[Reading Science] Is Korea's First Independently Developed Space Launch Vehicle a 1950s-Style 'Old-Fashioned' Model?

Rising Interest in Space Rockets as New Space Era Begins
Korea's Independently Developed 'Nuriho' Rocket Launch Success in October Draws Attention
Fuel and Propulsion Methods Similar to 1950s Soviet Model
"First Independent Development Limits Considered Industrial Base, Reliability, and Development Difficulty"

[Asia Economy Reporter Kim Bong-su] The 'New Space' era is unfolding, where the private sector leads space development and space tourism is becoming more active. Low Earth orbit will be filled with tens of thousands of satellites through space internet and 6G high-speed communication networks, and the day when cities are built on the Moon and Mars is not far off. At the right time, South Korea is also making a full-scale entry into space development. The first domestically produced space launch vehicle, 'Nuriho,' is scheduled for launch this October. If successful, South Korea will join the 'Space Club,' which only 10 countries worldwide belong to. The shackles that limited launch vehicle development under the Korea-US summit agreement have also been lifted. Accordingly, attention is focused on space launch vehicles, or rockets, which are considered the most basic infrastructure for space development. There are also views that disparage Nuriho, Korea's first independently developed latest rocket, as 'old-fashioned' for adopting technology from 70 years ago. This shows how far there is to go.

◇Principle of Space Rockets

Anyone can understand it by knowing Newton's Third Law of Motion, the 'principle of action and reaction.' Fuel is burned to generate gas, which is expelled, and the force propels the rocket into space beyond Earth. The problem is that to break through the tremendous pressure caused by Earth's gravity and the weight of the launch vehicle and escape the atmosphere, an enormous speed and force are required. The minimum speed needed to escape Earth's orbit is 11.2 km/s. To expel the propellant (fuel + oxidizer) under such strong pressure, the tanks must be thick and sturdy, and the more thrust generated, the heavier the engine inevitably becomes.

Precise technology is also necessary. A representative case is the Challenger space shuttle disaster on January 28, 1986 (local time), which exploded 73 seconds after launch, killing all seven crew members. The tragedy was caused by frozen rubber rings due to sudden cold weather, which allowed exhaust gases to leak. The third launch of Naroho, developed by Korea using Russian technology, was also postponed due to a problem with the 'O-ring' connecting the bottom of the rocket to the launch pad. This shows that perfect technology that does not tolerate even the smallest mistake is required.

◇The 'Father' of Space Rockets

Robert Goddard, an American physicist and rocket scientist, is known as the 'father of rockets' for successfully conducting the world's first flight test of a launch vehicle using a liquid propellant rocket engine in 1926. He is credited with laying the foundation of modern rocket technology by researching various rocket engines and launch vehicles and registering 214 patents. However, his claim that space travel by rocket was possible was harshly criticized by contemporary American media such as The New York Times, which said he ignored the common knowledge that thrust does not occur in a vacuum, known even to high school students. The U.S. government only recognized the value of his technology posthumously in 1960, paying his heirs $1 million in royalties and acquiring his rocket patents, which were later used in NASA's satellite launch and lunar exploration projects. Werner von Braun, a German rocket scientist who developed the V-2 rocket during World War II, also surrendered to the U.S. after the war and made significant achievements by developing early space launch vehicles like Saturn for NASA. Sergey Pavlovich Korolev is regarded as the leading figure who directed the development of multiple space launch vehicles such as the R-7 during the period when the former Soviet Union was ahead of the U.S.

◇Types of Space Rockets

Rockets can be classified by propellant type into liquid, solid, electric, nuclear energy, beam energy, and others. Among these, solid rockets are mainly used for low Earth orbit and military purposes, while liquid rockets with adjustable thrust are most commonly used for space development. Structurally, depending on how fuel and oxidizer are delivered to the combustion chamber, rockets are divided into pressure-fed and turbopump-fed types. Pressure-fed rockets require separate pressurization devices, making them heavier. Turbopump-fed rockets use turbopumps installed at the connections between fuel and oxidizer tanks and the combustion chamber to increase pressure.

This method is further divided into the 'gas generator cycle (open cycle)' and the 'staged combustion cycle (closed cycle).' To operate the turbopump, a small separate combustion chamber (preburner) is needed besides the main combustion chamber. In the open-cycle gas generator system, the exhaust gas from the preburner is expelled outside, whereas in the closed-cycle staged combustion system, the exhaust gas is sent back to the main combustion chamber for reuse.

◇Where Does Korean Technology Stand?

The engine of Korea's first domestically produced space launch vehicle, Nuriho (KSLV-II), uses a gas generator cycle method with kerosene and liquid oxygen as propellants. Some criticize this as using outdated technology developed by the former Soviet Union in the 1950s. It is true that the gas generator cycle method was developed during the Soviet era.

The pressure generated by the combustion chamber is only about 100 bar, much lower than the more advanced closed-cycle staged combustion method (200?300 bar). The closed-cycle staged combustion is like a 'boiler that burns twice,' making it more fuel-efficient. Using kerosene as fuel also has the disadvantage of producing a large amount of soot.

Rockets using propellants other than kerosene are increasing. A representative example is SpaceX's Raptor engine, which uses liquid methane as propellant in the Starship project. Another private space company, Blue Origin, has been developing the BE-4, a liquid methane staged combustion cycle engine, since 2011. Korea is aware of this and is already developing a closed-cycle staged combustion engine. The Korea Aerospace Research Institute (KARI) has been actively researching this technology since 2010 and plans to produce a prototype for technology verification by 2024.

However, the Nuriho project, which began development in 2008, did not apply such technology. KARI is known to plan to apply advanced technologies through a performance improvement project after completing two launches of Nuriho by next year.

Jin Seung-bo, Planning and Coordination Team Leader of KARI's Korean Launch Vehicle Development Project Headquarters, explained, "We selected the open-cycle gas generator cycle engine considering the domestic industrial status, development difficulty, reliability, and cost while developing purely with domestic technology without any technical assistance. Fuel efficiency, thrust, and specific impulse have been improved by applying the latest technology. Even considering that SpaceX's commercial Falcon 9 uses the Merlin engine with kerosene, it is not appropriate to judge rocket technology solely based on fuel type and injection method."