Fusion releases more energy, but fusion reactors do not exist yet. My goal was to describe a ship ot of technology that has been demonstrated. Fission reactors are up to about 50% efficiency now actually. And super conducting linacs are being built that claim 50% efficiency. Beam currents of about one amp are considered possible now. And of course, nuclear energy is only a few percent efficient at mass/energy conversion. Sadly, it is just the best we can do with existing technology.
I hope fusion power works someday, but the history of this subject is not good. Magnetic confinement of plasma has been described as "like trying to confine jello with rubber bands". And you hve to reach enormous temperatures, much much higher than the core of the Sun (inside the Sun, fusion takes place at a very slow rate per unit volume). I don't know enough about this to say if its is just a fundimentally hard probelm, or if it is just something that academic researchers have been fiddling with ineptly. Sometimes when the economics is right, professional engineers step in and make something work in a surprisingly short time (I see this all the time in the computer field).
The problem with NERVA-style rockets (heating hydrogen in a reactor) is that they do not produce relativistic exhaust velocity. It think nuclear-electric drives like ion or VASIMR drives probably have much higher specific impulse. But to reach a star, I believe you have to try to get an exhaust velocity at some healthy fraction of the speed of light. The ORION drive might not be a bad idea, exhaust velocities of 1000 km/sec or so are apparently feasible, but that's still not reletivistic.
Taylor and Wheeler's book does some math on an idealized case:
1. Theoretically, the most powerful propulsion system is one with an exhaust velocity of the speed of light. The ideal spacecraft would convert fuel mass into pure energy, for example by anihilation of matter and antimatter, and emit the resulting gamma rays in a tight beam behind the ship (there is no theoretically known way to do that though).
2. Assume a time dilation factor of 10. This would allow an astronaut to travel 500 light years in 50 years (in his timeframe).
3. If he wants to just get to the star, without stopping, a 100 ton (empty weight) ship will have to carry 2000 tons of fuel. If he wants to decelerate to a stop, he will have to carry 40,000 tons of fuel. if he wants to also come back to Earth and stop, he will have to carry 32 million tons of fuel!
4. At full velocity, his ship will impinge on about 3.0E+15 atoms per second, each of which will strike the ship with an energy of 9 GeV. This is about 300 times the flux of a powerful proton accelerator. So serious radiation shielding will be needed.
But what I'm getting at is, how do you reach a nearby star without resorting to crank science or concepts that are totally beyond modern engineering possibility (antimatter, etc). And maybe the answer is that it can't be done with what we have now.
QUOTE (Richard Trigaux @ May 25 2006, 03:48 AM)
Yes true, the equilibrium temp in deep space is about 3°K, far enough to allow for supraconduction in most common materials. But is it really useable? First, these temps are available only far beyond Pluto, that means that the probe will need years before switching its engine on. Second, at such low temperatures, it will be enough of something radiating some heat (and it will be) to heat all the thing up, so that an equilibrium temp cannot be expected below 20-30°K or more, not allowing for common materials. That still makes supraconduction much easier to operate than on Earth. But the ideal would be that better semiconductors are found...
Pluto is still just on our front door step, for a journey to a star. Niobium is an old-style superconductor, which requires very low temperatures, but it is being used in new accelerators now because the new high-temperatures superconductors cannot yet withstand high intensity electromagnetic fields or very high frequencies. The linacs are essentially superconductiong cavity resonators, where a radio-frequency amplifier maintains a standing-wave. Here's an actual one, people are building these things now:Click to view attachment