China’s Chang Zheng 3B launches Fengyun 4B weather satellite

by William Graham

China launched a new geostationary weather satellite on Thursday June 3 local time, following a 24 hour delay. The Fengyun 4B satellite lifted off from the Xichang Satellite Launch Center at 00:17 China Standard Time (16:17 UTC on Wednesday June 2), aboard a Chang Zheng 3B/G3 carrier rocket.

Fengyun, meaning “wind and cloud,” is a series of meteorological satellites which provide China and the international community with data for terrestrial and space weather forecasting. The program includes both polar-orbiting and geostationary satellites, with the Fengyun 4B (FY-4B) satellite to be part of the geostationary element of the constellation. Fengyun satellites are operated by the National Satellite Meteorological Center, part of the China Meteorological Administration.

The first two Fengyun spacecraft, FY-1A and 1B, were deployed by Chang Zheng 4A rockets in 1988 and 1990 respectively. These were developmental versions of the low Earth orbit satellite, with FY-1A failing a little over a month after launch due to attitude control issues, and FY-1B operating for its planned two-year period.

The operational FY-1C and FY-1D satellites were launched by Chang Zheng 4B rockets in 1999 and 2002, with FY-1C later serving as the target for China’s 2007 anti-satellite weapons test, having ceased operations in 2004.

Alongside the FY-1 satellites, China developed their first generation of geostationary satellites, Fengyun 2. The first of these satellites was lost in a pre-launch accident in 1994, with the successful launch of Fengyun 2A taking place three years later aboard a Chang Zheng 3 rocket. Eight satellites were launched between 1997 and 2018, of which the last three remain in service.

The Fengyun 3 and 4 satellites represent a second generation constellation, replacing the FY-1 and FY-2 series respectively. Fengyun 3 was introduced first, with FY-3A riding a Chang Zheng 4C to orbit in May 2008, with three more FY-3 spacecraft following it in 2010, 2013 and 2017.

Long March 3B launches the Fengyun-4A satellite in December 2016 – via Xinhua

While FY-4A has since been retired, the rest of these satellites are still in service. The first new geostationary satellite – FY-4A – was deployed in December 2016 as a pathfinder for the second-generation spacecraft and remains in operation. The FY-4B satellite will be the first operational launch of the second-generation geostationary satellite.

The Fengyun 4 satellites are being constructed by the Shanghai Institute of Spaceflight Technology, based around the SAST-5000 platform. Unlike the spin-stabilized Fengyun 2 series, Fengyun 4 employs three-axis stabilization to maintain orientation. Each satellite has a mass of approximately 5,300 kilograms (11,700 lb), and is designed to operate for at least five to seven years.

The FY-4B satellite carries the same suite of four instruments as FY-4A, which will gather data to help forecasters studying weather systems on Earth as well as helping to study and predict space weather.

Fengyun 4B’s primary instrument, the Geostationary Interferometric Infrared Sounder (GIIRS) provides a new capability over the Fengyun 2 series. This is a vertical sounding instrument with a Michelson interferometer, operating in visible-light, short to mid infrared, and longwave infrared channels. It is designed to measure temperature, humidity, and cloud cover and composition to build a three-dimensional map of atmospheric conditions. The GIIRS instrument aboard Fengyun 4B has been redesigned compared to the instrument on Fengyun 4A, providing increased resolution over the pathfinder satellite.

The Advanced Geosynchronous Radiation Imager (AGRI) is a successor to the S-VISSR spin-scan radiometer aboard the Fengyun 2 series. AGRI uses an off-axis telescope with two scan mirrors to image the Earth across fourteen spectral bands. These include three visible and near infrared bands, three shortwave infrared bands, two midwave infrared bands, two bands in the absorption spectrum of atmospheric water vapor, and four longwave infrared bands. Overall AGRI can image at wavelengths between 0.45 and 13.8 microns, with a peak resolution of 500 meters in the visible-light band.

Render of the Fengyun-4B satellite – via the Chinese Meteorological Administration

By producing images at different wavelengths, AGRI can highlight different features in the atmosphere or on the Earth’s surface that might be relevant to forecasters. This includes aerosols, cloud and fog cover, vegetation, snow, fire, and water vapor in the atmosphere. Longwave infrared images can also be used to monitor the temperature of the ocean surface. AGRI takes about fifteen minutes to produce a full-disc image of the Earth and can also produce images of smaller regions with scan times between one and five minutes.

