한국항공우주연구원

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 performance test like quenching

Liquid 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 test

According 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 thrust

The 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 technology

The 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 engines

KARI 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 Korea 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

Therefore, the inner walls of the propellant tank, considered to be the launch vehicle body, are made in a form called isogrid structure to withstand such pressure and load. It is a method of efficiently increasing the cylindrical structure’s rigidity compared to the weight with a pattern of repeated triangular grid reinforcement structure. The isotropic lattice structures are derived from extremely difficult techniques that require repetitive calculations and analysis. Unlike advanced countries such as the United States and Russia, which have a long history of space development, Korea had to find the optimal isotropic grid structure through repetitive calculations and numerical analysis since it was the country's first time to develop a propellant tank for a launch vehicle.

Design and manufacturing optimized for liquid flow

Kerosene 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.