Fuel of the Week: Aluminium

The Neumann Drive has several advantages over legacy solar-electric drive technologies.

Because it uses solid fuel, it saves on the tankage, pumps, pipes and all the other things you need for dealing with compressed xenon in a space environment – it is perfectly possible to just leave the metal fuel rods in a vacuum.

Because it throws the high-energy particles away from the working parts of the drive, it is more likely to have a longer operating life than a given design for Hall Effect or Gridded Ion Thrusters, which push ions either through their grid or past the accelerators, wearing them down over time.

With the right fuels, it is far more fuel efficient than competing models, with specific impulse over 14,000s in the case of magnesium, and around 5,000s with the case of molybdenum, as compared to about 2,000s for a Hall Effect Thruster or about 3,800s for a Gridded Ion Thruster.

But the biggest advantage for the Neumann Drive over the legacy drives is that it can use fuel that doesn’t need to be moved up from Earth.

Aluminium is one of those fuels, because much of the approximately 7,000 tons of space junk is aluminium. A large proportion of that space junk is sitting just above geostationary orbit, in the so-called ‘graveyard orbit’.

Let’s use the old Satellite Business Systems-6 (SBS-6) as an example. SBS-6 was launched on an Ariane rocket in 1990, and between getting to a stable geostationary orbit, and the years of station keeping and then moving to its current graveyard orbit, it used all of its fuel. This took it down to its current, empty “dry mass” of 1.514 tons, and it’s likely to be more than half aluminium by mass.

If somebody moved a space furnace into the graveyard orbit above geostationary, and the owners of SBS-6 were co-operative, then you could recycle this aluminium into fuel rods.

These fuel rods will be markedly inferior to those you could bring from Earth, but being already in space means you don’t have to pay the very high price for a chemical rocket to get it up there. Even if the results are less than the best, the price-per-pound could be worthwhile. But just how bad/good could they be?

Let’s assume a mostly-aerospace aluminium miscellaneous mixed fuel rod is 20% worse in every category than pure aluminium usually is (testing space-grade aerospace alloys is a high priority for us, but we don’t have any results we can release – so let’s simply assume it’s even worse than a material that we regard as pretty damn crap to start with).

In testing, pure aluminium fuel rods got a specific impulse of 2,323 +/- 325 seconds and a thrust-to-power ratio of 10.8 +/ 0.5 microNewtons per watt. This means we’re assuming you get about 9 microNewtons per watt of power put in the rod, and it’ll have a specific impulse of around 1,800.

That means you need about the same amount of fuel to get somewhere as if you’d used a Hall Effect Thruster system, though the HET will need about one-eighth of the mass of solar panels to keep it moving.

A satellite in geostationary orbit needs 50 m/s of acceleration each year to stay there. Lets assume we have a 3 ton satellite. Therefore, we will need 150 000 Newton-seconds of impulse each year to stay in the right orbit.

Let’s assume we have 10 kW of effective power to the Neumann Drive. 10 kW at 9 microNewtons per watt makes 90 milliNewtons of time-averaged thrust, or 5.4 Newton-seconds every minute. This makes 7, 776 Newton-seconds every day, so running the system for 19.5 days per year will keep your 3t in orbit.

At a specific power of around 150 watts per kilo, our hypothetical junk-fuelled Orbit Maintenance Tug will need around 70 kilos of Spectrolab ITJ panels for 10 kW of power.

Assuming the Orbit Maintenance Tug is a total of 500 kilos, then with a specific impulse of 1,800s, 250 kilos of Mixed Space Junk fuel rods has a total momentum change over time of 1,217 meters per second, or around 25 years of station keeping, with the fuel obtained from slow-moving objects about 150 km away from geostationary orbit around Earth.

This means construction of the Space Furnace, and its use to recycle satellites in the graveyard orbit will allow the practically unlimited life extension for all satellites currently in geostationary orbit.

This is why the ability to use fuels already in space is the greatest advantage of Neumann Drives over legacy technologies.

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