Fuel of the week: Magnesium


Magnesium is an interesting fuel for several reasons. The first one is that it is a metal that, because of it’s combination of light weight, high strength and ability to be easily formed, is extensively used in aerospace applications. The second is that it has been identified in various places in the solar system, usually as the mineral Olivine. The third is because its specific impulse – how much momentum change you can get out of a given mass of fuel – is utterly insane. All of which combine to make Magnesium one of the best fuels around for the Neumann Drive.

This is the fuel that gets you to Mars and back on a tank of fuel, and we will now give you the numbers to show you how.

In a Neumann Drive, Magnesium needs to be treated fairly delicately – it appears to have a “sweet spot” of four pulses per second at 40 joules per pulse. At this rate, the fuel has a power efficiency of 11 micronewtons per watt, and over a month will produce 7,200 newton-seconds of momentum exchange and use just 52 grams of fuel to do that.

What this means in practice is that the power demands for a Neumann Drive running magnesium are quite low. Three kilos of off-the-shelf Spectrolab space-rated Improved Triple Junction solar panels will support one Neumann Drive running magnesium… even in Mars orbit. Likewise, only needing 40 joules per pulse means we can get away with a smaller capacitor bank than when using some other fuels.

If we use four kilos of Spectrolab ITJ panels to allow for degradation over time and efficient use as far out as Mars, and share power management resources among Neumann Drives so each has a “slice” of two kilos of shared capacitor-bank and two kilos of shared inductor per Neumann Drive, and each Neumann Drive thruster unit masses four kilos by itself, then each Neumann Drive will have a total mass of twelve kilograms, including shared supporting systems.

Add six kilograms of fuel – which is enough for 115 months of continued operation, or just short of two five-year missions.

We therefore are committing 18 kilos to create 7,200 newton-seconds of momentum exchange each month for a “burn period” of just short of ten years.

If we have 100 of these, this adds to 1.8 tons. Then add 700 kilos of structure, cooling, flight computers and payload, then the final spacecraft has a fully-fuelled/”wet” mass of 2.5 tons, and a empty/”dry” mass of 1.9 tons.

When we are in enough sunlight all the time, the drives create 720,000 newton-seconds of momentum change each month, which will represent 288 meters per second per month of acceleration while the craft is fully fuelled.

Low Earth Orbit to Mars Orbit needs approximately 15 kilometres per second, when using low-thrust solar-electric propulsion. At a minimum of 288 meters per second per month of acceleration when fully loaded and 115 months of fuel supply, we have 33,120 meters per second of delta-vee budget (in reality, it will be better than this, as the craft will accelerate better as more fuel is used and it gets lighter. To keep the maths simple, assume we picked up around 300 kilos of moon rock at Phobos or something).

If we are using no fancy navigation (like the Mars gravity assist that the Dawn probe used to help get to Ceres, or the Earth swing-by that Hayabusa used on it’s way to 25143 Itokawa), this shows a return trip to Mars orbit, Deimos or Phobos is easily achievable with these assumptions about engineering.

Recent development of the magnetic exhaust nozzle has improved this situation, as less material is being used per pulse to create the same thrust.

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