George Lucas gave us the midichlorians, which no one wanted. Blame the Atomic Rockets website and Winchell Chung for making me think about these things. Allow me to hang a lampshade on the dreadnought in the room.
Maneuver drives: give them some electricity and they let you go anywhere. No messy rockets or delta-v or limits except light speed. Here's the rub: if you get your ship up to great speed,. It becomes an angel of death. Anything in its way dies. When a 20 ton launch can destroy a planet almost as fast as you can detect and track it what good is a navy or a government that can't protect you?
(The blogger notes: fusion rockets are no picnic either.)
Obviously that isn't the case. We had navies since Book 1. We had governments, no less than three Imperiums last time I checked, since Book 4.
The M-drive and the J-drive are both indispensable to Traveller and both shoot the game's hard science rep to hell. The J-drive lets you effectively travel faster than light giving Einstein the finger. The M-drive lets you go anywhere slower than light without using nasty rockets and dragging massive fuel tanks around and gives Newton the finger. It would be somewhat logical to say they are related in some way. The devices that shoot our understanding of physics operate according to some similar principles.
The J-drive only works properly 100 diameters away from a planet or other massive object. Never mind why.
What if an M-drive has an exhaust in a fashion? As the M-drive runs it creates an energy field like static electricity. This field increases in strength the longer you run the drives. Fly around within the 100 diameter limit and this energy dissipates continuously and you'd never know it was there.
But if you cruise around outside the 100D limit then that energy builds up. When you reach the 100D limit it is discharged. If you hit the 100D limit too fast a'o with too high a charge not only is your energy build up high you discharge it even faster. Therefore ships try to reach the 100 diameter limit at low speeds. Note that flying inside the limit doesn't build up a charge. Ships going between planets in a system better make sure they arrive at their destination world at a low speed.
If ships don't watch their speed and hit the limit at a few kilometers a second some part may burn out. At tens of kilometers a part may blow out. At hundreds of kilometers a part will blow out and probably take several walls with it. You get the idea. Hit it at any higher speeds and you are going to vaporize your ship. Any matter exiting the field takes some of the energy with it proportionate to its mass. No fair making a run at the Throne Planet, dropping a load of ball bearings then jumping away. They'll disintegrate too.
So we can have a navy and an Imperium! Any space stations are going to be near planets and dwarf planets. Colonies and stations beyond that limit are flat out. You can't defend them against terrorists or extortion. Ships arriving at a planet will take several hours to move a few kilometers inside the 100D limit to discharge their energies safely.
What about things like missiles or micro-meteors striking the ship? A ship that is not a sphere would probably have its 100 diameter limit around 50 times its longest dimension. So a Scout's limit is about 1500 meters. An object smaller than a ship would probably trigger the effect when it reached that limit at bear the brunt of some of the energy being released and affecting it. This makes M-drives very handy for interstellar travel. At high speeds the field will disintegrate or deflect anything you'd find the hard way at relativistic speeds. No need to lug a small moon for shielding!
Pre-gravitic STL ships might be viewed as very scary, especially those that can get to relativistic speeds. they don't have to deal with the technobabble and can hit a target at incredible speed. If I was a nutcase and found a Bussard ramship I'd sell it to a collector and use the funds to bankroll my evil plans but that's just me.
This does mean that missiles are all proximity fused. They fire a spread of very durable projectiles that survive the field effect long enough to reach the target. Or the damage effect is still relatively low at tens or hundreds of KPS to let the missile reach its target though damaged. I'm not redoing the combat systems.
But let's take a look at a long time staple of the Traveller 'Verse: spinal mounts. Now I like many built my Trillion Credit Squadron (tm). Hell I participated in the first TSC tournament at GenCon. How did I do? Unimportant. Off topic. Meson guns were recently taken apart as not working on the Atomic Rockets website (and maybe other places.) What if instead of a particle accelerator they used M-drive tech to accelerate tiny projectiles to near light speeds? The projectiles would take along their own field of double talk energy and explode when they hit the enemy ship. Even better if you had a long needle ships with parts sticking out of its sphere of energy. Instead of meson screens you'd have some sort of grav shield that replicates the 100D effect far away from the ship. Or one of these monsters could be used to accelerate ships. Just build a THICK thruster plate on the back (like the Orion project.) Aim the projectiles very carefully and off you go. You get sped up to high speed without using your own fuel and presumably decelerated if you head for a civilized star system. Note you still couldn't use one of these beasts to bombard a vacuum world unlike the original meson guns.
I admit it's more double talk. In my defense:
1) it gives some kind of limits to the M-drive
2) it adds a layer of color to my Traveller setting
3) better to know your double talk before that suicidal 8-term Scout with all 2's in their stats tries to turn the sector capital starport into a s'more.