Heat shields made in space out of rocky source material - cheap way to get cargo to Earth?

Question

Let's say there's an operation one day producing items from asteroid and lunar material in orbit, and they want to ship goods to Earth. They try making cheap heat shields by processing rock and regolith material. they put the shields on really simple capsules. The processing doesn't chemically change the material, it just sinters it, or melts it and sprays it. They want to beat the price of a ship coming from Earth to collect cargo and take it to the surface.

The simple capsules ditch in the ocean using parachutes. They are just a metal skin sufficient to protect the cargo, a beacon, and parachutes. The heat shield is discarded after entry, the rest is reused. Capsules are disassembled and sent back to the orbital operation.

The operation is able to make shapes of rock particles that can be quite porous, of arbitrary size and shape. The capsule needs to survive entry, we'll say if it survives, whatever it's shipping does too. Cargo mass is 2 metric tons.

Edit I apologize for yet another edit, but the existing answer, while helpful, considered rock and regolith, not rock that's been processed, so I made edits above to be clear that this is about processed rock and regolith.

Also, I searched and actually found a study on this (which really surprised me). It's a NIAC study from 2012, lead author Michael Hogue So, it seems that sintered regolith will work for this, and the focus changes to the rest of the task of making this cheap. From page 36:

Within the scope of the testing to date, the feasibility of using extraterrestrial regoliths as the construction material for atmospheric entry heat shields has been confirmed from the results of the acetylene flame and arc jet testing. While some of the arc jet-tested samples were heavily ablated, they provided adequate low temperatures on their rear surfaces. These rear surface peak temperatures were recorded several minutes after arc jet test termination.

How should the capsule be designed to work best with this? What would be the best way to make use of such capsules to make the full cost of shipping from an operation in Earth orbit as cheap as possible?

Answer 1

A successful ablative heatshield should evaporate as necessary during reentry. It should not break, crack or disintegrate.

The ablative heatshields used for the Apollo mission were made of a phenolic resin carefully injected into a fiberglas honeycomb structure.

The fiberglas disintegrates at a higher temperature than the resin, that is important to avoid cracks within the heatshield.

I don't think there is a cheap rocky material with similar properties. If original rock is used, you have to avoid pieces with hidden fissures or cavities. If the heatshield is assembled using a lot of pieces, you have to fill the gaps between those parts with a heat proof and gas tight material.

A heatshield material should tolerate thermal stress when the outer layers are hot and the inner layer are cold. Brittle materials like rock would crack under thermal stress, you need an elastic and flexible material.

Answer 2

I remember a nice bit from the (marvellous) book Mining the Sky, by John S. Lewis.

There were studies in how to reduce dependency on goods launched from Earth and closing as many resource cycles with in situ lunar resources. A significant part of lunar regolith is made of ilmenite, mainly containing, among other more minor (albeit valuable) things, titanium, and iron oxyde.

You can start by crushing and sifting your mineral to the required granularity; then cooking it to extract oxygen from it. Of course, my description is extremely short and the engineering problems are not to be ignored but just got yourself a source of oxygen and ultrapure metallic iron. And ultrapure iron has neat mechanical properties not encountered in regular iron from what I understand. We usually use steel because it's way easier to make down there on earth, but from my understanding ultrapure iron would work great.

What's left after you got the nice stuff? Titanium oxide. Rutile. Rutile is a refractory material. You can mold your rutile grains in a giant microwave oven the shape of a reentry shield, obtaining an instant, quite literally cheap as dirt, reentry shield. I can't remember the name of the guy who performed the experimentation but I can find it again if you want. And voilà, a reentry shield out of industrial byproduct. [EDIT]: This experiment was performed by Professor Tom Meek, from the University of Teennessee.

And as I said, you also have a steady source of ultrapure iron to build your single-use moon-to-earth reentry capsule.

I haven't touched the subject of ergol for the moon-earth transit but that also can be sorted. [Edit]: on the top of my head, you can extract aluminum from anorthite, another readily available mineral on the moon, and burn it with oxygen, to launch a rocket. Sure you may find water in permanently shielded craters at the poles and electrolyse it, but we'd probably be better off keeping such a limited and valuable resource (both water itself, and hydrogen which is extremely rare in lunar regolith) where it is rather than using it as rocket fuel, whereas anorthite is maddeningly plentyfull. Besides, the poles make for a suboptimal launch location for rockets.

From asteroids, it would be even easier. A thing like ilmenite would be not dissimilar to the "worthless" slag after you extracted all the easy to extract valuables.