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Comment by dogma1138 | Hacker News Reader

The helium is used to keep the pressure on the oxygen tank constant once the tank starts to empty (basically the helium is used to push the oxygen down).

It's probably inside for mechanical reasons pumping liquid helium is a nightmare it sips through pretty much everything (it can even go through glass when sufficiently cooled), in this case you don't really need to worry about all that stuff since you simply let the helium out and the difference in density would sort everything.

As for the reason it's not used to "cool the helium" it's because helium boils at -268.9c the liquid oxygen is cooled to -182.9c or to ~-206c in cryogenic mode, this is still way above the boiling temperature of helium. This would be about as effective at cooling, as cooling boiling oil with boiling water, incidentally it's also about as explosive ;)

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The helium tank is inside the oxygen tank to keep it cold. For a given pressure and tank size, more helium is stored in the tank at lower temperatures.

For an ideal gas, PV = NkT, where P is pressure, V is volume, N is the number of atoms, T is temperature, and k is a constant. It looks like the O2 temperature for this version of Falcon is 66 K. If we take T_out ~ 273 K just for the sake of the discussion,

N_in / N_out = T_out / T_in ~ 4.

It's a significant difference.

That's not correct notation for the ideal gas law.

Ideal gas law doesn't apply to real gasses, (actually in some cases especially helium).

Ideal gas law does not apply to liquids.

Ideal gas law only works at low pressure and high temperatures.

Overall the ideal gas law is not a law it's an approximation that works in a few cases, it has no baring in this case.

You are right that the ideal gas law doesn't describe real gases but it can be a reasonable approximation for some applications, even at fairly low temperatures, provided the density isn't high.

Helium is the closest thing we have to and "ideal gas" as far as the ideal gas law goes when it's at STP.

It can still somewhat behave as an ideal gas as low as 50K or so granted it is not under pressure.

Once you increase the pressure the ideal gas law breaks down pretty much completely, and is not applicable to liquid helium at about 6000PSI.

People also forget that the tank is made out usually titanium or aluminum (i think aluminum can't hold helium but am not sure about that) wrapped in a composite material usually Kevlar/Epoxy or some sort of carbon fiber composite, extreme low temperatures are well a bit tricky for these materials.

NASA had issues with that during the shuttle program, and it seems that even the new Orion has encountered issues with the effects of low temperature on COPV's which required them to develop new resins and composites http://www.compositesworld.com/articles/orion-composite-tank...

A temperature difference of 63 °C is still an awful lot easier to deal with than a temperature difference of 280 °C

That isn't how physics work, when something boils it boils. Also when you factor in heat transfer coefficients then the liquid oxygen in tank is actually "heating" the liquid helium pretty substantially as it has considerably higher heat conductivity than air. ;) The analogy of boiling water into boiling oil is pretty close one as far as the process goes, the boiling oil would evaporate the boiling water instantly and things go boom, the liquid oxygen would evaporate the liquid helium instantly and things go boom.

The helium is in the gas phase and at thermal equilibrium with the surrounding oxygen.

And 757 times it's liquid volume, hence the boom part.

No... I took too long to write my comment.

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