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While a rocket is waiting on the platform during the final countdown, it often seems to have clouds of steam or something similar escaping from it.

Is this normal, and what are these clouds?

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This occurs when cryogenic boosters are used (Liquid Oxygen / Liquid Hydrogen) - as these warm up on the launch pad, some of the liquids boil and to release pressure, bleeder valves allow this gas to escape. If it remained within the boosters, there is a risk of the pressure blowing seals or damaging other components.

enter image description here (from http://www.nasaspaceflight.com/2013/02/hot-fire-success-orbitals-antares/)

Most of what you can see is water vapour as it condenses around the O2 and H2, which are still extremely cold despite being in gaseous form.

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    $\begingroup$ Most boosters use LOX, which produces fog/clouds when overpressure is bled off. If kerosene is the fuel, it likely won't need to be bled off. If hydrogen is the fuel, the bleed may be burned off rather than risk an explosion of accumulated hydrogen. If simply vented, it would also produce a cloud of condensed moisture. Some liquid fueled rockets such as the Titan II used room-temperature hypergolic fuels which normally wouldn't need to be vented. $\endgroup$ – Phil Perry Jul 11 '14 at 16:48
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Typical spacecraft fuels/oxidizers are super-cooled. The vapour visible is formed due to the extreme temperature difference.

The Apollo 11 flight journal has this to say

Filling these tanks with such cold contents requires a little finesse. Initially, the LOX is fed at a slow rate which furiously boils as it contacts the relatively warm tank structure. The vapourisation of the LOX takes heat away until a pool of liquid begins to form. When enough liquid has collected, filling steps up to a fast rate until the tanks are nearly full and the slow rate is reestablished to top them off. From then on, until three minutes before launch, the level is replenished as the volatile LOX continues to boil off from heat leaking into the tank.]

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    $\begingroup$ On the Saturn V, if memory serves, the first stage LOX tank was uninsulated, and accumulated a large load of ice, which started shedding when the engines fired up. The second and third stages' LH2 and LOX tanks were insulated, and experienced little external ice buildup. $\endgroup$ – Phil Perry Jul 11 '14 at 16:51
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    $\begingroup$ Here's a great set of pictures of the Saturn V launching showing that ice sheet falling off during launch: en.wikipedia.org/wiki/Saturn_V#/media/… $\endgroup$ – Sarah Bailey Nov 30 '16 at 19:31

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