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Looking at the Wikipedia page for the J-2 (one engine was used on the Saturn V third stage, and 5 engines on the second stage) it gets about 232Klbs thrust in vacuum, but at sea level, only 109KLbs thrust.

That is a pretty wild performance swing.

Why such a difference? If you compare the Vacuum Merlin's vs the sea level Merlin's performance you get a much smaller swing in performance.

I am guessing it is because there never was a sea level optimized J-2 engine (it was meant as an upper stage engine) so the sea level number may be spurious or only from test stands.

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    $\begingroup$ This, in a nutshell, is the problem that linear aerospike engines are supposed to solve. $\endgroup$ – Travis Bear Feb 14 '14 at 5:34
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Designed for high altitude low ambient pressure, I assume. The primary optimization is in the nozzle:

For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure: if the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted.

To maintain this ideal of equality between the exhaust's exit pressure and the ambient pressure, the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on (and reducing the exit pressure and temperature). This increase is difficult to arrange in a lightweight fashion, although is routinely done with other forms of jet engines. In rocketry a lightweight compromise nozzle is generally used and some reduction in atmospheric performance occurs when used at other than the 'design altitude' or when throttled.

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  • $\begingroup$ So you expect a reduction in behavior for a vacuum optimized engine/nozzle, but this is a 50% cut in performance. More than usual. $\endgroup$ – geoffc Feb 13 '14 at 19:03
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    $\begingroup$ J-2 fires at a very low chamber pressure (30 bar/435 psi) compared to something like the SSME (206 bar/3000 psi); I think that's the main factor in the sea level specific impulse dropoff. $\endgroup$ – Russell Borogove Aug 11 '15 at 22:58
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There is another important variable besides chamber pressure and nozzle design to consider--how the engine pumps the propellants into the chamber. The J-2 engine, like the lower stage (and much more powerful but lower ISP kerosene burning F-1 engine) used a gas generator (basically burning oxygen and fuel, here hydrogen) to drive a turbine that pumped the oxygen and hydrogen into itself and the main chamber. The F-1 (using kerosene instead of hydrogen) was designed with high enough gas generator pressure to drive the pumps at launch, but the J-2, designed for vacuum operations, used a lower gas generator pressure to drive its pumps. The very low sea level ISP reflects not just the usual "back up" of rockets by sea level air pressure but also a turbo pump rendered anemic by the same back pressure.

Just today I downloaded software called RPA that simulates rocket chambers and nozzle performance for a wide variety of propellant types and chamber pressures and other conditions. My attempt to simulate the J-2A engine (an upgrade of the J-2) suggests, if I did it correctly, that a reasonable sea level ISP for the J series engines with the same type of chamber and nozzle would be a bit under 300 seconds, a reduction of 31 percent. In terms of thrust in Klbs, that would suggest a bit under 160, so at just under 110 poor pump performance at sea level apparently dings it another 22 percent.

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  • $\begingroup$ What's the J2's gas generator chamber pressure? It would have to be mighty low for an ambient pressure difference of 15 psi to be a large fraction. $\endgroup$ – Organic Marble Aug 31 '16 at 20:14
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    $\begingroup$ Where did you get the RPA sortware? We have a meta post that is collecting this sort of thing. $\endgroup$ – Jerard Puckett Sep 2 '16 at 16:27
  • $\begingroup$ @JerardPuckett Main page, RPA page itself $\endgroup$ – Peter Nazarenko Oct 22 '20 at 15:03

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