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Getting rocket propellant from other sources than Earth is by many viewed as an important step towards further exploration of space. One such source could be our closest neighbour, the Moon.

The problem is, almost all the surface of the Moon is just this endless ocean of regolith, a mixed blend of minerals where ores are scattered all around the place. That means it does mostly contain iron and aluminium in addition to oxygen.

While it is easy to find the performance details of powdered aluminium and liquid oxygen, data on how iron performs is more difficult to come by.

There are lots of practical problems, I know, like that both the fuel and the exhaust are solids, but from a theoretical point of view, it would be nice to know:

What is the $I_{sp}$ of powdered iron/LOX?

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  • $\begingroup$ I don't have the data, but the rule of thumb is: heavier element - higher thrust, lower ISp. $\endgroup$ – SF. Jun 25 '16 at 22:23
  • $\begingroup$ There are some regions at the poles that may have good deposits of water, carbon dioxide, hydrocarbons and ammonia. But I'm guessing you already know that. $\endgroup$ – HopDavid Jun 26 '16 at 0:12
  • $\begingroup$ @HopDavid Yes, I was only looking into what other possibilities that exists, as the exact nature of the deposits is not known. $\endgroup$ – Hohmannfan Jun 26 '16 at 0:14
  • $\begingroup$ Why would you even build such a rocket? Aluminum is lighter and more energetic. $\endgroup$ – Loren Pechtel Jun 27 '16 at 2:45
  • $\begingroup$ @LorenPechtel iron can be gotten much more easily. It's estimated maybe half a percent of the regolith is metallic iron, and reaction of ilmenite with hydrogen produces iron, water, and titanium dioxide, and the hydrogen can then be recycled by splitting the water molecules. So if you can produce it locally easily enough, it might be worth it. $\endgroup$ – kim holder Apr 18 '17 at 21:15
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Figure 5 on page 23 of this paper shows the Isp of tri-propellants including iron, where the curve goes out to 95% Fe and 5% H2. Just eyeballing it, the curves might converge at around 190 seconds. On the other hand, the curve might go to hell at 100%, with no light hydrogen for the heat of the burning iron to expel. Note that even at 5% H2 by weight, there are still three hydrogen atoms for each iron atom.

Isp as a function of Fe %

The reason the curve might go to hell is that the Isp is proportional to the inverse square root of the molecular weight.

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