Fuel of the Week: Tin

In science, learning from your failures can be more important than success.

Tin is a really bad fuel for Neumann Drives, but the way it behaves tells us a lot of useful things.

Before anything else, tin is a soft metal with a low melting point – it will melt at 233 degrees Celcius, which is a little hotter than you want to bake chickens at in a home oven.

A Neumann Drive works by passing an electric current through a solid fuel rod used as the cathode, after triggering the arc with a surface flashover from the trigger pin. Thus there are two main things that can reduce efficiency – you let the current pulse run too long, so that the cathode spots run off the edge of the front face of the cathode and arc sideways to the anode (“side-arcing”), or you use a level and duration of current that melts part of the metal fuel rod. This last one means that rather than producing ions that leave the cathode at tens of kilometres per second, you get pools of molten tin that produce droplets and clouds of neutral tin vapour, neither of which move very fast at all.

The image below shows the result of this for tin.

tin_graph_a

At low currents applied quickly, tin is merely a terrible Neumann Drive fuel, with specific impulses approaching the glorious heights of 600s – about twice as fuel efficient as a solid chemical-fuelled rocket, or about a third as fuel efficient as a solar-electric xenon-fuelled Hall-Effect Thruster. However, when higher currents are applied for longer, performance goes into the toilet and beyond.

Using sixty joule pulses over 300 microseconds, performance drops to below what can be achieved using household sugar and potassium chlorate, the so called ‘sugar rocket’ of amateur rocketry.

Now, the point is not to make jokes about tin being worse than fuels that are made in kitchens using coffee grinders – it is about the why performance gets worse, which is that parts of the fuel rod itself get melted.

This is why we are so interested in actively cooling the cathode, as then we may be able to maintain the high performance of good fuels like molybdenum as we crank up the pulse rate, moving to higher power levels and thus get more thrust out of the same number of drives.

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