Hi Guys

So you want to nuke to 0.2 c? Hard to do. It's all a matter of exhaust velocity.

Fusion pulses as used by Daedalus managed 0.03 c using D/He3 fusion. The theoretical maximum is 0.088 c - so Daedalus had an efficiency of only ~ 11%. With some fiddling and refinement the exhaust might hit 0.06 c.

To slow down you want a magnetic sail, rather than squaring your mass-ratio. Hence you need to start off with a mass-ratio of 28 - about 810,000 tonnes of fusion fuel for that 30,000 tonne starship.

Can fusion do better???

The theoretical maximum you can squeeze out of fusion is 0.9% of the mass-energy in the reaction - fusing it all the way to iron(!) Using Larry Niven's fusion shield to turn it all into a fast and tight particle jet you get ~ 0.134 c. Much better, but hard to do.

Proton-proton fusion - and its follow on reaction chains [D-D and D-He3] - gets 0.67% of mass as energy which is a ~ 0.116 c jet-speed. But proton-fusion is slow. Carbon catalysis can speed up the reaction and a jet-speed of ~ 0.1 c is possible.

But such performance is way beyond our fusion bomb technologies.

If you've ever seen the designs for Enzmann starships, they carry huge spheres of frozen deuterium for fuel, with mass-ratios of ~ 100 (Yes... 100 !!!) And they could only do a piddling 0.1 c...

So how can you go faster??? Without using "magic" or antimatter? Ramjets are "magic" since we can't see any physically reasonable way of doing them.

Antimatter is inefficient - tho great for interplanetary missions where you only need tiny, tiny amounts.

Well... laser-sails have been mentioned and they have good performance. But another idea that hasn't been mentioned is mass-beam propulsion. Instead of photons streams of small pellets are used.

Not as crazy as it sounds and for low speeds <0.3 c we could do it with simple extensions of current technology. Accelerating to 0.2 c at 20 gee [crew floating in oxygenated flurocarbons to take the gees] the mass-beam range needed is only ~ 63 AU

For longer ranges/speeds the pellets need advanced guidance, but this isn't absurd either.

To deccelerate all you need is a magnetic sail. And at 0.2 c I don't think anyone on-board is coming back to Earth.

Adam