Specific Impulse of a Rocket Engine- Here’s All You Need to Know
Thanansan Kuganesan
Head of Engineering | Advanced Manufacturing | Earth Observation | Space Technology
One can say that their car mileage is 15km/L. To elaborate, the distance a car can travel on a single gallon or litre of fuel in a city is referred to as its mileage. In short, these units are used to measure a car’s efficiency compared to other cars.
Similarly, rocket engines have special parameters to measure their capabilities. One of the parameters used to measure its efficiency is the ‘specific impulse’, which is notated as Isp , where I represent Impulse, and sp denotes the idea of ‘specific’. Measuring and maintaining a healthy value of a specific impulse in a rocket ensures efficiency and reliance because it has the propensity to discharge better thrust for the same rocket propellant.
This article aims to help the reader understand the ins and outs of a specific impulse of a rocket engine in order to have a quick, comprehensive understanding of how the Specific Impulse varies and how it can be measured.?
What Is the Specific Impulse of a Rocket?
The Specific Impulse? roughly says how efficiently the rocket will eject the fuel; therefore, in other words, the more efficient the specific impulse is, the more speed the engine can eject the fuel.? This will allow the engineers? to control the use of fuel based on their unique requirements of the rocket’s specifications and capacities.
A Step-by-Step Guide to Understanding Specific Impulse
Before understanding the mechanism behind specific impulse let us have a vivid understanding on the mechanism of a rocket launch. Firstly, rockets expel the propellant at high speed by burning the fuel and oxidiser, which produces a thrust in an opposite direction that lifts the rocket against its mass and the atmospheric drag. Here are some terms and equations that will help you understand how the specific impulse is calculated.
Mass Flow Rate
Mass flow rate can be determined by the rate of the flow that achieves by burn and expel propellent. It is calculated by how many kilograms of propellent is expelled in each second. It can be defined as;
Weight Flow Rate
Weight flow rate will be measured by multiplying the mass flow rate by the gravitational constant g where the weight is being used to calculate the specific impulse.
Thrust
The Thrust is defined by the velocity and the mass expelled from the nozzle.?
Specific Impulse
Thus, the specific impulse can be defined as the proportion of the speed of expelling the propellant from the nozzle and measured in second. A higher specific impulse is the higher speed of ejection of the propellant. This speed is monitored to measure the quality of the engine, in other terms, how fast the engines burn fuel to lift off the Rocket.
Solid Rocket, Liquid Rocket and Their Specific Impulse
If we look at a space shuttle, there are two types of rocket engines. One is the main engine RS-25, which burns the liquid hydrogen as fuel and liquid oxygen as an oxidiser and can eject the propellant at a 3,700 m/s rate.
In the space shuttle, NASA used two solid rocket engines as boosters to facilitate vertical takeoff. This was because a solid rocket engine produces more thrust than a liquid engine despite the specific impulse. This leads to a question:
Why can’t rocket use a solid rocket instead of a liquid for all missions?
Though the solid rocket is a simpler design than a liquid, it cannot be controlled once it starts to burn the fuel. We can control the speed and fuel-burning ratio and gimbal the liquid rocket engine to manoeuvre the spacecraft, but this quality is absent in solid fuel-burning rockets.
Here the specific impulse is not measured for the thrust but for the efficiency of how fast the engine can burn the fuel and oxidiser. This is the most important KPI of a rocket engine, as the faster and more efficient burn rate can accelerate to gain altitude and velocity with less fuel.
The following table shows some of the famous rocket engine’s specific impulses to help you understand engine efficiency better.
Sea Level Specific Impulse VS Vacuum Specific Impulse
Rockets are tested in testing facilities- one as shown in the picture. These testing facilities have the capability to measure all parameters while they are tested under complicated yet well-controlled measures. The specialists of the testing facility monitor the specific impulse and other KPIs of the engines.
There is nothing to push back the expelling gas in a vacuum as, unlike at sea level, the ambient atmosphere (1 bar/100 kPa) is available to create a push at sea level. That is why the specific impulse in the vacuum is always higher than the sea level.
Suppose we use the engine in the upper stages of the rocket beyond the atmosphere (after 100 km from the sea). In that case, the specific impulse in the vacuum should be calculated based on the engine’s performance beyond the atmosphere- in a vacuum or space. For example, when the shuttle is orbiting on a low-earth orbit, the specific impulse should be specifically calculated based on the vacuum and not based on the expected specific impulse at sea level.
Specific Impulse An Important Parameter- But Why?
To understand why the specific impulse is a critical parameter, let us first look at this rocket equation.
velocity (△v) plays an important role in maintaining the rocket's speed. For instance, if we want to double the rocket's speed, we have two options. Firstly, increase the rocket's mass four times at the start, or double the 'velocity of the propellant').
However, the first option is an inefficient way to accelerate the speed; rocket designs are trying to reduce weight by building lightweight models. Therefore, the second option, which is increasing the speed of the propellant ejection, is the most convenient option to increase the rocket speed. This ejection speed is proportional to the specific impulse. Thus, this is one of the reasons why the specific impulse is one of the significant KPIs of a rocket engine.
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