Nasa recently did a test firing of an RS-25 rocket, and released a video of it. Here's a frame grab:

Static firing of Aerojet Rocketdyne RS-25

Note that pool of water at the bottom right, with a pipe leading to it, and clouds of steam coming off the end near the pipe?

Steaming pool near engine firing

It looks like a disposal pool of some sort, and the whiteness of the pipe and behavior of the steam/fog cloud suggests that the disposed material is cryogenic. But, RS-25s use liquid hydrogen and oxygen, and disposing of either of those by dumping into a pond would get entertaining fast.

Any idea what's going on here?

  • $\begingroup$ Saturn-V had a LH2 vent pool, where they bubbled the hydrogen gas slowly over a pool as I recall. But this is not 'slowly' from the pic. Something very cold, in reasonably large volumes. $\endgroup$
    – geoffc
    Commented Feb 22, 2018 at 14:24
  • $\begingroup$ Could it be very cold nitrogen gas from liquid nitrogen used for cooling? But the white cloud should be very fine water droplets. Nitrogen gas is very dry, but if warm and humid air is mixed with very cold nitrogen gas some of the water vapor is turned to the mist. $\endgroup$
    – Uwe
    Commented Feb 22, 2018 at 14:46
  • 2
    $\begingroup$ Does the color and behavior of the plume at the top of the picture, coming from the test stand itself, also suggest cryogenic material? $\endgroup$ Commented Feb 22, 2018 at 16:45

1 Answer 1


The containment pond at the Stennis Space center, Test Stand A-1, is used to hold runoff from contact-cooling water (where water comes into contact with something other than the insides of a clean pipe). You can see it's not particularly deep and does not itself runoff or drain anywhere (except to groundwater and the nearby river).

Google Map view of the Test Stand A-1; the containment pond you're asking about is in the area surrounded by trees above the test stand. (The pipe ending in the dry area above and to the left of the test stand is a Flare, for disposing of excess fuel.)

Stennis Space Center - Test Stand A-1

The test stands use a lot of water for everything from transporting the largest parts by barge, cooling the deflector, even sound dampening (literally, that's not a pun). Most of the steam from the flame trench is the reaction of the fuel, hydrogen and oxygen; they burn clean producing water (steam) and not smoke. The remaining steam is from the heating of the cooling water; heated water that is not evaporated is pumped into the containment pond.

Here's the Stennis Space Center B-2 Test Stand, being tested while under construction:

Test Stand B-2 runoff

Here's a typical feed pipe:

Cooling Water Supply

Here's a video and webpage of the cooling system being tested at the Kennedy Space Center, Florida. They use a lot of water but it is safely recycled in the form of steam (later becoming rain) or by settling out mild contaminants like rust and paint.

Non-contact cooling water can be returned to the river once it reaches ambient temperature, cooling water that has come in contact with the deflector and even the exhaust, needs to sit in a settlement pond before the water is permitted to leach back into the river.

Watch and listen as Richard Hammond explains how bubbles suppress sound underwater and how (6 minutes later) in air the opposite is demonstrated:

Worth watching twice. See also this question: What is the purpose of the jets of water often under rocket engines during launch?

  • $\begingroup$ So, the two pipes that leave the test stand to the NNE and NNW carry water heated by the exhaust, for cooling and later disposal? Any idea why the NNE pipe ends in a large pool, while the NNW pipe ends in an apparently dry area? $\endgroup$ Commented Feb 22, 2018 at 18:10
  • 1
    $\begingroup$ @DanielGriscom - The dry area is a Flare: en.wikipedia.org/wiki/Gas_flare -- Read the text: "The flaring of associated gas may occur at the top of a vertical flare stack (as in the adjacent photo) or it may occur in a ground-level flare in an earthen pit." and here: "... as 'over steaming' can occur resulting in reduced combustion efficiency and higher emissions. To keep the flare system functional, a small amount of gas is continuously burned, like a pilot light, so that the system is always ready for its primary purpose as an over-pressure safety system.". That's what that is. $\endgroup$
    – Rob
    Commented Feb 22, 2018 at 18:59
  • $\begingroup$ Thanks. I've proposed an edit that integrates your comments with your answer. $\endgroup$ Commented Feb 22, 2018 at 20:34
  • $\begingroup$ One more question: that pond is steaming before the engine test even begins. Where did all that heat come from? $\endgroup$ Commented Feb 22, 2018 at 20:35

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