The Korea Aerospace Research Institute (KARI) successfully developed the engine—the heart of the Korean Launch Vehicle (NURI)—using domestic technology. Except for a few components, such as bearings for the engine turbopump, all parts were localized. KARI achieved the engineering design and manufacturing of the complex liquid rocket engine entirely with Korean expertise.
Combustion Chamber
A device that generates propulsion by expelling high-temperature, high-pressure gas produced from the combustion of fuel and oxidizer through a nozzle.
Turbopump
A device that supplies fuel and oxidizer to the combustion chamber at high pressure.
Gas Generator
A component that drives the turbopump turbine by producing high-pressure gas through combustion.
Valves and other propellant feed system components
Liquid rocket engines must endure intense internal chemical reactions while being designed with the minimum possible weight to achieve the maximum performance relative to their mass. As a result, numerous inherent risks are associated with their operation. Therefore, securing reliability is the top priority when applying newly developed engines to rockets, and extensive repeated testing is essential.
The 75-ton-class liquid engine for NURI, developed with domestic technology, overcame early technical challenges, such as combustion instability, and verified its combustion performance during flight through a test launch in 2018. To ensure the performance and reliability of the 75-ton-class engine, the Korea Aerospace Research Institute (KARI) has conducted continuous engine combustion tests.
From the initial test in April 2016 through November 2020, a total of 25 engines were assembled and tested. The first and second units were produced to evaluate the functionality and performance of each component and to establish key engine sequences. Based on these early tests, engines from the third unit onward were configured to closely resemble the flight model. Subsequent acceptance tests for the test launch vehicle certification models and flight models were successfully completed, leading to their delivery for launch operations and ongoing engine verification testing. As of November 2020, 25 units of the 75-ton-class engine had been produced, with a total of 168 tests conducted and a cumulative burn time of 16,690 seconds. The longest continuous burn duration recorded for a single engine was 260 seconds.
In addition, development of the 7-ton-class engine began with power pack tests in April 2015, identifying potential issues before full engine testing and providing information for the layout design of the initial prototype. The engine assembly process was established through fabrication and integration efforts, and the first combustion test was successfully conducted in July 2015. The 7-ton-class engine proceeded through a stable manufacturing and testing process, leading to the assembly of the certification model for the third stage, followed by the commencement of flight model assembly. Because the third-stage liquid engine remains inactive during the operation of the first and second stages while being subjected to severe vibration, vibration environment testing was completed to verify its structural integrity and operational functionality. A total of 11 units of the 7-ton-class engine have been developed, with 89 tests conducted and a cumulative burn time of 16,385.7 seconds.
Combustion testing can be categorized into ground combustion tests and high-altitude combustion tests, depending on the operating environment. Ground combustion testing verifies whether the engine operates normally at approximately a 50 km altitude—the operational range for NURI’s first stage. High-altitude combustion testing focuses on ensuring that the engines function properly above 50 km in altitude, the operational domain for NURI’s second and third stages. Propulsion systems operating at high altitudes, such as the second-stage 75-ton-class engine and the third-stage 7-ton-class engine, encounter much lower ambient pressure and higher nozzle expansion ratios compared to ground conditions. As a result, conducting combustion tests at ground level often leads to “flow separation,” where the exhaust flow detaches irregularly from the nozzle walls, making it difficult to accurately measure thrust performance. Therefore, to validate propulsion system performance on the ground, it is essential to simulate actual flight environments during testing and accurately verify ignition and combustion characteristics.
The first stage of NURI uses a clustering method, combining four 75-ton-class liquid engines to generate 300 tons of thrust. Clustering is a technique where multiple engines are grouped to achieve the required thrust. While clustering allows for greater thrust while maintaining structural stability, it presents significant challenges in control compared to operating a single engine. The difficulty increases as more engines are grouped. In engine clustering, all engines must produce thrust uniformly, functioning as if they were a single engine. To achieve this, fuel and oxidizer must be supplied to all engines under identical conditions. Maintaining the same temperature, pressure, and flow rate across all engines ensures the engines perform identically, behaving as one. In addition, the reliability of critical components such as turbopumps, piping, and combustion chambers must be ensured. It is also essential to develop the capability to precisely monitor whether all engines are producing nominal thrust and to measure the margin of thrust deviation. Moreover, maintaining horizontal alignment and balance among the engines during simultaneous ignition and combustion is a critical technology for achieving a highly reliable clustered engine configuration.
