Been reading the old Prime Directive RPG. I got it stupid cheap ($2.00 on clearance at Half Price Books). I have a thing for both little-known systems and non-canon stuff (be it Star Trek, Star Wars, or comics), and this ticks both boxes.
I get why this game might be off-putting to many. Too much Trek for non-Trek fans, not enough Trek for Trekkers. See, it's set in the universe of the Star Fleet Battles wargame. So, take The Original Series, add The Animated Series, then add a dozen wars, season with a dash of Niven, and blend. Garnish with smooth-headed Klingons, and voilá!
The result is a very militaristic take on the original Star Trek universe.
The game was designed from the ground up to simulate the tropes of the TV show, despite being far more combat-oriented. But aside from the trappings of Trek, the game has a unique RPG system underneath the hood.
Prime Directive uses pools of 6-sided dice, the number of which is determined by stat and/or skill. Every action has a "Tricode", or 3 ascending target numbers (e.g., 4/6/8 for minimal/moderate/complete success). The highest number rolled out of the dice in the pool is the one that counts. 6's explode, and are re-rolled, with the new roll -1 being added to the first roll of 6. There are also rules for failures and "botches", of course. In combat situations, the same roll determines both the success of a given attack, and its damage. Each weapon has a maximum damage capacity, and the Success Level decides how much of it is applied. Initiative is a little hard to grok at first. The characters roll, usually against the base initiative Tricode of 4/6/8, with the same roll being read two different ways to determine both the order of action (determined by overall number of successes, which aren't counted for other roll types), and how many/what type of actions each character can do in their turn (determined by Success Level). So, it is possible that a character might not get to act first, but would get to do more. There's sometimes more to it (such as modifiers), but that's the gist.
Characters have both stun and lethal damage pools.
There is a great analysis of the system here:https://rpggeek.com/thread/731070/share ... ode-system
And please don't think I'm lazy for not wanting to
a) quote that thread so extensively that I may as well copy/paste the whole thing
b) copy/paste the whole thing
That thread contains a finer dissection than I could give. Check it out for a real nuts n' bolts breakdown of the system.
As for the setting, aside from what is detailed above, the characters are primarily part of "Prime Teams", basically a Special Forces version of Away Teams (though later supplements expanded the theater of play).
Now, whether you like this take on Star Trek or not (and I am NOT wading into that quagmire), it seems to me that this system is sound, and would probably be fun to play. It also seems to me that, with little alteration, the Tricode System could be used as a damn good generic sci-fi ruleset. Well, except for the complete absence of space combat/ship construction/etc. rules. Presumably, the designers intended players to use SFB for that. But, hey, Star Frontiers didn't have those rules out of the box, either, and it did just fine. Prime Directive does have enough to get you started nicely. Weapon and equipment lists, tons of skills, even psionics, for both Vulcans and non-Vulcans (these were also greatly expanded in later books).
There are several optional rules simplifications throughout, for those who want a less complex game. But, overall, Prime Directive lends itself to a more "cinematic" style of play (though it can certainly be deadly!). I find it particularly interesting that the designers choose to break from the complex rigidity of the Star Fleet Battles game for its role-playing counterpart.
I think a re-skinning (or maybe de-skinning, in this case) of these rules would make a fine retro-clone.
And, for that matter, I see no reason why you couldn't use these rules to run more traditional Star Trek adventures, with a bit of tweaking. I could also see this same basic system used for other genres, just as Tunnels & Trolls begat Mercenaries, Spies & Private Eyes.
Prime Directive, AKA PD One, is available from drivethrurpg. Not the best scans, to be honest, but at least it's available.
Later iterations of Prime Directive would use GURPS or D20, but to me, neither has the unique charm of Tricode, and neither was custom-built to convey the setting.