If fusion doesn't provide a substantial energy gain, it's not worth the effort. Existing ion thrusters are quite efficient (60+%) at translating electrical power to exhaust kinetic energy.
Indeed, but they offer very low thrust. For manned space travel, you may need something that can give you a higher acceleration, even if at the expense of efficiency.
Also, fusion reactors designed for power generation have very different goals than fusion rockets. With power generation, particles leaving the reactor may be considered wasted energy. With rockets, the whole idea is to have particles leaving the reactor in a certain direction taking as much energy as possible with them. You just point the jet at the direction opposite to the one you want to go.
Well, controlled fusion hasn't reached energy breakeven at all, even taking into account the kinetic energy of the reaction products. But the low thrust of current ion engines is mostly due to power restrictions (https://groups.google.com/group/sci.space.science/msg/0cb332...). If your spaceship has a mass of one ton and you want an acceleration of 1 centigee with an exhaust velocity of 30 km/s, you will need at least:
0.5 * thrust * velocity = 0.5 * (0.1 m/s^2 * 1000 kg) * 30 km/s = 0.5 * 100 N * 3E4 m/s = 150E4 W = 1.5 MW
"kinetic energy" means nothing alone, thrust and specific impulse are better metrics to look at. Doing a lot better than ion thrusters at either without being too much worse in the other and energy efficiency would be interesting.
However, the article quotes an exhaust velocity of 30km/s (or ISP of 3000s), and using 200kW doesn't leave much room to beat http://en.wikipedia.org/wiki/HiPEP on thrust without counting on net energy gain. The linked slides also claim net energy gain.
Yes, I agree. My point was that you cannot do substantially better than ion engines without having energy gain and, in fact, you are probably going to do much worse due to the weight of the "fusion hardware".
I'm quite optimistic about magnetized inertial fusion. But the idea of doing the job much better than the Z-machine, with something lightweight enough to carry into space and in less than 10 years seems to me... unlikely, to put it mildly.
Ion drives are highly efficient, but have ridiculously low thrust. Which doesn't help at all when you are trying to reach some point quickly.
If you can construct an ion drive with enough thrust to match this proposed fusion drive (or even a NERVA: http://en.wikipedia.org/wiki/Nuclear_thermal_rocket), talk to NASA, I am sure they will be interested in buying several from you.
> Which doesn't help at all when you are trying to reach some point quickly.
Sure it does. You just leave it turned on. This thruster assumes 6 days of thrust then 24 days of coasting. With an ion thruster you leave it on for all 30 days.
There are no high thrust systems that can just be left on - even nuclear ones are used for a short period then turned off. The idea of an ion thruster is that you leave them on, and achieve the same total thrust, over the same time.
> With an ion thruster you leave it on for all 30 days.
With a thrust of - at best - 5 newtons, you won't achieve the goal of getting to Mars faster. You may get there cheaper and using less fuel, but for pure speed you lose. Ion thrusters are good for very long trips when you're going to leave the engine on for months, or for trips where the total time doesn't matter much, only fuel economy (e.g. for cargo shipments or probes).
5 newtons won't get you there in 1 month, but according to http://en.wikipedia.org/wiki/VASIMR it will in 5 months which is still pretty good, and much better than a standard rocket.
Also, fusion reactors designed for power generation have very different goals than fusion rockets. With power generation, particles leaving the reactor may be considered wasted energy. With rockets, the whole idea is to have particles leaving the reactor in a certain direction taking as much energy as possible with them. You just point the jet at the direction opposite to the one you want to go.