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Transportation to spaceThe United States and Russia have secured space launch vehicle technology since the 1950s. Europe, Japan, China, and India are also pursuing space development, such as launching satellites, space probes, and transporting space cargo, by securing space launch vehicle technology. All of the satellites developed in Korea have been launched using foreign space launch vehicles. As a latecomer in the research and development of space launch vehicle, Korea does not own a space launch vehicle yet. Since countries restrict cross-border technology transfer for space launch vehicles, it takes much time and development cost and many trials and errors due to technical difficulties. With the recent introduction of innovative recycled launch vehicles by US private space enterprise Space X, Europe and Japan are also developing low-cost and high-efficiency launch vehicles. Moreover, many startups around the world are developing ultra-small launch vehicles capable of launching nanosatellites. The global commercial space launch vehicle market is expected to expand as the number of space development countries increases and more small satellites are developed.
Development of space launch vehicle with domestic technologyKARI has cultivated its rocket design and manufacturing capabilities by developing single-stage solid propulsion science rocket (KSR-I, 1993), double-stage solid propulsion mid-sized science rocket (KSR-II, 1998), and Korea’s first liquid propulsion science rocket (KSR-UUU, 2002). It acquired space launch vehicle know-how and experience by developing Naro (successfully launched in 2013), the double-stage space launch vehicle consisting of a first-stage liquid engine and a second-stage solid engine, through international cooperation with Russia. It is currently developing a 3-stage Korean launch vehicle (Nuri) with domestic technology to launch a 1.5t-class application satellite into a solar-synchronous orbit at an altitude of about 600~800km. The Nuri is a space transportation vehicle necessary for Korea to become a space powerhouse and a key vehicle for stable space development. KARI plans to launch a domestically developed satellite using the Nuri between 2022 and 2027. With the Nuri development, Korea has finally secured the three elements of space development: the satellite, the launch vehicle, and the launch site. They will enable Korea to launch its satellite at any desired time.
Status of Rocket Development in Korea
|Subject||KSR-I||KSR-II||KSR-III||Naro (KSLV-I)||Korean Launch Vehicle (KSLV-II)|
|Purpose||Localization of single-stage non-guided scientific observation rockets and exploration of the ozone layer over the Korean Peninsula||Localization of double-stage solid propulsion scientific observation rockets with initial altitude control function||Securing base technology for independent development of liquid propulsion rockets and small satellite launch vehicles||Securing technology and experience for independently developing launch vehicles that can carry a 100kg- class satellite into low-earth orbit||Securing development know-how of a launch vehicle that can carry a 1.5-ton application satellite into low-orbit|
|Development period||1990.7 ～ 1993.10||1993.11 ～ 1998.6||1997.12 ～ 2003.2||2002.8 ～ 2013.4||2010.3 ～ 2022.3|
|Development cost (KRW 100 million)||28.5||52||780||5,025||19,572|
Comprehensive verification of safety and performance of 75-ton enginesThe test launch vehicle is a single-stage launch vehicle to check the performance of the 65-ton liquid engine, which is the main engine and is applied to the first and second stages of the Nuri, through actual flights. The test launch vehicle launched in the afternoon of November 28, 2018 from the Naro Space Center in Goheung-gun, Jeonnam exceeded the target time of 140 seconds and burned for 151 seconds, reaching a maximum altitude of 209 km and then falling to the point of about 429 km on the southern open sea. The test confirmed the soundness of the performance of ground systems such as structures, electronics, controls, heat/aerodynamics, launch pad, and tracking system, which are subsystems that make up a launch vehicle, including liquid engines developed with domestic technology. With the successful launch of the test launch vehicle, Korea became the world’s 7th country to have the technology to develop medium and large liquid rocket engines of 75 tons or more. The test launch vehicle's development and launch were named one of the Top Ten Outstanding Research Outcomes in 2018 by the National Research Council of Science and Technology.
Engine development with domestic technologyKARI developed an engine, considered the heart of the Korean launch vehicle (Nuri), with domestic technology. Except for some such as the bearings used in the engine turbopump, all parts were successfully localized. The complex liquid engines of space launch vehicles were engineered and manufactured with Korean technology. Even developed countries possessing long-time accumulated engine development know-how, professionals, and infrastructure require a considerable amount of physical time to develop a new engine. The average development period for major liquid rocket engines using non-toxic propellants is 9.17 years. An engine using a liquid oxygen-kerosene (fuel oil) combination requires an average of 8.50 years. An engine with liquid oxygen-liquid hydrogen combination requires an average of 9.83 years.
