Posts Tagged ‘SI’

Energy and Calories

The latter is an obsolete unit for energy anyway. The SI unit is Joules. 1 J = 1 N * 1 m = 1 kg * 1 m²/s² = etc etc – see that’s the whole idea of the SI system – you don’t have any coefficients.

Yet using Calories further makes it easier to construct misconceptions like the advertisement for Hydrive energy drinks: they give you lots of energy and are healthy because they have low Calories.


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Pump Power

There’s been some discussion of pump power. It is extremely easy to calculate it when using SI units.

A flow front in a tube has a power:


W power, F force, v speed.

The force is


Where A is cross sectional area and P the pressure.



Where V is volume so its time derivative is the volume flow.

Thus we can combine these and get


1 kg/s at water density is a volume flow of 1E-3 m^3/s. If pressure is 10 bars, that’s 1 MPa (Megapascal, 1 Pascal is 1 N/m^2) in SI, and thus the power is 1000 W or 1 kW.

1 kW for 90 seconds means 25 Watt hours (I know, I know, this is a horrible unit of energy). With modern LiFe batteries that have 100 Wh/kg energy densities (J/kg could often be more useful measure), a quarter of a kg battery could power the rocket for 90 seconds. With an exhaust velocity of 2 km/s, 1 kg/s gives a thrust of 2 kN. Thus the rocket could be in the 200 kg mass category. The battery mass seems negligible.

Of course, in real life the pump efficiency is a small number and thus plays a big factor, and the pump times are longer too.

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