There’s two broad ways to think about how to obtain sublight travel. The first is the obvious way that humanity has been using since we first aimed for the stars - reaction drives. Basically, you have your rocket, and you throw some mass in a direction. In a frictionless environment with no other forces involved, your rocket must travel in a direction opposite that of where you threw your mass. Basically a long rewording of the Newton law that every action must have an equal and opposite reaction. Conveniently, there are many ways to achieve this goal. Rockets, ion engines, and photon engines.
Inconveniently, reaction thrusters require reaction mass. Changing what direction you are going in requires even more mass. Creating an acceleration requires mass, and then reversing that acceleration when you get to where you’re going requires even more mass. Eventually, if you want to go any distance in a reasonable amount of time, you will wind up with a ship that is almost entirely reaction mass. Those reaction thrusters which aren’t, such as photon drives, instead have the problem of requiring staggering amounts of power. Wikipedia tells me it requires about 300 MW to produce a newton of thrust from a photon drive. For comparison, a Nimitz class carrier carries two 550 MW reactors to truck around the ocean in. One newton is about .2 pounds force undergoing acceleration by Earth gravity.
Not exactly the kind of acceleration one should get excited about.
While having ships that are basically tiny habitation modules on top of gargantuan fuel tanks is certainly plausible, it’s not a direction I’m interested in going in. Likewise, simply scaling up power supplies to be able to provide the ridiculous amounts of power a photon drive requires to be viable is not a very good option. Throwing that kind of power would have other consequences. For example, what’s the difference between a photon drive and a laser? If your photon drive is providing thrust, it is also putting out enough power to basically be a very serious kind of weapon. I will admit to having not done the math, but I suspect that once we get up to high thrust, we’re also spraying enough photons to melt planets. Neither of these is desirable.
What’s a writer to do? Well, I could provide sufficiently advanced technology such that mass modification or space modification become possible. Store the fuel in a pocket reality (TARDIS fuel tank), or shrink its mass down to be really small, only expanding it when it’s needed for thrust. These ideas both seem to head towards a higher technology future than the one I wish to play around in, though I will admit I am keeping both of these ideas in the back of my head.
However, more likely, my final path will be to keep the reaction thrusters around for manoeuvring and certain other special circumstances. They would perhaps have military uses. However, for the most part, my intention is to use a reactionless drive of some sort, which I am calling for now a wave drive.
Now that I’ve lost the hard science fiction fans, I shall detail how the reactionless drive should work.
Basically, I plan on having the reactionless drive provide some thrust based on power input and the mass of the ship. Coming from the vessel in question having zero acceleration, power is put into the wave drive, which will begin to generate a propulsion field. This propulsion field will generate an acceleration, which will initially be quite high. As power continues to be applied, the acceleration provided will drop, while the field builds up. At top speed, the field stabilizes at a point where it grows no further. At the same time, acceleration stops. Power must continue to be applied, however, as the ship now travels at some constant velocity. If power is interrupted or dropped, the field will being to collapse. As it does that, inertial kickback from the drive will cause a negative acceleration on the ship. If power is completely removed, the field will die away, and as it does away, the craft will undergo negative acceleration, until a negative acceleration equal to the initial positive acceleration has been applied to the craft.
That’s a whole lot of words to say put power in drive, ship speeds up to some steady state velocity. Drop power, ship drops back down to its initial velocity. Higher power provides faster acceleration. Higher mass causes a slower acceleration. The drive’s capacity will dictate its final acceleration potential.
This drive has a few positive effects. One big one is it lets me get rid of the mass problem. Just use this fancy reactionless drive to truck around, and so long as you’ve got fuel for your power plant, you’ve got fuel for propulsion. Another nice one is it prevents continuous acceleration. Ships using my reactionless drive not only cannot get up to lightspeed, they also can’t easily get up to relativistic velocities with absolutely gargantuan drives. This allows me to avoid some of the nastier to write around problems relativity presents me with, such as event observation skew and time dilation. I also plan on calibrating them such to avoid a common problem reactionless drives have, namely that of having dirt cheap planet crackers. Basically, if you have a reactionless drive with no restrictions on its acceleration and that is cheaply powered, you have a missile that can be arbitrarily ramped up to some speed such that it can hit a planet with planet-shattering force ( recall that force is equal to mass times acceleration ).
It is not without problems. One, it will need to be carefully calibrated to serve the needs of narrative while at the same time avoiding unintended consequences of such technology being available, such as the aforementioned dirt cheap planet crackers. Also, I have not yet fully determined how the wave drive will interact with the reaction drives that my ships will also have (never know when you need the outboard motor). In a Newton space, this question would be easier, but my ships must, out of necessity, be operating in space - and thus must pay attention to relativity, and concepts such as relative velocity. Given an absence of absolute velocity, it only becomes useful to talk about accelerations. So, basically, when the wave drive is done providing acceleration, and it is sitting at zero acceleration, how do I handle the captain of the ship flipping on a reaction drive. I haven’t made a decision here yet.
Still, the basic idea is to make it possible for a ship to travel across a space the size of a star system in a reasonable amount of time. Our own solar system is (very approximately) five light-hours from the sun to Pluto. Being able to make this trip in days, at most, would be preferable, though I admit I need to run the math to see if that’s still fast enough to make the aforementioned dirt-cheap planet crackers. While defending against such things is arguably possible ( though the cost of a true planetary defense system for something the size of a planet or solar system would be staggering ), I’d rather avoid the problem up front by not making them to begin with. If the velocity winds up being a problem, I will probably place more limitations on the drive, like perhaps making it so gravity wells interfere with it so much that once you’re in orbital distance, you can’t use it anymore. I will worry about that when I actually have the time to run the numbers.
However, the short version is I want to make sublight high enough speed for achieving the narrative purpose of allowing ships to zipper across solar systems in the span of days. I also want to avoid having to have ships carry gargantuan fuel tanks, or gargantuan power supplies, and avoid having cheap planet crackers. Some analysis will be needed to see if the wave drive provides me with all of these goals neatly, but it’s a start.