Single-system Hexmaps for Stars Without Number
One of my 'white whale' RPG campaign goals is to run a hard(-ish) sci-fi rocketpunk campaign in a setting similar to The Expanse or the settings sketched out on the Atomic Rockets website, using a slightly hacked version of Stars Without Number.
SWN assumes a hexcrawl with destination stars/planets randomly allocated to Hexes (example here). Players use faster-than-light 'spike drives' to travel between them. The Engines of Babylon supplement has rules for non-ftl ships that use 'range bands' to track distance & fuel expenditure. It's perfectly fine, but I'm a nerd about these things and want to do better. In particular, the range band system loses the open-endedness, intelligibility, and old-school feel of the hex crawl.
So a long time ago I found this on one of the Atomic Rockets Project Rho sites, where Winchell Chung breaks down some old board games that do hard-sf space travel well. Rocket Flight, an out-of-print board game, had robust rules for realistic spaceflight on a hex map.
As Chung explains,
The game gem in Rocket Flight is the strategic map. Unlike other games strategic space maps, it is not a map of, well, space. It is a map of energy states.
The Rocket Flight strategic map does not have hexes that represent X thousands of kilometers of space. Instead, each hex represents a delta V (i.e, change in velocity) of three km/s. So by measuring the distance between planets in hexes, one determines the delta V required to move the spacecraft to that planet.
This is convenient, since that is the units that spacecraft propulsion is rated in.
Therefore a map gridded in delta V models the situation much better than one gridded in distances.
Genius, pure genius.
Each hex is an entire unique orbit, or an abstraction of being partway along a transfer orbit between two stable orbits. This is a bit tricky to wrap ones head around but I don't think it's much worse than spike-drive hyperspace as described in SWN 'canon'.
Rocket Flight rules map very nicely onto existing SWN travel rules where the hex range of one's ship is determined by amount of fuel and engine quality. Project Rho even provides a handy table for calculating fuel expenditure per hex based on engine quality (specific impulse) and dry mass:
For SWN, this could be simplified a bit; maybe have one high-thrust, low SI drive (Methane NTR?), a high-SI low-thrust drive (hydrogen NTR?), and a high-thrust high-SI zubrin NSWR or Orion drive that's extremely dangerous to use (military only?). Then have a dry mass rating for each ship size and a table of fuel costs for each hex of range desired. Since bolting on extra remass tanks1 affects mass ratio non-linearly, costs would rise exponentially for each additional hex, incentivizing frequent refueling stops (and thus adventures).
At the most basic, it boils down to tracking abstracted 'delta-V points'. When refueling, decide how many points you want to load up on and consult the table for your ship type to see the cost. When underway, you can either spend extra points to go short distances fast, or dole them out to go long distances slowly.
With that said, I see a few potential issues:
Time - as far as I can tell, Rocket Flight assumes all spacecraft move one hex/turn, with the option to burn 2^n units of propellant to move n hexes in a turn. I'm not sure how long a turn is supposed to be, but based on the distance from Earth to Mars I'm guessing something like two months? This gets wonky for things like Earth-Luna, which should only take a day or two.2 My inclination would be to introduce some fudging here with imaginary high-thrust drives such that travel turns are one week (as in SWN3), and maybe add a rule that travel within a moon system takes one day/hex.
Map size - the above map is pretty close to a typical SWN sector, but only extends out to Jupiter. Adding Saturn would (afaik) require a pretty long extension. It might make sense to make gas giant systems their own hexmaps and link them pointcrawl-style flat travel cost.
In hard-sf space travel, it's tougher to justify fancy piloting - so SWN's conciet that players investing in the Pilot skill can get places cheaper and faster doesn't really apply. Ideas:
Landing on anything that isn't a spaceport probably still requires skill, or docking with a derelict, or matching orbits with a piece of valuable space-debris. Same with fancy maneuvers like gravitational slingshots. Combat (in civilian ships without specialized combat computers) would definitely take a good pilot.
For a slightly dystopian or cyberpunk bent, reduce Pilot to a more niche skill for piloting planes and helicopters and use Administer to affect travel times (for settings with hyper-regulated bureaucratic space shipping lanes or something).
Go full rocketpunk with slide rules, analog computers, and hot-shot rocketeers who can crunch out orbital dynamics in their heads (by rolling the Pilot skill).
Ultimately, I think it's delightful that a hexmap can approximate realistic space travel at all. I'd argue that tracking 'delta-V' points is about as book-keeping intensive as standard SWN travel and strikes a good balance between realism and convenience. I'm honestly surprised I haven't seen similar systems in use before, though it maybe I'm not looking in the right places.
I'll be keeping this system in my back pocket and might typeset it into a proper modular ruleset for my next SWN game.
I'm assuming for this system that most ships are semi-modular and can add or remove fuel tanks as needed.↩
I'm guessing that for Rocket Flight, resource management > strict time tracking, hence taking x weeks or months to get to Luna is less important than that can be done in one turn or less and requires 5 km/s of remass.↩
It's actually six days for some reason, but close enough.↩