Specific Impulse (AI) is a measure of how efficiently a rocket or engine uses fuel. By definition, this is the total surge delivered per unit of power consumed, and is equivalent in size to the generated draft divided by the mass flow rate. If kilograms are used as a propellant unit, then the specific impulse is measured in speed. If weight in Newtons or lbf is used instead, a certain value is expressed in time, most often in seconds.
Multiplying the flow rate by standard gravity converts the AI into mass.
Tsiolkovsky's equation
The specific impulse of a higher mass engine is more effectively used to create forward thrust. And in the case when a rocket is used, then less fuel is required. It is he who is needed for this delta-v. According to the Tsiolkovsky equation, in the specific impulse of a rocket engine, the motor is more efficient at gaining altitude, distance and speed. This performance is less important in jet models. Which use wings and outdoor air for combustion. And they carry a payload that is much heavier than fuel.
The specific impulse includes the movement created by external air, which is used for combustion and is depleted by spent fuel. Jet engines use the outside atmosphere for this. And so they have a much higher MI than rocket engines. This concept, from the point of view of the spent mass of fuel, has units of measurement of distance over time. Which are an artificial quantity called the "effective exhaust gas velocity." This is higher than the actual swiftness of the exhaust. Because the mass of combustion air is not taken into account. Actual and effective exhaust speeds are the same in rocket engines that do not use air or, for example, water.
General considerations
The amount of fuel is usually measured in units of mass. If it is used, then the specific impulse is an impulse on EM, which, as the size analysis shows, has units of speed. And therefore, MDs are often measured in meters per second. And often called effective exhaust swiftness. However, if mass is used, the specific impulse of the fuel divided by the force is a unit of time. And therefore, specific shocks are measured in seconds.
This rule is the main one in the modern world, it is widely used with a coefficient r 0 (constant from gravitational acceleration on the Earth's surface).
It is worth noting that the rate of change of a rocket's motivation (including its fuel) per unit time is equal to the specific impulse of thrust.
Specificity
The higher the thrust, the less fuel is required to create a given thrust for a certain time. In this regard, the more effective the liquid, the greater its MD. However, this should not be confused with energy efficiency, which can decrease with an increase in shock, since the specific impulse of the engine, which gives high results, requires a lot of energy for this.
In addition, it is important to distinguish and not confuse traction and a specific push. MI is created per unit of fuel consumed. And traction is the instantaneous or peak force that is generated by a particular device. In many cases, propulsion systems with very high specific impulse - some ion installations reach 10,000 seconds - create low traction.
When calculating the shock, only fuel that is transported with the vehicle before use is taken into account. Therefore, for a chemical rocket, the mass will include both fuel and an oxidizing agent. For air breathing engines, only the amount of fluid is taken into account, not the mass of air passing through the engine.
Atmospheric resistance and the inability of the installation to maintain a high specific impulse at a high burning rate is precisely the reason why all fuel is not used as quickly as possible.
A heavier engine with good UI may not be as effective at climbing, distance or speed as a lightweight device with low performance
If it weren’t for air resistance and reduced fuel consumption during the flight, the MD would be a direct measure of engine efficiency in converting mass to forward motion.
Specific Impulse in Seconds
The most common unit for a particular push is H * s. Both in the context of SI, and in those cases when imperial or ordinary values are used. The advantage of seconds is that the unit and numerical value are the same for all systems and are essentially universal. Almost all manufacturers indicate their engine characteristics in seconds. And such a device is also useful for determining the specifics of an airplane device.
Using meters per second to find the effective exhaust speed is also quite common. This unit is intuitive when describing rocket engines, although the effective exhaust speed of the devices can vary significantly from the actual one. This, most likely, can be associated with fuel and oxidizer, which are discharged overboard after turning on the turbopumps. For air-breathing jet engines, an effective exhaust speed does not make physical sense. Although it can be used for comparison purposes.
Units
Values expressed in N * s (in kilograms) are not uncommon and are numerically equal to the effective exhaust velocity in m / s (from Newton’s second law and its definition).
Another equivalent unit is specific fuel consumption. It has measurement values such as g (kN · s) or pound / hour. Any of these units is inversely proportional to the specific impulse. And fuel consumption is widely used to describe the characteristics of jet engines.
General definition
For all vehicles, the specific impulse (push per unit weight of fuel on Earth) in seconds can be determined by the following equation.
To clarify the situation, it is important to clarify that:
- F - is the standard gravity, which is nominally declared as power on the surface of the Earth, in m / s 2 (or foot / s squared).
