Japanese H-IIA rocket launches GCOM-C mission

by William Graham

Japan launched the second satellite of its Global Change Observation Mission Saturday. The GCOM-C satellite lifted off from the Tanegashima Space Centre atop an H-IIA rocket at the start of a 22-minute window that opened at 10:26:22 local time (01:26 UTC).

Saturday’s launch, also lofting the Super-Low Altitude Test Satellite (SLATS), was Japan’s seventh of the year. It came two and a half months after the rocket’s previous mission delivered the fourth QZSS navigation satellite into orbit.

The Global Change Observation Mission (GCOM) is a project that is being undertaken by the Japan Aerospace Exploration Agency (JAXA) to study long-term changes in Earth’s climate and water cycle. The project’s first satellite – Shizuku, or GCOM-W – was launched in May 2012 with an expected five-year operational lifespan, and remains in service. Shizuku is dedicated to monitoring Earth’s water cycle, while the GCOM-C satellite that is being launched on Saturday will focus on climate change. Once in orbit, the satellite will be renamed Shikisai.

GCOM – photo by NEC and JAXA

GCOM-C, which is also known as GCOM-C1, is a 2,093-kilogram (4,614 lb) spacecraft that is expected to operate for at least five years. The satellite carries an imaging payload that will allow it to monitor aspects of Earth’s climate. Its images will be used to study distributions of aerosols, water vapor and clouds in the atmosphere, to monitor the color and temperature of the oceans, snow and ice cover on land and to monitor vegetation and land usage.

The Second-Generation Global Imager (SGLI) is a 400-kilogram (880 lb) instrument capable of producing visible-light and infrared images of the Earth. The instrument is a successor to the Global Imager (GLI) instrument aboard the Midori 2 (ADEOS-II) satellite, which was launched in December 2002 but ceased operations just ten months later after its power system failed.

SGLI itself consists of two radiometers – Visible Near Infrared (VNIR) and Infrared Scanner (IRS). The former covers visible light and the near infrared, while the latter operates in the mid-infrared part of the spectrum. SGLI can image the Earth at resolutions of between 250 and 1,000 meters (820 to 3,201 feet).

The GCOM-C spacecraft will be placed into an approximately circular sun-synchronous orbit at an altitude of 798 kilometers (496 miles, 431 nautical miles) and an inclination of 98.6 degrees.

JAXA initially planned to launch three GCOM-C and three GCOM-W satellites – with a new satellite in each series launching four years after its predecessor. This would allow data to be gathered over a longer period of time, with an overlap of at least a year between new satellites launching and older ones being retired, to ensure continuity of data and allow calibration of the new spacecraft. GCOM-W has now been in orbit for over five years, however, and JAXA have not yet announced any concrete plans to deploy a successor.

The Super-Low Altitude Test Satellite, or SLATS, is an approximately-400-kilogram (880 lb) miniature satellite that was deployed into a lower orbit after GCOM-C separates from the carrier rocket. SLATS, which will be renamed Tsubame – meaning Swallow – after deployment, is a technology demonstration mission that will test the use an ion engine to allow the satellite to operate in a very low orbit without re-entering the atmosphere.

SLATS – Photo from JAXA

After its initial deployment, SLATS will lower its orbit to between 180 and 268 kilometers (112-167 miles, 97-145 nautical miles) using four one-newton (0.2 pounds-force) monopropellant hydrazine thrusters. At this altitude, atmospheric drag would normally cause a satellite’s orbit to decay relatively quickly. However, SLATS will use its xenon ion engine to overcome this.

The satellite carries three experiments: the Atomic Oxygen Fluence Sensor (AOFS), the Material Degradation Monitor (MDM) and the Optical Sensor (OPS). AOFS uses eight quartz crystal microbalances to monitor the mass of polyimide film samples mounted to the satellite. The mass will decrease as the film reacts with oxygen atoms in the upper atmosphere, allowing the amount of atomic oxygen present to be calculated. MDM will use a camera to monitor material samples mounted on the underside of the satellite, studying how they are affected by exposure to space.

