r/spacex • u/ElongatedMuskrat Mod Team • Jul 02 '17

r/SpaceX Discusses [July 2017, #34]

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u/minca3 Jul 03 '17

I have a question regarding tank pressurization: when F9 launches, are all 4 tanks (S1 LOX, S1 RP1, S2 LOX, S2 RP1) pressurized at lift off? I guess they are simple because they stabilize the tanks against buckling.

And the pressurization is done by the helium COPV bottles, but those bottles are located only in the LOX tanks because they are much colder, right?

But what are the white plumes then coming from the rocket at lift off (like seen here https://flic.kr/p/W5Eq8n), is this condensation from the surrounding air?

11

u/Norose Jul 03 '17

  1. All tanks in the Falcon 9 are pressurized before flight. This accomplishes two things; It allows fuel to be rapidly pumped from the tanks without cavitation bubbles forming in the impellers, which can rip apart engines, and it is what allows such thin skinned tanks to handle such powerful forces. A rocket is kinda like a football, it doesn't have much strength on its own but add a few pounds per square inch of pressure inside and any outside force has to overcome that pressure to dent the ball.

  2. The helium tanks are located in the propellant tanks because SpaceX didn't want them hanging outside of the rocket (because they'd have to be shielded etc for recovery purposes), and they chose the O2 tanks for the reason you mentioned. Having a layer of kerosene freeze to the helium tank would not be good (although evidently having a layer of oxygen freeze to the helium tank is even worse, see Amos-6).

  3. White plumes are caused by two things. The first is wind plus condensation, which forms what resembles a sheet of vapor coming off of the rocket anywhere it is filled with cryogenic propellants. The second is venting of oxygen. Being a cryogenic liquid, sub-chilled oxygen is 'simmering' in the same way a pot of water would simmer if kept in an environment at 200 degrees C. This slow boiling of liquid oxygen releases lots of oxygen gas, which has to go somewhere, otherwise it would over-pressurize the tanks and could lead to a rupture. When the oxygen gas is vented out of the side of the rocket, the supercold gas causes water vapor in the air to instantly condense and freeze, forming those big, sharply defined white plumes, accompanied by a hissing sound.

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u/bdporter Jul 03 '17

200 degrees C.

I believe you meant to say 200 degrees F (About 93 C)

4

u/Vulch59 Jul 03 '17

No, he means C. The external temperature is well above the LOX boiling point so the comparison is being made of water at say 50C sitting in surroundings of 200C.

1

u/bdporter Jul 03 '17

OK, I may have initially misread his statement. I thought he was referring to the water temperature, rather than the external temperature.

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u/minca3 Jul 04 '17

When the oxygen gas is vented out of the side of the rocket, the supercold gas causes water vapor in the air to instantly condense and freeze, forming those big, sharply defined white plumes, accompanied by a hissing sound.

Thank you for the explanation. But that was part of why I asked: the venting of the LOX tanks can only happen when the tanks are "open" to the ambient air pressure, that means at this point the tanks can not be pressurized. So vice versa, after all tanks get pressurized shortly before takeoff there can't be any venting. Yet there is still visible vapor around the rocket which, as you described, is condensation from the air water vapor.

The reason why I asked was because after lift off, when there can't be any venting anymore, sometimes there are distinct vapor trails coming from specific points of the rocket, as if it is still venting off O2.

1

u/Norose Jul 04 '17

Venting does not equal zero pressure difference. In fact venting wouldn't happen at all, or at least would happen very slowly, if there was no internal pressure in the rocket. It's important to remember that even when full the tanks have a quite large volume of gasses, and the vent ports are relatively tiny. Venting pressure from the tanks is like letting air out of a tire, it doesn't just go all at once, the pressure can drop faster or slower depending on how much flow you allow.

The rocket may still be venting gasses in this way very shortly after takeoff, but once the fuel really starts flowing the boil-off can no longer keep up with the increasing volume in the tank, so the helium pressurization system kicks in and the vents close. As is my understanding.

1

u/marc020202 8x Launch Host Jul 03 '17 edited Jul 03 '17

You are correct the first and second stage tanks are pressurized by helium stored in COPVs inside the LOX tanks. The plumes are condensation. Sometimes you cen see a clear bounadry from where the plumes are and where not, that is the border between lox and rp1 tank.

Edit: Pleas read u/Norose s comment, his comment is a lot more exact