We know that ships' drives (normal space - not Witchdrive) can run indefinitely without refuelling.
I assumed it was because no gravity + no friction implies eternal travel?
Energy would only be needed to steer.
No, space doesn't work like that - there is gravity as long as you're anywhere near a planet or a star, which heavily restricts how real spacecraft behave, and also, a rocket's fuel is a huge component of its mass and most of a Saturn V's take-off weight was fuel + somewhere to put it. In order to leave Earth orbit and head for the Moon you had to crank it up to 25,000 mph, but you were bleeding off speed every inch of the way up to the Moon (barring the last thousand miles or so where the Moon's gravity finally took over) which is why it took about three days to cover 248,000 miles instead of less than ten hours.
Elsewhere in the solar system, you will keep travelling in a straight line indefinitely - except, as Einstein tells us, it'll be a straight line in curved space. To us mere mortals that looks a lot more like an ellipse, and you won't leave the solar system or even reach a radically different planet without a lot of power input or, more practically for our space probes, a gravity slingshot whereby we can siphon off an insignificant amount of a planet's orbital velocity and give it to the probe. Fun fact: it's easier to reach Pluto than the Sun; that's counterintuitive but perfectly true.
As you can see, physics in the OoVerse works differently. Whatever speed you want to be doing, you need a certain power output, implying that OoSpace isn't frictionless (except, as you can see, for cargo barrels, which bat along at whatever speed they were dropped at). Also, rotational momentum isn't conserved either. From what I remember from about 1985, you had the option in Elite to conserve momentum and rotation... and it was darned hard to fly a ship that way.
Whatever speed you're doing, your drive is at a certain setting, and you can see the evidence from an external view. But you could fly for ever without running your drive fuel out, for all the game says otherwise, and since your ship doesn't change mass while all this is going on, it's much easier to get a drive-equipped ship (such as an Adder or a Moray) from surface to orbit: just stand the ship on its tail and, as long as you can go up at all in a vertical climb, you can reach space. (The narrator of the Sidewinder
series comments on this from time to time.)
And once again, yeah, space elevators.
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