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As you climb at launch, atmospheric pressure falls, causing thrust to rise.

The thrust gain I'm talking about here is due to the increased ratio of nozzle pressure to atmospheric pressure. I'm going by things I've read. I'm not a rocket scientist.

And here's where I'm confused.

Specific impulse increases with altitude also, and since thrust is a function of specific impulse, this would seem to mean that thrust rises still further from increase in specific impulse...

...Or is the first thrust gain mentioned above due to the increase in specific impulse with altitude, so that if you account for the change in specific impulse, then you've automatically accounted for the change in thrust?

Not a rocket scientist, have only a tenuous grasp on rockets, please hold your spears. I'm just trying to get a handle on this one particular thing. Thanks!

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    Read the linked answer, but thrust is lower at low alittudes due to back pressure, and propellant flow is the same, hence the lower Isp. "thrust is a function of specific impulse" is backwards; Isp is a parameter derived from thrust and flowrate. – Organic Marble Apr 01 '21 at 16:18
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    Thanks! I think I see: thrust drops with decreasing altitude due to back pressure, and that drop reflects in the drop in specific impulse. So if you capture the change in one, you capture the change in the other, it seems---unless the specific impulse depends on still other variables not accounted for in the change in thrust. –  Apr 01 '21 at 16:30
  • That sounds right to me. – Organic Marble Apr 01 '21 at 16:31
  • You might be interested in this, where I helped somebody calculate Merlin thrust vs altitude. https://space.stackexchange.com/a/46548/6944 – Organic Marble Apr 01 '21 at 16:33
  • I have to wonder: the thrust of a Merlin 1D goes from 190,000 lbf at sea level to 205,000 lbf in vacuum, a change of 8%. But the specific impulse goes from 282 s at sea level to 311 s in vacuum, a change of 10%. I expected the change in thrust to match the change in specific impulse. Could the difference be attributed to error in the numbers reported (taken from Wiki)? Or are there other factors causing one to fall at a different rate from the other? –  Apr 01 '21 at 16:35
  • Thanks! This is actually just what I needed. You're the best. –  Apr 01 '21 at 16:36
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    @user39728 The 8% vs 10% is either rounding errors or other inconsistencies in data source. Especially with the Merlin, there have been a lot of minor revisions to the engine, and specs have probably gotten out of sync. – Russell Borogove Apr 01 '21 at 18:06

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...Or is the first thrust gain mentioned above due to the increase in specific impulse with altitude, so that if you account for the change in specific impulse, then you've automatically accounted for the change in thrust?

That's correct. The thrust change is because of the rising specific impulse. Thrust is essentially propellant mass flow rate times specific impulse.

Russell Borogove
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