The door to space did not open easily. The Nuri, which made its first test flight on the 21st, reached its target orbit of 700 km and proceeded to separate the satellite model normally, but it was one step short of a perfect success. The combustion time of the 3-stage engine, which is a 7-ton engine, was prematurely terminated in 475 seconds, 46 seconds shorter than the target time of 521 seconds, so it did not reach the orbital input speed of 7.5 km.
The Nuri took off at 5 pm on the 21st, igniting the first stage engine from the second launch pad at the Naro Space Center in Goheung, Jeollanam-do. The first stage engine climbed to an altitude of 59 km 127 seconds after launch, and the ignition was completed and separated. Immediately, the two-speed engine started to ignite. 233 seconds after launch, the fairing covering the satellite base was separated, and 274 seconds after launch, the second-stage engine finished ignition and separated while climbing to an altitude of 258 km. However, the separated satellite model could not land in orbit due to the lack of combustion time of the 3-stage engine. It was confirmed that the satellite model, which lacked speed, soon deviated from orbit and crashed into the waters of southern Australia.
The Ministry of Science and ICT will set up an accident investigation committee to start analyzing the technical cause of the insufficient combustion time of the three-stage engine. Koh Jung-hwan, head of the Korea Aerospace Research Institute’s Korean launch vehicle development division, said at a post-launch briefing, “All processes, from the separation of the satellite to the separation of the satellite, were performed accurately. The cause of the engine’s end of combustion may be a lack of internal pressure in the fuel tank or a malfunction of the command to end combustion, but a conclusion can only be reached after analyzing the data sufficiently.”
According to General Manager Koh, the 7-ton liquid engine used in the third-stage engine is more difficult to develop than the 75-ton liquid engine used in the first-stage engine. This is because the fuel nozzle design conditions are demanding and harsh. The 7-ton liquid engine used in the third stage of the Nuriho was tested with 12 test engines by August 2021. A total of 93 combustion tests were performed, and the cumulative combustion test time reached 16,925.7 seconds. Nevertheless, it was a cause of failure due to malfunction. Oh Seung-hyeop, head of projectile propulsion engine development department, said, “There are about 30 types of propellant supply valves in the three-stage engine system, and there are more than 43 parts. . Experts say that it is a great achievement just to actually fly with the clustering technology that binds four 75-ton liquid engines, but pointed out that additional analysis is needed for the part where the third-stage engine did not burn for the target time. Jang Young-geun, a professor of aerospace and mechanical engineering at Korea Aerospace University, said, “If combustion suddenly stopped unexpectedly, there is a possibility that some of the valves in the engine have malfunctioned.”
However, since this experiment was successful almost to the last stage, there is a prospect that it will be able to fully compensate for this in the next retry. Director Ko said, “We were able to confirm during the flight that the attitude control of the projectile and the guidance algorithm for entering the first target trajectory were carried out accurately.” He also added, “The cause of the short combustion time of the three-speed engine can be easily found, and we will overcome it without fail to achieve perfect results in May next year.”
Reporter Kim Min-soo, Dong-A Science reborn@donga.com
Dong-Jun Seo, reporter for Dong-A Science bios@donga.com
Donga Science Correspondent Cho Seung-han shinjsh@donga.com
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