[GonzoGOP]

Drive lasers and concentrated beams of charged particles operating continuously over months might be a better bet as while the initial acceleration forces are small, overtime the craft will attain enormous velocities at “c” or close to it since acceleration, while tiny in force is continuous until the whole craft is traveling as fast as the photons leaving the drive engines.

There are two problems with the drive lasers. First you have to build them, and that isn't something we know how to do. They require vast amounts of orbital infrastructure (in contrast a city sized Orion could be surfaced launched if you can get it past the EPA). Finally in order to decelerate you need drive lasers on the other end.

Stopping an Orion wouldn't be easy either. Nuclear bombs have a use by date, and after a few hundred years they aren't going to go bang any more. So the ship would need to have some way of reprocessing the tritium pits used to detonate the bombs used for the deceleration phase. A NSW engine would work for deceleration. The problem with those isn't getting them going, it is keeping them under control. If any of the fuel leaks it immediately goes critical. When you remember that we are talking about a corrosive liquid that you need to store for four centuries the problem starts to become clear.

A beamed energy craft may be the answer to intersteller colonization as their top speed is vastly higher than a nuclear salt-water rocket or Orion drive. The top end is actually limited to less than .5 c because since the photon is only going at c it hits with less force as the ship goes faster and faster. In addition when the photon reflects off the sail it carries energy away with it. Still .5 c is a heck of a lot faster than the .035 c that is the limit for an Orion or NSW engine. Considering human life spans a 40 year trip is a lot more attractive than a 400 year trip. You also don't have to share your ship with a portable Chernobyl as you would with a NSW drive.

So when you find someplace you want to go send ahead a robot ship with a laser. Then when it is in place you send the colonists on a much faster light sail craft.

[SunkenCiv]

The problem of deceleration exists also for the beamed energy craft, so it’ll probably be built and used to send probes to nearby stars. By the time it reaches those stars, people not yet born probably will have developed something better, like the stuff currently in use by various ETs.