Contained here are some notes on the sort of technology available in a Yags SciFi setting.

Most of the following discussions are about reaction drives - those that work by throwing something out the back to push a space craft forwards. All require 'fuel' (actually, reaction mass, but it's the same thing for chemical rockets and fuel is easier to write), of which any vehicle will have a finite supply. This means that any reaction drive will have a maximum speed - though it can keep on accelerating, eventually it runs out of fuel. For spacecraft, this is generally known as Delta V (dV) - the maximum change in velocity possible.

A ship with a Delta V of 20km/s could accelerate up to 5km/s in one direction, coast for a bit, then turn around and decelerate by 5km/s to come to a stop, for a total of 10km/s. It then has enough fuel to accelerate back up to 5km/s back towards home and stop again using up the last of its fuel. Though it could get up to 20km/s by burning all its fuel, doing so would leave it with no more fuel to stop or manoeuvre. A missile might do this, but a space craft with crew on board probably wouldn't.

Acceleration is another factor that must be considered, and is the one most people think of when considering space ship drives. Acceleration is how quickly you can get from not moving to moving fast. A ship with an acceleration of 1g (9.8m/s/s, though we'll assume 10m/s/s for simplicity) can go from a standing start to 36km/s in an hour. This does assume that the ship has enough dV to accelerate this long.

Note that the Space Shuttle could accelerate at several g's, but that was only for a few minutes. It's total dV was in the order of 10km/s - 90% of which was needed just to get into Low Earth Orbit (LEO) to dock with the space station. Deceleration was managed by atmospheric drag, since it didn't have enough fuel to do much more than reduce it's orbit back into the atmosphere.

Chemical rockets are the simplest form of space propulsion, and are assumed to be possible from TL 6 onwards. On Earth, they didn't start being used until TL 7, but the technology was there if there had been the necessary will. Technically, the basic physics was understood in TL 3 or 4, but the ability to build a big enough rocket to actually get into space would come along much later.

The problem with chemical rockets is that they are limited to chemical energy, which greatly limits their dV. Even at TL 8 or 9, dV for rocket ships will be limited to a few 10s of km/s.

Ion drives are a form of electric propulsion that uses some energy source to propel a reaction mass out the back at high speed.