Muon-catalyzed fusion for propulsion

I've liked fusion for propulsion since the first time I saw it discussed.

Of course, if it ever works well enough for net energy, it becomes self-sustaining and that will help us to kill two birds with one stone: one on the ground with energy production, and one in space for getting places.

An interesting idea expressed in the link above is what if you can get useful thrust out of it even without achieving breakeven?

You could use fission to provide the energy for making muons, and then the muons would create the fusion.  The fusion products will be high-velocity alpha particles that would provide the thrust.  A side benefit is the production of neutrons could be used to breed U-233 from Thorium 232 and to keep the fuel process going on that side.  A type of synergy between the two processes.

My worry is that you couldn't make enough muons in order to make enough alpha particles for thrust.  You'd be supplying much more energy than what you are getting out of the process, but you'd be getting alpha particle zooming along at a fraction of the speed of light.

Anyway, the scientific breakeven for muons is a bit over 300 reactions per muon, according to this chart.  Economic  breakeven is much higher at 900 or so.  I'm wondering that if it would be worth it from a rocket's thrust standpoint to get close to 300?  It won't be economical, but that's not what we are trying to accomplish.  We are trying to get useful thrust out a rocket without it weighing too much and without it causing too much radiation.

http://books.google.com/books?id=s8a5PphYZJ8C&pg=PA194&dq=muon+catalyzed+fusion+efficiency&hl=en#v=onepage&q=muon%20catalyzed%20fusion%20efficiency&f=false

Update:

Muon catalyzed fusion won't work with Boron-11, so it will not be aneutronic.  This means there will have to be neutron shielding, or there'll be radioactivity and other problems with the mechanical stuff inside the fusion chamber.  I suppose you could also utilize the neutrons to make more U-233.  But that wouldn't work with the tubing that carries the Thorium floride gas.  Hmm.  Maybe you could just line the innards with Thorium and Boron and let it soak up the neutrons.  Replace it as needed and harvest the U233, but that stuff is bad stuff.