KARI reaches orbit on second test flight of domestic Nuri rocket

The Korean Aerospace Research Institute (KARI) has conducted its second launch of the Korea Space Launch Vehicle (KSLV)-II rocket, also known as Nuri. This was Nuri's return to flight after nearly reaching orbit on its previous mission. The rocket lifted off from Launch Complex-2 (LC-2) at the Naro Space Center on June 21 at 16:00 Korea Time (07:00 UTC).

This launch was Nuri's second demonstration flight and KARI's fifth orbital launch attempt overall. On this flight, Nuri carried a 1.2 metric ton mass simulator, the 162.5 kg Performance Verification Satellite (PVSAT), four CubeSats, and a dummy CubeSat successfully to orbit.

Rocket and payload overview:

Nuri is South Korea's second-generation orbital launch vehicle. The rocket is the country's first domestically-built rocket to reach orbit. South Korea's first orbital rocket was the KSLV-I, also known as Naro-1.

Naro-1 was a joint South Korean/Russian-developed small-satellite orbital launch system. The first stage was a modified Russian-built Universal Rocket Module (URM)-1 using an RP-1/LOX RD-151 engine. Its second stage used a South Korean-built solid-fuel rocket motor. With two stages, the 144 mT, 2.9-meter diameter, and 33-meter tall rocket could take a 100-kg payload to low-Earth orbit (LEO).

Launch of Naro-1. (Credit: KARI)

Naro-1 completed its first flight on August 25, 2009, carrying the STSat-2A spacecraft. Shortly after the first stage shut down, the flight failed to reach orbit as the payload fairing did not separate. Due to the extra mass of the fairing, the second stage failed to reach orbital velocity.

The second launch of Naro-1 took place just under a year later on June 10, 2010. For this mission, the rocket attempted to take the STSat-2B satellite to orbit. However, 137 seconds after liftoff, the launch vehicle was lost. The cause of the failure is still disputed.

Two and a half years later, the third and final Naro-1 successfully reached orbit with the STSat-2C satellite. As South Korea's first rocket to reach orbit, it became the 11th and most recent country to reach orbit. After this flight, Naro-1 was retired in favor of a larger, domestically-developed launch vehicle.

Nuri is a three-stage rocket capable of lifting 1,500 kg to sun-synchronous orbit (SSO). The first stage is powered by four Jet-A/LOX KRE-075 engines. Each engine produces 735 kN of thrust at 298 seconds of specific impulse while in a vacuum. Altogether, the four engines produce 2,942 kN of thrust and burn for 127 seconds. This stage is 3.5 meters in diameter and stands 21.6 meters tall.

The second stage is powered by a vacuum-optimized Jet-A/LOX KRE-075 engine. This vacuum-optimized engine produces 788 kN of thrust with a specific impulse of 315.4 seconds. While in flight, this stage burns for 148 seconds. Compared to the first stage, the second stage is smaller, at 2.6 meters in diameter and stands 13.6 meters tall.

Nuri's third stage is the smallest and final stage for the rocket. The stage is powered by a single Jet-A/LOX KRE-007 vacuum engine. This engine produces 68.7 kN of thrust with a specific impulse of 325 seconds. During a launch, this stage burns for roughly 500 seconds and places its payload(s) into the intended orbit.

With the payload fairing, the rocket stands 47.2-meters tall, 3.5-meters in diameter, and has a mass of 200 mT.

The engines on Nuri each use a gas-generator cycle to power their turbopump. Each engine completed testing on the ground in preparation for flight onboard Nuri. To qualify the KRE-075 engine as well as several elements of the Nuri rocket, KARI launched the KSLV-II Test Launch Vehicle (KSLV-II TLV) on November 28, 2018.

The small-scale suborbital launch vehicle successfully took flight, reaching a 209-kilometer apogee, and its single KRE-075 engine burned for 151 seconds. After this test flight, KARI later qualified the KRE-007 engine for flight. Following multiple test campaigns, the first Nuri rocket was ready for its maiden launch.

