Why do liquid fuel rockets vent gas?

December 30, 2007 12:02 PM

Why do liquid-fuel rockets vent gas while on the launch pad?

Ever since I was a kid, I've wondered about this, and have never received a satisfactory answer. I've read Time-Life books and Wikipedia, which all say something vague like "liquid oxygen boils, and the excess gas is vented off".

My best guess is that the vapor pressure of liquid oxygen is high, and that to conserve weight the LOX storage tanks must be relatively thin, and thus to avoid over-pressure the gaseous oxygen is simply vented off.

The question intrigues me because you don't see this kind of venting occurring with other liquefied gasses.

posted by Tube to technology (7 comments total)

In rocketry, you want to pack as high an amount of energy into as little space as possible. Oxygen and hydrogen as fuel, in their liquid form is respectable in this regard. However those gases are only liquid at either very high pressure, or very low temperature (or a combination), so in order to get a tank full of those fuels in liquid form, you need to either

a) Compress your gas until it's liquid and inject it into a tank, able to withstand the pressure from the compressed gas, or
b) cool your gas until it's liquid and then pump it into a tank, able to hold the weight but at much lesser pressure than option a.
Actually i think they use a combination of those methods.

However, if you just let the tank sit there, the fuel is going to take on energy (heat) from the surroundings and will heat up. This will increase the pressure within, and if you only designed your tank to take a certain pressure you don't want that. Thus the boil of. The phase change from a liquid to a gas is an endothermic process (it takes energy). This energy comes from the liquid itself which is cooled down. Thus you maintain a reasonable pressure, although you have to continuously top off your fueltank with new fuel.
posted by Catfry at 12:54 PM on December 30, 2007

Space is not the issue, mass is the issue. If they were concerned about space, they wouldn't be using hydrogen and oxygen, because liquid hydrogen is ridiculously undense.

Mass is the most important thing, because you have to lift it. The tanks holding the liquid hydrogen and liquid oxygen are reasonably well insulated, but too much insulation would add dead weight to the rocket, and they've decided that a better tradeoff is to accept a certain amount of boil-off in exchange for reducing the weight of the tanks.
posted by Steven C. Den Beste at 1:02 PM on December 30, 2007

Actually I guess they don't even try to maintain adequate pressure and temperature in the tank. It is simply too difficult to either insulate or reinforce it sufficiently. My guess is the fuel starts to boil as soon as it is loaded, and when entering gas form, naturally it rises to the top of the tank where it is sucked out. However enough of the fuel remains liquid for long enough for the purposes of the vehicle, as long as they keep filling it up, until launch. I don't know why they just vent the gas and don't try to recycle it somehow?
posted by Catfry at 1:05 PM on December 30, 2007

You're right.
posted by Catfry at 1:10 PM on December 30, 2007

Catfry writes "I don't know why they just vent the gas and don't try to recycle it somehow?"

After designing a coupler to collect the exhausted gas they'd have to worry about it not working at launch. Plus they'd need the equipment to recompress the gas hanging around the launch pad.
posted by Mitheral at 1:36 PM on December 30, 2007

I suspect the answer you're looking for is that they're not really venting that gas that you're seeing. That some venting is actually occurring is a bit of a red-herring because the "venting" you are wondering about is really condensation from the intense cold of the tanks. The water vapour in the atmosphere instantly condenses into mist/fog as it nears the tanks, and then visibly falls away through convection.

you don't see this kind of venting occurring with other liquefied gasses.

Yes you do. You're probably thinking of stuff like LPG, which has a boiling temperature of -42 degrees Celcius. Liquid oxygen by comparison, is -180. That's the difference between merely a cold day in an area where people live, vs temperature that is closer to absolute zero than to populated places!

You make even a solid chunk of metal that cold, and you'll still see it "venting gas" as mist continually appears and drops away from it.

In the case of the rockets, the vented gas will also be condensing the water in the air, thus making the effect even more pronounced, so it's a bit of both, but the point is that it's the temperature behind what you're seeing, rather than the venting.
posted by -harlequin- at 5:24 PM on December 30, 2007

liquefied gasses, yes
posted by caddis at 5:40 PM on December 30, 2007