[ctdonath]

Tangent: fusion will require carrying...how much hydrogen? I once calculated that trying to gather it along the way (interstellar hydrogen) is pointless, as a 1m^2 swath from here to Alpha Centauri would net 0.01g of the stuff.

ETA: Yup, I was considering the Ramjet design. Scale of the scoop overwhelmed itself trying to acquire enough to function. I was just startled at how little interstellar hydrogen there is; I knew it wouldn't be much, but 0.01g in ten million cubic kilometers was even less than I'd assumed.

[lutusp]

> ... how much hydrogen? I once calculated that trying to gather it along the way (interstellar hydrogen) ...

FWIW this is an old sci-fi idea / plot device called a Bussard ramjet:

http://en.wikipedia.org/wiki/Bussard_ramjet

But this isn't meant to dispute your calculations. The linked article comes to the same conclusion: it's not feasible, too little interstellar hydrogen.

[ackydoodles]

Most notional implementations of the Bussard Ramjet depend upon some sort of heretofore undiscovered force field rather than a physical structure to collect hydrogen, because size matters.

Naturally, a very large force field ramjet will deplete interstellar hydrogen along the main routes between stars, leading to a situation known as "peak hydrogen", and possibly prompting the invasion of developing hydrogen-rich planets on some sort of flimsy pretext.

Most scientists working on imaginary spacecraft projects therefore eschew the Bussard, correctly anticipating the moment when, shortly after the leader gives a speech on the deck of a starship in front of a huge "Hydrogen Accomplished" banner, the price of hydrogen rises to over $5 a gallon and starships are all left to rust in the front yard.

[lutusp]

Yep. And shortly after his victory speech, the leader is heard to loudly complain that he's been "misunderestimated".

[dennisgorelik]

> from here to Alpha Centauri would net 0.01g of the stuff

... and yet that 0.01g of stuff would produce lots of friction and heat if spaceship flies with 10%+ of light speed...