China’s Chang Zheng 3B/E launches ChinaSat 6D

by Trevor Sesnic

China has successfully launched the ChinaSat 6D satellite into geostationary transfer orbit Friday via their Chang Zheng 3B/E rocket. The launch took place at 12:00 UTC (8:00 PM local time) from Launch Complex 2 at the Xichang Satellite Launch Center.

The ChinaSat 6D satellite, also known as Zhongxing 6D, was deployed into orbit by the China Academy of Launch Vehicle Technology (CALT). The Chang Zheng 3B/E (CZ-3B/E) – or Long March 3B/E – rocket is the most powerful of the Chang Zheng 3 series of rockets that China uses for most of its geostationary satellite launches.

ChinaSat 6D was built by the state-owned China Academy of Space Technology and will be operated by China Satcom. Equipped with 25 C-band transponders, the satellite will provide users in China with reliable high-bit rate uplink and downlink for radio and TV stations from its station at a longitude of 125 degrees East.

The satellite is powered by two large solar arrays which charge onboard batteries. It is expected to operate for at least 15 years. 

Having been deployed successfully into geostationary transfer orbit (GTO), it will spend the initial months raising itself into geostationary orbit (GEO) and undergoing on-orbit testing.

ChinaSat 6D undergoing assembly (credit: China Satcom)

ChinaSat 6D will replace the ChinaSat 6A satellite, which was launched in September 2010 and was formerly known as SinoSat-6. ChinaSat 6A had been expected to remain in service until 2025; however, a fault with its helium pressurization system has shortened its operational life and required that a replacement be launched sooner than had been planned.

Friday’s launch is the first to deliver a new satellite for the ChinaSat constellation since the ChinaSat 6C mission, which lifted off on September 3, 2019, aboard a CZ-3B/E.

The Chang Zheng 3B/E is an upgraded version of the three-stage Chang Zheng 3B, with enhancements including a slightly larger and more powerful first stage and upgraded side-mounted boosters. The first stage measures 24.76 meters in length and 3.35 meters in diameter and is equipped with four YF-21C engines.

The YF-21C is an upgraded version of the earlier YF-20C engine. It is an open-cycle engine that burns hypergolic propellants: unsymmetrical dimethylhydrazine (UDMH) propellant oxidized by dinitrogen tetroxide (N2O4). Each engine produces about 740 kN of thrust at sea level with a specific impulse (isp) – the measure of its efficiency – of 259 seconds. In a vacuum, the engines can deliver 820 kN of thrust with an isp of 289 seconds. To steer the first stage, each engine can gimbal by ten degrees.

Chang Zheng 3B prior to integration (credit: Xie Qiyong)

Attached to the first stage are four liquid-fueled boosters, each of which are 16.1 meters long with a diameter of 2.25 meters. A single YF-25 engine – a version of the YF-20 engine modified specifically for use on the boosters – powers each of the boosters, providing 740 kN of thrust per booster with a specific impulse of 260 seconds at sea level.

With both the first stage and the four boosters firing at liftoff, Chang Zheng 3B/E is powered aloft by 5,920 kN of thrust.

Chang Zheng 3B/E’s second stage measures 12.92 meters in length and 3.35 meters in diameter. Also running on N2O4 and UDMH, this stage is equipped with a single YF-24E engine cluster. This consists of a single YF-22E main engine and four YF-23E vernier engines for roll, pitch, and yaw control.

The YF-22E is a vacuum optimized version of the YF-20, producing roughly 820 kN of thrust while the Vernier thrusters produce 47 kN of thrust each.

To allow it to reach geostationary orbit, the CZ-3B/E uses a high-efficiency third stage burning liquid hydrogen (LH2) and liquid oxygen (LOX). This is 12.38 meters long and 3.0 meters in diameter. Equipped with a pair of YF-75 open-cycle engines, the stage produces about 170 kN of thrust with 438 seconds of ISP and can fire for up to 478 seconds.

CZ-3B on the launch pad at Xichang prior to Friday’s mission (credit: Xie Qiyong)

The Chang Zheng 3B and 3B/E can also fly in a four-stage configuration with an optional hypergolic-propellant YZ-1 upper stage. This can be used to perform orbit circularization to inject satellites directly into their operational orbits and is primarily used on missions carrying Beidou navigation satellites.

The ChinaSat 6D launch is the first of two that China will carry out in quick succession, with another rocket – most likely a Chang Zheng 4C – scheduled to lift off from the Taiyuan Satellite Launch Center at approximately 18:15 UTC.

(Lead image: Chang Zheng 3B lifts off from Xichang)

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