What is the temperature and pressure of the exhaust that comes out of the rocket during the last three seconds of countdown (smoke that stays near ground)?

How much carbon dioxide is there in that smoke (approximately)?

enter image description here

  • $\begingroup$ For the CO2 question, there's some information here : space.stackexchange.com/questions/13082/… $\endgroup$
    – Hobbes
    Jul 29, 2018 at 14:32
  • 2
    $\begingroup$ The nozzles of rocket engines are optimized for exhaust with maximum speed at minimum pressure. Pressure is therefore only a bit higher than atmospheric pressure. High pressure of exhaust would be bad for efficiency. $\endgroup$
    – Uwe
    Jul 29, 2018 at 15:48
  • 4
    $\begingroup$ Lower than sea level atmospheric at the exit, actually. $\endgroup$ Jul 29, 2018 at 18:13
  • $\begingroup$ A lot of that "smoke" is actually steam produced when the hot exhaust hits the water from the sound suppression system. $\endgroup$
    – zeta-band
    Jul 31, 2018 at 0:02

1 Answer 1


The temperature and pressure inside the engine's combustion chamber is very high -- in the ballpark of 3400º C and 100 atmospheres for the Falcon Heavy's Merlin engines.

However, the bell-shaped nozzle of a rocket engine expands the exhaust stream, which both cools it and reduces its pressure. Ideally, for best performance, you want the exit pressure to match the ambient air pressure you're exhausting into; at sea level that usually means the exhaust is actually below ambient pressure because it's optimized for higher altitude flight.

The exit pressure for Merlin is about 0.7 atmosphere; I'm not sure about the temperature but it's probably around 1500ºC.

  • $\begingroup$ Does this mean that you have to have a ~140 times higher surface area at the end of the nozzle compared to the engines throat? $\endgroup$
    – DaGroove
    Jul 30, 2018 at 10:03
  • 5
    $\begingroup$ @DaGroove Mass flow is constant along the nozzle. Exit speed increases a lot and the density doesn't decrease lineary with pressure because of the temperature lowers. Therefore the nozzle ratio is a lot smaller than 140. Nozzle ratio is about 16. $\endgroup$
    – Christoph
    Jul 30, 2018 at 10:23
  • $\begingroup$ @Christoph ah, that makes sense. 140 seemed like an awful lot to me, even for vac nozzles. Thx! $\endgroup$
    – DaGroove
    Jul 30, 2018 at 10:50
  • $\begingroup$ Expanding cools, yes. But wow going from 100 to 0.7 atmosphere (a factor of ~140) only drops 3700 K to 1800 K (a factor of ~2)? Is that the temperature of the brightly radiating soot particles (which have some thermal inertia and may even still be combusting as embers) or the temperature of the H2O and CO2 reaction product gases? There's no reason that they would have to be the same temperature - the timescale's too short for full equilibrium. $\endgroup$
    – uhoh
    May 7, 2023 at 20:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.