Nuri: reaching orbital speed takes the whole ascent

Nuri, or KSLV-II, is a three-stage launcher developed through Korean design, manufacturing and testing work. It followed the experience of Naro but is not simply a renamed example of the earlier two-stage vehicle. Its first stage clusters four liquid engines; the second uses one engine of the same thrust class, and the third uses a smaller liquid engine.

KARI’s engine account describes extensive domestic development while acknowledging some imported components. That is more precise than assuming that national development means every bearing or material came from the same country. Nuri also required suitable test facilities and changes at the launch site. Its larger vehicle and different propulsion system could not be supported merely by changing the name on Naro’s ground equipment.

Nuri’s first test flight on October 21, 2021 reached its target altitude but failed to place its simulated satellite in orbit. KARI’s result account identifies an early third-stage engine shutdown and insufficient velocity. The second flight, on June 21, 2022, achieved orbital insertion — open definition. These dated tests illustrate why reaching space and establishing the intended orbit are different outcomes.

At a chosen altitude, a vehicle needs an appropriate velocity and direction to follow the desired orbital path. A high point alone does not establish that path. During ascent, successive stages accelerate the remaining vehicle after earlier hardware is discarded. An underperforming final stage can therefore prevent the intended insertion even when much of the preceding sequence worked.

The May 2023 third flight carried NEXTSat-2 and small secondary payloads. It represented a different payload task from the first flight’s simulator. A simulator can test launch performance without doing the same science or applications as an operational spacecraft. South Korea’s lunar exploration is a separate program, represented by Danuri. Conversely, carrying a real satellite does not by itself establish that every payload has completed its later mission.

In a fictional planning exercise, a launcher’s allowed payload allocation — open definition is 1,500 kilograms. The spacecraft account for 1,200 kilograms and their deployment equipment for 100, leaving 200 kilograms within that assumed allocation. These teaching values are not a performance prediction for Nuri. Orbit, configuration and operating constraints determine real capability. Following both the physical ascent and the payload accounting makes Nuri’s development easier to understand than judging a launch only by the height of its flame.