Vacuum vs. sea-level rocket engine Isp

I am wondering if a rocket engine's performance in the atmosphere
could be improved by changing the operating point, such as mixture
ratio or chamber pressure.

My understanding is that under identical conditions in the chamber and
nozzle we will get less thrust in the atmosphere because of the air
pressure acting on the outside of the rocket engine.

I also think that in the atmosphere we should get higher pressures in
the nozzle because the combustion gases expand to a higher pressure.
This effect will partially compensate for the effect of the external
presure.

This picture gets more complex if there are feedback loops that
maintain constant conditions.

I think that if we maintained a higher pressure during atmospheric
flight such as to compensate for the effect of the atmospheric
pressure we could get a constant thrust and improve atmospheric Isp. I
think that as we increase the pressure we will get improved Isp, and
if the pressure is limited by the structural load on the chamber and
nozzle, the structure would only care about the pressure difference
and so the pressure could be raised.

I know about the aerospikes etc, I don't think it is practical to make
a variable geometry nozzle.

Zoltan

Zoltan Szakaly wrote:

I am wondering if a rocket engine's performance in the atmosphere
could be improved by changing the operating point, such as mixture
ratio or chamber pressure.

My understanding is that under identical conditions in the chamber and
nozzle we will get less thrust in the atmosphere because of the air
pressure acting on the outside of the rocket engine.

I also think that in the atmosphere we should get higher pressures in
the nozzle because the combustion gases expand to a higher pressure.
This effect will partially compensate for the effect of the external
presure.

This picture gets more complex if there are feedback loops that
maintain constant conditions.

I think that if we maintained a higher pressure during atmospheric
flight such as to compensate for the effect of the atmospheric
pressure we could get a constant thrust and improve atmospheric Isp. I
think that as we increase the pressure we will get improved Isp, and
if the pressure is limited by the structural load on the chamber and
nozzle, the structure would only care about the pressure difference
and so the pressure could be raised.

I know about the aerospikes etc, I don't think it is practical to make
a variable geometry nozzle.

Zoltan

Well, raising chamber pressure is exactly what they do to get good
sea level performance. That is one of the reasons that the Shuttle
main engines operate at 3000 psi.

You need to look at any good rocket propulsion text because
this is something that is very fundamental.

Mike Walsh