Designing rockets and the Tsiolkovsky rocket equation

[Urwumpe]

I repeat:

Seconds are not a SI unit - and not even a correct imperial unit.

ISP(SL) is not specific impulse in SI, but sea-level specific impulse.

Don't mix imperial and SI units. It causes what you experience: Errors.

If the ISP is measured in SI units, the unit next to the value would be either m/s (average exhaust velocity, see gas-generator rocket engines, why it is not THE exhaust velocity) or N*s/kg = specific impulse (N*s = impulse, kg = mass, so it is impulse per kg fuel mass, or the specific impulse of the propulsion system)

 

[DanP]

:suicide:

So it was SL, not SI!

Argh. In that case SI Isp is 2636, which makes mass flow 4370kg/s, and order is restored to the world.

Thank you very much for your help and sorry for being such an idiot

 

[Quick_Nick]

Actually I'm pretty sure(and Wikipedia explains it to a degree) that ISP in seconds is used as imperial and SI. If you use lb/s for mass flow, ft/s² for acceleration due to gravity, and poundals(1 lb·ft/s²) for force, the ISP in seconds is equal to using kg/s for mass flow, m/s² for acceleration due to gravity, and newtons(1 kg·m/s²) for force.
N·s/kg is the 'effective exhaust velocity' in m/s. Divide by the acceleration due to gravity to get seconds.(Ve(in m/s) = Isp(in s) * g(in m/s²))
Much is explained on Wikipedia: http://en.wikipedia.org/wiki/Specific_impulse
The likely most important equation here is immediately under the "Specific impulse in seconds" section.