Making a Single-system Sector in Stars Without Number
Preface: I’m a big fan of hard sci-fi, and after getting a copy of Engines of Babylon (and binge-watching The Expanse for the second time), I’ve really been into the idea of running a game of SWN in single star system. The massive amount of text below is my attempt at developing a way to make a SWN sector fit into a single large star system, while still preserving as much of SWN’s retro hex-crawling sandbox magic as I can.
Using a Hexmap for a Star System
In my first attempts at making a one-system sector using the spaceflight rules in Engines of Babylon, one thing I really missed was the hexmap. Not only is it a great visual tool for players, it also gives the game a wonderful retro feel. The problem is that unlike relatively static stars, planets are constantly whirling around at different speeds, and thus difficult to pin down on a grid.
Fortunately, some clever person in 1995 wrote a game called Rocket Flight in which there was a scientifically accurate way to represent in-system spaceflight on a hexmap. It’s better explained here, but essentially one can use the hexes to represent energy levels (aka orbits). Moving vertically from the top represents moving further from the sun, and moving horizontally represents moving between different orbits that are roughly the same distance from the sun.
Since the basic performance of a rocket is measured in delta-v, The designers of Rocket Flight abstracted this to a system when ships spend fuel points to move from hex to hex. I’ll talk about applying this to SWN after I go over my approach to placing objects into this delta-v grid.
First of all, it makes sense to start by assuming that we’re working with a star that’s a bit bigger than our sun. We want a nice big goldilocks zone where we can fit lots of habitable planets.
If we go off of the example from Rocket Flight, we’ll run into problems from the fact that it uses a hexgrid bigger than SWN’s to represent a fairly small part of our solar system. If we want to fit in the 20+ planets of a SWN sector, we’re going to run into problems. As I see it, there are two options for getting around this:
2. Make the hexes represent larger amounts of energy, and relegate gas-giant systems to a single hex
I’m a fan of option 2, for a few reasons. For one thing, it maps nicely to regular SWN. Planets with inhabited moons and gas giant systems become like the multi-planet systems of a normal sector – like a multi-planet system with spike drives, we can treat movement within gas-giant systems as being fuel effective enough to not matter much for fuel consumption. This spares one from having to figure out what was going on with the Jupiter system in Rocket Flight. That said, a combination of both options may be optimal.
Before I start placing planets, I like to roll to put down an asteroid belt or two. For this, I just roll a d10 twice and put down asteroids in a big v-shape starting from the row number I rolled. These are helpful for visually orienting one’s self on the map, and are good for having a sense of where mining stations will be.
Now, I place planets like I would place stars in a typical sector, but only place 10-15. If a planet falls on an asteroid belt, I assume it is a Ceres-like planetoid. If two planets land on the same hex, I treat one as a moon of the other.
Next, I roll a d6 for each planet that doesn’t have a moon and isn’t in a belt. On a roll of 6, this planet is now a gas giant. I then roll 1d4+1 to determine how many extra habitable moons are in its system. (Increasing this roll to 1d6+1 or 1d8+2 is a good way to boost the number of worlds in the sector without overcrowding the hex grid).
When rolling a planet’s temperature, I roll the usual 2d6, but add +3 to the roll for all planets in the first row, +2 for the second, all the way down to -7 for the last. This gives a general trend of warmer to colder planets. I privileged colder worlds a bit, figuring that future space colonists would rather live on a frozen world than a burning one.
And from there, it’s basically like regular sector creation – roll atmospheres, biospheres, tags, societies, etc.
So how do we actually deal with travel between planets? It turns out that one can do a pretty simple port of what roughly are the rocket flight rules to SWN. As mentioned above, you can read a rundown of the Rocket Flight rules here. I’m going to attempt a rough implementation of them to SWN. I am not a rocket scientist, and I’ve never actually played Rocket Flight, so my rule of thumb has been to assume the designers generally knew what they were doing and that the rules are accurately reported on the linked page.
