[rewgs]

Ha! Funny, I've never considered it even though it's so incredibly obvious: the spacecraft needs to be able to slow down out of light speed. That might honestly be even harder than getting up to light speed, and that of course is almost certainly impossible.

Imagine the "runway" you would need. How many lightyears would it take just to slow down? And that's before you even consider how some sort of reverse propulsion mechanism would even begin to turn on without tearing the ship apart in a microsecond.

[hwc]

If you simply assume that your spacecraft uses its own engines to accelerate to cruising speed, then the Δv to decelerate is approximately equal to the Δv to accelerate.

Similarly, if the engines are operated at a constant acceleration, then the distance needed to decelerate is equal to the distance needed to accelerate.

Fuel(/reaction mass), on the other hand, us not equal. The fuel to be used to decelerate must also be accelerated first, meaning more fuel is used to accelerate than to decelerate.