The Optical Sensor is a small camera which will be used to image Earth’s surface. This will allow the satellite to determine the level of improvement in imaging resolution that can be achieved by placing the satellite into a lower orbit. SLATS carries ten kilograms of xenon, with its ion engine generating between 10 and 28 micronewtons of thrust. It will require upwards of 370 watts of power, provided by the satellite’s deployable solar panels which are capable of generating upwards of 1,140 watts. The spacecraft is expected to operate for at least two years.

Saturday’s launch marked the fiftieth flight of Japan’s H-II family of rockets. Japan’s first fully-indigenously-developed liquid-fuelled rocket, the H-II first flew in February 1994. It replaced the H-I, which used a license-built version of the American Thor rocket as its first stage and Japanese upper stages. It was Japan’s fourth liquid-fuelled orbital launch system overall, but the N-I and N-II that preceded H-I also used license-built components from the American Thor and Delta rockets.

The H-2A rocket for this mission – Photo from JAXA

The original H-II made seven flights between 1994 and 1999. Its core vehicle consisted of two stages burning cryogenic propellant, with two large solid rocket motors attached to the first stage. For the rocket’s third flight, which orbited the Himawari 5 weather satellite and the recoverable Space Flyer Unit (SFU) – which was returned to Earth by Space Shuttle Endeavour during the STS-72 mission – H-II also flew with two additional, smaller, solids.

The H-II’s last two launches both failed. Its penultimate launch, which carried an experimental communications satellite – COMETS – reached a lower-than-planned orbit due to a second stage malfunction. The final flight – the only one to use the H-IIS configuration which included an upgraded second stage with an LE-5B engine instead of the earlier LE-5A – failed to achieve orbit resulting in the loss of the MTSAT-1 spacecraft. An investigation discovered that a blade in one of the first stage turbopumps, responsible for feeding liquid hydrogen into the LE-7 engine’s combustion chamber, had fractured – causing the pump to fail and the engine to shut down prematurely.

Citing cost and reliability, Japan abandoned the H-II in favor of an upgraded rocket, the H-IIA. A launch that had been planned for 2001 was canceled – its payloads were eventually flown aboard H-IIA rockets and the final rocket was placed into storage at Tanegashima.

The H-IIA first flew in August 2001, introducing an upgraded first stage powered by an LE-7A engine, and a second stage powered by the LE-5B. Like the H-II, solid rocket motors would be used to provide additional thrust at liftoff, although the H-IIA’s SRB-A motors are smaller than the H-II’s. Depending on mission requirements, H-IIA can fly with two or four SRB-A motors – the 202 and 204 configurations. Two earlier configurations that used two SRB-A motors alongside two or four US-built Castor-4AXL boosters – the H-IIA 2022 and 2024 – have since been phased out.

H-IIA Launch – Photo By JAXA

During thirty-six flights before Saturday’s launch, the H-IIA has achieved thirty-five successes. Its only failure came in 2003 during the rocket’s sixth flight, which was aiming to deploy a pair of IGS reconnaissance satellites for the Japanese government. One of the SRB-A boosters failed to separate from the rocket, which was later destroyed by range safety after it became clear that it was not going to achieve orbit.

Since its introduction the H-IIA has carried critical missions for JAXA and the Japanese government, including reconnaissance satellites, scientific, technology demonstration, communication and weather satellites. H-IIA rockets launched the SELENE (Kaguya), PLANET-C (Akatsuki) and Hayabusa 2 missions to the Moon, Venus and asteroid (162173) Ryugu. In recent years the rocket has also begun to attract interest from commercial satellite operators, making its first dedicated commercial launch in late 2015 with the Telstar 12V communications satellite aboard.

The H-IIA has been upgraded continually, with changes including more powerful SRB-A3 boosters that were introduced in place of the earlier SRB-As and enhancements to the second stage that allow it to perform missions involving extended coast periods and multiple restarts. During Saturday’s launch the stage will be called upon to make three burns.

The H-II family has also spawned the H-IIB, a more powerful rocket with a wider, twin-engine, first stage. Used exclusively to launch H-II Transfer Vehicle (HTV) spacecraft to the International Space Station, the H-IIB has made six successful launches since its introduction in 2009.

A launch with the four booster version – Photo by JAXA.

