Liquid-fulled rocket engine efficency.

[flatmaster]

"Acceleration increased during S-IC flight for two reasons: decreasing propellant mass; and increasing thrust as F-1 engine efficiency improved in the thinner air at altitude."

This is from Wikipedia, so who knows. Why would a thiner atmosphere increase the thrust efficency of the engine? Something to do with the ullage space in the tanks not needing to be filled with atmosphere?

 

[Danger]

I don't know whether or not this would be considered 'engine efficiency', but obviously aerodynamic drag decreases with altitude.
Also, rocket nozzles have specific air-pressure ranges at which they work best. Bell-shapes work better lower, while conical ones are better in thinner air.

 

[FredGarvin]

It's mostly due to the pressure thrust. As altitude increases and atmospheric pressure decreases, there is a contribution to the thrust due to static pressure at the exit of the nozzle. Both thrust and Isp increase with altitude.

 

[flatmaster]

It's mostly due to the pressure thrust. As altitude increases and atmospheric pressure decreases, there is a contribution to the thrust due to static pressure at the exit of the nozzle. Both thrust and Isp increase with altitude.

Ah, this makes sense. You can imagine a 1atm pressurized container. Opening this on the ground does nothing. Opening this in space would create thrust. Thanks!