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In listening to a youtube video that was discussing the Ariane 5, the narrator noted briefly in passing that the Vulcain 2 engine produced more power thrust in a vacuum than it did at sea level. Not being a rocket scientist, I thought this was odd so looked it up and indeed, according to the Wikipedia entry for the Ariane 5 (screenshot below), the core stage produces 1390 kN in a vacuum and 960 kN at sea level.

Why is this? The Vulcain 2 is a liquid hydrogen engine with its own liquid oxygen oxydizer. So why would it matter whether it's in a vacuum or at sea level? Or does it instead reflect something else such as throttle control at sea level vs. low orbit?

Ariane 5 core stage specs

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  • $\begingroup$ You are mixing up "power" and "thrust". They are not the same thing. The power produced is more or less constant, but at the wrong alitutde more of that power will end up driving lateral expansion of the exhaust, or as heat in the exhaust, neither of which propels the rocket. $\endgroup$
    – Steve Linton
    Commented May 2, 2020 at 17:58
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    $\begingroup$ @SteveLinton: Actually, exhaust power does increase in vacuum. Rocket engines are heat engines, and an ambient atmosphere keeps the exhaust from expanding and cooling as much, reducing Carnot efficiency and mechanical power output (and thrust, and specific impulse, and pretty much every other measure of performance you can think of). $\endgroup$
    – Christopher James Huff
    Commented May 2, 2020 at 18:56

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It's because the ambient atmospheric pressure affects the thrust generated by the engine.

enter image description here

(Image source)

An engine nozzle has a "design altitude" at which the pressure of the exhaust at the exit plane matches the ambient pressure.

It isn't just the Ariane but all engines with fixed-geometry nozzles. Here is a graph showing the change in performance for the H-1 engine.

enter image description here

Sutton, Rocket Propulsion Elements, 4th edition, p. 37

Various tricks such as sliding "sleeves" over the nozzle that drop down and increase the area ratio as the vehicle ascends have been tried, but none have gained wide acceptance.

Check out this question From the General Thrust Equation towards Tsiolkovsky, how to explain dropping these terms along the way? and its answers if you want some math.

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    $\begingroup$ Wow! Does it consume the same amount of fuel at to produce more thrust? Or does the atmospheric pressure at sea level produce more “resistance” than manifests as less fuel and less thrust? $\endgroup$
    – RoboKaren
    Commented May 2, 2020 at 17:02
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    $\begingroup$ @RoboKaren At low altitudes it uses the same fuel for less thrust :( The "specific impulse" in the graph is a measure of "thrust per amount of fuel burned". $\endgroup$
    – Organic Marble
    Commented May 2, 2020 at 17:07

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