Understanding High Thrust of Vacuum Rocket Engines

[RocketAstro]

Why do vacuum rocket engine always producing high thrust than sea level rocket engine ?

for example spacex's merlin vacuum engine produce more thrust than merlin sea level engine. And it is due to the big nozzle with large area in vacuum engine because when the flow reaches mach 1 in the throat then if we increase the area the velocity will be increase (opposite to subsonic flow) and pressure and temp will decrease and that's why it is producing higher thrust ? and we can't that much big nozzle in sea level because it will highly over expanded and cause flow separation right ?

or they just increasing the nozzle exit area in vacuum for reducing the under expansion because the ambient pressure in vacuum is almost zero so what ever we do to the nozzle it will definitely under expanded but if we increase the area then we reduce the under expansion and improve efficiency and mass flow rate in the vacuum engine divergent area will be same as the mass flow rate in the sea level rocket engine's divergent area ?

and if thrust increases then the Isp also increases (thust/weight flow rate) than the sea level rocket engine right ? or any other thing is happening ? pls pls pls pls say

 

[Dullard]

The answer to your basic question is: The (ultimate) exit pressure is 15 PSI lower for the 'vacuum' engine. For engines with identical mass flow, this is a significant net thrust advantage (assuming that the divergent section is optimized for the exit conditions). Vacuum nozzles are longer in order to 'capture' the thrust available from what would be sub-atmospheric exhaust gas in a sea-level engine. You can see this effect in an ascending (from sea-level) engine - the exhaust plume increasingly 'expands' at the exit plane as atmospheric pressure decreases - this is the 'waste' that a vacuum nozzle is optimized to capture.

For some vacuum-operated engines (most solid apogee kick motors) there isn't room for an optimized nozzle (extra fuel is easier). Designers have to take steps (plume deflectors) to prevent the 'expanding' exhaust gas from damaging the spacecraft .

 

[RocketAstro]

The answer to your basic question is: The (ultimate) exit pressure is 15 PSI lower for the 'vacuum' engine. For engines with identical mass flow, this is a significant net thrust advantage (assuming that the divergent section is optimized for the exit conditions). Vacuum nozzles are longer in order to 'capture' the thrust available from what would be sub-atmospheric exhaust gas in a sea-level engine. You can see this effect in an ascending (from sea-level) engine - the exhaust plume increasingly 'expands' at the exit plane as atmospheric pressure decreases - this is the 'waste' that a vacuum nozzle is optimized to capture.

For some vacuum-operated engines (most solid apogee kick motors) there isn't room for an optimized nozzle (extra fuel is easier). Designers have to take steps (plume deflectors) to prevent the 'expanding' exhaust gas from damaging the spacecraft .

thanks for saying