Apollo 1

[JayUtah]

I went back to check the Apollo 204 Review Board -- spacecraft 012 indeed had Teflon insulation, not Kapton.  The change happened before the Block II.  But I do think  Block II had thicker insulation to combat damage from abrasion.

[Luke Pemberton]

I wonder if helium might be a good diluent gas in a spacecraft. Its heat conductivity is significantly higher than air, so it ought to suppress fire better than an equal percentage of nitrogen. Helium speech used to be a real problem in diving, but I believe it's now possible to use special vocoders to correct it.

This link:

http://history.nasa.gov/Apollo204/invest.html

SPACECRAFT ATMOSPHERE

The use of pure oxygen in American spacecraft has been the subject of much consideration. The use of a diluent gas, either nitrogen or helium, in large proportions would undoubtedly reduce the risk of fire to a significant degree. At the same time it would introduce other operational problems and risks. There is no obvious advantage of one diluent over the other, although much progress has been made in developing the complex technology required for controlling gas concentrations to maintain a proper mixture reliably. This technology is still far from being fully developed. Furthermore, there are many difficult operational problems that must be solved in a reliable manner in order to decrease rather than increase the risks before undertaking the use of a two-gas system.

[ka9q]

Well, all the newer NASA spacecraft are using air. We switched to it with the Shuttle. The ISS contains air, which makes working with the Russians a lot easier as they've always used it.

And I'm sure the fire hazard of a high O₂ concentration (regardless of pressure) had a lot to do with it.

To see what it takes to mate spacecraft with different atmospheres, look at the docking module flown on ASTP.

[Dalhousie]

I wonder if helium might be a good diluent gas in a spacecraft. Its heat conductivity is significantly higher than air, so it ought to suppress fire better than an equal percentage of nitrogen. Helium speech used to be a real problem in diving, but I believe it's now possible to use special vocoders to correct it.

I saw a documentary on Sealab showing some guys trying to light matches. It was impossible.

Helium was considered for MOL, I think.  the inability to strike matches on Sealab might have been as much due to the low O2 concentration as to heat conductivity.

[ka9q]

My guess is that the heat capacity of the gas is much more significant than the conductivity

I suspect conductivity plays the major role because it is much higher for helium than any other gaseous element than hydrogen.

He has a heat capacity at constant volume of 12.5 J/mol-K (same for all the noble gases, which are near-ideal). Thermal conductivity is 0.1513 W/m-K.

For N₂, a diatomic gas, heat capacity at constant volume is higher than He: 19.9 J/mol-K, but its molar mass is considerably higher (28 vs 4). OTOH, the number of moles would be the same regardless of gas for the same temperature, pressure and volume: PV = nRT. But conductivity is 25.83 mW/m-K, considerably less than He.

I know that some divers inflate their suits with argon because it has an even lower thermal conductivity than air. The tanks are hardwired and/or carefully marked so they can't accidentally breathe it; it's even more narcotic than nitrogen. Double-pane windows are often argon-filled.

SF₆ would be a poor choice: thermal conductivity is 11.627 mW/m-K, less than half that of nitrogen. Also, it is almost as narcotic as nitrous oxide. (I'm not sure those inhaling it for comic effect know this). And finally, while chemically almost inert, electric arcs can generate disulfur decafluoride, which is so toxic it was considered as a war gas.

[Bob B.]

Well, all the newer NASA spacecraft are using air. We switched to it with the Shuttle. The ISS contains air, which makes working with the Russians a lot easier as they've always used it.

Skylab also used an oxygen-nitrogen atmosphere, those it wasn't a normal Earthlike mixture.  It was 74% oxygen, 26% nitrogen at 5 psia.

[JayUtah]

I had a chemistry professor once explain to me that electron configuration has a bearing on heat conductivity, hence the behavior of the noble gases.  But it was at a party so I wasn't really paying attention.