JAXA currently aims to replace both the H-IIA and H-IIB with a new rocket, H-III, which is currently under development. The H-III is expected to fly around 2020, with the H-IIB retiring in 2019 after the ninth HTV mission and the H-IIA making its final flight by 2023.

H-II launches take place from Japan’s Tanegashima Space Centre. The Yoshinobu Launch Complex consists of two launch pads: Pad 1 was originally built for the H-II and is now used by the H-IIA, while Pad 2 was built as a backup pad for the H-IIA but is used instead by the H-IIB. Saturday’s launch was the fiftieth from the launch complex and the forty-fourth from Pad 1.

Saturday’s launch used the H-IIA’s 202 configuration: which consists of a two-stage liquid-fuelled core vehicle augmented by a pair of SRB-A3 solid rocket motors, which provide additional thrust during the early phases of first-stage flight. The two liquid-fuel stages burn liquid hydrogen, which is oxidized by liquid oxygen. A type 4S payload fairing will be used to encapsulate the two payloads.

The first stage is powered by a single LE-7A engine, which ignited a few seconds before liftoff. At the zero mark in the countdown – designated X-0 for Japanese launches – the two SRB-A3 boosters ignited and the H-IIA lifted off, beginning its ascent to orbit. The solids burned for the first 91 seconds of flight, burning out at an altitude of 41 kilometers (25 miles, 22 nautical miles) before separating seventeen seconds later. After the boosters have separated, the first stage continued powering the rocket towards space.

Four minutes and five seconds into the mission, with the rocket at an altitude of 167 kilometers (104 miles, 90 nautical miles) and traveling downrange at 1.9 kilometers per second (1.2 miles per second), the payload fairing separated from the nose of the vehicle. The fairing, which protects the satellites as the rocket passes through the dense lower regions of the atmosphere, was no longer needed once the rocket reaches space and can be discarded to reduce weight.

The first stage continued to burn until six minutes and 46 seconds mission elapsed time. Eight seconds after main engine cutoff – or MECO – the stage was jettisoned. Second stage ignition took place nine seconds later, beginning the first of three planned burns for the upper stage’s LE-5B engine. This burn lasted eight minutes and eleven seconds, placing GCOM-C into its planned deployment orbit. The satellite separated seventy-five seconds after the end of the burn.

Forty-one minutes and 25 seconds after GCOM-C separated, the second stage ignited again for an eight-second burn to lower the perigee – or lowest point – of its orbit. After another 47-minute, 52-second coast, the second stage fired again – also for eight seconds – as it lowered the apogee, or highest point, of the orbit. SLATS separated from the H-IIA seventy-one seconds after the end of the third burn, at one hour, 48 minutes and four seconds mission elapsed time.

Saturday’s launch was the sixth and final H-IIA mission of the year. This represented the most flights that H-IIA has made in a calendar year, exceeding its previous highest total of four, which was achieved in both 2006 and 2014. Overall, the launch is Japan’s seventh of 2017: the country also conducted an experimental launch in January using a modified sounding rocket in an unsuccessful attempt to orbit the TRICOM-1 satellite.

The total of seven launches is the most that Japan has conducted in a year, surpassing 2006’s six launches. The number could yet increase to eight with a final launch planned for next week. In a re-flight of January’s experimental mission, a three-stage version of the SS-520 rocket will be launched from the Uchinoura Space Centre carrying TRICOM-1R.

Japan’s launch was the first of up to three planned worldwide for Saturday. The second launch of the day, which was SpaceX Falcon 9 flying from California’s Vandenberg Air Force Base orbit ten Iridium-NEXT communications satellites, lifted off barely a minute after H-IIA’s launch window opens – although it was still Friday in California.

H-IIA launched at the beginning of its window and Falcon 9 lifted off on time, meaning it was the shortest time between two orbital launches for over 57 years, and within the margin of error for the shortest time ever. Two and a half hours later, China launched a Chang Zheng 2D rocket from its Jiuquan Satellite Launch Centre with a second Ludikancha Weixing spacecraft.

There had earlier been some doubt as to whether the Chinese launch was going to happen. However, this was later confirmed.

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