The liquid rocket engine for a space launch vehicle is the culmination of extreme technologies, and its development inevitably involves numerous technical challenges. First and foremost, it must control highly explosive fluids stably under extreme temperatures and high pressures. The NURI liquid rocket engine generates thrust by combusting kerosene and liquid oxygen at -183°C. Once combustion begins, the temperature inside the combustion chamber soars to 3,000°C. Within the confined space of the engine, fluids must be managed across an extreme temperature differential from -183°C to 3,000°C.
Above all, the complexity of its structure makes the development of liquid rocket engines particularly difficult. Furthermore, igniting a space launch vehicle’s liquid engine is a highly challenging process.
In less than a second, multiple valves and components that supply fuel and oxidizer must operate precisely in a strictly predefined sequence. NURI’s 75-ton-class engine combusts approximately 255 kg of fuel and oxidizer per second, and even a slight deviation in the ignition sequence can immediately lead to an explosion. It is similar to lighting a gas stove at home, when a delayed spark can cause an instantaneous explosive ignition.
While developing NURI, the Korea Aerospace Research Institute (KARI) concurrently pursued preliminary research on future space launch vehicles. In addition to NURI’s open-cycle liquid engine, KARI has been developing a 9-ton-class staged combustion cycle liquid engine, which, although significantly more difficult to develop, offers higher combustion efficiency.
Since 2016, KARI has conducted more than 60 tests—including a single continuous burn of up to 600 seconds—for the staged combustion cycle engine, a technology currently held by only a few advanced spacefaring nations such as the United States, Russia, and China. The testing has also verified the engine’s throttling capability, with thrust reducible by up to 40%.
The staged combustion cycle engine is planned to be utilized to enhance the performance of future NURI-class launch vehicles. Applying the 9-ton-class staged combustion engine to post-NURI launch vehicles could increase the satellite payload capacity from 1.5 tons to approximately 2.7 tons. Additionally, it would enable the deployment of multiple satellites into various orbits and support lunar exploration missions.
Furthermore, to improve the performance of the 75-ton-class engine, KARI is developing a new combustion chamber capable of withstanding higher combustion pressures than that of the current 75-ton-class design.
| Item | Specification |
|---|---|
| Vacuum Thrust (tonf) | 8 - 10 tonf |
| Specific Impulse in Vacuum, Isp (sec) | Minimum 350 seconds |
| Mixture Ratio (O/F) | 2.5 - 2.6 |
| Turbopump Inlet Pressure (bar) | 4/1.5 |
| Preburner Combustion Pressure | 210 bar or higher |
| Main Combustion Chamber Pressure | 90 bar or higher |
| Ignition/Restart Capability | Up to 3 starts |
| Propellants | Liquid Oxygen (LOX) / Kerosene |
| Engine Cycle | Staged Combustion Cycle |
| Weight (kg) | 300 kg or less |
| Operation Time | 600 seconds or longer |
| Fuel | Kerosene | Oxidizer | Liquid oxygen |
|---|---|---|---|
| Thrust at sea level | 68.85t/674.7kN | Specific impulse in vacuum | 304.8s |
KARI developed the engine—the core of NURI—entirely with domestic technology.
Except for a few components, such as the bearings used in the turbopump, full localization was achieved, completing the engineering design and production of the complex liquid rocket engine through indigenous capabilities.
Although technical difficulties, such as combustion instability, were encountered during the initial development phase, these challenges were overcome. Combustion performance during flight was successfully verified through the 2018 test launch. .