To simplify things, I’ll assume all human ships have roughly the same type of drive, and thus the same specific impulse. I’ll also keep the ship-design, combat and travel rules from the slowships section of Engines of Babylon with a couple of modifications.
In the real world, Delta-v (the ‘distances’ on our hex grid) is calculated from two things: exhaust velocity and the ratio between reaction mass and “dry mass” (everything else). (There are a few other relevant factors, but they can be ignored). The problem is that the ship design rules of EoB account for mass, but not mass ratio – a cruiser can load up on more mass bunkers than a frigate, and thus can go much farther despite having more structural mass to lug around. Now, big ships should have a slight advantage, because the square-cube law works in their favor – the storage space of a fuel tank increases cubically, while it’s mass (determined by its surface area) increases by squares – but at present the advantage is ridiculous.
My proposed solution is to assume every hull, before any fittings are added, is optimized for, say, a 30% mass ratio, giving each hull (excluding habs and stations) a basic amount of delta-v. In game terms, this means that I treat each hull as if it begins with, say, ten points of fuel. The mass-bunkers represent a conscious choice to devote extra mass to fuel storage.
Rocket Flight doesn’t specify how long one of its turns lasts, so I’ll call our basic unit of travel time one month. Then, going off of the description on the Project Rho page, to move one hex in one month, a ship must spend one fuel point. To move two hexes in one month or one hex in two weeks, the ship must spend four fuel points. To move three hexes in one month, eight fuel points. Four hexes, sixteen. (I think the jump from one to four fuel points represents the gap in fuel cost from Hohmann to Brachistochrone transfers, but I’m not sure). To stop midflight (to answer a distress signal, investigate a newly discovered asteroid, etc.) costs half the spent fuel, minimum one. To resume the journey, simply recalculate the trip from whatever hex the ship stopped in.
When stopping midflight, crews must make a physical effect save to withstand the high g-forces. [expanse link] I set the DC as 10+ the number of fuel points spent to stop. So a typical burn is an easy 11, but for ships that spent 16 fuel points to go four hexes will have a risky DC 18. PCs in “crash couches” or other gravity mitigating setups get a +5. Failure adds 1dx points of system strain where x is the number of fuel points spent to stop (1,2,4, or 8).
For gravitational slingshots, a ship ending its month-long “turn” on a hex with a planet can optionally move one extra hex in any direction except that in which they were moving. For gas giants, you get three extra hexes.
Since I’ve added quite a bit to the baseline amount of fuel each hull has, I’ll amend the E0B rules so that getting into orbit from a planet’s surface and vice versa costs 3 fuel points for an earth sized planet, 2 points for a smaller planet, and 1 point for a planetoid.
The last thing I want to talk about is a few ideas for recreating some of the basic notions of SWN lore in a one-system sector. Here are some issues I’ve found, and possible solutions:
- With no Jumpgates, the Scream can’t really happen. That said, there are other reasons why space travel might shut down for a few hundred years. Maybe some sort of horrible Kessler syndrome has made most orbits unsafe. Perhaps the system’s sun started emitting dangerous radiation for a couple centuries, making life only possible under the protection of planetary magnetic fields and radiation shielding.
- There’s also the fact that even with a scream-substitute, TL3 and above planets will be able to communicate with each other using radios (though with delays). This doesn’t make that much of a difference, but it’s worth being aware of. If the PCs discover something horrible on an outer planet, they can just send out a radio message and a few hours later it will be heard all over the system.
- Finally, in a single-system setting, there are no neighboring sectors, no vast Terran Mandate that it was once part of (unless you do allow for ftl travel to have existed at some point).
So there you have it. This is a very unfinished and untested system, so feedback, criticism, and suggestions are welcome. I had a very specific idea of space travel I wanted to capture with this, so I’m happy to answer questions about why I made the choices I did.
