IMO, we simply can't get there with chemical rockets as we know them. Continuous acceleration in space is just any kind of engine that keeps running and provides thrust. We're not talking cars here.
The estimation of the energy needed is easy to make sense of - what does it take to have a rocket (or more likely, some other engine) hover over the ground for 4 years using some kind of thrust?
When we have that capability, we can pretty much do non-suicidal interstellar transport. But it requires new technology and for me, that constitutes sci-fi. Doesn't mean a conversation can't be remotely serious though, IMHO.
But the point I actually wanted to make was, that whenever people talk of cruise-speed in interstellar transport, it seems to me like they haven't grasped the scope of interstellar travel with an intent of conquering it. Rather, they are implying it just can't be done - without arguing for it.
The term 'cruise-speed' itself becomes an annoying passive-aggressive attack on the notion of interstellar transport. That was my original reason for posting.
To almost completely eliminate fuel requirements in subsequent trips you would bring the self-replicating equipment necessary to scale up the industrial capacity to build an complimentary laser system in the destination star system. This would allow you to propel ships on return voyages and slow down ships coming to the planet.
The downside of course is that you've now built a death laser in each star system that could be used to wreak havoc. But then again in this scenario you've also built self-replicating space bots that can turn asteroids into stuff or planets into asteroids into stuff.
Is it not possible to plot an interstellar course that utilizes gravity for deceleration?