Ken Burnside said it best.Friends Don't Let Friends Use Reactionless Drives In Their Universes.
(Quote via Rocket Cat at Atomic Rockets)
Reactionless drives are a great example of a cure that is worse than the disease. true they get you magical accelerations. They expedite stories by not having your Space Patrol Cruiser (tm) take 17 months to get to Mars. After al in 17 months the evil mastermind might have put his plans into effect (evil masterminds are very proactive.)
The downside: with reactionless drives any space war becomes the final war for the participants. It has no limit to delta vee so even a ship's boat becomes a world wrecker moving at .999 c. Maybe races develop space travel, eventually discover reactionless drives and destroy each other with loads of jelly beans fired at relativistic velocities. After all when your sensor operate at light speed detecting incoming moving at .999 lightspeed doesn't give you a lot of time to file your will or otherwise react.
Oh yes. Reactionless drives pretty much invalidate most of physics as it is currently understood. Apparently giving the finger to Newton pisses a lot of smart people off even if people keep on playing Traveller in all its avatars. For every action there must be an equal and opposite reaction. Not just a good idea, it's the law.
What can we do about reactionless drives since everyone likes them? I offered one setting: Ghost Drives. You want a magical drive fine. It is powered by evil disgruntled spirits that may get loose and ruin your day if you overuse their powers. Go read the pages. But that was scratching the tip of the iceberg.
The first and simplest way to fix reaction drives is to say they are not reactionless. Maybe they shoot out dark matter or neutrinos or some form of matter or energy that doesn't interact with normal matter at all. The drawbacks are that you need a honking big power plant to run a rocket with a delta vee in the hundred or thousands of KPS. The heart of a star gets that hot but not much else. That kind of super science can raise other questions like what society is like with essentially free energy. Also you don't want to be near a power plant like that when it blows up. Instead of a kinetic weapon your ship becomes a fusion bomb. Local governments love having those in civilian hands.
Your rocket is now a torchship. You have a very high but magically infinite delta vee. You still need to carry propellant. Damned if I know what you'd carry fuel that is intangible to normal matter in.
The other way a reactionless drive might work is by thrusting against planetary bodies with a sort of reversed tractor beam (a repellor). No propellant needed. You still need fuel or energy for it to work. Also if it is a beam you're training on a planet that is pushing your ship then it will weaken with distance (actually the square of the distance which is way worse). So you feisty little blockade runner that can pull 4 gees at ten diameters from Earth will only pull .04 gees at 100 diameters (jump limit). This fits games like Traveller well where ships usually appear near a planet and fight or drop cargo. A trip will start with a period of high acceleration, a long coasting period and frantic deceleration at the destination planet.
Reactionless drives might work by manipulating a fluid filling hyperspace (or warp space or whatever you call it). By thrusting against this medium they move. The medium also exerts drag on a ship giving it a maximum velocity.YMMV.
Another cheat is to limit your power systems. Say there is a minimum size for fusion power plants (a biiiiiig minimum). Your dinky little free trader couldn't mount one even if you got all the government licenses. Instead you use fuel cells or solar panels with al their lovely side effects. You run out of power you stop accelerating. I leave it to the referee to figure out why people can't get fission reactors and find uranium in the vast universe. Because that will let you thrust for years.
As a final note some drives in the real world are effectively reactionless: solar and magnetic sails are being developed now but their accelerations are minuscule resulting in journeys of months at the very least. Perhaps what we really need to develop is a montage device. You turn it on, everybody works to a soundtrack and boom it is months later.