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I try to understand what the exhaust cloud on the ground from the start of Starship consists of.

Of course it was unusually mineral-rich, but I try to understand whether we saw much else, apart from pulverized launchpad foundation.

Burning methane and oxygen creates water. If that condenses, we have a visible cloud. But it can only condense when it is fairly cold, something below 100 °C (373 K), or in high pressure. It may temporarily condense near shock waves, but that is very local in space and time.

I assume moderate outside temperature. How hot is this big cloud at a rocket launch? Initially, the exhaust is very hot. In other cases, it may be significantly cooled by a water deluge system, that also adds a lot of water, resulting in a big white cloud of condensed water droplets. the Starship launch pad does not have a full water deluge system. It uses a much smaller amount water in a similar way, but I assume I can disregard that in this context.

Shouldn't at least the central part of the exhaust cloud be hot enough to not condense water, and be transparent?

If it should, I assume it was masked by dust from the foundation. No, wait, dust of the foundation.

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    $\begingroup$ Wherever hot exhaust gas with lots of water vapor hits normal temperature air, the local water concentration will be above the dew point. $\endgroup$
    – Jon Custer
    May 4 at 13:43
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    $\begingroup$ The temperature of rocket exhaust is not extreme. The massive expansion through the nozzle does quite a bit of cooling to the static temp. Raptor has a sea level expansion ratio of ~35 and a chamber temp of 1400F. This means the static temperature is ~200F. Just dont stick your hand in. When you stop the flow moving, it remembers how hot it should be. $\endgroup$
    – A McKelvy
    May 5 at 15:23
  • $\begingroup$ I should note though that as soon as it leaves the nozzle, the shear layer will be extremely hot. As well as anything the exhaust touches. But as far as the gas in the jet core is concerned, it's pretty chilly. Engines operating at lower chamber temps even need to be wary of exhaust condensation in the nozzle. $\endgroup$
    – A McKelvy
    May 5 at 15:26
  • $\begingroup$ @OrganicMarble see my comment there I think a more definitive answer about the temperature of the gas component of the exhaust is necessary, and pre-closing this question prevents such answers from being posted. $\endgroup$
    – uhoh
    May 7 at 20:29

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