[ka9q]

I suspect it's molecular weight more than anything else, since there's a strong inverse correlation between it and thermal conductivity. It's probably for the same reason that the average molecular speed and the speed of sound are greater in lighter gases at the same temperature.

Per-mole heat capacity on the other hand is the same for all monatomic gases (the noble gases, which are close to ideal). Diatomic gases have more degrees of freedom and can store more energy, so they have higher heat capacity, and more complex gas molecules have more still.
 

[Peter B]

O2 at one bar (atmospheric pressure) is toxic, therefore in a pure O2 environment the pressure is reduced to 1/5th atmospheric normal. There would be advantages in decompressing and recompressing a craft. Using an air mix does introduce the problems of narcosis, but reduces the problems of fire in a pure O2 system.

O₂ at 1 bar is not acutely toxic, as shown by the fact that every Mercury, Gemini and Apollo astronaut breathed it starting hours before launch when they suited up. It may have some subtle effects when breathed for a long period of time, e.g., several weeks.

The pressure is dropped mainly to reduce the stresses on the spacecraft hull (remember how the LM was built). At 5 psia, the ppO₂ is still well above that of sea level air on earth.

Pure O₂ does become acutely toxic above pressures of 2-3 bar. Convulsions occur, which are usually fatal in a diver. This severely limits the use of pure O₂ in diving, although it's useful as a final mix to help flush out N₂ at shallow depth during decompression.

"Technical" mixtures for deep sea diving must limit the partial pressures of both N₂ (to avoid narcosis) and O₂ (to avoid toxicity). This is almost always done by adding helium. I was surprised to find that on especially deep dives hydrogen is also used. It turns out that it's safe to mix with oxygen provided the O₂ percentage is kept below about 4%; that keeps it outside the flammability range. These small O₂ percentages are used only at great depths.

I've just started re-reading Chaikin's "A Man on the Moon", and I've just read his description of the Apollo 1 fire.

One of the things the book mentions is that, in response to Grissom's grumble that the comms were misbehaving, both Joe Shea and Deke Slayton considered joining the crew in the CM on the day of the fire. But they didn't follow through because of difficulties with including them in the spacecraft comms.

As I read I wondered whether the CM's atmosphere would've been a problem for whoever sat in there. What I'm reading here suggests it wouldn't have been. Is that the case, or would the interloper have had to wear breathing equipment of some sort?

[ka9q]

Until the fire another person would have been fine in the cabin since it was filled with pure oxygen at a few tenths of a bar above ambient. The astronauts themselves were breathing oxygen at a pressure slightly above that in the cabin.

[Luke Pemberton]

One of the things the book mentions is that, in response to Grissom's grumble that the comms were misbehaving, both Joe Shea and Deke Slayton considered joining the crew in the CM on the day of the fire. But they didn't follow through because of difficulties with including them in the spacecraft comms.

Oh, if only to beat the Blunder here and show how easy it is to make rubbish up.

<bs conspiracy mode>
Yeah, right. More likely NASA briefed them not to go in the CM because they wanted to bump off the crew. Slayton and Shea were also in on the conspiracy so NASA had to call them off entering the CM.
</bs conspiracy mode>

[Obviousman]

One of the things the book mentions is that, in response to Grissom's grumble that the comms were misbehaving, both Joe Shea and Deke Slayton considered joining the crew in the CM on the day of the fire. But they didn't follow through because of difficulties with including them in the spacecraft comms.

Oh, if only to beat the Blunder here and show how easy it is to make rubbish up.

<bs conspiracy mode>
Yeah, right. More likely NASA briefed them not to go in the CM because they wanted to bump off the crew. Slayton and Shea were also in on the conspiracy so NASA had to call them off entering the CM.
</bs conspiracy mode>

Of course, the only way to include them would be to have a separate cable running through the hatch which would have meant the pressure test could not happen and therefore the incident would have never occurred.

Those facts would never enter the 'Worsty Westy' thinking.