In short, the reactionless drive 'issue' boils down to this:
1) So far, in Real Life, it appears that in a vacuum and microgravity (or close enough) you need to expel reaction mass to generate thrust. Burn a rocket and shoot out the exhaust; use magnetic bottles to eject ionized gas; have hydrogen bombs detonated by a shock plate; whatever. In short, to move mass X through space you somehow hurl mass Y in the other direction and the reaction generates thrust.
The RF Resonant Cavity Thruster which is being tested in space about now might change the 'in Real Life' part of this post. We live in interesting times
The issue is this - with this as true travel through space is hard, expensive, short-range, and slow because you have to use mass to move mass.
2) In fiction lots and lots of people use reactionless drives - ships move, but they don't have to throw mass 'that way'.
3) So some people say 'Yay, reactionless drives are fun! They let my book/game/whatever be Horatio Hornblower in space!'
Other people say, 'Boo, reactionless drives are no fun! They break my ability to accept the book/game/whatever and mean that everyone should just be hurling planets at one another at 99.9999% C!'
As much as some would like to label the latter group 'pearl-clutching ninnies' in the what was perhaps the very first fictional portrayal of reactionless drives (Doc Smith's Lensman books) the characters did, indeed, escalate until they were destroying planets by hitting them who two other planets.
From opposite directions.
Both doing 99.9999999% C.
And both were made of antimatter.
The father of Space Opera wrote BIG stories.So there is a risk there.
You'd think that such a rather nerdy, niche, obscure issue wouldn't be that big a deal, right? I mean, it isn't as if people obsess over things like food or fuel sources in fantasy rpgs, right?
But reactionless drives are a Big Deal in SF TRPGs, so much so that one side of the debate has the slogan 'friends don't let friends use reactionless drives'.
Part of the problem is classic Traveller.
If you are among the few guys who might read this blog who don't know what Traveller is, hoo-boy: you are missing out.
As I remember, Traveller hit the FLGS in Spring of 1977. Dad owed me a huge favor involving a situation straight out of a 1980's sitcom
...but that is a story for another time...so I got it that week and started reading it.
Two weeks after I got it, I saw Star Wars for the first time.
Traveller is a pretty crunchy game. The original books are full of mathematical formulae you need for play, including an intro to the use of vectors. The ship building rules, planet generation rules, sub-sector generation rules, etc. are essentially minigames. The game is developed enough that you can run a full game that is all about being explorers based on a remote, agrarian frontier world: you slip out into barely-explored space and come back with valuable knowledge and rare items. Or you can run a full game that is all about being mercenaries based on a remote, agrarian frontier world; you are guns-for-hire for the brush wars that erupt far from centralized power. Heck, you can run a full game that is all about being merchants based on a remote, agrarian frontier world; you are trying to corner the market on farm machinery!
Oh, yeah - the trade system is another mini-game.
Anyway, Traveller supports SF RPG play from asteroid prospectors trying to earn enough for more oxygen to intrigue among galactic nobles at imperial court where entire solar systems are used as currency and everything in between. A seminal game in the early days of tabletop RPGs.
And it uses reactionless drives.
I can remember the debates about this from Back in the Day, and they were pretty serious on the old Traveller Mailing List from time to time. I remember particularly when T4 was coming out with new ship construction rules.
Personally, I have never had an issue with reactionless drives for one simple reason - we are surrounded by 'reactionless acceleration' all the time.
"But, Rick!," I hear you say, "Gravity involves mass! The mass of the attractors!"
Yeah. I know.
As a little aside, I have fond memories of my Physics 360 prof telling us a humorous aside as we discussed gravity. He was quoting someone else (whose name I don't think he mentioned) and I am paraphrasing,
"The Medieval world used the concept of Crystal Spheres to predict the movement of the sun, moon, planets, and stars and were very, very accurate about it. If you pushed a Scholastic to tell you what it was that made the celestial objects move he couldn't tell you exactly what it was - he could measure its effects, he could make very accurate predictions about the future, etc. but what it was? He only had measurements and formulae. So he said it was the angels."
"Today people laugh about that, and say 'it is gravity!' But all we have done is give the angels a new name. We can measure its effects; we can make very accurate predictions; but as to what it really is? Could be angels."
Anyway, the idea of a gravity-based drive being 'reactionless' is actually kinda' goofy. The reaction mass is just other places.
Here is an analogy - beam powered propulsion. This is the 'planet based laser pushing a vessel with a light sail' idea. In this case the reaction mass is the planet that holds the laser - the vessel doesn't carry reaction mass for the main trip.
With sub-light thrust using gravity fields the 'reaction mass' is, well, the rest of the universe, really. Even some of the biggest proponents of 'reactionless drives are broken' admit this (not all - just some).
We know Traveller uses artificial gravity (it is explicitly mentioned in the books) and even use a form of defensive gravity generator, the repulsor. So I assume that the drive systems in spaceships are gravity-based in any spacefaring civilization in Traveller unless otherwise noted.
One of the things I like about classic traveller in particular is a lot of things are implied, giving a GM plenty of room to move around. Look at gravity technology in books 1 through 8 of Classic Traveller and you see a lot of discussion about artificial gravity in use. Indirectly, usually. It really lets you go off on your own and has some interesting little quirks. G-carriers, air/rafts, repulsors, grav belts - artificial gravity tools are all over the background clutter of the CT game.
Looking at the development of ships by tech level I decided to add something to a campaign I wrote up in 1986, re-wrote in 1988, and eventually never got to run. I have ported it over to my about to be launched campaign. That is....
High Guard stats:
Repulsor Shield Tech Level Table
Tech Level- 11 12 13 14 15
Shield Rank- 2 3 4 5 6
this is the maximum shield rating available at each tech level
Repulsor Shield Displacement Table
Rank 1 2 3 4 5 6
Displacement- 2 5 8 11 14 17
this is the percentage of the ship required for the shield generator
Repulsor Shield Cost Table
Rating- 1 2 3 4 5 6
Cost- 1.5 0.7 0.5 0.5 0.5 0.5
in millions of credits
Repulsor Shield Power Requirement
is calculated by: R 0.01M
Where R equals the rating of the repulsor screen and M is the total displacement of the ship.
Feel free to point out any errors I am making as I am recreating these really experimental devices from memories about 30 years old!
If you do more than glance at this, you'll realize that repulsor shields are just another maneuver drive! Rather than provide thrust for the ship, though, they push away any inbound missiles. In combat a ship with active repulsor shields applies its shield rating against all incoming missiles! This is, naturally, in addition to any counter-fire, dedicated repulsor bays, and nuclear dampers.
The downside is that a ship with active repulsor shields cannot launch or recover any sub-craft, regardless of size, and also cannot fire any missiles or even use deadfall ordnance.