Metal foam for space constructs

Question

So I'm writing a setting featuring asteroid mining à la Deep Space Industries, where the material mined is used for construction in space rather than brought to Earth. To maximize efficiency, I'm thinking of the main material for construction being steel foam (foamed steel?), but I don't know how feasible it is, and I don't know the math to find out. So I come to ask this:

Could steel foam be used to build a Stanford Torus-style space habitat with a radius of at least 1.8 kilometers and a width of around 500 meters, rotating at 1 rpm?

Also, could you include the equations you used in the answer?

Answer 1

From Wikipedia:

A metal foam is a cellular structure consisting of a solid metal, frequently aluminium, as well as a large volume fraction of gas-filled pores. The pores can be sealed (closed-cell foam), or they can form an interconnected network (open-cell foam). The defining characteristic of metal foams is a very high porosity: typically 75–95% of the volume consists of void spaces making these ultralight materials. The strength of foamed metal possesses a power law relationship to its density; i.e., a 20% dense material is more than twice as strong as a 10% dense material.

Metallic foams typically retain some physical properties of their base material. Foam made from non-flammable metal will remain non-flammable and the foam is generally recyclable back to its base material. Coefficient of thermal expansion will also remain similar while thermal conductivity will likely be reduced.[1]

This indicates that metallic foam could be used in your structure, but there would be various penalties to pay. The first and most obvious one is the power law relationship between density and strength. While foamed metal will be amazingly light, it is not amazingly strong. Rotating structures like a space colony will be under a lot of tension, so foamed metals should not be structural elements, or at least not unless they are backed by a network of cables or nets to provide strength in tension.

The other issue in space is that the environment is highly radioactive, so you need a solid mass of "stuff" to block incoming cosmic and solar radiation. The Millennial Project: Colonizing the Galaxy in Eight Easy Steps by Marshall T. Savage (my go to book for this kind of thing, highly recommended) states that you would need 5 metres of water or an equivalent amount of concrete or regolith to keep radiation levels inside the colony at Earth levels. Bluntly, a metal foam is not going to have nearly the stopping power of a similarly sized piece of solid metal. You might use the metal foam as the matrix for some sort of composite material, with the pores of the metal foam holding something else to absorb incoming radiation, but this detracts from the lightweight property of metal foam, and would probably be insanely difficult to do (fill all the pores with some other material).

Metal foams could be non load bearing elements, like wall panels, with good thermal and acoustic insulation properties and very light and easy to work with, but for the main structural elements, not so much.

Answer 2

There is an advantage to metal foams in some cases. If you want to increase the strength in resisting bending or twisting loads the increased thickness (for the same mass of metal) is a huge advantage.

There are likely some components in a space station that would benefit from metal foam. However, most components will not have to resisting twisting or bending and and primarily have to support stretching or compression loads, for which foamed materials are somewhat worse than convention materials. A beam in compression that is subject to buckling is an example of bending load that would make sense for foamed materials.

Foamed steel is really not commercially available today, but may be easier to manufacture in null gravity.

So, a minority of the components would benefit from foamed steel. But the majority clearly would not.

If you are lifting mass from earth to build your station, there is really no reason to us foamed steel. Some composites have better strength and there would be have other advantages as well. Radiation shielding would be one of these advantages. Thermoset plastics are often very convenient compared to what foamed steel would require though they are not as strong.

If you are making steel in bulk from asteroids, etc. The foamed steel becomes much more attractive since we have not struck oil in space and don't expect to do so anytime soon.

Now to the stated question. Could you make your orbital using foamed steel. Yes, I believe you could. I don't have data for the actual strength of foamed steel (not really on the market), but based on what I learned in materials engineering I would guess that it is weaker than standard steel components for 2 reasons. 1) It would be hard to control the quenching process in foamed steel to get the best strength possible. 2) Cracks tend to form in materials near sharp bends in the material -- the sharp corners have higher stress regions. Since foamed steel would be nothing but sharp corners, I would expect it to fail more quickly than equivalent bulk steel.

IIRC, the Stanford torus was calculation to be with a normal safety margin even if the radius was extended to 4 km (a little over double). Given that, I would assume the even the reduced strength of foamed steel would not break the design safety. But, without actual strength data, this is only a best guess.

Foamed aluminum is likely a better choice as it is stronger per unit mass and we can actually produce foamed aluminum with reasonable ease today. However, if you want your station to last more than a few years, it is back to steel. Aluminum has a very nasty habit of falling apart after a number of expansion and contraction cycles. Steel does not do this, depending upon the alloy. As long as the deformation in steel is below a certain threshhold, you can stress cycle it millions of times without forming cracks and breaking. No matter how small the deformation, aluminum will always form cracks and break given enough stress cycle. In space, there will always be stress cycle arising from the thermal cycle, which can be as frequent as 90 minutes in low earth orbit.