Specific impulse, orSpecific impulse(English: specific impulse), specific impulse is defined as unitpropellantThe impulse produced by the quantity of.If usingweightDescribe the amount of propellant, and the specific impulse hastimedimension, the international unit is seconds (s);If usingqualityDescribe the quantity of propellant, and the specific impulse is expressed in the speed dimension,International unitIs meters per second (m/s).Since the specific impulse can also be written as the thrust generated by consuming unit propellant per unit time, it is also called the specific impulseSpecific thrust(English: specific trust).
Specific impulse is used to measure rocket orairplane engineImportant physical parameters of efficiency.Its size is related to the propellant chemical energycombustion efficiency It is related to nozzle efficiency.The higher the specific impulse of the engine, the greater the velocity increment that the propellant can produce under the same conditions,Engine efficiencyThe higher.
The specific impulse is defined as the impulse produced by the unit propellant quantity, which is represented by the symbol
express.There are two ways to describe the quantity of propellant, one is weight description, the other is quality description.The corresponding definition formula is as follows:
Weight description
Quality description
Including:
: Impulse generated by propellant
: Weight of propellant
: Mass of propellant
It should be noted that the general concept of "weight" is related to the gravitational field near the earth's surface, but in space or orbit, "weight" only means mass times standardGravitational acceleration(or standard gravity)
, i.e
，
Is a conventional constant, precisely defined as
, does not change with the position of the gravitational field.[1]
Extended definition
Expand the definition.utilize
，
as well as
The following formula can be obtained:
Weight description
Quality description
Including:
: engine thrust,
: propellant mass flow rate, i.e
，
: standard gravity acceleration, precisely defined as
。[1]
and
It is also the reason why specific impulse is also called "specific thrust".
What is defined above is the instantaneous specific impulse. We can also write the specific impulse under the time average for the time integral:
Weight description
Quality description
Among them,
Indicates the engine operating time.Thrust in the above equation
And propellant mass flow rate
Can vary over time.[2]
It can be found that under the description of weight and mass, the expression of specific impulse only differs by a constant coefficient
。
For liquid engines, it is easy to measure the thrust and propellant flow rate at each moment to calculate the instantaneous or average specific impulse;However, it is difficult to calculate the propellant flow rate for solid motors, so the average specific impulse is usually calculated by the following formula:
Weight description
Quality description
Among them,
Indicates when the engine is operating
Mass of propellant consumed internally.[2]
Units and conversions
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According to the definition of specific impulse, it is easy to write the dimension of specific impulse.Common units areInternational unit（rice，Kg，second）And commonly used in the United StatesImperialEngineering unit（foot，pound, seconds).
Any two values in the same row in the table have the same meaning, such as the second and fourth columns in the second row
。
remarks:
1 pound is precisely defined as 0.45359237 kg;
One foot is exactly 0.3048 meters.[3]
influence factor
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Is measurementrocket engineThe important parameter of the property, in seconds (s).The specific impulse has a great influence on the range of a rocket. The higher the specific impulse,rangeThe farther away.To obtain high specific impulsepropellant, the propellant is required to have highchemical energy, highcombustion efficiency And highnozzle efficiency ，nozzleThe shape directly affects the specific impulse.
The higher the specific impulse, the greater the total power obtained by the rocket, and the faster the final speedSolid rocket motorThe specific impulse of the liquid rocket main engine is 300 to 453 secondsIon engineA larger specific impulse can be obtained.According to the above definition, the unit of specific impulse is meters per second m/s, which is the same as the unit of velocity V: FT/M=MV/M=V, which is one gravity acceleration g: FT/M=MgT/M=gT different from the above simplified unit.For example, "the specific impulse can reach 290 seconds" means that the speed of fuel ejected by the rocket can reach 290 * g=2900 m/s.The specific impulse is in direct proportion to the speed of the fuel ejected from the rocket. To increase the specific impulse, we must find ways to increase itrocketThe speed of the ejected fuel.The limit of specific impulse is that the fuel injection speed reaches the speed of light, which is 3 × 10 ^ 8 seconds in seconds.
