Specific impulse ($I_{\mathrm{sp}}$)

Specific impulse measures the impulse delivered by a rocket per unit propellant consumed. In its conventional seconds form, divide total axial thrust impulse by propellant mass $m_p$ and standard gravity $g_0$:For constant conditions this becomes $I_{\mathrm{sp}}=F/(\dot{m}{g}_0)$, where $\dot{m}$ is the positive expelled mass flow rate. Multiplying by $g_0$ gives the effective exhaust velocity. The conventional value $g_0=9.80665\mathrm{m}{\mathrm{s}}^{-2}$ is a unit convention, not a requirement that the engine operate near Earth.^{[ref] [ref]}

For a worked example, a steady $6000\mathrm{N}$ engine consuming $2\mathrm{kg}{\mathrm{s}}^{-1}$ has $I_{\mathrm{sp}}\approx 305.9\mathrm{s}$ and effective exhaust velocity $3000\mathrm{m}{\mathrm{s}}^{-1}$. A ten-second firing supplies $60000\mathrm{N}\mathrm{s}$ using $20\mathrm{kg}$ of propellant. The unit of seconds does not mean that this engine can burn for 305.9 seconds: duration also depends on the available propellant and its flow rate.

In a laboratory illustration, integrate the force recorded on a calibrated engine stand and measure the mass consumed during the same firing. Report the ambient pressure, since nozzle pressure forces contribute to thrust. At fixed specific impulse, doubling the mass flow rate doubles thrust; a large specific impulse alone does not specify a large force. For varying performance, the burn-integrated value is weighted by propellant flow, rather than generally equaling a simple time average.