Liquid engine configuration- Combustor: A device that produces propulsion by ejecting high-temperature, high-pressure gas generated by the combustion reaction of fuel and oxidizer through nozzles - Turbopump: A device that supplies high-pressure fuel and oxidant to the combustion chamber - Gas generator: A drive that drives the turbopump turbine through the combustion reaction of high-pressure gas - Supply system parts such as valves
|Engine System||=||Combustor||+||Turbopump||+||Gas Generator||+||Supply System Parts|
Engine performance test like quenchingLiquid rocket engines have many risk factors because they must be built to provide the best performance while violent chemical reactions occur inside the liquid engine at the same time. Therefore, securing reliability is most important, and repeated tests are essential to apply the newly developed engine to rockets. The Nuri's 75-ton liquid engine, developed with domestic technology, overcame technical difficulties such as combustion instability in the early stages of development and confirmed the flight process's combustion performance through a test launch in 2018. KARI has continuously conducted engine combustion tests to ensure the performance and reliability of the 75-ton engines. The 75-ton engine used in the Nuri’s first stage should burn for 123 seconds, and the second-stage 75-ton engine for high altitude needs to continue combustion for 142.2 seconds. A total of 25 75-ton engines were assembled and tested in April 2016 until November 2020. Units 1 and 2 were manufactured to determine each component's operability and performance and the various sequences of the important engines. As a result, unit 3 and later one have had a shape similar to that of the flight model. Afterward, KARI delivered the test launch vehicle certified model and flight model after successful acceptance tests and continuously conducted the engine verification test. As of November 2020, 25 75-ton engines had been produced, and they have gone through a cumulative combustion time of 16,690 seconds in 168 tests. The single longest combustion time was 260 seconds. Moreover, the initial layout of the 7-ton engine was designed by identifying and correcting problems through the power pack tests from April 2015 before the engine test. After that, the engine assembly process was established through manufacturing and assembly, and the first combustion test was successfully conducted in July of the same year. The 7-ton engine's flight model assembly began after the manufacturing and test following the three-stage certified model. Since the three-stage liquid engine does not operate during the first and second stages of flight but is exposed to a vibrating environment, a vibration environment test was conducted to verify structural stability and operability during combustion. Eleven 7-ton engines were developed, achieving cumulative combustion time of 16,385.7 seconds in 89 tests.
Enduring extreme conditions: the engine combustion testAccording to the engine’s operating environment, combustion tests can be divided into ground combustion tests and high-altitude combustion tests. The ground combustion test checks whether the engine operates normally at an altitude of about 50 km, which is the operating section of the first stage of the Nuri. The key to the high-altitude combustion test is to verify that the engine operates normally at an altitude of 50 km or higher, which is the operating section of the second and third stages of the Nuri. The propulsion engines operating in high-altitude environments, such as double-stage 75-ton engines for the 2nd stage and the 7-ton engines for the 3rd stage, have a very high nozzle expansion ratio since the atmospheric pressure of the operating altitude is much lower than that of the ground. As a result, it is difficult to measure the thrust accurately through a combustion test on the ground because of the flow separation phenomenon wherein the nozzle fluid does not flow well but appears irregularly away from the wall. Therefore, it is necessary to simulate the actual flight environment on the ground and check the exact thrust characteristics such as ignition and combustion to prove the propulsion engine's performance on the ground.
Clustering of four engines for a powerful thrustThe Nuri’s first stage has adopted a clustering method that generates 300 tons of thrust by bundling four 75-ton liquid engines. Clustering is a method of bundling multiple engines together to generate the necessary thrust. Although it can exhibit strong power while maintaining structural stability, the clustering method is difficult to control compared to a one-engine operation. Control becomes more difficult as there are more engines bundled. In engine clustering, multiple engines must produce the same thrust as if they were one engine. Fuel and oxidant must be supplied to all engines with the same requirements to ensure such. It is necessary to maintain constant temperature, pressure, and flow rate for the four engines to perform the same as if they were one engine. Moreover, the engine components such as the turbopump, piping, and combustor must have high reliability. It is also important to secure a technology that can determine whether the four engines' thrust is normal and how much the engine thrust error is. Maintaining the engines' level and balance is an important factor in preventing the four engines from interfering with each other when they are ignited simultaneously and in developing a highly reliable engine.
