Actually, Firefly/Serenity COULD be a Traveller campaign in an Alternate TU. So could the VorKosigan series by Bujold, or the Sten Series by Cole & Bunch. Or, just about anything by Niven, Pournelle, Heinlein, or Azimov. Space Above and Beyond also could easily be done in traveller.
Calling Traveller "Hard Sci Fi" is like Calling the Weekly World News a credible newspaper... there are those who believe it, but they're rare and not in touch with consensual reality.
It's pretty hard science for the space opera genre, but it's got more than the allowed "3 breaks from reality" for hard science fiction.
There have, Traditionally, been several styles of traveller games:
- Tramp Merchant Crew
- Mercs in space
- Troubleshooters for Hire
- Political Machination
- TCS*
Tram merchant crews run much like Serenity. Small group, struggling to make the bills. Ethically challenged individuals can make lots of money by doing the odd jobs. Even the "routine day to day" can have its hairy (and roleplay worthy) moments.
Mercs play, well, it ranges from the squad for a nail mission to the cadre of regiments. A few years ago, I ran a Traveller Game (using MegaTraveller) where the PC's were the cadre (Officers and SNCO's) of an Imperial Marine Drop Regiment (Battalion Combat Team, really, but hey.) A little automation later, and I had the entire regiment statted out, and the unit totals figured for the MT Mass Combat rules.
Troubleshooters for hire is a mode that several of the Classic Traveller adventures use. Patron has a problem, and is seeking ethically deficient folk to cure it. It varies from Nail Missions worthy of battle-hardened mercs, to deliver the package.
The Political game: It has been a not-uncommon variant to have PC's as high level officers and functionaries of small pocket empires. (T4's support for this was the most in-depth; TNE was the most playable for smaller colonial systems and pocket empires.) When you are playing the Big Fish, the style of play is VERY different. Kind of like Diplomacy, but with characters. May include a TCS element, as well.
The TCS game: You are the fleet admiral for a pocket empire. Here's your budget. Wipe out our enemy (run by another player). No character play, save perhaps some improv, this is the wargame mode.
Now, as to prior rulesets:
CT: CLassic Traveller.
- Brutal combat, but first shots seldom killed. All injuries caused immediate (and temporary) stat losses. No Hit Points, since damage applied to stats.
- No task system, but combat to hit was 8+ on 2d+ mods.
- Trade system was "Seek lot, if lot found, find out what it is and how much it costs, negotiation presumed by table rolls."
- Alternate trade system (Bk 7) dropped the "find out what it is" step completely.
- Two (very different) critical hit based ship combat systems and two matching ship design systems.
- Character Generation as mini-game.
- Stat:Skill use is about 1:5 but varies ***
Stat:Skill Acquisition is 1:1
MT: MegaTraveller
- Damage to hit points resolved to Stat Damage, as I like to put it, "After the Adrenaline Wears Off."
- Ship combat was a variant of the CT Bk 5 system.
- Craft design included everything from motorcycles to Battleships, but was incompatible with CT designs.
- Characters have an additional skill per term available due to special duty.
- CGen still a mini-game, but set up to allow more choices.
- Separate Damage and Penetration stats allowed for consolidation of units into a single "unit" stat without all units needing to be same scale.
- Used Bk 7 Trade system
- Setting integrated to core rules
- Extra skills absorbed by larger skill list.
- Stat:Skill Use is 5:1
Stat:Skill Aquisition is 1:1
TNE: Traveller-The New Era
- Rescaled attributes and skills completely.
- CGen minigame far less random
- Skills per term are descending scale, but first term in a career quite heavy. Worked out pretty much to about 1 level higher than CT/MT
- Very different task ssytem using 1d20.
- Hit point system for damage
- Detailed craft design system (incompatible with both CT and MT)
- Used the CT Bk7 Trade system.
- Starship combat very fiddly
- Very humanocentric ruleset.
- Setting has been destroyed by "AI Virus"
- Design systems tried to feel more hard sci-fi, not in core rules
- Stat:skill use is 1:1
Stat:Skill Aquisition is 1:2
T4: Marc Miller's Traveller
- Returned to original attribute scale
- Totally different skill scaling; one per year, rather than 1 per term.
- Shorter Skill list than MT or TNE
- Combat system does damage to atts.
- Armor highly effective
- PC"s all widely skilled
- CGen mini-game has "Roll or pick" at almost all decision points.
- Task system flawed.
- Strong emphasis on stats within task system.
- design systems expanded from TNE, not in core rules
- Stat:Skill use is 1:1
Stat:Skill Aquisition is 1:1
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* Trillion Credit Squadron. Adventure 5.
** Critical Hit systems do not track non-funtion-reducing hits.
*** Use is the ratio of how many points of Stat equal how many points of skill. Acquisition is how many skills levels are a stat equivalent to for CG purposes.