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Ariane 5   Kourou


Specific Impulse

Specific impulse is the parameter used to measure the efficiency of a rocket jet engine, and is basically the ratio of the useful jet impulse Ft (Ns) produced divided by  the fuel  flow-rate (mass per second in Kg/sec) required to produce that impulse .

Specific impulse Isp is the index of performance for rocket engines and is defined by the following ratio :



Where the numerator is the total impulse obtained over the operating time of the rocket engine. Usually referred to as the ‘Burn time’. Defined by the time period from t0 to t1.seconds.

Accompanied by a denominator which represents the total fuel mass flow necessary to produce the impulse during the same time period.

‘g’ is the acceleration due to gravity gearth at sea level m/s2

The above expression can be simplified to the ratio given below if the flow rate and thrust are constant over the burn time.

For liquid fueled rocket engines this is usually the case,  so they can be represented  by the ratio of the thrust force produced  at the exit of the nozzle F    divided by the product of  gearth  at sea level  multiplied by the mass flow rate of fuel used to produce the thrust.

Note: F is measured at  tearth

Specfic  impulse       Isp =      sec


Where F is the Jet Thrust force at the outlet of the nozzle F = q * v2         N   Newtons   

‘q’             is the volumetric flowrate * density r  to give mass flow-rate of fluid leaving the nozzle Kgs/sec

v2               is the exhaust jet velocity at the outlet of the nozzle m/sec

Mdt          is the mass flowrate of fuel


Dimensional analysis of Isp

Isp  = Sec


Isp = Sec


Research Page

esa comparison of specific impulse for liquid propergols and electric propulsion 

Once you are out in space and have overcome aerodynamic drag of the atmosphere then the total impulse produced over a long period by a certain mass of fuel become significant.

Definition of Tsiolkovskis Formula

Tsiolkovski modified end function



© All rights reserved by John Gwynn                                                                Water Rocket Explorer

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Ariane 5 Envisat 2


" Great things are not done by impulse, but a series of small things brought together".
Vincent Van Gogh

Ironic quotation which says that success is the result of assembling many  simple existng systems or ideas in

a new original way.