After a relatively smooth countdown – besides a one-hour delay for additional checks of the rocket – Nuri took flight from LC-2 on October 21 at 08:00 UTC on its first orbital attempt. After liftoff, the vehicle successfully passed most of its flight milestones. The first and second stages performed perfectly with the fairing separating as planned.

The third stage ignited and performed well for 475 seconds into its 521-second nominal burn time. A premature engine shutdown 475 seconds into the flight caused the stage to end up just short of orbital velocity. Its 1.5 mT mass simulator still successfully separated from the third stage, reaching its target 700 km altitude.

After an extensive look at the data collected during the flight, a few abnormal events were discovered. At T+ 36 seconds, off-nominal vibrations were detected in the third-stage tank truss support structure and payload adapter. Helium then began to leak from the third stage helium tank increasing the pressure in the LOX tank.

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32 seconds later, the third stage LOX tank pressure began to drop, and the surface temperature of the upper half of the tank plummeted. Finally, at T+115.8 seconds into the flight, helium tank pressure began to fall and the LOX tank pressure rose once again. Even with the increase and decrease of the third stage LOX tank pressures, the stage was still able to operate for some time.

In December 2021, the cause of the failure was announced. Due to the increasing acceleration during flight – and therefore also increasing the buoyancy of any components in the LOX tank – an anchor for the third stage LOX helium tank failed. Due to the anchor failing, the helium tank became detached and started to move around the LOX tank. With the helium tank moving around, it damaged tanks and caused cracks leading to further helium and LOX leakage.

Eventually, the leakage became severe enough to no longer provide LOX to the KRE-007 engine. With LOX not being fed to the engine, a premature shutdown of the engine took place, ending the flight.

After the investigations were complete, changes were made to the anchor to compensate for the increased buoyancy seen during the flight. The second flight of Nuri was delayed from May 19, 2022, to mid-June 2022, to implement these changes to the vehicle.

With the lessons learned from the first flight, the second flight carried five active and two passive payloads. The largest of the payloads onboard Nuri was the 1.2 mT mass simulator. This mass simulator is similar to the 1.5 mT simulator seen on Nuri's first flight. However, on the top was a payload adaptor for the PVSAT.

PVSAT is a test satellite to verify domestically-built satellite hardware as well as Nuri's performance in flight. The satellite is relatively small, at roughly 0.93 x 0.9 x 0.89 meters in size, with a mass of 162.5 kg. Solar cells will be used to power the spacecraft. Equipped on the satellite are five domestically-built components to be tested in the space environment.

Models of the PVSAT (right) and mass simulator (left, with PVSAT mounted on top). Notice the five CubeSat bays on PVSAT. (Credit: KARI)

First is a one-kilogram Electrically-Heated Thermoelectric Generator (ETG) to generate electricity using temperature differences. This technology could be used for lunar exploration and development programs. Another instrument is a nine-kilogram Control Moment Gyroscope (CMG) to act as an actuator for high-speed attitude control. The CMG will be used to test gyroscope technology for future missions.

A 400 g hemispherical S-band antenna will be used as a redundant antenna. This antenna will be used to test the performance of an S-band antenna for telemetry/command transmissions. A Video Camera System (VCS) will be used on the spacecraft. The VCS will be used to record the ejection of the CubeSats – which will take place from the spacecraft. The captured data and video is transmitted back to Earth using the S-band antenna.

On the satellite are five CubeSat deployers, four of which hold university-built CubeSats. The largest Cubesat is the 6U STEP Cube Lab-II which has a mass of 9.6 kg. This CubeSat is equipped with an optical mid-infrared/infrared camera as Korea's first electro-optical mid-infrared multi-band Earth observation mission. The second-largest is the 3U SNUGLITE-II with a mass of 3.8 kg. This CubeSat will be used as an educational amateur radio satellite with a GPS receiver.

Next is the 3U Multi-spectral Imaging for Monitoring Aerosol by Nanosatellite (MIMAN) massing 3.7 kg. MIMAN will use multispectral imaging to monitor aerosols. Repeater Arrangement & Disaster Early View (RANDEV) is the fourth and final active CubeSat. This 3U/3.2 kg CubeSat will be used to collect images of potential hazards from volcanos, coasts, and clouds.

