Who Are You, Nuriho? ... Previewing the 'Top 10 Questions' [Nuriho 2nd Launch]

On the 15th, one day before the second launch of the Korean launch vehicle Nuri (KSLV-II), Nuri is being erected on the launch pad at Naro Space Center in Goheung-gun, Jeollanam-do. Photo by Korea Aerospace Research Institute

[Asia Economy Reporter Kim Bong-su] On the afternoon of the 16th, Korea's first indigenous space launch vehicle, Nuriho (KSLV-II), will be launched for the second time. Since 2010, a total of 1.9572 trillion KRW has been invested in the 'largest space development project since the era of Dangun,' attracting the attention of the entire nation. We have addressed the questions surrounding the second launch of Nuriho.

◇ Nuriho, Who Are You?

Nuriho is also known by its English abbreviation KSLV-II, which stands for 'Korea Space Launch Vehicle-II.' It means Korea's second space launch vehicle. It is called the 'second' because the first, Naroho (KSLV-I), was successfully launched in 2013. However, since Naroho imported its first-stage engine from Russia, it is not considered a 'Korean-type launch vehicle.' Nuriho was officially developed from 2010 with an investment of 1.9572 trillion KRW. A total of three units were produced, with the first and second launches conducted, and the third unit reserved as a 'spare.' If the second launch fails, it is undecided whether the spare unit will be used to complete the mission or if it will be used as the first vehicle for a separate advancement project starting next year.

Nuriho is a three-stage space rocket capable of placing a medium-sized satellite weighing 1.5 tons, equivalent to a mid-sized car, into low Earth orbit (600?800 km). The first stage at the bottom consists of four 75-ton class liquid engines combined to produce 300 tons of thrust. The second stage has one 75-ton class liquid engine, and the third stage uses a 7-ton class liquid engine. The total length is 47.2 meters, equivalent to a 15-story apartment building, weighing 200 tons when fueled (20 tons when empty), with a maximum diameter of 3.5 meters.

◇ Truly Domestic Technology

Korea began its history of domestic launch vehicles in the early 1990s by developing the single-stage solid science rocket KSR-I. This was followed by the two-stage solid science rocket KSR-II, the first liquid-propelled science rocket KSR-III, and then the Naroho, developed jointly with Russia, marking the start of serious space launch vehicle research. At that time, the Korea Aerospace Research Institute (KARI) acquired technology from Russia and succeeded in independently developing a 30-ton class liquid engine. A famous anecdote is that during the Naroho development announcement, Russia handed over a 'model engine' for the first stage, which turned out to be an actual 170-ton class staged combustion cycle Angara engine. This served as a reference during the development of Nuriho's 75-ton liquid engine. Some have questioned whether Nuriho can be considered 'domestic technology' due to this. However, experts argue that Russia did not actively transfer technology at the time, and the technology and real rocket engines obtained were only through indirect observation, so it is unfair to belittle Nuriho's domestic technology development. A person involved in Naroho's development said, "Russia took KARI engineers on factory tours but did not allow access to core technologies or critical parts for security reasons. They were surprised that we managed to implement things ourselves that we had only seen and heard from them without hands-on experience." Former senior researcher Lee Chun-geun of the Korea Institute of Science and Technology Policy, an expert in launch vehicles, said, "No country in the world has built a space launch vehicle with 100% domestic technology. Even Russia and the U.S. referenced German V2 rocket technology. Even if there were blueprints or consultations, the nature of the technology makes actual implementation very difficult. Although it took some time, our engineers succeeded in building Nuriho from the engine to the propellant tanks from start to finish, which can only be called 'domestic technology.'"

The Korean launch vehicle Nuriho (KSLV-II) is being moved from the assembly building to the launch pad at Naro Space Center in Goheung-gun, Jeollanam-do on the 15th. Photo by Korea Aerospace Research Institute

◇ Remaining Procedures on Launch Day?