The Lightning Mapping Imager (LMI) is a near-infrared CCD imaging system which will continuously monitor the Earth for lightning. With a resolution of up to 6.8 kilometers, LMI will record the number and intensity of lightning flashes between clouds and the ground, between different clouds, and within individual clouds. Depending on the time of year, LMI will be used to study storms over mainland China or over the Indian Ocean.

The Space Environment Package (SEP) consists of a high energy particle detector, radiation dosimeter, fluxgate magnetometer, and a surface charging sensor. These will be used to help study the impacts of space weather with the Earth, including geomagnetic and heliophysics interactions.

As well as gathering data with its own instruments, Fengyun 4B hosts a data collection system that will receive and relay data from remote ground research stations – both in China and surrounding regions within the satellite’s field of view.

An image captured by the Fengyun-4A satellite in February 2017 – via the Chinese Meteorological Administration

China used a Chang Zheng 3B/G3 rocket to carry Fengyun 4B into orbit. Also known as the Long March 3B/G3, this is a three-stage rocket that forms part of the legacy Chang Zheng family of vehicles originally derived from the Dongfeng 5 missile.

This family, which includes several configurations of the Chang Zheng 2, 3 and 4 vehicles, currently accounts for most of China’s orbital launches – although in the long term they are expected to be supplanted by newer designs such as the Chang Zheng 5, 6, 7 and 8.

The Chang Zheng 3 subfamily are specialized for launching geosynchronous satellites, incorporating a cryogenic upper stage. Development began in the late 1970s as a three-stage version of the existing Chang Zheng 2 vehicle, with the first Chang Zheng 3 rocket flying in January 1984 but failing due to a problem during third stage flight. The first successful launch was made a little over two months later. The stretched Chang Zheng 3A, which also featured a widened upper stage, made its debut in February 1994, doubling the rocket’s payload to geostationary transfer orbit.

In the 1990s, China had ambitions of winning a large share of the commercial satellite launch market. Even with its upgrades, the CZ-3A was not powerful enough to carry most of the contemporary geostationary satellites, so the Chang Zheng 3B was developed as well. This added four liquid-fueled boosters to the rocket’s first stage, as well as a further stretch to the second stage of the vehicle.

Chang Zheng 3B made its maiden flight in February 1996, however as soon as the rocket left the ground it was obvious that something was wrong. Before it had even cleared the tower, the CZ-3B pitched over. After flying horizontally, it pitched down with its engines still firing and struck the ground about 1.9 kilometers away from the launch pad.

The impact site was close to Xichang’s main gate. Official reports indicated that six people were killed and another 57 injured in an accommodation area close to the gate, however western reporters have estimated that the true death toll may have been much higher. The failure more or less ended China’s aspirations in the commercial launch industry, as the US State Department refused to grant further export licenses for satellites with any American components to China, after concerns that US firms participating in the investigation into this failure, as well as a Chang Zheng 2E failure a few years beforehand, had contributed to China’s development of more reliable missile systems.

Long March 3B lofts a pair of BeiDou-3 navigation satellites in December 2019

Chang Zheng 3B returned to flight in August 1997, making two flights that year and two in 1998 to fly out the remaining US-built satellites that had already been authorized to launch. The rocket would then not fly again until 2005 when it launched APStar-6, a Chinese-owned communications satellite build by European manufacturer Alcatal Alenia Space (now Thales Alenia Space) without any US-sourced components to avoid running afoul of the export ban. Further commercial launches, with Thales-manufactured satellites and an increasing number of Chinese-built spacecraft, followed.

In recent years, the CZ-3B design has undergone a series of upgrades aimed at increasing its payload capacity. The first upgrade, CZ-3B/G2, was introduced in 2007 incorporating stretches to the first stage and boosters. Another version, CZ-3B/G3, made its debut in 2011 and has been primarily used for Chinese government launches.

Two other versions have made small numbers of launches: the CZ-3B/G1 was used to launch two pairs of Beidou-2 navigation satellites in 2012, although subsequent Beidou launches have used G3 rockets with an additional YZ-1 third stage, while the CZ-3B/G5 was used for a launch to sun-synchronous orbit last year with the Gaofen 14 satellite.