The final CubeSat on PVSAT was a 3U dummy satellite.

Nuri stands on LC-2 before its maiden launch. (Credit: KARI)

PVSAT was mounted onto the 1.3 mT mass simulator in early June. Soon afterward, the payloads were encapsulated in the payload fairing. With the payloads encapsulated, the third stage and payloads were transferred to be integrated with the rest of the rocket. By June 12, all preparations for the launch were completed.

Launch:

Final launch preparations begin the day before launch with Nuri's transfer to the launch site. From the assembly building, the vehicle moves at 1.5 km/h traveling a distance of 1.8 kilometers. This journey took roughly an hour and 20 minutes. After arriving at LC-2, the rocket began to be raised vertical at ~10:00 Korea time.

Over the day, connections to the launch pad and the rocket were created. Electrical, TVC, Communications, and hydraulic checks are also completed. Final preparations are completed around 21:00 Korea Time.

Launch day preparations begin at 9:00 am (T-7 hours) Korea time with inspections of the launch pad and rockets. At T-5 hours and 30 minutes, checks on the propellant tanks are completed. Two hours later, ground personnel are evacuated from the pad to begin fueling. T-2 hours before launch, propellant loading begins.

Propellant loading finished at T-1 hours and 20 minutes. At T-1 hour, the launch stand was moved to its horizontal launch position. 30 minutes before launch, the final system checkouts and the go/no-go poll are completed. The automatic launch operations (PLO) begin 10 minutes before launch.

Nuri lifts off on its maiden flight. (Credit: KARI)

At T-0, the first stage engines ignite and the vehicle lifts off. Shortly after liftoff, the vehicle begins a pitch maneuver to reach its launch azimuth of 170 degrees. Just under a minute into the flight, the vehicle reaches Mach 1.

After the propellant is depleted, the first stage shuts down approximately two minutes and seven seconds into flight. In quick succession, the first stage separates from the second stage and the second stage engine ignites.

At nearly four minutes into the flight, the payload fairing separates exposing the payloads to space. 34 seconds later, the second stage shuts down its engine and separates from the third stage. Shortly after, the third stage ignited and conducted a dogleg maneuver to reach a new azimuth of 191 degrees.

Approximately 13 minutes after launch, the third stage shuts down. Nuri successfully reached a circular orbit of 700 km inclined at 98.2 degrees, having an orbital velocity of 7.5 km/s. 100 seconds after the engine cutoff, the PVSAT separates from the rocket. Then the stage pitched to 90 degrees and – 70 seconds after PVSAT separated – the mass simulator separated.

42 minutes after launch, the PVSAT begins communicating with ground stations. After verifying the satellite is healthy, it will begin deploying the CubeSats every two days starting on June 23. On June 23, the STEP Cube Lab-II will be the first to separate starting its one-year mission. Two days later, RANDEV will separate and begin its six-month mission. Next on June 27 is the SNUGLITE-II satellite starting its one-year mission. Finally, on June 29 the MIMAN will separate beginning its six-month mission.

After this launch, KARI has a busy few months. On August 2 (EDT), a SpaceX Falcon 9 will launch KARI's Danuri lunar orbiter. This Falcon 9 will feature a flight-proven booster and will send the lunar orbiter on a trek to the Moon.

Danuri in final processing before its delivery. (Credit: KARI)

KARI will also begin to prepare Nuri for its third launch. This third launch is expected to take place in January 2023, likely carrying NextSat 2. Following this mission, Nuri has several launches planned for multiple years up to 2027.

Using Nuri, KARI will use the lessons learned to develop the tentatively named KSLV-III. KSLV-III will be KARI's next-generation rocket, designed to send around 3.5 mT to geostationary transfer orbit. This two-stage rocket will use five 100-ton thrust RP-1/LOX engines on the first stage with two 10-ton thrust RP-1/LOX engines on the second stage. Many ideas for the rocket have been proposed, including landing the first stage similar to Falcon 9. This rocket is expected to launch in the 2030s.

(Lead Photo: Nuri lifts off on its second flight, and the first to successfully reach orbit. Credit: KARI)