Eli5 how decreasing the diameter of the nozzle (of a rocket for example) makes the rocket go faster? It’s still the same amount of gas exiting the rocket no matter the diameter, so why is there a difference?
In: 1
The subject is a bit complex, but I’ll try…
At a given atmospheric pressure, there is an ideal nozzle size – one that allows the exhaust to expand just enough so the pressure at the edge of the nozzle is exactly the pressure of the atmosphere.
The nozzles for second stages are pretty simple as they generally only run in vacuum. The ideal size would be very big but they are limited to practical sizes, but are pretty big in general.
Unfortunately, rocket nozzles on boosters are a compromise as the start out at sea level and must operate in vacuum as well. If you run a full vacuum nozzle it sea level the air pressure from the outside will cause a disruption of the exhaust flow inside the nozzle and will likely cause the nozzle to break apart. Which would be bad.
So you design an engine with a nozzle small enough so that doesn’t happen at sea level, and therefore booster engines have nozzles that are pretty tiny. They are good at the start of the flight, but when they get partly out of the atmosphere the nozzles are too small to be effective. If you watch a SpaceX Falcon 9 launch you’ll see the rocket exhaust is going straight down at the launch, but right before the first stage is done, you can see that a lot of the exhaust is going out to the side – that is because the nozzle is too small for that altitude. A nozzle that is too small is less efficient but it’s not dangerous the way a nozzle that is too large would be.
Latest Answers