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Comment by Aardwolf | Hacker News Reader

Why max to 0.1c? Is there friction in space or so?

Sort of... the faster you go, the more energy you need to accelerate even faster. The Orion design, IIRC, basically involves sitting on a big iron plate and tossing nuclear bombs under it. Each bomb can only impart a certain amount of energy and at some point you're not going to accelerate any faster unless you shed some mass.

You will shed some mass in the form of the bombs you're blowing up, and whatever you break off the bottom of the plate you're blowing them up against, and I think the design requires a front shield too which will also deteriorate over time and shed mass, but those all probably balance out around the 0.1c velocity.

Last I read up on such ideas, the better design was to use a sail in front of the ship to catch the push from each explosion, then you only need the one radiation shield. I think it also allows for making a larger surface to catch the explosive push, and it depends on materials absorbing loads through tension rather than through compression.

Part of the problem is that you need to decelerate at the end. Maybe it's possible to use a gravity assist to decelerate from 0.1c inside the destination solar system, but otherwise you need to turn around and thrust the other way at about the half-way point.

Another part of the problem is the faster you want to go, the more fuel you need, which makes your ship more massive.

Use a mag-sail as a "brake" to slow down at the destination star.

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