Reactionless thrusters are an oxymoron. Isaac Newton spoiled it for everyone with his Laws of Motion (tm). To get anywhere you exert an equal and opposite reaction on mass around you or that you expel.
The problem is that this method sucks for space travel stories. Read Ken Burnside's The Hot Equations, or darn near any part of Winchell Chung's Atomic Rockets website if you doubt it. High acceleration ships have to be all propellant just to get to orbit. Deep space ships get wherever they're going slowly in order to wring the most out of their stores of propellant. There's no rocketing to the rescue of the Martian Princess. A Mars transit will take months. It's not quite so bad to get to Luna but rescuing the Lunar Princess is just silly. Similarly your secret invasion of Earth will be underway for months and detectable by a High School Science project winner.
Okay an Orion drive is high specific impulse and high thrust. However most referees will be hesitant about giving a load of nukes to player characters even if they 'promise' to only use them for propulsion. I would. restricting it to government or military use means either the characters are limping along in their ion rocket or chemical engine and easy to catch by the cops or they get hold of their own Orion to level the field and become a minor nuclear power. I don't like to give my players a grenade launcher.
So enter the reactionless drive. It turns energy into thrust with no unpleasant radiation or huge propellant tanks. Most action oriented video series have such a drive because a show where the characters are constantly cooped up in their ship is hard to keep interesting. Novels can pull it off better. Unrealistically short travel times are the standard for roleplaying games as well.
Here are a few alternatives to lay on your reactionless drives though. I mean expecting a drive that violates the laws of nature with no consequences is a bit optimistic (if you don't count people using gigs to ram planets at near lightspeed.)
Heat: My favorite. Even if you break the Laws of Motion, thermodynamics is still a bitch. You need to dump waste heat meaning radiators that can be damaged. For extra crunch you can have engineering constraints limit acceleration with radiators deployed. Ad Astra games do a great jobe of dealing with radiators and their consequences.
Reactionless Reaction Drives: Your drive basically uses a planet or other massive body to thrust against. This limits its range. You might be able to thrust at five gees but the acceleration falls off the further you get from a planet or star. This limits your maximum velocity.
No Reaction Drives: Not as bad as it seems if you have a warp drive or a jump drive with very accurate navigation. Who cares if your nerva drive will take a year to get to Jupiter? Do a micro-jump or warp to get there in a week or arbitrarily short time. Any planet in the solar system is only a week away. This also allows some strategic stealth, if you can't detect a ship going ftl. You might also allow a limited warp drive ( 10% light speed will get you to Pluto in a little over a day from the inner system.) If you're being very realistic you'd have to have landers and orbital craft and such strange and wonderful infra structure as space elevators and bolo transfers.
Mostly Reactionless: the drive requires some reaction mass, though not as much as a reaction drive. The tyranny of the rocket is replaced by a constitutional monarchy. Instead of every gram counting every kilo or ton counts. You can report for duty with some baggage and need not be shaved bald and starved to a maximum weight a little above that of a chubby skeleton. Keep in mind with this dodge that all vehicles will be propelled by thrusters, they'll be way more fuel efficient than anything in the real world. That's barring exhaust issues or other nasty side effects.
Note that you can use any or all of the above in whatever way suits you.
Some other side effects or clauses:
Reactionless drives work by folding space in unnatural ways letting things in that do bad stuff to ships.
Reactionless drives put out radiation and lots of it.
Ever wonder how ghosts that are essentially massless entities manage to move stuff? Me too. Your drive works by binding psionic, non-biological entities to its programming. Cue the philosophers and SJWs and again what happens when they get loose (and they will)?