Revisitation of the muon-catalyzed fusion as a space=propulsion idea
An idea came to mind, and given the fact that I know very little of the field, it is only an idea. Maybe it is an idea worth pursuing, or maybe not.
The question is this: how many muons would you need to make sufficient number of reactions that could provide the necessary thrust that could make this useful? It seems that the Earth receives 10k muons per meter of muons. Would 10k's worth of muons produce much in the way of energy?
A muon can catalyze maybe 200 reactions before they get "stuck", and can't help any further. That's a couple million reactions per minute ( 10k per minute of muons/meter). To increase the muons, just increase the area that they are "collected". But how? Well, muons are electrically charged, so they can be diverted with a magnetic charge.
The muons travel at relativistic speed. Which means maybe you can't move them far.
Muons can be created on Earth using big devices. What I propose is that somehow, you can get the muons in quantity by either making them using cosmic rays, or guiding them to where you want them. Cosmic rays are moving at relativistic speed as well. That means you don't need to use a machine to accelerate them. If you can guide those cosmic rays where you want them in order to make muons... that's the question. Cosmic rays are made of charged particles, which means they can be guided. But how much and how far, and to what effect?
I suppose someone has already thought of all this. But if you can make the charged particles go where you want, and do it in sufficient quantity, you can make muons with little energy expenditure. Perhaps you could focus a beam from 10's of meters to something the size of a pin. Then use it to blast away at something that produces bunches of muons which are used to catalyze a fusion reaction. The reaction creates alpha particles, which could be ejected as thrust.
The math seems imposing. You need a LOT of muons. Because to catalyze just 1 mole of hydrogen would take 10 to the 23rd power of reactions. Therefore, it doesn't seem feasible just on this intital calculation. Even 1 square kilometer may not be enough to do the necessary quantity of reactions to catalyze just one gram of hydrogen in a reasonable amount of time.
What would 1 gram of hydrogen produce in terms of thrust? That seems like a pretty large number. But my math could be wrong, so I won't hazard a guess here. I'd guess that you'd need more than a gram of mass to get a spacecraft's velocity up to the speeds that would shorten the length of a journey by an appreciable amount.
It would be a big apparatus, so I think the idea isn't feasible. Oh, well. It was a thought.
Originally posted in 2014:
2.2.18:
An idea occurred to me last night that the fusion concept that almost works could be used for this propulsion system. The fusion concept is called muon-catalyzed fusion.
The thermal system Waddington discusses would not be necessary. Instead, you want to use the alpha particles directly for thrust. Unfortunately, most of the energy release is from the neutrons, which are not handy, unless you want to breed fission fuel. The neutrons could be put to good use, though.
Since an energy source would be necessary, and that could be achieved with the SAFE nuclear fission reactor design, which NASA is dusting off the shelf.
However, there is a respectable amount of energy in the alpha particles, and this should be usable for thrust. Doubtful that this could be used for a SSTO rocket, though.
The main benefit could be larger payload fractions due to the much higher ISP, which reduces the amount of fuel required to run the propulsion system.
If the amount of thrust is sufficient, you could also transit the distance between planets in a much shorter time frame. If these two benefits are realized, human colonization would be much simplified.
The original 2014 post follows:
Fusion propulsion that is.
An idea occurred to me as I was reading about breakeven in fusion research. This topic came up maybe a couple years ago when Trent Waddington at QuantumG blog speculated a bit about using fusion propulsion even without breakeven. He noted that you can get kinetic energy out of it even without breakeven. The alpha particles would supply the reaction mass and at great velocity since they are traveling at a fraction of the speed of light.
Now at merely 10% of the breakeven point, you'd have enough alpha particles emitted that could give some decent thrust, or so I speculate.
Another thought arose as to the Bremsstrahlung radiation which is such a problem with nuclear fusion as a power source. The thought is this: could you use that for propulsion in space? Like directing the X-rays in the opposite direction of movement like from light pressure from the sun or from lasers.
If you could get enough light pressure and alpha particles from your fusion device, it may be enough to power a spacecraft through the solar system. Just 1% of the electrical energy input into the system may be enough. ( honking big speculation alert ).
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