Star system generation in Classic Traveller favors creating shirtsleeve worlds. The majority of systems that the rules generate are the sort of places you can walk around on. Sometimes you need a filter mask, sometimes you need specialized clothing, but in general they are very livable places. This is all very well understood.

I want to talk about some variant rules that generate systems that don’t fall into this pattern. These are rules that I developed for my own games, to fit the sort of maps that I wanted to play around in. In general, I don’t like to see a lot of shirtsleeve worlds. I like to see hostile systems, and I like games that largely occur in space and in space habitats. I’ve been using the Book 6 expanded system generation rules for decades, so all of these rules are based on that. If you don’t have a copy of Book 6, these rules also exist in similar forms in MegaTraveller, T5, etc.

Expanded System Generation

Book 6 provides a set of rules for generating a complete star system, filled with planets. There are a couple of methods of doing this: the Continuation Method, and the Expanded Method. The Continuation Method starts with a pregenerated main world, and generates the rest of the system around it. The benefit of this is it lets you use an existing subsector. The Expanded Method, on the other hand, generates the star system from scratch. There are some statistical differences between the two, the big one being that the expanded rules are slightly less likely to give you a shirtsleeve world. They still do favor generating shirtsleeve worlds, and they fit very nicely into the feel of a standard Traveller map.

Stellar Class Tweaks

Book 6 generates stellar classes for stars, so you might have a class B star that’s bright and blue in a system, or a planet might huddle close to a dim class M red star. However, the same tables are used for both the continuation and the expanded method, and so they are biased towards sunlike stars set up for shirtsleeve worlds. I’ve come up with a variant table that favors the class M stars that are far more common in the galaxy.

Note that this table will never generate class O or B stars. Those are rare, and should be hand-placed.

Atmosphere Tweaks

The expanded system generation rules apply a small negative DM to the atmosphere type of words outside of the habitable zone of their star. However, they still generate a lot of breathable atmospheres all over the system. One of the things I wanted to address is this. Since I was effectively redefining the atmosphere rules, I wanted to make sure I understood what the various atmospheres meant to me.

Vaccuum and trace atmospheres: These are the familiar class 0 and 1 atmospheres. They are, effectively, what they say on the tin. In terms of survival, both effectively require you to wear a vacc suit.

Shirtsleeve worlds: Atmosphere class 2-9. Some of these require filtermasks, some require compressors, some require both. Yet, breathing apparatus aside, you can walk around in ordinary clothes. There is an implication that the temperature is sufficient to allow someone to survive in ordinary clothes, though “ordinary clothes” might include things like stillsuits, cold weather clothing sufficient for Antarctica, etc.

Exotic: Class A. Exotic atmospheres are unbreathable, but otherwise these are still shirtsleeve worlds. You might have to wear heavy insulative clothes to deal with temperatures, but otherwise you can get by with what are effectively scuba tanks.

Corrosive: Atmosphere class B. Corrosive atmospheres require the use of protective suits or vacc suits. Protective suits are closed environment suits with their own breathing systems, and are basically vacc suits for use in atmospheres. I used to believe corrosive atmospheres were literally corrosive – acid droplets in the air, that sort of thing. In retrospect, looking at them from simply a game-rules point of view, they are merely environments which are so hostile that you need a protective suit. The moon titan, for instance, has an atmosphere that’s largely nitrogen, which is not corrosive at all. I would call it a class B atmosphere, though, because it’s so cold that you couldn’t survive on the surface without a protective suit.

Insidious: Atmosphere class C. Insidious atmospheres are akin to corrosive ones, but they will defeat any personal protective measures in 2-12 hours. The surface of Venus, for example, is hot enough to destroy every lander that made it there within a matter of hours. No matter how good your protective suit, eventually that heat will seep through it and bake you. I would call that a classic insidious atmosphere. But imagine a world with a thick atmosphere of hydrogen – a failed gas giant, perhaps. Hydrogen gets everywhere. It will seep through any material by osmosis. An explorer in a space suit will eventually start seeing hydrogen seeping into their suit, forming a flammable mixture with their breathing air. Because of that, I would categorize that atmosphere as insidious, as well.

All of that, along with my desire to avoid shirtsleeve worlds outside of the habitable zone, led me to develop these tables for the inner and outer atmosphere:

Inner planet atmosphere

Outer planet atmosphere

Brown Dwarfs

This is the easiest variant rule to make. The Book 6 expanded system generation rules creates star systems with stars of class O-M – stars that shine by fusion. Adding brown dwarfs allows you to add additional star systems, typically without shirtsleeve worlds. To add brown dwarfs, just decide on a ratio of brown dwarfs to regular stars – anything from 1 brown dwarf per 6 stars all the way to 1:1. This makes it easy to determine which systems have fusing stars and which have brown dwarfs. For the most part, brown dwarfs don’t have a habitable zone, so all planets generated should follow the “outer planets” rules.