The Nuriho vehicle for the second launch was moved to the second launch pad at Naro Space Center as of the afternoon of the 15th, erected vertically, and is undergoing final inspection with the umbilical connected. The launch is scheduled for around 4 p.m. on the 16th. On the morning of the launch day, the final launch time will be determined considering weather conditions such as wind speed, clouds, lightning over Naro Island, and the possibility of collision with space objects. If postponed, a one-week backup period will be set starting the next day. Currently, Nuriho has been fully assembled in the launch vehicle assembly building at Naro Space Center, transported to the launch pad using a transporter, erected vertically, and fixed on the launch pad. The umbilical, which functions like an umbilical cord, is connected to conduct comprehensive checks of fuel and electrical systems.

On launch day, fuel and oxidizer will be loaded starting four hours before launch, and if the final inspection shows no issues, the countdown and automatic launch operation will begin 10 minutes before launch. At launch time, the first stage's 300-ton class engine (four 75-ton engines) will ignite, and the hold-down clamps will be released. After a few seconds, once sufficient thrust is generated, the vehicle will lift off, the umbilical plate connecting it to the launch pad will separate, and the rocket will ascend.

◇ Post-Launch Process and Debris?

The launch of Nuriho is overseen by the Mission Director Center (MDC) within Naro Space Center. To track the launched Nuriho, tracking radars and telemetry antennas are installed and operated at Naro Space Center and Jeju Island. Tracking in the latter part of the flight is handled by telemetry antennas installed in Palau, Philippines. Notably, Naro Space Center operates tracking radars capable of tracking the launch vehicle up to 3,000 km to obtain real-time location information, and telemetry equipment that can monitor flight trajectory and operational status up to 2,000 km. Nuriho will fly approximately 100 km away from Jeju Island and Japan's Fukue Island. The first stage is expected to fall about 413 km from the launch site, the second stage about 2,800 km away, and the fairing about 1,514 km offshore. On launch day, land within a 3 km radius around the Nuriho launch pad will be restricted. At sea, a 24 km wide and 78 km long area along the flight path will be closed to all vessels. In the air, a no-fly zone 44 km wide and 95 km long along the flight path will be established.

◇ Criteria for Success

For Nuriho to be considered successful, the engines must ignite normally and liftoff as scheduled, and the separation of each stage and the fairing must occur at the correct time and altitude. Approximately 127 seconds after launch, the first stage separates at an altitude of 57 km; 233 seconds after launch, the fairing separates at 191 km; and 274 seconds after launch, the second stage separates at 258 km. Most importantly, the performance verification satellite and satellite mock-up must enter orbit. About 15 minutes (exactly 897 seconds) after launch, at the target altitude of 700 km, the performance verification satellite will separate, followed by the satellite mock-up about 70 seconds later. If all proceeds normally, KARI engineers plan to analyze data for about 30 minutes and then officially declare the launch a success.

◇ Differences Between the First and Second Nuriho Launches?

During the first launch, the third stage's 7-ton liquid engine shut down about 46 seconds earlier than expected, resulting in insufficient thrust at the final moment and failure to reach the target orbit with the satellite mock-up. Subsequent investigation revealed a crack in the oxidizer tank of the third stage engine, causing fuel leakage. The cause was a faulty helium tank fixing device that loosened during flight. Consequently, the second launch schedule was postponed by about a month from mid-last month. After about two months of root cause analysis, KARI began redesigning and verifying parts in January. To reinforce the structure of the first and second stages, the oxidizer tank design was changed to a fixed support type. They also verified the tanks by submerging them in liquid nitrogen. Repairing the already assembled third stage was difficult, requiring disassembly and reassembly. KARI Launch Vehicle Project Manager Ko Jung-hwan explained, "We had to open the oxidizer tank lid and enter it to replace parts. There were many other components inside, making it a complicated task, but all replacement and reassembly work was completed by the end of April."

Unlike the first launch, which carried only the satellite mock-up, the second launch will carry a performance verification satellite including four CubeSats and the satellite mock-up together into orbit. The performance verification satellite, specially made to test Nuriho's performance, weighs about 200 kg, and the satellite mock-up weighs 1.3 tons. When Nuriho reaches an altitude of 700 km, the performance verification satellite will separate first, followed by the satellite mock-up. Two hours later, the performance verification satellite will maintain continuous communication with ground stations, and four hours later, the satellite's attitude information will be confirmed at the Antarctic King Sejong Station. On the following day, more precise status checks will be conducted through smooth communication with ground stations. One week after launch, the performance verification satellite will deploy the four CubeSats it carries inside.