The fruit of complex and precise space technologyThe space launch vehicle's liquid engine is the output of extreme technology, and many technical difficulties arose during the development process. First of all, the highly explosive fluid must be stably controlled at extreme temperatures and high pressures. The Nuri’s liquid rocket engine reacts with kerosene and liquid oxygen at -183°C to generate propulsion through combustion. When combustion starts, the engine combustion chamber’s internal temperature soars to 3,000°C. The fluid with extreme temperature difference between -183° and 3,000°C must be operated within the confined engine space. Above all, developing a liquid rocket engine is difficult because of its complex structure. It is also hard to ignite a space launch vehicle’s liquid engine. Ignition requires valves and components supplying fuel and oxidant in an extremely short time of less than a second to operate correctly in a precisely defined sequence. Nuri's 75-ton engine burns 255 kg of fuel and oxidant per second, and even a slight startup sequence error can lead to an explosion. The same principle of the gas stove used at home is at work; when it is turned on, an explosive ignition occurs instantly when the spark rises even with a slight delay.
Advanced research and development of multistage combustion cycle enginesKARI is also conducting preliminary research on future space launch vehicles while developing the Nuri. It is developing a 9-ton multistage combustion-cycle liquid engine with high combustion efficiency in parallel, although it is more difficult to develop than the existing open-cycle liquid engine. KARI has completed more than 60 tests, including a single combustion test for a maximum of 600 seconds since 2016 to develop a multistage combustion-cycle engine for which only some advanced countries such as the United States, Russia, and China have the technology. It confirmed the ability to lower the engine thrust by up to 40%. The multistage combustion cycle engine will be used to improve the performance of the Korean launch vehicle (Nuri) in the future. Applying the 9-ton multistage combustion cycle engine to the space launch vehicle to be developed after the Nuri is expected to increase the payload weight from 1.5 tons to 2.7 tons. Moreover, it can put two or more satellites into various orbits and even launch a lunar probe. KARI is also developing a combustor that can withstand higher combustion pressure than the existing 75-ton class combustor to improve the 75-ton class engine's performance.
Performance of Multistage Combustion Cycle Engine
|Vacuum thrust (tonf)||8 - 10 tonf|
|Vacuum non-thrust, lsp (sec)||Last 350 seconds or more|
|Mixing ratio (O/F)||2.5 - 2.6|
|Turbo pump inlet pressure(bar)||4/1.5|
|Combustion pressure of the smoke absorber||210 bar More|
|Main Combustor Combustion Pressure||90 bar More|
|Engine cycle||Staged Combustion|
|Weight(kg)||300 kg Less than|
|Operating hours||600 second Less than|
Development of propellant tank with advanced technologyThe Nuri is a space launch vehicle that uses cryogenic oxidizing agents and room temperature fuel as propellants. Most parts of the launch vehicle consist of a propellant tank containing fuel and oxidant, and the propellant tank accounts for 70-80% of the projectile volume. Reducing the propellant tank weight leads directly to the launch vehicle’s performance improvement. Therefore, the propellant tank is made of lightweight yet durable aluminum alloy and configured with a cylinder corresponding to the body and a dome corresponding to the head. Nuri's propellant tank has maximum height of 10m and diameter of 3.5 m but is only 2.5-3 mm thick. Nevertheless, the propellant tank must be able to withstand 4 to 6 times the atmospheric pressure applied to the inside of the propellant tank and the load from inertia and aerodynamic forces during the flight. In particular, since the propellant tank is a cylindrical structure whose column is longer than the cross-section, the buckling phenomenon occurs when the load reaches a certain level.
Design and manufacturing optimized for liquid flowKerosene and liquid oxygen acting as oxidant are the liquid fuel of the launch vehicle, and their constant flows inside the tank exert an impact. Even in automobiles and large ships using oil as fuel, this flow makes it difficult to control the altitude, and devices for reducing this phenomenon are installed. It is the same for the Nuri’s propellant tank. Because it is much larger, and it contains large amounts of fuel and oxidants, strong flow inside the tank can have a devastating effect. For this reason, it is necessary to find and apply a design and manufacturing method optimized for the tank size and flow of fuel and oxidizing agent. In addition, the Nuri uses liquid oxygen as an oxidizing agent. Since the liquid oxygen temperature is -183℃ in cryogenic state, and the vaporization rate is very high, the tank plays an important role in injecting and storing it.
04Research on future launch vehicle technology Development of large and small launch vehicles using the Nuri as a platformMore Close