"Morgans" wrote in message Plus jets are more
efficient at altitude, so more air again, right?.

No, less air, because the density of the ambient air is less as altitude
rises. Less air in the front means less air out the back (though the
pressure ratio can be the same). Jet engines produce less thrust at
altitude. There is less cooling air which means that maximum exhaust
temperature is reached at a lower thrust. The efficiency gains come from the
forward speed of the engine (sort of a ram effect) and the lower aerodynamic
drag at altitude (higher true airspeed).

Another thing that is being overlooked, is the HP rating of the engine.

Turbojets have no torque and therefore have no horsepower. There is an
equation for 'equivalent horsepower' which involves an airspeed of around
375 mph.

I'm no expert on this stuff, but I think my thinking (and guestimates) are
about right.

If you are more confused now than before, you get an A+!

D.