The Rocketdyne F-1 was to be updated with a new turbopump and gas generator assembly, without other major component redesign. This was to result in a thrust increase from around 1,522,000 pounds force for the F-1 to 1,800,000 pounds for the F-1A. Specific impulse at sea level was to have grown from 265 second to between 269 and 271 seconds, and chamber pressure would be increased from 1015psi to 1,161psi.

However, the combustion chamber dimensions and nozzle area ratio were not altered; so why does the higher-pressure F-1A feature a lower listed vacuum specific impulse of 303 seconds compared to the F-1 at 304s to 305s?

I am aware Astronautix applies a 'six second improvement in specific impulse' for both vacuum and sea level Isp - meaning 270s SL and 310s Vac. However other reports disagree; and during my searching the opposition has grown in number.

Why would a pressure improvement with no changes in chamber dimensions cause lower efficiency?

Could it have been a case of testing at sea level (F-1A was never flown), where vacuum trials were actually conducted at imperfect low air pressure rather than none?

  • $\begingroup$ If a higher fraction of the propellants flowed through the gas generator it could result in lower specific impulse, but I have no numbers on this. In gas generator cycle engines the propellants burned in the gas generator don't contribute to the thrust and thus lower the efficiency of the engine. $\endgroup$ Dec 18, 2016 at 0:07
  • $\begingroup$ I suppose this could work for many GG-type rocket engines; but F-1 gas generator exhaust was routed into the lower portion of the nozzle rather than completely dumped overboard as with F-1B (this probably wouldn't add much thrust anyway). However, if this was the case, wouldn't the engine's efficiency be lower at sea level as well? If the gas generator assembly draws a greater percentage of propellant mass flow, shouldn't the Isp loss continue across entire F-1A operation? $\endgroup$ Dec 18, 2016 at 4:42
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    $\begingroup$ Your question is a bit rambling, I was trying to address only this part: "Why would a pressure improvement with no changes in chamber dimensions cause lower efficiency?" $\endgroup$ Dec 18, 2016 at 4:44
  • $\begingroup$ Yeah, sorry. Most of my questions are like that... But nevertheless, both the F-1 and F-1A have the same chamber and nozzle dimensions, and both route GG exhaust into the nozzle extension. While the F-1A may use more propellant for the pump, this goes towards higher flow and chamber pressure that, when combined with the same area ratio, seems like it should result in higher Isp. $\endgroup$ Dec 18, 2016 at 5:08

1 Answer 1


It may be odd to answer my own question, but for anyone wondering…


The F-1A Task Assignment Program report featured above lists sea level specific impulse at 268.8s at the nominal 1,800klbf thrust level and 2.27:1 O:F ratio. Ideal chamber pressure was in the region of 1146psi, well above the 982-1015psi of the base F-1 design. Maximum thrust during testing under sea level conditions was around 1840klbf, corresponding to a specific impulse closer to 270s.

Vacuum performance of the F-1A increased specific impulse to 306.2s with a 2.484:1 O:F ratio, resulting in a thrust increase to 2,054,500 pounds force. Effective chamber pressure could have risen to 1172psi as a result of the increase in combined gas generator power from 44MW to 56.7MW (77,000+ hp).

The information that motivated my asking this question appears to have been based on simulated altitude testing that could not reduce ambient pressure quite low enough to give ideal vacuum performance. Another possibility is that the static testing did not incorporate a change in mixture ratio (which also occurred passively on regular Saturn V F-1 operation) and resulted in a less efficient vacuum burn.

In summary, the increased chamber pressure with no changes in nozzle dimensions results in higher specific impulse at sea level and in a vacuum – as expected. The vacuum Isp of 306.2s compares favourably to the F-1’s 304-305s, just as the sea level specific impulse of 268.8-269.7s compares favourably to the F-1’s 265s.

Just lettin’ you know.

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    $\begingroup$ Answering your own questions is encouraged, +1 $\endgroup$ Dec 25, 2018 at 14:21
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    $\begingroup$ Just as well! Thanks for the response. The link in the answer is quite interesting, too - shows just how variable the performance of the F-1 engines could be. I feel like it was more susceptible to flight conditions than other engines, but this could just be more apparent because of how massive it was. $\endgroup$ Dec 25, 2018 at 16:28

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