Fuel of the week: Molybdenum


Molybdenum is your go-faster fuel. It is a heavy refractory metal, with a high melting point. It has been identified in the solar system, but it’s likely to be a parts-per-million component of nickel-iron asteroids.

You’ll need to bring it from Earth, but this is the fuel that gets people to Mars.

In a Neumann Drive, Moly can be hit hard and hit often – it appears to have a linear efficiency pattern that means we can happily use five pulses per second at 125 Joules per pulse for three hundred microseconds, and we’re confident that aggressive cooling of the cathode can increase the number of pulses per second. At this rate, the fuel has a power efficiency of 20 micronewtons per watt, and over a month will produce 32,400 Newton-seconds of momentum exchange and use five hundred and twenty grams of fuel to do that.

What this means in practice is that the power demands for a Neumann Drive running Mo are fairly high – you need 3 kilos of off-the-shelf Spectrolab space-rated Improved Triple Junction solar panels will support one Neumann Drive running moly while near to Earth, and about double that if you plan on operating near Mars. Likewise, using 125 joules per pulse means we need a good sized capacitor bank.

If we use 6 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 three kilos of shared capacitor and two kilos of shared inductor per Neumann Drive, and each Neumann Drive thruster unit masses four kilos by itself, then each Neumann Drive optimised for molybdenum will have a total mass of fifteen kilograms, including shared supporting systems.

Add six kilograms of fuel, which should be enough for just over 12 months of continued operation.

We therefore are committing 21 kilos to create 32,400 newton-seconds of momentum exchange each month for a “burn period” of just over a year.

If we have 100 of these (adding to 2.1 tons) and 700 kilos of structure, cooling, flight computers and payload; then the spacecraft adds to a total of a fully-fuelled “wet” mass of 2.8 tons, and an unfuelled “dry” mass of 2.1 tons. When we are in enough sunlight all the time, the drives create 3,240,000 newton-seconds of momentum change each month, which will represent 1,150 metres per second per month of acceleration while the craft is fully fuelled, and 1,542 metres per second per month of acceleration when running on empty.

For the sake of a first-cut approximation, let’s assume an average acceleration based on average mass of the craft, of 2.45 tons – what does it do running Moly when fully loaded with half the fuel tank gone.

With 3,240,000 newton-seconds pushing 2.45 tons, you get an acceleration of 1.32 kilometres per second per month.

Low Earth Orbit to Mars orbit needs approximately 15 kilometres per second of delta-v when using low-thrust solar-electric propulsion. We have an average acceleration per month of 1.32 km/sec, and 12 months of fuel, we have a delta-vee budget of 15.89 km/sec.

Clearly, this is for a people-to-Mars mission, which introduces some complications. Assuming your people are leaving from Earth and you want them in good condition at the other end, you probably don’t want to be taking them through the Van Allen Radiation Belts via solar-electric propulsion, because even if you avoid the really dangerous parts, spending time in the radiation trapped by Earth’s magnetic field isn’t good for humans.

A better plan would be to send the humans directly to a transfer station at L5 via a fast chemical rocket, spending the fuel to keep them healthy, while their stuff goes in advance from where it was dropped off in LEO

If the rocket that got their stuff from the surface drops their it well in advance to some conveniently-located International Space Station or other, then it can be moved slowly by Neumann Drive to a second station at L5. The humans go directly to the second station via a much faster chemical rocket – something that spends the extra fuel to take three days in the Van Allen Belts rather than three months.

As an aside, this second station can and should be the mark one of any Mars Orbital Habitat, going through its multi-year shakedown cruise somewhere it can get spare parts delivered. Once most of the bugs are shaken out, the mark two gets built, assembled in orbit and moved to Mars orbit.

The humans are then reunited with their stuff, and then take the moly-fuelled Neumann Drive craft from the second station to Mars, but from a point where we have pretty much escaped Earth’s gravity field. This will take less kilometres per second of delta-vee, which means we can shave the fuel down, which gives us lower mass, which gives us more acceleration and so on. If you assume we need 10 kilometres per second, then you only need 4 kilos of moly per drive, which drops average craft mass from 2.45t to 2.25t, sending average acceleration goes up to 1.44 km/sec per month, which means a touch over 7 months from the transfer point to Mars orbit.

If you’re happy to spend the ton or so of chemical fuel to get each human from Earth to the transport point fast, then the humans will experience an eight month trip, split between chemical propulsion to get them through the Van Allen belts and solar electric propulsion to get them to Mars orbit.

Recent work on the magnetic exhaust nozzle has improved this situation, as less material is being used per pulse to create the same thrust, but the higher atomic weight of moly seems to make the magnetic exhaust nozzle less effective than other fuels we’ve tried.

Aggressive cooling of the cathode should be possible, and let us jack up the number of pulses per second when we have the surplus power. You need roughly double the mass of solar panels to create the same number of watts near Mars, so if you match your solar requirements to what you’ll need near Mars, then you have a bunch of spare power closer to the Sun. But we aren’t even at benchtop testing for that. Theoretically, it should mean the above design gets to use 10 pulses per second near Earth, and 5 near Mars, which means you should use the same amount of fuel but get there faster. How much faster, we don’t know, as we haven’t melted enough cathodes to find out, but we’re working on that.

Regarding power systems, the Spectrolab ITJs are actually their entry-level models, and you can get better from them, which will shave the base solar need from six kilos to five. For people-to-Mars, its probably worth buying their expensive ones (if you need to ask the price, you cant afford it). Azurspace are achieving 350 watts per kilo specific power with their cells, but that’s cells not panels, so we did the numbers using Spectrolabs’ panels. Interesting work is also being done by various parties with thin-film solar on low mass substrates, and there’s mutterings about a kilowatt per kilo being possible soon.

We’re also assuming standard capacitors. Interesting things are being done by Skeleton with ultracapacitors, and we’re confident graphene-based ultracaps will continue to improve.

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