- g - is the mass flow rate in kg / s, which appears to be negative with respect to the rate of change in the mass of the vehicle over time (as fuel is pushed out).
Measurement
The English unit, the pound, is more commonly used than other quantities. And also when applying this value per second to the flow rate, during conversion, the constant r 0 becomes unnecessary. As it becomes dimensionally equivalent pounds divided by r 0.
I sp in seconds - this is the time during which the device can generate a specific thrust of the rocket engine, given the amount of fuel whose weight is equal to attraction.
The advantage of this formulation is that it can be used for missiles where the entire reaction mass is carried on board, as well as for aircraft where most of the reaction mass is taken from the atmosphere. In addition, it gives a result that is independent of the units used.
Specific impulse as speed (effective exhaust swiftness)
Due to the geocentric coefficient g 0 in the equation, many prefer to determine rocket thrust (in particular) in terms of thrust per unit mass of fuel flow. This is an equally valid (and in a sense somewhat simpler) way of determining the efficiency of the specific impulse of rocket fuel. If we consider other options, then the situation will be almost the same everywhere. Missiles of a specific specific impulse are simply the effective exhaust speed relative to the device. The two attributes of a particular push are proportional to each other and related as follows.
To use the formula, you must understand that:
- I is the specific impulse in seconds.
- v is the shock measured in m / s. Which is equal to the effective exhaust velocity measured in m / s (or ft / s, depending on g).
- g is the standard for gravity, 9.80665 m / s 2. In Imperial units, 32.174 ft / s 2.
This equation also holds true for jet engines, but is rarely used in practice.
It is worth paying attention that sometimes different symbols are used. For example, c is also considered for exhaust speed. While the symbol sp can be logically used for MD in units of N · s / kg. To avoid confusion, it is advisable to reserve it for a specific value, measured in seconds before the start of the description.
This is due to the thrust or force of the specific impulse of the rocket engine, the formula.
Here m is the mass fuel consumption, which is the rate of decrease in the value of the vehicle.
Minimization
The rocket must carry all its fuel. Therefore, the mass of unburned food must be accelerated along with the device itself. Minimizing the amount of fuel needed to achieve this push is critical to creating effective missiles.
The specific impulse formula of Tsiolkovsky shows that for a rocket with a given empty mass and a certain amount of fuel, a general change in speed can be achieved in proportion to the effective speed of the outflow.
A spacecraft without a propulsion moves in an orbit determined by its trajectory and any gravitational field. Deviations from the corresponding velocity template (they are called Δ v) are achieved by the exhaustiveness of the exhaust gases in mass in the direction opposite to the necessary changes.
Actual swiftness versus effective speed
It is worth noting here that these two concepts can differ significantly. For example, when a rocket launches in the atmosphere, air pressure outside the engine causes a braking force. Which reduces the specific impulse and the effective exhaust speed is reduced, while the actual speed is practically unchanged. In addition, sometimes rocket engines have a separate turbine gas nozzle. Then, to calculate the effective exhaust velocity, it is required to average two mass flows, and also take into account any atmospheric pressure.
Increased efficiency
For air-breathing jet engines, in particular turbofans, the actual discharge speed and effective speed differ by several orders of magnitude. This is due to the fact that when using air as a reaction mass, a significant additional impulse is achieved. This allows better coordination of airspeed and exhaust speed, which saves energy and fuel. And significantly increases the effective component while reducing actual swiftness.
Energy efficiency
For rockets and rocket-like engines, such as ion models, sp implies lower energy efficiency.
In this formula, v e is the actual jet velocity.
Therefore, the required force is proportional to each exhaust speed. At higher speeds, the required power is much stronger for the same traction, which leads to less energy efficiency per unit.
Nevertheless, the total energy for the mission depends on the total fuel use, as well as how much energy is required per unit. Huge amounts of reaction mass are needed for the low exhaust velocity relative to the delta-v mission. In fact, for this reason, a very low exhaust speed is not energy efficient. But it turns out that not one type has the highest rates.
Variable
Theoretically, for a given delta-v, in space, among all fixed values of the exhaust velocity, the value of v e = 0.6275 is the most energy-efficient for a given final mass. To find out more, you can view the energy in the propulsion system of the spacecraft.
However, variable exhaust speeds can be even more energy efficient. For example, if a rocket is accelerated with some positive initial speed using the exhaust swiftness, which is equal to the speed of the product, no energy is lost as the kinetic component of the reaction mass. As it becomes stationary.