Originally Posted by malloyd (Post 1609365)

No. That's true if the thrust is provided by reaction to exhausting the fuel, but that's not actually the definition of specific impulse, which is the thrust of the engine divided by the weight of the fuel it burns. If you move a very large volume of air very slowly you can get lots of thrust for very little energy (and hence fuel) use. Turbofan specific impulses can break 100,000 Ns/kg, helicopter rotors routinely reach millions. It's more common to quote these as the reciprocal ("thrust specific fuel consumption") and multiply by 3600 to get them in per hour but its the same thing.