Anyways, check out the OP of the Age of Rebirth 2 thread to see some of the tech you can use during this one.

In addendum to that I talk a bit about plausible space propulsions.

1.) Nuclear thrusters
The ship uses the heat of nuclear energy and some kind of ablative material to produce thurst.
There are two methods.
A.) Orion Drive and its derivatives
Using small nuclear bombs and detonate them in succession to gain thurst. The smaller the bomb, the more frequent you can use it and with better containment. 
Primitive space warships may literally detonate nukes behind themselves and accelerate in erratic pulses. This method is anything but subtle and strains the ship considerably.
More sophisticated methods use tiny pellets of micro fusion explosives initiated by burning antimatter filled fulerenes. This method is likely the more widespread. It's pretty much like chemical rockets in terms of mechanism except much more effective.
More compact warships may use these.

Drawbacks are less efficiency compared to the other type (little to no "gears" to switch) and that your propellant is basically a huge stash of thermonuclear bombs. Battle damage could have catastrophic consequences.

B.) Torchships
You have a gigantic thermonuclear reactor which heats up your propellant and use that as your propulsion. Basically the space equivalent of nuclear subs.
Advantages for torchsips is that you can mix various propellants, adjust the output or perform other methods in order to alter the exhaust velocity of your propellant. Lower exhaust velocity means more thrust at the same energy expense. On the other hand exhaust velocities limit your maximum theoretical Delta-V in space.
So yeah, if done right a rector-powered thruster can be much more fuel efficient than the above. On the other hand the said reactor would occupy a considerably portion of the ship, reducing the useful mass. Long story short it generally only worths to do at large scales.
Another obvious advantages of Torchships is that you can use your crazy reactor output to power big weapons.
A theoretical 1000-ton torchship would require a reactor with roughly 45 GW output. Now imagine your space battleship pumping even fraction of this energy to a laser.
Yeah, a direct hit could bring nasty results.
On the other hand building these ships would be ridiculously expensive (basically combine a super-large aircraft carrier with all the space shuttle programs and add the building of a huge nuclear power plant to it). Even the most powerful nations shouldn't be able to possess more than a few 1000-ton range space battleships.
On the other hand since it requires less anti-matter and generally more efficient this ship could travel to places more economically than Type-A.
Potentially you can build mini-torchships too, but like I said with the limited usable mass their utility is rather dubious.


Alright, but while these things are impressive in general nuclear propulsion would be limited to military use.
That is because these ships would be so damn powerful that their misuse as weapons is inevitable.
Type 1A is basically carries nukes as propellant. It's a flying weapon of mass destruction.
Type 1B has enough reactor output to power all kinds of terrible weapons also with some tempering the reactor itself can be turned into a bomb of mass destruction.
You just don't want these to be anywhere around civilians.

So what for the rest?
Well, there's a nice method which circumwents the issue.

2.) Laser Propulsion
Laser propulsion basically means that you burn the propellant by external means. You raise the ship into orbit either by rockets, space elevators or even the laser apparatus itself.
After that a hi-power but somewhat low intensity array of lasers would burn your propellant.
This method would eliminate the need for expensive power generation systems and thanks to the comparatively cheap ground-based fusion powerplants the cost of operating the lasers should be fairly economical, too.

The laser arrays themselves would be gigantic (think of a city-sized plant and laser emitters reaching kilometers deep into the ground) and strictly owned by the government. Not all countries would be capable of building such gigantic laser arrays but for this reason they can be easily lent to other nations (thus generating pretty fat stacks of revenue for the owner).

The above reasons make each of these laser array systems the treasure of the respective nation. They are the most heavily guarded part of the entire country. In addition if necessary these laser arrays can be tuned and altered for defensive purposes. Their powerful lasers can be focused to destroy incoming ballistic missiles or even deter enemy spaceships. This is another big reason why these gigantic installations are so precious.

Smaller laser arrays are possible to build of course and these can be used to propel smaller lightcrafts (that's the typical name of these spaceships, remember it). And like the main array these can similarly offer some aid in defense.

After hearing the effectiveness of these lasers you may wonder, why not use these to propel warships?
Well, first off a ship restricted to follow a laser beam is hardly maneuverable. This is the biggest reason.
Second, the laser beam can be intercepted thus cutting off power from the warship.
Third, lightcrafts are designed different from warships and have some qualities which aren't desirable for them.
Fourth, it's unlikely that any nation has enough laser projectors to cover all parts of space and power their warships on patrol.
So while it's efficient there are several issues for it being used for war.
On the other hand this method may be viable for smaller vessels controlled by Torchships.
Think of a space battleship shooting compact torpedoes or drone fighters while both controlling and powering them via laser transmissions.
This is a potential and much cheaper alternative to using Type 1A methods (albeit this makes the smallcrafts more predictable).
So yeah, that could work.


Alright, this would be it for now.
Basically you have 3 distinct futuristic propulsion methods with their set of advantages and drawbacks.
I could get much deeper into the details but in general this sorts of knowledge would be enough.
I may make a similar post about the basics of hard sci-fi space warfare on a later date.