Heat shields

[TechnicalK3rbal]

What does the Soyuz reentry module use as (the material for) a heat shield?

Edit: Yes, I Google'd it.

Edit Edit: I know its ablative.

Edited March 26, 2014 by TechnicalK3rbal

[SargeRho]

Soyuz uses an Ablative heat shield, not sure what it's made of. But basically, it boils away during reentry, carrying the heat away from the spacecraft in the process.

[Tex]

I would think that ablative shielding is hugely risky... Think about it: what if reentry takes longer than planned? Some sort of navigational failure...

But anyway, I did find similar findings on the heat shield.

[Kryten]

I would think that ablative shielding is hugely risky... Think about it: what if reentry takes longer than planned?

There's an engineering margin. If you go beyond the engineering margin, you die. That's going to be the case for any kind of heat-shield, just replace 'eating through the entire shield' with 'melting' for non-ablative ones.

[Seret]

Re-entry is risky, full stop.

[Guest]

On Soyuz, though, the heat shield is massively overengineered, because it was conceived as a lunar ship. As such, you could probably make it back from the Moon with it, though the actual "lunar Soyuz" (the LOK) had it even thicker. Soyuz did, in fact, reenter in an off-nominal mode on a couple of occasions, so it can survive most reentry screwups, including reentering with the PAO still attached (and hatch-first as a result, it turns out the coupler burns off before the hatch gives, and the pod reorients itself after that)

[TerLoki]

Soyuz did, in fact, reenter in an off-nominal mode on a couple of occasions, so it can survive most reentry screwups, including reentering with the PAO still attached (and hatch-first as a result, it turns out the coupler burns off before the hatch gives, and the pod reorients itself after that)

Yeah, thanks to that big ol' lead weight they stuck in the bottom of the capsule. Gotta love Russian simplicity sometimes.

[christok]

I don't have any data on the Soyuz specifically but I can almost guarantee that it must be graphite--the usual suspect.

Graphite requires 28700 Btu/lb to vapourise, compared to 13400 Btu/lb for beryllium oxide*, 3865 Btu/lb for titanium and 1870 Btu/lb for tungsten. Given the very high energies that need to be dissapated (e.g. 13500 Btu/lb for a velocity of 26000 ft/s, although the vast majority of that goes to heating the atmosphere instead of the spacecraft) it should be clear why. The melting point of graphite is also above that of any of the materials listed.

I've copied the numbers above from W.L. Hankey's Re-entry aerodynamics, 1988. AIAA. p. 4. (Sorry but I'm too lazy to convert to SI.)

* A rather good heat shield but probably a bad choice due to the toxicity of Be. Not to mention the superiority of graphite and the cost.

[lajoswinkler]

I really doubt the shield literally boils away and I'm sure it mostly sublimes. I found that, in Apollo capsules, it was a composite of phenolic resin and fiberglass, which turns into a matrix of carbon and silica as it's heated by radiation from the rammed air in front of it.

Graphite is a rather good heat conductor. What you want is something very poor conducting and with high melting points, like mineral fibres, but it's best to rely on composites.

[jwenting]

or wood... Which has been used effectively as an ablative heat shield.

Especially when soaked through with something else that boils away, delaying the wood heating up.

[Javster]

Yeah, thanks to that big ol' lead weight they stuck in the bottom of the capsule. Gotta love Russian simplicity sometimes.

It was the opposite when they tested the Moscow-Washington DC hotline after the Cuban Missile Crisis: the US sent all the letters and numbers, Russia sent back a poetic description of the sun rising in Moscow

[Tommygun]

Have any spacecraft ever used tungsten as part of a heat shield?

[SargeRho]

Unlikely. I think Tungsten is a fairly good heat conductor. Ceramics are just about the best you can get for a reusable heat shield.

[lajoswinkler]

Have any spacecraft ever used tungsten as part of a heat shield?

Tungsten is a very dense metal. It conducts heat very well. It wouldn't be useful as a heat shield.

[SciMan]

Tungsten? I Agree, not the best material for the bulk of a heatshield. From what I know of it:

Tungsten (W from Wolframite, a tungsten-containing ore)

the majority of uses for it rely on it doing one or both of two things it does VERY well.

1. Pure Tungsten metal, having the highest melting/boiling temperatures of any PURE element, is ideal for use in spark plugs, spark gap electrodes, and many common welding electrodes {TIG welding = Tungsten Inert Gas welding})

2. Tungsten forms an extremely hard and abrasive wear-resistant carbide. Tungsten carbide is likely the most common material used for the cutting face of metal-working tools, and is the foundation material for other cutting edge materials used to make cutting edges

For a heatshield, we're interested in #1. However, we have a slight problem. It doesn't take much heat energy to make Tungsten melt or boil, even tho it takes a high temperature to do so. Graphite (carbon) takes far more heat to melt or boil than Tungsten does, but at a lower temperature.

Here's how I understand enthalpy of phase transitions, which is what makes that actually perfectly reasonable:

Lets say we have a 1kg cube of Tungsten, at 10° C, and a 1kg cube of Graphite (carbon), also at 10° C. perfectly thermally insulated from each other and the outside world (closed system)

Now let's say we heat both cubes with separate 1kw heat sources.

You should ignore temperature limits of the heating system, black-body radiation of the cubes, and non-uniform heating effects.

Both cubes started at the same temperature of 10° C, and are gaining heat energy at the same rate. (1000 joules per second)

Both cubes have the same mass, and started at the same temperature.

It takes less energy to melt or boil 1kg of Tungsten than it does to melt and boil 1kg of Graphite.

The numbers for "how hard is it to heat up material X" are called "heat of melting", "heat of vaporization", and "heat capacity", IIRC. (might not be technically correct, but should be word-synonym close)

That means that the Tungsten cube will reach its melting and boiling points with less total heat energy input, even tho the graphite cube has the lower melting and boiling points.

Incidentally, Water has both Tungsten and Graphite beat hands-down on all three of those numbers I mentioned. It's great at conducting heat, too.

I hope that helped someone figure out this particular quirk of the way the world works (or how we think it works, at least).

It might not make sense right away, but it usually* makes sense when you look at it closer.

I just don't get quantum physics. The closer I try to look at that mess, the less it makes sense to me.

Photon energy as wavelength not velocity? Sure, that's why blue or white LED's should theoretically drain batteries faster than red or green ones.

On the other hand, I'm not going to pretend to understand how statistics got involved, or how "observing" something can make it change state.

[christok]

Thermal conductivity isn't all bad. It's good if at least the leading edge of the heat shield conducts heat somewhat, which creates a heat-sink from the point of view of any hotspots. And letting the heat be conducted deeper into the heatshield creates a better sink (as long as you don't let the sensitive parts of the spacecraft get too hot, of course).

Heat shields get rid of energy in three ways. Ablation, radiation and heat sinking. The latter doesn't directly get rid of the energy, of course, but gives radiation more time to do its work. Copper is actually discussed as an almost doable shielding material in the book I quoted, in the form of a big heat sink and radiator. It isn't practically viable by any means but an interesting concept nonetheless.

lajoswinkler, interesting info on the Apollo shield. I did not know about the silica content. You are correct about the refractory materials used in heat shields sublimating. The table I looked at had the sublimation temperatures listed for materials which are below the triple point at 1 atmosphere under the heading "melting point". I didn't notice, so take your pick if you want to blame me or the author .