In this blog post, we will examine the principles of rocket propulsion and the characteristics of liquid-fueled and solid-fueled rockets, focusing on the Naro Rocket.
The Launch of the Naro Rocket and the Principles of Rocket Propulsion
In 2022, following the successful launch of the “Nuri (KSLV-II),” developed entirely with its own technology, South Korea continues to advance its space transportation capabilities. At the starting point of these achievements was the Naro (KSLV-I), South Korea’s first space launch vehicle. Launched on January 30, 2013, the Naro consisted of a two-stage solid-fuel kick motor developed by South Korea and a first-stage liquid-fuel engine manufactured by Russia’s Khrunichev, and it successfully placed a science and technology satellite into its target orbit. This success spurred South Korea to accelerate its development of an independent space launch vehicle and laid a crucial foundation that led to the subsequent development of the “Nuri” rocket.
The Naro rocket, which holds such significant meaning in South Korea’s space development history, serves as the propulsion system for launching satellites into space. Many people likely remember the live news broadcast of the Naro rocket’s launch. The sight of massive flames erupting from the rear of the rocket as it soars into the sky amid a deafening roar is truly awe-inspiring.
This phenomenon can be explained by Newton’s Third Law—the law of action and reaction—which is taught in middle school science. The law of action and reaction states that when two objects interact, the forces they exert on each other are equal in magnitude but opposite in direction. A rocket gains thrust by rapidly expelling high-temperature gas generated by its engine through a nozzle, and moves forward as a result of the reaction force. This is the same principle by which a balloon filled with air flies by expelling air through its opening. However, while a balloon expels air due to the elasticity of rubber, a rocket burns propellant to convert chemical energy into thermal energy and gains thrust by expelling the high-temperature gas produced during this process. The high-temperature gas is further accelerated as it passes through a nozzle with a small cross-sectional area and is expelled outward, causing the rocket to move forward as a result of the reaction force.
This propulsion method, based on reaction, shares the same basic principle as an airplane’s jet engine. However, the combustion process requires oxygen; while airplanes draw in outside air to use the oxygen in the atmosphere, rockets must fly in outer space, where there is almost no air, so they carry their own oxidizer along with the fuel.
How do liquid-fueled rockets differ from solid-fueled rockets?
Rockets can be classified as liquid-fueled or solid-fueled based on the form of the propellant they use. The Naro rocket used a liquid engine in its first stage and a solid kick motor in its second stage.
A liquid rocket engine consists of a combustion chamber, a nozzle, propellant tanks, and propellant supply and flow control systems. When liquid propellant is injected into the combustion chamber and burned, high-temperature gas is generated; this gas is rapidly expelled through the nozzle, generating thrust. The liquid propellant used in this process is mostly a bipropellant system, in which the fuel and oxidizer are stored and supplied separately. The Naro rocket used kerosene, a kerosene-based fuel, as its liquid fuel. Kerosene can be stored safely, is easy to handle, and is relatively inexpensive, making it suitable for use in large quantities. As with most liquid rockets, liquid oxygen was used as the oxidizer. Although liquid oxygen must be stored at cryogenic temperatures, its excellent oxidizing performance makes it widely used in rocket propulsion.
Solid-fuel rockets also share the same basic principle as liquid rockets in that they generate thrust by expelling high-temperature gas. However, they differ in that they directly combust solid propellant rather than supplying liquid propellant. Solid propellants use metal powders, such as aluminum, as fuel components to achieve high thrust and energy density, while oxidizers such as ammonium perchlorate (AP) and ammonium nitrate (AN) are used. Solid-fuel rocket engines have a simpler structure and fewer moving parts than liquid-fuel rockets, making them relatively inexpensive to manufacture and allowing for immediate launch when needed. On the other hand, once ignited, it is difficult to adjust thrust or shut down the engine during flight, so they are primarily used as auxiliary propulsion systems for space launch vehicles or in military missiles.
How will rocket technology evolve in the future?
Current rocket technology is advancing toward simultaneously achieving cost-effectiveness, reusability, and high propulsion efficiency. In particular, the U.S. private space company SpaceX is significantly reducing launch costs by applying a “clustering” method—which combines multiple engines to form a single large propulsion system—while also commercializing launch vehicle reusability technology. South Korea’s “Nuri” rocket also employed a method of clustering multiple 75-metric-ton-class liquid engines to form its first-stage booster.
The Naro rocket holds historical significance as South Korea’s first space launch vehicle and served as the foundation for the subsequent development of the “Nuri” rocket and the establishment of the country’s independent space transportation capabilities. We look forward to the continued advancement of South Korea’s rocket technology, which we hope will yield meaningful achievements in an even wider range of space exploration fields, including lunar and deep-space exploration.