The Korean launch vehicle Nuriho (KSLV-II) is being moved from the assembly building to the launch pad at Naro Space Center in Goheung-gun, Jeollanam-do on the 15th. Photo by Korea Aerospace Research Institute

◇ Performance Verification Satellite and Four CubeSats Also in the Spotlight

The performance verification satellite not only serves as a payload but also conducts various scientific experiments and performance tests. It carries a thermoelectric generator (ETG) produced by the Korea Atomic Energy Research Institute, weighing less than 1 kg and measuring only 85 mm in diameter and 125 mm in height. This advanced product generates electricity using temperature differences and is planned for use in future lunar landing explorations. It also includes a control moment gyro (CMG) developed by the private company JusTech, which is essential for attitude control of space launch vehicles such as satellites and probes and is part of Korea's core space technology development projects. Additionally, an S-band antenna for communication between the satellite and ground stations in space is attached to the performance verification satellite to test its performance. This is also a critical technology under the core space technology development project. The satellite also carries a CubeSat deployment mechanism and a Video Camera System (VCS) for imaging and transmission.

The CubeSats housed inside the performance verification satellite, which will be sequentially deployed, are also of interest. They include Korea's first multi-band Earth observation satellite (STEP Cube Lab-II) with electro-optical, mid-infrared, and long-infrared sensors developed by Chosun University; 'SNUGLITE-II,' a satellite from Seoul National University that collects Earth atmospheric data using precise satellite navigation system (GPS) carrier signals; KAIST's hyperspectral camera Earth observation satellite RANDEV; and Yonsei University's fine dust monitoring satellite MIMAN. These CubeSats will be sequentially deployed from the performance verification satellite starting on the 23rd at two-day intervals to begin their missions.

◇ Performance of the 75-ton Liquid Engine

The four 75-ton liquid engines mounted on the first stage measure 1.9 meters wide and 3.0 meters tall. The second stage carries a slightly larger engine of the same output, measuring 2.2 meters wide and 4.0 meters tall. Since only one engine is mounted on the second stage, it ensures more stable thrust. Initially, the engine was 25% heavier than the current version but was later lightened. Before the first launch in October last year, a total of 33 engines were produced and tested 184 times for a total of 18,290 seconds to increase reliability. However, in 2017, unstable combustion caused vibrations, which were resolved after six months of research. This 'monster' consumes 1 ton (1,016 kg) of oxidizer and fuel per second. It uses kerosene fuel at 314 kg per second (equivalent to two 200-liter drums), and during the 130 seconds of first-stage operation, about 260 drums of fuel are consumed. The combustion pressure reaches 60 times atmospheric pressure (60 bar), and the combustion gas temperature reaches 3,500 degrees Celsius. The oxidizer, liquid oxygen, used to burn kerosene is cryogenic at minus 183 degrees Celsius.

◇ Criteria for Being Among the World's Top 7 Space Powers

The criterion is whether a country possesses a medium-class launch vehicle capable of placing commercial satellites weighing over 1 ton into low Earth orbit. Currently, nine countries have space launch vehicles: the United States, Russia, China, Japan, the European Union (EU), India, Iran, North Korea, and Israel. However, the launch vehicles possessed by Iran, North Korea, and Israel have capabilities limited to about 300 kg. Therefore, if Korea's second Nuriho launch succeeds, Korea will become the seventh country in the world capable of independently launching practical satellites. Moreover, Korea has independently developed all core technologies, including space launch vehicle engine development facilities, large propellant tank manufacturing technology, and advanced launch pad construction, covering design, manufacturing, testing, and launch operations.

◇ The White Smoke at Liftoff Is 'Water Vapor'

At the moment of liftoff, a huge white cloud of smoke billows from the engine area. This is water vapor. When the first stage engine ignites, the combustion gas temperature reaches about 3,500 degrees Celsius, emitting extremely high heat. To cool this, the launch pad's lower section sprays large amounts of water. The water sprayed collides with the hot combustion gases, generating massive amounts of water vapor. Also, white powder scattering around at liftoff is visible. This is ice. Because the propellant temperature inside the vehicle reaches minus 180 degrees Celsius, the gases cool and form ice. The vibration at liftoff causes this ice to break and scatter.