Re-entry and heat shield physics and engineering.

[nicp]

I have little to contribute regarding heat shields beyond what is freely available on Wikipedia or NASA documentation.
I haven't seen a general discussion here about the topic, so my apologies if I have missed it.
Oh, and I don't want a discussion that is very much about Starship.

So - for suborbital, the X-15 got away with ablation and Inconel-X. That stuff is dense, right? And you aren't going to use that heatshield again.
Ditto Mercury, Gemini and Apollo (I won't mention Soviet, I don't know enough), - no inconel, but the heatshield ablates (boils away), while being blunt (they all are) and then compression heats the atmosphere which largely flows away from your spacecraft.

Then we get onto Shuttle (or Buran, Starship, X-37) with refractory heat shields, with lightweight silica tiles (so far) that can take the mostly (?) radiant heat and survive. Obviously delicate and a pain in the rear to work with. (did anyone ever suggest silicosis for workers in this context? I have no idea if that is a problem and I hope and expect it has been considered).

Then we get onto transpiration cooling - and I have no knowledge of any use of this on spacecraft, though I believe there are suggestions that this is used on the F9 booster (with water?) for re-entry.

Transpiration is, of course having a liquid (water, fuel, perhaps oxidizer) you can push out through little holes (pores) much like plants do to cool themselves. The small (?) quantity of liquid, perhaps with a high specific heat (water) takes the heat and goes away.

What triggered this line of thought was the successful - pretty much - IFT-4 - from near   low Earth orbit and it barely made it.
Sure, it will be improved.
Now SpaceX talk about the Moon and Mars, but a few keystrokes on my calculator tell me that the energy returning from the Moon is about 1.9 times low Earth orbit. (11^2 / 8^2, 11 being close to Earth escape in km/s, 8 being LEO).

Can a refractory heat shield ever do this?
SpaceX are smart and have ended up with, pretty much, the Shuttle heatshield. Probably 'cos NASA guys are smart too. Well of course they are.

N

EDIT: Low earth orbit, not near.

[rfdesigner]

Then we get onto transpiration cooling - and I have no knowledge of any use of this on spacecraft, though I believe there are suggestions that this is used on the F9 booster (with water?) for re-entry.

Transpiration is, of course having a liquid (water, fuel, perhaps oxidizer) you can push out through little holes (pores) much like plants do to cool themselves. The small (?) quantity of liquid, perhaps with a high specific heat (water) takes the heat and goes away.

What triggered this line of thought was the successful - pretty much - IFT-4 - from near Earth orbit and it barely made it.
Sure, it will be improved.
Now SpaceX talk about the Moon and Mars, but a few keystrokes on my calculator tell me that the energy returning from the Moon is about 1.9 times low Earth orbit. (11^2 / 8^2, 11 being close to Earth escape in km/s, 8 being LEO).

Can a refractory heat shield ever do this?
SpaceX are smart and have ended up with, pretty much, the Shuttle heatshield. Probably 'cos NASA guys are smart too. Well of course they are.

N

Like you I have a lifetime in engineering, not quite retired though, and this is not my field of expertise beyond what I've gleaned from these boards and elsewhere on the net.

Transpiration: have a look at stoke aerospace...    yet to fly, but is using this on their second stage, there's quite a good everyday astronaut video where tim dodd visits the facility and discusses what they're doing.

Re higher re-entry speeds, the solution is a longer gentler re-entry, giving the shield time to radiate away the energy, they can adjust the re-entry profile from higher velocities such that the peak heating is controlled and doesn't go past what can be handled, yet the spacecraft isn't lost.  If you've ever played Kerbal Space Program, you will probably have tried aerobraking (if not the do give it a go), interplanetary aerocapture usually requires a first pass that just about reduces the speed to the point where the craft is captured into a very elongated orbit, then each pass thereafter reduces the speed a little until the craft can reenter without burning up.

[BaradleyM]

my guess as I am neither an engineer or an expert. the reason to look at something
 other then transpirational may have some thing to do with the mass to orbit.
extra equipment means more weight and when you still need to figure
"how much you are carrying to orbit." and the fact that it impacts
 you else where then it may not make sense.

[edzieba]

Transpiration: have a look at stoke aerospace...    yet to fly, but is using this on their second stage, there's quite a good everyday astronaut video where tim dodd visits the facility and discusses what they're doing.

Stoke are not using transpiration, they are cooling a metallic surface with LH2 flow on the backside. The gas generator powering the pump dumps exhaust products out the vent in the middle, but that's a bonus rather than the primary mechanism of action.

There's a lot more to TPS selection than just total orbital energy. e.g. an ablative TPS will want to keep total heat input down but can tolerate high peak heating (and needs above a minimum level of heating to work at all, too low and the TPS will fail to ablate), but a radiatively cooled TPS (e.g. STS tiles) can handle a much lower maximum temperature but can sustain that for a long time to tolerate more total heating. Actively cooled systems can tune tolerance based on flow rate and coolant mass, which gives even more knobs to turn for overall system optimisation (e.g. carrying more coolant means greater heating tolerance, but also carries more mass into entry which demands a larger entry body, both of which drive increases in launcher dimensions and mass, etc).