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Rocket Motor Specific Impulse

Engine Engineering Motor rocket Rockets

specific impulseIf you've exhausted the capacity of small manufactured rocket motors and are itching to send your rockets into the wispy upper reaches of the atmosphere and beyond, you have to do some real rocket engineering analysis. Nearly every discussion of rocket performance starts with your Rocket Motor Specific Impulse or Isp.

In general, model rocket engines are scaled based on their Impulse. For example, Estes sizes their 1/4A - G engines as follows:


Total Impulse (Metric Standard)


0.313-0.625 N·s


0.626-1.25 N·s


1.26-2.50 N·s


2.51-5.0 N·s


5.01-10 N·s


10.01-20 N·s


20.01-40 N·s


40.01-80 N·s


80.01-160 N·s

From Wikipedia

Total Impulse is a cumulative measure of the Thrust generated by the motor over it’s firing time. It can be easily approximated by using the average thrust, with the equation: [caption id="attachment_4367" align="alignnone" width="142"]Equation of Total Impulse Equation of Total Impulse[/caption]

These average values include: start-up, shut-down, and variable thrust (which are all very common in solid motors), so it’s important to note that Tavg might be very different from an engine's maximum Thrust. Average thrust is denoted on the rocket name code as the first number (example: a C6-3 rocket would have a Tavg of 6 Newtons). The drawback of Total Impulse is that its dependent on the size of the rocket, particularly the amount of propellant being used. If you want a pure performance metric for the engine itself, regardless of the rocket, you need calculate your Specific Impulse.

Specific Impulse or Isp can be thought of as the efficiency of a rocket engine, similar to MPG in a car. It is the measure of the amount of thrust generated for a given flow rate of propellant. Also, just as MPG is not directly related to a car’s Horsepower (and usually is inversely related), Isp is usually inversely related to the Thrust of an engine. This means that different types of engines have different Isp’s For example, solid motors have low Isp values but can generate massive amounts of thrust (which is very useful for launch applications pushing through the thick lower atmosphere). On the other end of the spectrum electronic propulsion devices generate a few Newtons or less of force, but are extremely efficient and can be run for days or even years for inter-planetary or stellar travel. Isp is calculated a number of ways, the most common using average thrust and propellant flow values: [caption id="attachment_4369" align="alignnone" width="183"]Specific Impulse Specific Impulse[/caption]

Where m(dot) is your propellant mass flow rate and g is sea level acceleration of gravity. You can also calculate Isp by dividing the engines total impulse by the weight of the propellant:

[caption id="attachment_4371" align="alignnone" width="193"]Isp using Total Impulse Isp using Total Impulse[/caption] All of these values are dependent on each other. What’s great about Isp is that it’s a similar measurement between all engines, even air breathing engines. This means you can directly compare the efficiencies of all rocket motors from class A scale motors all the way to Space Shuttle Boosters:



Space Shuttle Booster


Space Shuttle Main Engine


F-1 (Saturn V motor, largest moter ever built)


Estes B6 Motor


Russian 11B97 Electric/Xenon Propulsion system


(All values from

It can be seen that there is a pretty wide range of Isp’s for rocket motors, with the solid motors on the low end of the Isp and electric propulsion with much higher Isp values. The Estes motor is significantly lower than the other full sized space motors listed above. The reason for this is that Estes motors use a black powder propellant which while inefficient is very stable and safe. Knowing how to determine Isp is not only important for determining efficiency, it’s also crucial when sizing your rocket, determining how much fuel is required for a given mission, and really affects every aspect of designing your rocket. This is the first crucial step in designing your own rocket.

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