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3 helium tank issues in 3 years 2014 https://en.wikipedia.org/wiki/Falcon_9_Flight_10 https://en.wikipedia.org/wiki/Falcon_9_Flight_10 (delayed by SpaceX due t

3 helium tank issues in 3 years 2014 https://en.wikipedia.org/wiki/Falcon_9_Flight_10 https://en.wikipedia.org/wiki/Falcon_9_Flight_10 (delayed by SpaceX due to a first stage helium leak) 2015 https://en.wikipedia.org/wiki/SpaceX_CRS-7 https://en.wikipedia.org/wiki/SpaceX_CRS-7 (failure of a strut which secured a high-pressure helium bottle inside the second stage's liquid oxygen tank) 2016 (a large breach in the cryogenic helium system of the second stage liquid oxygen tank took place)

tropo 10y ago
I note that the helium tank is inside the oxygen tank. I'm sure there is a reason, but there have got to be some major downsides.
- meepmorp 10y ago
  It's probably to help keep the helium cold. Since you already have to maintain a large reservoir of a cryogenic fluid, why not make it do a little extra work to cool a substance with a lower boiling point.
  - dogma1138 10y ago
    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 ;)
    - yazaddaruvala 10y ago
      No... I took too long to write my comment.
    - makomk 10y ago
      A temperature difference of 63 °C is still an awful lot easier to deal with than a temperature difference of 280 °C
      - dogma1138 10y ago
        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.
        Ankaios 10y ago
        The helium is in the gas phase and at thermal equilibrium with the surrounding oxygen.
        dogma1138 10y ago
        And 757 times it's liquid volume, hence the boom part.
    - Ankaios 10y ago
      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.
  - yazaddaruvala 10y ago
    FWIW: I am not a rocket scientist. I'd be willing to bet it is to reduce the efficiency loss due to vacuum. The fuel tank (oxygen tank) is going to empty itself. However, you can't have a vacuum in the tank. So do you acknowledge that the rate of flow of fuel exiting the tank will be slowing down over time or do you keep the tank "full" to ensure constant pressure? Helium, is inert, less dense than oxygen and cheap. It's probably the best space filling collection of atoms, and simultaneously it's density[1] forces the oxygen further "down"[0] the fuel tank. [0] down is some direction relative to thrust. [1] density according to: "weight" generated by thrust.
- ansible 10y ago
  I'm addition to thermal issues others have noted... One reason is to keep the pipes as short as possible. Another reason is for internal volume efficiency. You could put the helium tank and other support equipment outside the LOX tank. However, there will naturally be some void spaces around all this stuff. This results in the second stage being larger and heavier. Putting it all inside the tank means that all the nooks and crannies can be filled with LOX.
djsumdog 10y ago
So these high pressure tanks must be an incredibly hard/complex problem?
- BinaryIdiot 10y ago
  Alternatively they're not incredibly hard / complex so not enough attention is being given to testing, manufacturing, etc. Though I'm curious which way it is.
- mikeyouse 10y ago
  They're a complicated problem, but I can't help but wonder if SpaceX underestimated the difficulty. They used to have a 3rd patty supplier for the helium tanks but they decided to manufacture them in-house a few years ago. As far as I know, all 3 tank problems have happened since they started producing their own.
  - mikeash 10y ago
    The strut which failed in 2015 was outsourced. A tiny fraction of the struts from the factory failed at well under the specificed maximum load, and one of them ended up in that rocket.
- supercoder 10y ago
  That or spacex overlooked the complexity
ranman 10y ago
You're describing 3 extremely different issues though -- it's easy for something going wrong to involve helium tanks because it's something that's used in the rocket that can lead to an overpressure event.

tropo 10y ago

I note that the helium tank is inside the oxygen tank. I'm sure there is a reason, but there have got to be some major downsides.

meepmorp 10y ago
It's probably to help keep the helium cold. Since you already have to maintain a large reservoir of a cryogenic fluid, why not make it do a little extra work to cool a substance with a lower boiling point.
- dogma1138 10y ago
  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 ;)
  - yazaddaruvala 10y ago
    No... I took too long to write my comment.
  - makomk 10y ago
    A temperature difference of 63 °C is still an awful lot easier to deal with than a temperature difference of 280 °C
    - dogma1138 10y ago
      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.
      - Ankaios 10y ago
        The helium is in the gas phase and at thermal equilibrium with the surrounding oxygen.
        dogma1138 10y ago
        And 757 times it's liquid volume, hence the boom part.
  - Ankaios 10y ago
    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.
- yazaddaruvala 10y ago
  FWIW: I am not a rocket scientist. I'd be willing to bet it is to reduce the efficiency loss due to vacuum. The fuel tank (oxygen tank) is going to empty itself. However, you can't have a vacuum in the tank. So do you acknowledge that the rate of flow of fuel exiting the tank will be slowing down over time or do you keep the tank "full" to ensure constant pressure? Helium, is inert, less dense than oxygen and cheap. It's probably the best space filling collection of atoms, and simultaneously it's density[1] forces the oxygen further "down"[0] the fuel tank. [0] down is some direction relative to thrust. [1] density according to: "weight" generated by thrust.
ansible 10y ago
I'm addition to thermal issues others have noted... One reason is to keep the pipes as short as possible. Another reason is for internal volume efficiency. You could put the helium tank and other support equipment outside the LOX tank. However, there will naturally be some void spaces around all this stuff. This results in the second stage being larger and heavier. Putting it all inside the tank means that all the nooks and crannies can be filled with LOX.

djsumdog 10y ago

So these high pressure tanks must be an incredibly hard/complex problem?

BinaryIdiot 10y ago
Alternatively they're not incredibly hard / complex so not enough attention is being given to testing, manufacturing, etc. Though I'm curious which way it is.
mikeyouse 10y ago
They're a complicated problem, but I can't help but wonder if SpaceX underestimated the difficulty. They used to have a 3rd patty supplier for the helium tanks but they decided to manufacture them in-house a few years ago. As far as I know, all 3 tank problems have happened since they started producing their own.
- mikeash 10y ago
  The strut which failed in 2015 was outsourced. A tiny fraction of the struts from the factory failed at well under the specificed maximum load, and one of them ended up in that rocket.
supercoder 10y ago
That or spacex overlooked the complexity

ranman 10y ago

You're describing 3 extremely different issues though -- it's easy for something going wrong to involve helium tanks because it's something that's used in the rocket that can lead to an overpressure event.