Pretty sure the implied inertial frame is the one relative to the spaceship, and not any other encapsulating frames.

But I linked that mainly for the table just down the page a little comparing various efficiency measures of different propulsion systems. I'm pretty sure that chemical rockets are the least efficient we've ever implemented or imagined, barely sufficient for travel within the solar system, and certainly not efficient enough for any trips, round or one-way, beyond our solar system.