why is it that it took me 3 hours to make it to a planet yesterday and almost 6 today? is travel time random or what?

Every ship has different speed and consumption. When you combine two ships with different speed, the speed of the slower ship will be used.
You can check the speed of the ship by clicking on it's picture or name on the shipyard.
You can also upgrade engines to make the ships faster. There are 3 engines. Check out the technology screen to see what kind of engine a ship uses.
Note that the some ships upgrades it's engine from combustion to impulse after you reach a certain lvl.