Does an engine's Isp rating always include all mass flow rates, including those for electrical or mechanical power generations?

Discussions here have convinced me to ask this separately.

Isp or mass-specific impulse is a ratio of thrust (force) to mass flow rate (kg/s). It comes in two flavors, seconds and meters/second depending on if it's multiplied by $g$.

Is Isp understood to be calculated using all mass flow rates including those that might be combusted to generate electrical power or mechanical power to drive turbo pumps and potentially other things?

I suppose we can ignore the kerosene used to run hydraulic systems, even though in some cases those systems are not closed!

• Not really an answer but an indication: I recently compared efficiency (stated Isp vs theoretical Isp at stated chamber pressure) and found staged combustion engines always having a few percent higher efficiency when compared to gas-generator cycles. As chamber and nozzle design for both cycles shouldn't differ that much (does it?) I guess that they all included the massflow for their turbine in their calculation. I compared only around 4 engines of each type. Sep 7 '18 at 10:52
• Aha! The plot thickens...
– uhoh
Sep 7 '18 at 10:56
• You might need to be a bit more specific. The so called "tag data" for SSMEs - used for flight predictions - was generated on a test stand from actual measurements of flow rate and sea-level thrust. So of course an Isp calculation from this would include all factors. Are you asking about some other case where the Isp is not derived from testing? Definitions might vary, but SSME used inlet flows, so any leakage (and autogenous pressurization fluid) resulted in an reduction to the Isp. In the sim we could see the Isp drop when the press valves opened (a tiny amount). The tag #s were averaged. Sep 7 '18 at 13:05
• @OrganicMarble It will be hard for me to be more specific, I'd intended to refer to Isp values "in general" and a component of the answer might then be "it depends", in which case I might say "what about the numbers listed in Wikipedia, the ones people sometimes quote in answer or question posts here?"
– uhoh
Sep 7 '18 at 14:42

Isp is generally derived from measurements (of thrust and mass flow rate). Thus, tautologically, it includes whatever mass flows are measured. For Isp to be a useful metric -- i.e. to use it in the rocket equation, or to compute thrust in a simulation -- it needs to include any significant, ongoing mass expenditure.

Conventionally, in open gas-generator cycle engines, that includes propellant mass flow through the gas generator and turbopump and out via a low-efficiency exhaust nozzle. As @Christoph notes, this means that gas-generator engines will show a few percent lower Isp than otherwise comparable closed-cycle engines, which is as it should be.

The accounting for something like an open-cycle hydraulic system for the gimbals is a bit of an edge case, but I would guess it's not usually accounted in Isp calculation.

• This seems fairly conclusive, or at least sufficient for me; if I understand correctly, the answer is pretty much "yes".
– uhoh
Sep 7 '18 at 14:42
• @uhoh, the other way around. An engine is designed for optimal performance at a specific flow rate. Throttling the engine (if it is throttleable) will almost surely result in reduced specific impulse. Cycling a fixed flow rate engine (turning it on and off over a brief interval of time), as is oftentimes done with attitude thrusters, will almost surely result in reduced specific impulse compared to continuous operations. Sep 7 '18 at 17:16
• @DavidHammen Did you intend this comment for a different post? Sep 7 '18 at 17:19
• @RussellBorogove - I misread the question. I thought uhoh was asking whether specific impulse is constant, which it most certainly is not, even after discounting for differences in external pressure (e.g., sea level vs vacuum). Sep 7 '18 at 17:24
• @DavidHammen I did mention the impact of throttling on Isp in this comment on the question linked in this question. You may have read that moments earlier?
– uhoh
Sep 7 '18 at 18:11