Okay reaction drives. I was hoping someone else would do this write up as it involves the e funstion and such but it is time to man up.
There are reaction drives in Classic Traveller. You just have to dig deep to find them. The first mention of using propellant in space movement comes from the 1977 LBBs. It said that non-starships used 10 kilograms of fuel per gee of acceleration per turn regardless of mass or cargo carried (this may be the first time Traveller formally abstracted mass and volume into the kludge that we came to know and love.) The second instance was in Special Supplement 3: Missiles in Traveller (included in JTAS #21). There it was specified that a 50 kg missile required one kilogram of fuel for burn (one gee for one combat turn).
Hell of a difference. You'd think from SS3 a ship would require 2% f its mass for a burn or two tons of propellant for a Scout Courier to boost at one gee for one turn. Using LBB 2 a non-starship requires 10 kg regardless of mass which is a little too rules light for me. Let's call it .01 mass for vessels because you can build bigger engines more efficiently according to Striker.
That's a minimum specific impulse of 100,000 for those who keep track of such things (50,000 for missile engines) and all I can say is yikes!* Ion engines have an efficiency like that but they are very low thrust to keep from melting. Orion nuclear engines are in that ballpark but again we have the specter of using a spacecraft as a WMD and they are also big assed things, not safe for driving little bitty missiles. Lunching your own missiles and hitting a self destruct switch also becomes hard to tell apart. I'd accept that it is future technology but according to SS3 these engines start around TL 8 (which we are poking with a short stick that we have advantageous strength for).
Actually a gravity control system to speed up rocket exhausts would be a way to use gravity generators to move a ship efficiently without causing Newton to haunt us. The engine fires its exhaust through a gravity ring where it is sped up to an ungodly speed and expelled. Think of it like a mass driver but not so picky about what it accelerates, instead of ferrous material you could shoot dirty socks out of this thing though your mileage might suffer. Stick with hydrogen. It honors tradition. You don't need to worry about the engine melting with all that power because ... science and we never used heat radiators in Traveller anyway.
Another possibility is the oft overlooked nuclear damper. What that implies is a technology that lets you manipulate the strong and weak nuclear forces. I'm not even educated enough to bullshit about what that would imply in chemistry, medicine and physics. I do know those forces are real though short ranged. Perhaps instead of a new force (artificial gravity) maneuver drives, air/rafts and grav plates actually use the nuclear forces?
Artificial gravity is pretty short ranged in canon. An air/raft can take you to orbit but not much further. Grav plate effects do not stack in a noticeable way, otherwise a three deck ship would have 1 gee on the bottom deck, 2 gees in the middle deck and 3 gees at the top deck. Maybe artificial gravity is just slang for a strong nuclear force amplifier allowing it to work at several meters instead of trillionths of centimeters?
In Traveller as it stands dampers are used to keep nukes from going boom and little else until a gadawful high tech level when they become disintegrators. What if early breakthroughs in damper tech allowed you to construct an engine using these powerful but short ranged forces? Instead of heating your propellant or using gravity or magnetic fields to accelerate it, you used the strong nuclear force?
One burn equals 10 kilometers per second by the way. A ship with 20% propellant will be able to accelerate to 100 km/s and decelerate at its destination. That will let you travel an AU in 17-18 days. This is incredibly fast the way NASA measures things. Saying that, starships will probably still use their jump drives for interplanetary journeys more than half an AU or so. But starships can still close the distance from jump limit to low orbit in a few hours to a day if you're hoarding fuel for a getaway.
Shuttles and small craft suddenly become crucial to many operations. Taking off from an Earth sized world requires 10 km/s. That isn't telling the whole story, you have to overcome a planet's gravity and make up losses to atmospheric drag as well. So about two burns will do it. Those add up fast and for merchants everything is money and that propellant you carry with you takes up space you need for mission requirements and cargo to make money. Eight percent of your ship for fuel just for taking off and landing is going to eat up your profits. So most starports will have an orbital element and shuttle pilots will be kept busy. Air/rafts, which can ascend to orbit will have increased use. They don't need any propellant to operate and are ideal for transporting a few passengers and small packages. Reaction drive shuttles will be faster and have more range than the lifter vehicles of course and probably carry more.
Starports will have runways on any planet with an atmosphere allowing streamlined ships to land safely on their bellies rather than deal with the stress of tail landings. Best of all planets will have a reason to invest in things like orbital towers/elevators and bolo transports. In fact the companies building these huge constructions might have a vested interest in suppressing reactionless thrusters.
*The specific impulse is probably even higher. One 'ton' of fuel does mass one ton. Apparently a ship's displacement ton is the volume a ton (mass) of liquid hydrogen takes up (13.5 cubic meters) and masses about 10 tons. Well yikes again. I have a problem with this figure as well (http://twilightgm.blogspot.com/2014/09/when-tanks-fly.html) and usually go with about 2.5 tons mass per ton of displacement.