Many years ago, I read a science fiction story "Rocket Ship Galileo" in which a nuclear powered spacecraft used zinc as a propellant. Apart from the fanciful nature of the story itself, I did wonder if the propulsion concept - creating thrust by using nuclear thermal energy to vaporize zinc - had any basis in solid science and engineering. More specifically, do the properties of zinc lend it to effective use as a nuclear thermal propellant? There would seem to be factors which might make it unsuitable such as melting point, which would create difficulties/complexities in a fuel handling system. Apart from such issues, would such a propellant offer any benefits such as a high ISP, for example?
Zinc has too high molar mass to find any practical application in Nuclear Thermal Rockets (NTR) as propellant fluid. An additional limitation in solid core NTR is the difficulty of achieving propellant temperatures higher than 2700°C because there are no such materials that have satisfactory mechanical properties at higher temperatures. Zinc has a relatively low boiling point, 907°C and low vaporization energy (for metal), but also 32.7 times higher molar mass than hydrogen. Since in most solid core NTR designs the temperature is limited to a max. 2700°C - the performance of such rocket with Zinc as propellant would be at the level of ancient black powder. Other NTR designs like pulsed, liquid or gas core are concepts, but still same limitations of Zinc high molar mass apply and light gasses like Hydrogen a Helium are far better options.
If there are solids which could be used for NTR as propellants, they should be searched among hydrides of light elements like Li, Be, B (LiH, LiBH4, BeH2, LiBeH3, Li2BeH4). They all have high hydrogen content and in general per volume can store 30-70% more Hydrogen than LH2. Beryllium is extremely toxic and expensive and it's unlikely that it could compete with LiH and LiBH4 although BeH2 has some attractive properties. LiBH4 has most advantages like low melting point 268°C and decent liquid region of 112°C prior decomposition. But it would be challenging to fully dissociate LiBH4 into individual elements. LiH on the other hand has much higher melting point (still lower than Zinc) but it can be fully decomposed to individual elements. Hydrogen can be used first and lithium (which boils at 1330°C) in second stage. Average molar mass of gases is acceptable 5.3g/mol, possible even 4.67 if Li6 isotope is used. Additional benefit of using Li6 isotope is its high neutron capture cross-section and fission which liberates 4.8MeV per atom while splitting into Tritium and Helium. Of course such NTR could only be used in space due to the toxic and corrosive nature of lithium compounds produced by it's combustion in the atmosphere.
Apart from such issues, would such a propellant offer any benefits such as a high ISP, for example?
There's one and only one property that makes zinc to occasionally show up in NTR contexts.
Everything else about zinc as a propellant is terrible.
The ISP is very bad. Handling what's under normal conditions a solid is bad.
But the density is a nice 7.14 g/cm3 for the solid, 6.57 g/cm3 for the several hundred °C liquid, presumably a bit lower for a slush mix of powder and water.
A higher density allows stuffing more propellant in the tanks, giving a delta-v advantage for situations that aren't mass limited.
... which is pretty much nowhere that a NTR would be useful.
More mass at lower velocities also means a higher T/W ratio for the same reactor power.
... which is also not really a use case for a NTR.