I know the role and mechanism of ablator, I know some examples, but I still don't know "what makes a material a good ablator". Certainly combustion/evaporation temperature below the expected work temperatures, and low thermal conductivity, but I'm fairly sure that's not all. So - what properties make a good ablator?
Here are some desirable properties of an ablator. The first ones are they you mentioned, with some additional detail.
- A sublimation point lower than the operation temperature is a must. An ablator that does not evaporate is not an ablator. The shield has to loose some mass in order to get rid of the added energy, in contrast to for example the tiles of the space shuttle. They where designed to not evaporate, and instead store heat in heat sinks.
- A low thermal conductivity is similarly important. While the shield can not stand re-entry conditions for ever, the point is to delay the problem for a few minutes during the descent. High heat capacity is related (but not directly dependent), and is together with the conductivity pointing towards low molecular mass, non-metals, and organic materials. Note also that internal voids are also excellent for insulation, giving a spongy or foamy material an edge.
- In addition, to actually consume energy, a combustion reaction with the air should be endothermic, and any sublimation of material associated with a high enthalpy of fusion.
- A high vapour pressure is important, as most of the shielding effect is not directly from the shield itself, but rather from the gas it emits, creating a cushion diverting the airflow.
- Mechanical robustness is in itself important, as a fragile material being shredded to pieces in the extreme conditions is of no use. This does not technically have to be a property of the ablator per se, as an underlying honeycomb lattice or aramid rings in the material can take care of that. (It is still a plus though.)
- For technical reasons, the materials in question are not practical to cast, so the materials must have good solidifying properties, like epoxy.