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?