Bismuth was a material we had high hopes for. It has been used as propellant in other electric spacedrives, notably in the work done by Massey on Bismuth-fuelled Hall Effect Thrusters at Michigan Technological University.
There are reasons to think Bismuth would have been a good fuel for the Neumann Drive – it’s a heavy element, non-radioactive and with a low ionisation energy and a potentially useful electron cross-section.
We were hoping for decent results.
We were horribly, awfully, wrong.
In testing, we hit a Bismuth cathode with 15 000 pulses, which eroded just short of fifteen grams of material. At a pulse rate of four per second, that’s about an hour of operation.
Magnesium erodes fifty two grams in a month.
Our best results for Bismuth involved hitting it with 100 microsecond pulses, with about twenty joules per pulse – that’s half the time and half the energy of magnesium, a fuel which we regard as needing to be treated gently for optimum performance.
These best results involved specific impulse of about 140, which is substantially worse than that achieved by amateur rocket enthusiasts using hydrogen peroxide as a monopropellant.
At higher energies, for example 40 joule pulses over 200 microseconds, it went much worse. We achieved a specific impulse of about fifty, which means it is competitive with the engineer teeing up the lump of bismuth up to hit in the opposite direction of travel with a golf club.
Friends don’t let friends use bismuth in Neumann Drives.