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You wrote, Neutron density is put at 10^17 kg/m^3 = 10^8 g/cm^3.

Very heavy particles if you can project them in a stream/

Yeah. But that last part kind of ignores what we know of physics. You can't use the electromagnetic force to confine neutrons into a beam and any matter you use as a reflector or containment vessel is going to tend to absorb the neutrons ... if they don't just pass right through the material.

Also the efficiency of a rocket exhaust is measured in specific impulse (Isp:, which is a function of the mass and exhaust velocity. Higher mass exhaust products must be expelled at correspondingly higher exhaust velocities to achieve the same efficiency.

Thermal nuclear rocket designs tend to heat hydrogen to a plasma and expel that. The high heat of the exhaust helps produce a high Isp; but the fact that the propellant is low mass allows you to achieve high efficiency without requiring temperatures (or pressures) that might cause issues with known materials. If you increase the mass of the exhaust propellant you also need to increase the temperature and pressure, which creates new problems...

Trying to confine a neutron stream adds yet another problem to the mix - one we don't have the science for, AFAIK.

- Joel
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