reach their target..

earth rotates at 2000 mph (at the equator) you would need to be going faster 65 miles up.
I don't know about revolution speed around the sun, but if you leave for a minute, its going to be pretty far away when you head back.
67,000 mph in solar orbit. 18.5 miles/second

what if they want to make trading runs to the empire's furthest city? or the other continents? Thankfully this group stays mostly focused, so they are may be more focused on the moon, than orbital shenanigans

Some of my rules are pulled from 2e Spelljammer books, some from google.

How would you handle orbital travel?
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unless the "object" that you need to get 1 mile away from is the "atmospheric envelope" of the object, not just the surface.
NASA says edge of space is 62 miles up. I will use that rather than the ship air/gravity envelope rules.

Also as a side-note since a 10-ton ship provides normal gravity, a small moon & rocky Jupiter-size planet should both have the same gravitational force. although it is possible that some planets of celestial bodies have a "Light gravity" keyword.


naughty word NASA. You are using a fantasy game that allows unenclosed ships to fly through space. Why would you even want to use mundane real-world physics for this? Go with the 1-2 mile air envelope (unless you have mountains taller than that for some reason) and ditch the real-world space physics for distances, size and whatnot and go with values that are more reasonable for space fantasy.

Also, the sea ship speed (4 MPH) for a flying ship is stupid as well. In atmosphere, flying ships should be faster than most land land vessels. For my own game, I have most Spelljammers moving 30+ MPH and they are as useful as Eberron's air vessels, or better.


Mod Squad
Staff member
How would you handle orbital travel?

I'm a physicist. I've done the math behind orbital mechanics.

In a fantasy game, I'd handle it by... magic! You go from point A to point B. It takes some time that's set either by "speed of plot" or by some number of days travel dependent on how long I want it to take for people to get between those points to make the setting work out.

In a sci-fi game, I'd handle it similarly, because I'm likely the only one at the table who has the math to handle actual orbital mechanics.

Tip: FORGET real world space. It has bugger-all to do with Spelljammer.

Spelljammer rules are NOT designed to fit in any way with the real world, other than MAYBE the distances. Nothing else is grounded in reality. Not gravity, speed, acceleration, atmospheres, navigation, propulsion. ALL of that s... is made up fantasy stuff that simply makes dealing with fantasy-space at least mostly practical for an RPG.

You say you're using a combination of rules from 2E and 5E, and that it's a homebrew world so I assume there's some house-ruling happening not just to fix problems with mixing the two rule sets but to deal with other spelljammer-related stuff you want to address as well. I know absolutely nothing about how 5E handles spelljammer, but I'm quite well-versed in 2E spelljammer and have written hundreds of pages on it - mostly looking at issues that the 2E rules failed to look at whatsoever (and from what I've gathered trickling down, 5E just tossed a fair amount of 2E SJ out the window and didn't replace it - that's problematic).

Frankly, I think you're looking at the wrong problems. It takes as long as it takes to fly from the ground into space - it's whatever set of rules you choose for that or whatever you decide it's going to be otherwise. Distance between planets doesn't matter as you can simply decide that spelljammers can go faster to cover inter-planetary distances easier, or slower so that those journeys themselves take a lot more time. You can decide that it will take 5 minutes to get to the moon or 5 months. It doesn't much matter what it takes. That's up to you. If you want the adventures to be about the journeys themselves, have it take a long time with many interesting things along the way. If you want the adventures to be about what happens at the destination, well you have a lot of work ahead prepping all the places they might want to travel to and why they might want to go there next.

For the most part I think 2E spelljammer did just fine with speeds and interplanetary distance. Interplanetary speed was, IIRC 1,000,000 miles per day, so yeah, once you're out of the influence of a planet you can punch it and in 1 second you're... 11 or 12 miles away. At that speed in the time it would take a planet comparable to Earth to revolve once (one day..), your spelljammer can fly A MILLION MILES. Forget the real-world issues of space travel. Pay attention to the impact that the rules you have are going to have on the campaign setting you're inserting them into.

Your ground-bound campaign for instance. If the players didn't have a map of the whole globe before - they sure as hell don't need one now. In 2E spelljammer, for an Earth-sized planet, a spelljammer could reach ANY spot on that globe in no more than about 30 minutes IIRC. You could move goods from the other side of the planet as fast as you can load and unload the ship because travel time is damn near irrelevant. The issue comes in people not REALIZING that, hey, once "in orbit" you can see all the undiscovered islands, entire continents, weather, rivers, oceans, terrain that people may not have even theorized existed simply because they could never travel the vast distances needed to find it. Well, the moment you fly off in a spelljammer that time and distance problem is gone. You can have MANY campaigns that are nothing more than flying around over an existing ground campaign and discovering all that stuff... being Volo, and Columbus, and Captain Kirk all at once. You don't even NEED to fly off to moons, other planets or other systems. But the exploration of your single starting world may or may not be what you wanted to do with spelljammer.

IS that the campaign you wanted? You say they're headed off to drop a magical nuke on the moon that NOBODY has ever even been to before. Nuke the site from orbit. Okay. THEN what? Your concern isn't orbital velocity - it's what the hell the PC's are going to do next and why they would want to do it. Issues of orbital velocity you should already have wiped off your list of concerns because YOU'RE RUNNING A SPELLJAMMER GAME NOW. That isn't supposed to be a mystery to solve now - but what LIVES on the moon they're about to blow up is. Or what USED to live there that might be upset that the PC's just destroyed it when they come back from visiting Aunt Marglebargle on Pluto. What's between the planet and the moon? Monsters? Inhabited asteroids with bars full of worm-guys from MIB? Vortexes to suck the ship out to the furthest planet in the system? Space pirates? The ashes of Gene Roddenberry? THAT is your focus.

If you get caught up in the basic mechanics/fantasy physics of planets, moons, stars, comets, whatever you're going to overlook the fact that players are STILL going to want to do something other than make fantasy physics calculations. They want enemies to kill, treasure to loot, strange new worlds and new civilizations, and the stellar wars that they've been fighting for a thousand years while your original planet went about it's business in relatively peaceful ignorance.

Spelljammer is going to take any old campaign you had and turn it on its ear. Figure out the NEW campaign you're going to run - which I'm sure the players hope ISN'T about orbital mechanics.

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