Brown Dwarf Stellar Class

Reducing Atmospheres

My current Traveller game sets up a universe that draws a lot from C.J. Cherryh’s Alliance-Union universe, and one of the big things about that setting is that planets with breathable atmospheres are rare. This variant simply says “roll a die for any breathable atmosphere to see if it’s actually an exotic atmosphere.” With my current setting, I made 90% of the breathable worlds into class A atmospheres.

Putting it all together

And lastly, here is a subsector that uses all of these rules. None of the planets have been populated, but you can see how there is only a single world with a breathable atmosphere in the whole subsector, and that atmosphere still requires a filtermask. The rest of the planets either have exotic or no atmosphere, and no shirtsleeve world exists around any of the brown dwarfs.

#Hex Name             UWP       Remarks          {Ix}  (Ex)    [Cx]   N     B  Z PBG W  A  Stellar
#--- ---------------- --------- ---------------- ----- ------- ------ ----- -- - --- -- -- -------
0103 all-0103         X000000-0 As Ba Va                                         000 6     FVI VI 
0104 all-0104         X000000-0 As Ba Va                                         000 3     MVI VI 
0105 all-0105         X5A7000-0 Ba                                               004 9     GV V   
0108 all-0108         X4AA000-0 Ba Wa                                            003 6     KVI VI 
0203 all-0203         X000000-0 As Ba Va                                         004 11    GV V   
0204 all-0204         X000000-0 As Ba Va                                         003 7     MV V   
0205 all-0205         X000000-0 As Ba Va                                         011 8     LDD    
0301 all-0301         X000000-0 As Ba Va                                         020 6     MV V   
0303 all-0303         X5A3000-0 Ba Po                                            003 10    FV V   
0306 all-0306         X5A0000-0 Ba De Po                                         004 9     GV V   
0308 all-0308         X000000-0 As Ba Va                                         012 9     GDD    
0309 all-0309         X5A7000-0 Ba                                               014 10    GIV IV 
0310 all-0310         X000000-0 As Ba Va                                         003 9     YDD    
0401 all-0401         X000000-0 As Ba Va                                         000 1     MV V   
0402 all-0402         X000000-0 As Ba Va                                         002 9     GV V   
0403 all-0403         X000000-0 As Ba Va                                         000 1     MDD    
0409 all-0409         X000000-0 As Ba Va                                         000 1     MV V   
0502 all-0502         X000000-0 As Ba Va                                         002 8     MV V   
0504 all-0504         X000000-0 As Ba Va                                         003 9     YDD    
0506 all-0506         X6A7000-0 Ba                                               004 10    MV V   
0508 all-0508         X5A9000-0 Ba                                               003 10    MII II 
0510 all-0510         X6A3000-0 Ba                                               003 8     MV V   
0601 all-0601         X000000-0 As Ba Va                                         000 4     YDD    
0609 all-0609         X000000-0 As Ba Va                                         002 6     GV V   
0610 all-0610         X553000-0 Ba Po                                            005 10    MV V   
0703 all-0703         X000000-0 As Ba Va                                         004 10    AV V   
0704 all-0704         X000000-0 As Ba Va                                         000 1     KVI VI 
0802 all-0802         X000000-0 As Ba Va                                         001 3     MV V   
0805 all-0805         X000000-0 As Ba Va                                         013 11    TDD    
0807 all-0807         X000000-0 As Ba Va                                         000 1     MDD    
0808 all-0808         X000000-0 As Ba Va                                         021 7     MV V

Ken Pick wrote a fantastic article for Freelance Traveller talking about using external mounts for cargo ships in Classic Traveller. I make heavy use of this in my own small ship settings. It’s fairly easy to do with large ships – loading up the 2000 ton Type TJ with a few thousand tons of external pods – but it gets more difficult with smaller ships. A 400 ton ship, for instance, that can carry 400 tons in pods is easy to make, but a 400 ton ship that can carry 800 tons in pods runs afoul of the Drive Potential Table, because it doesn’t have a line for a 1200 ton ship.

Rather than using Book 5 High Guard rules for ship construction, I wanted to stick with simple Book 2 ship construction, so I extended the Drive Potential Table, adding entries for 300 ton, 1200 ton, 1600 ton, and 6000 ton ships. For the 300 ton line, I divided the 600 ton line by 2. For the others, I multiplied the performance of the “half” entries by two, so 1200 ton performance is based on the 600 ton line, etc. The 6000 ton entry doesn’t include a build time, because it’s intended more to provide a line for a Type TJ with 4000 tons of external pods.

There are some oddities in the table, especially when you get into the heavier hulls, and I didn’t try to change those. They’re part of the character of the original game, and I wanted to stick close to that.