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As such, "specific impulse" could be taken to mean either "per reaction mass", as with a rocket, or "per fuel burned" as with cars. The latter is the traditional and common choice. In sum, specific impulse is not practically comparable between different types of engines. In any case, specific impulse can be taken as a measure of efficiency.
Specific impulse (usually abbreviated I sp) is a measure of how effectively a rocket uses propellant or jet engine uses fuel. By definition, it is the total impulse (or change in momentum) delivered per unit of propellant consumed [4] and is dimensionally equivalent to the generated thrust divided by the propellant mass flow rate or weight flow rate. [5]
Impulse has the same units and dimensions (MLT −1) as momentum. In the International System of Units, these are kg⋅m/s = N⋅s. In English engineering units, they are slug⋅ft/s = lbf⋅s. The term "impulse" is also used to refer to a fast-acting force or impact.
This figure is inversely proportional to specific impulse, which is the amount of thrust produced per unit fuel consumed. TSFC or SFC for thrust engines (e.g. turbojets , turbofans , ramjets , rockets , etc.) is the mass of fuel needed to provide the net thrust for a given period e.g. lb/(h·lbf) (pounds of fuel per hour-pound of thrust) or g ...
The higher the specific impulse, the less propellant is required to provide the desired impulse. The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be ...
The designation for a specific motor looks like C6-3.In this example, the letter (C) represents the total impulse range of the motor, the number (6) before the dash represents the average thrust in newtons, and the number (3) after the dash represents the delay in seconds from propelling charge burnout to the firing of the ejection charge (a gas generator composition, usually black powder ...
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VASIMR, short for Variable Specific Impulse Magnetoplasma Rocket, uses radio waves to ionize a propellant into a plasma. A magnetic field then accelerates the plasma out of the engine, generating thrust. A 200-megawatt VASIMR engine could reduce the time to travel from Earth to Jupiter or Saturn from six years to fourteen months, and from Earth ...