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Is it viable (or even possible), to use a neutron source as a form of propulsion for a spacecraft? I imagine this drive would have similar trust/specific impulse characteristics as electric propulsion methods (ion/hall effect drives), but with higher specific impulses.

From some basic searching, I've found that D-D reactions produce neutrons with energies of ~2.5MeV and D-T ~14MeV, and there are commercial D-D neutron generators available with an output of 1 x 1010 - 1 x 1011 n/s (1 x 1013 with tritium), so it seems we can currently produce high numbers of fast neutrons on demand.

There is no mention of the amount of deuterium, titanium and energy required to achieve this output, so I couldn't even begin to guess the specific impulse or trust to weight ratio of such a configuration, but I'm curious to know would it be feasible.

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    $\begingroup$ Directing the neutrons to generate net thrust is a bit of a problem - unless you use fairly high accelerating voltages the neutrons pretty much head out into $4\pi$ steradians. At that high a voltage, you'd be better off just using the ions and skipping the hit from the reaction cross section. $\endgroup$
    – Jon Custer
    Commented Oct 24, 2017 at 12:49
  • $\begingroup$ @JonCuster - How about if you just absorbed the neutrons in one hemisphere and let the other fly away? Unless there is some sort of neutron mirror. $\endgroup$
    – mmesser314
    Commented Oct 24, 2017 at 12:55
  • $\begingroup$ @mmesser314 - sure, you can play games like that. Or, you just use a nice ion thruster. Note that 1 nanoAmpere is about $10^{10}$ charged particles per second, all going in the same direction already, without any nasty radiation side effects from neutrons. $\endgroup$
    – Jon Custer
    Commented Oct 24, 2017 at 12:59
  • $\begingroup$ For long voyages the interesting stat would be the thrust/weight ratio of the fuel, not the total number of particles per second. $\endgroup$
    – JMLCarter
    Commented Oct 24, 2017 at 13:17
  • $\begingroup$ @JMLCarter - for specific impulse, neutrons aren't that good compared with heavy ions. Trying to use a nuclear reaction (with a peak cross section of ~5 barns), rather than the primary beam is just a losing effort. $\endgroup$
    – Jon Custer
    Commented Oct 24, 2017 at 13:24

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To reach MeV energies in DD you need accelerations, which means a lot of energy to accelerate the Ds and a lot of weight/space-taken-up, as said in comments ions would be more efficient, because they can be manipulated. Neutrons have no handles to generate beams with, except absorption , another loss of energy.

NASA already has ion propulsion systems for space technology.

Electrons produced by the discharge cathode are attracted to the dis- charge chamber walls, which are charged to a high positive potential by the voltage applied by the thruster’s discharge power supply. Neutral propellant is injected into the discharge chamber, where the electrons bombard the propellant to produce positively charged ions and release more electrons. High-strength magnets prevent electrons from freely reaching the discharge channel walls. This lengthens the time that electrons reside in the discharge chamber and increases the probability of an ionizing event.

The positively charged ions migrate toward grids that contain thousands of very precisely aligned holes (apertures) at the aft end of the ion thruster. The first grid is the positively charged electrode (screen grid). A very high positive voltage is applied to the screen grid, but it is configured to force the discharge plasma to reside at a high voltage. As ions pass between the grids, they are accelerated toward a negatively charged electrode (the accelerator grid) to very high speeds (up to 90,000 mph).

The positively charged ions are accelerated out of the thruster as an ion beam, which produces thrust. The neutralizer, another hollow cathode, expels an equal amount of electrons to make the total charge of the exhaust beam neutral. Without a neutralizer, the spacecraft would build up a negative charge and eventually ions would be drawn back to the spacecraft, reducing thrust and causing spacecraft erosion.

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  • $\begingroup$ Also neutrons would likely require more shielding weight, so while maybe possible, I'm really not convinced it is a good idea, specially compared to ion propulsion. $\endgroup$
    – Vendetta
    Commented Oct 24, 2017 at 15:50

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