Specific Impulse - Units

Units

Imperial and SI units for various rocket motor performance measurements.
Specific impulse (by weight) Specific impulse (by mass) Effective exhaust velocity Specific fuel consumption
SI =X seconds =9.8066 X N•s/kg =9.8066 X m/s =(101,972/X) g/kN•s
Imperial units =X seconds =X lbf•s/lb =32.16 X ft/s =(3,600/X) lb/lbf•h

By far the most common unit used for specific impulse today is the second, and this is used both in the SI world as well as where Imperial units are used. Its chief advantages are that its units and numerical value are identical everywhere, and essentially everyone understands it. Nearly all manufacturers quote their engine performance in these units and it is also useful for specifying aircraft engine performance.

The effective exhaust velocity in units of m/s is also in reasonably common usage. For rocket engines it is reasonably intuitive, although for many rocket engines the effective exhaust speed is considerably different from the actual exhaust speed due to, for example, fuel and oxidizer that is dumped overboard after powering turbopumps. For airbreathing engines the effective exhaust velocity is not physically meaningful, although it can be used for comparison purposes nevertheless.

The N•s/kg is not uncommonly seen, and is numerically equal to the effective exhaust velocity in m/s (from Newton's second law and the definition of the Newton.)

Another equivalent unit is specific fuel consumption. This has units of g/kN.s or lbf/lb•h and is inversely proportional to specific impulse. Specific fuel consumption is used extensively for describing the performance of air-breathing jet engines.

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