Editing Openai/691a2d9f-c044-8012-bccf-942bfce6334d (section)

===== Once two things are true: =====
# The block has escaped the Sun (positive specific orbital energy, on an interstellar trajectory).
# Its asymptotic direction is within a small angular error of your desired “corridor” toward Alpha Centauri.

…then you’re done. After that, the block just:
* coasts inertially in deep space
* with almost no external perturbations
* keeping its relative alignment quite well over light-years.

You don’t need:
* continuous thrust
* active station-keeping for millennia
* a 1000-year reliable engine

You need:
* big burns early (injection + maybe perihelion)
* tiny, occasional burns for the first tens to low hundreds of years to tame errors

A sane design: the engine is designed for, say, 100 years of intermittent use, not 100,000. That is still very hard, but not “absurd sci-fi magic” hard.