The final member of the Chang Zheng 3 series is the Chang Zheng 3C. This is a modification of the Chang Zheng 3B, with two boosters instead of four to optimize it for smaller payloads.

Wednesday’s launch was the seventy-fifth for the Chang Zheng 3B and the 132nd overall for the Chang Zheng 3 family. Of its previous seventy-four missions, the Chang Zheng 3B has completed seventy successfully, with two failures – including the maiden flight – and two partial failures which reached lower-than-planned orbits but where the payloads could be salvaged.

The most recent failure – a total loss – occurred during last year’s launch of the Palapa-N1 satellite. The cause of the failure has not been formally announced, however it has been indicated that the anomaly occurred during second and third stage separation, or at third stage ignition. Seven CZ-3B rockets have flown successfully since the failure.

For Wednesday’s launch of Fengyun 4B, the CZ-3B rocket flew from Pad 2 of the Xichang Satellite Launch Center in China’s Southwestern Sichuan Province.

Xichang was first used for orbital launches in 1984, with the introduction of the Chang Zheng 3. The site’s southern location compared to other launch sites at Jiuquan and Taiyuan makes it better suited to geostationary launches, where a satellite needs to end up orbiting over the equator.

A jettisoned Long March 3B booster after landing downrange from Xichang

The center’s inland location, however, is not ideal for launching satellites as debris from the rocket – including spent boosters and discarded payload fairings – can fall into populated areas downrange of the launch site. Photos or videos of these falling onto or around houses and other buildings have not been uncommon following launches from Xichang.

Launches from Xichang will slowly be phased out as China replaces the Chang Zheng 3 family with newer rockets. The Chang Zheng 5, 7 and 8 rockets, which are expected to take over geostationary missions, fly from the Wenchang Satellite Launch Center on Hainan Island. This site is coastal – meaning debris will fall into the sea instead of on land – and closer to the equator than Xichang.

Xichang has two launch pads for large orbital rockets, which can be used by Chang Zheng 2, 3 or 4 rockets. Pad 3 was originally built for the Chang Zheng 3, and was later used by CZ-2C vehicles. Between 2005 and 2007 this complex was completely rebuilt for the Chang Zheng 3A, with further upgrades made around 2014 allowing it to host CZ-3B and 3C launches. A small number of Chang Zheng 2C rockets, and a single Chang Zheng 4C, have flown from the pad since refurbishment.

Pad 2, which will be used for the Fengyun 4B launch, was built in the late 1980s. It was initially used by the Chang Zheng 2E rocket, a modified version of the Chang Zheng 2D with boosters and a solid-fuel upper stage which was designed to fly commercial missions prior to the introduction of the Chang Zheng 3B. At their introduction, the Chang Zheng 3A, 3B and C3 flew exclusively from Pad 2, with the former moving to Pad 3 following its 2007 rebuild. Since the most recent upgrades to Pad 3, all three configurations can fly from either pad.

Because of its heritage as a descendent of the Dongfeng 5 missile, Chang Zheng 3B’s first and second stages are fueled by hypergolic propellants: unsymmetrical dimethylhydrazine and dinitrogen tetroxide. The first stage is powered by a YF-21C powerplant, consisting of four individual YF-20C engines built into a single module. Each of the four boosters clustered around the first stage has a single YF-25 engine, a derivative of the YF-20 which burns the same propellant mixture.

Chang Zheng 3B/G3’s second stage is powered by a YF-24E unit, consisting of a YF-22 main engine – a YF-20 variant optimized for high-altitude operation – and a YF-23 vernier engine with four combustion chambers that can be gimballed to provide attitude control. The rocket’s third stage burns liquid hydrogen and liquid oxygen in its twin YF-75 engines.

Wednesday’s launch delivered Fengyun 4B into a geostationary transfer orbit (GTO), an intermediate elliptical trajectory from which the satellite will maneuver itself into its final geostationary orbit.

The Fengyun launch was China’s sixteenth mission of 2021 and the third for the Chang Zheng 3B. China has several further launches scheduled later in June, including the Shenzhou 12 mission which will carry the first crew to China’s new space station. The next launch from Xichang is likely to be a Chang Zheng 2C flight, also targeting the middle of the month.

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