China prepares first cargo launch to new station with Tianzhou 2

China will step up preparations for the arrival of a first crew aboard their new space station with the launch of the Tianzhou 2 cargo spacecraft with hardware and provisions to support astronauts on an initial three-month stay aboard the outpost. Liftoff is expected at 17:35:30 UTC on Wednesday, 19 May — which is 01:35:30 China Standard Time on Thursday, 20 May — aboard a Chang Zheng 7 rocket.

Tianzhou 2 marks the second flight of the uncrewed Tianzhou spacecraft, which China will use to resupply its space station, and the first such mission in support of a crew.

Tianzhou history and mission

Tianzhou 1 launched to the Tiangong 2 station in April 2017 and conducted a series of docking demonstrations and propellant transfer tests over the course of a five-month mission. This test flight was conducted after the departure of Tiangong 2’s only visiting crew – and ended with the Tianzhou spacecraft undocking and deorbiting to a destructive reentry.

The Tianzhou spacecraft is similar in function to the Progress spacecraft used by Russia (and formerly the Soviet Union) to resupply the last two Salyut stations, Mir, and the International Space Station. Cargo spacecraft play a vital role for long-duration space station expeditions by allowing provisions, replacement parts, and new scientific equipment and/or experiments to be delivered to the crew periodically. Other spacecraft fulfilling this role for the International Space Station include the US Cygnus and Cargo Dragon and the Japanese Kounotori (HTV) crafts — and formerly the European Automated Transfer Vehicle (ATV).

Like all of these spacecraft except for the Cargo Dragon, Tianzhou is designed for a one-way trip to the Chinese space station. At the end of its mission, it will be deorbited and burn up as it reenters the Earth’s atmosphere.

Tianzhou 2’s launch will be the first mission to dock with China’s new space station, following the successful launch of the Tianhe core module last month. The arrival of Tianzhou 2 and the supplies it is bearing will be the last major milestone before the first crewed launch to the station, currently slated for 10 June with the Shenzhou 12 mission.

Tianzhou 2 will dock at the aft port of the Tianhe module and will remain there through most of the three-month Shenzhou 12 expedition. It is carrying 4,690 kilograms of supplies and equipment for the station and its first crew, including a pair of spacesuits, crew provisions, and equipment for installation activities inside and outside of the outpost. The spacecraft is also carrying 1,950 kilograms of propellant, which can be used for maneuvering or transferred to the Tianhe core module.

Towards the end of the Shenzhou 12 crew’s stay aboard the space station, Tianzhou 2 is expected to undock to perform a rendezvous test. After Shenzhou 12 departs the station, Tianzhou 2 will re-dock at the forward port that Shenzhou previously occupied. Once re-docked, a propellant transfer test will be conducted. 

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Shortly after Shenzhou 13 arrives at the station in October, the crew will test the station’s robotic arm by relocating Tianzhou 2. After the robotic arm test, Tianzhou 2 will depart the space station loaded with trash and unneeded hardware for disposal.

The Tianzhou 2 mission will occur during the “Critical Technology Validation Phase” of the new space station program. This is the first of three phases of operation, lasting from the launch of Tianhe up to the departure of the second crew aboard Shenzhou 13 early next year. This will encompass checkout and preparation of the Tianhe module before the second, or “Construction Phase”, commences — during which the Wentian and Mengtain science modules will arrive. Once these have been installed, tested, and configured, the station will enter its final “Operational Phase” in 2023.

Launch

Tianzhou will launch aboard a Chang Zheng 7 (CZ-7) – also known as Long March 7 – a two-stage rocket which forms part of China’s new-generation fleet of modular rockets.

The vehicle’s first stage consists of a 3.35 meter core powered by a pair of YF-100 engines. This is surrounded by four side-mounted boosters, each with a single additional YF-100. The second stage retains the same diameter as the first stage and is powered by a single YF-115 engine.

Tianzhou 2’s launch will mark the fifth flight for the Chang Zheng 7 rocket overall and the third to use the CZ-7 configuration. The other two launches used the CZ-7A variant, which adds a cryogenic-propellant third stage to enable geostationary launches.

The first flight of the Chang Zheng 7 came in June 2016, carrying a scaled-down prototype crew capsule to support testing for future human spaceflight projects. The rocket’s second flight, ten months later, carried Tianzhou 1 into orbit. Two CZ-7A launches took place in March 2020 and March 2021 with experimental geosynchronous satellites aboard, with the 2020 launch failing to reach orbit due to an issue around the time of first stage separation. This remains the only failure for the CZ-7 series, with an investigation determining the most likely cause to be a premature cutoff of one of the four boosters, although other reports suggested the vehicle disintegrated shortly after second stage ignition.

The Chang Zheng 7 rocket that will carry out Tianzhou 2’s launch is numbered Y3, reflecting its status as the third flight of the two-stage CZ-7. CZ-7A rockets are numbered separately.

The CZ-7 is part of a wider family of rockets that China has introduced over the last few years, which will eventually replace their ageing fleet of Chang Zheng 2, 3 and 4 rockets that currently account for most of the country’s launches. The newer family of rockets are designed around a series of modular core stages: for example, the 3.35 meter core that is used as the first stage of the CZ-7 also serves as the first stage of the Chang Zheng 8 and as the side-mounted boosters for the Chang Zheng 5. A shortened single-engine version is used as the first stage of the Chang Zheng 6.

Unlike the earlier Chang Zheng rockets, which used storable and highly toxic hypergolic propellants, CZ-7’s engines burn rocket-grade kerosene, oxidised by liquid oxygen. This is a cleaner, less toxic and more efficient combination which reflects the fact the rockets are purpose-built for satellite launches, whereas the previous-generation rockets were derived from ballistic missiles where the main consideration was keeping them ready to launch at a moment’s notice for extended periods of time.

Tianzhou 2’s launch will take place from the Wenchang Spacecraft Launch Site on Hainan Island off China’s southern coast. Originally a sounding rocket launch facility, Wenchang was rebuilt as China’s fourth orbital launch site to support the country’s next-generation rockets. From its coastal location, rockets can fly out over the South China Sea, reaching a wide range of orbits without dropping spent stages over land.

Marine space closure notices have been issued confirming ~17:30 UTC launch time on May 19. There are rumor of a 6.5 hours rendezvous profile, putting docking to the CSS around midnight UTC on May 20. pic.twitter.com/Vnj4bgS8vI

— Cosmic Penguin (@Cosmic_Penguin) May 18, 2021

China nevertheless attracted criticism last month after the core stage of the Chang Zheng 5B rocket, launched from Wenchang to deploy Tianhe, was left in orbit to make an uncontrolled re-entry. While spent rocket stages being discarded in orbit is not unusual, the Chang Zheng 5B’s core stage was significantly larger, leading to concerns that debris might survive and reach the ground. In the end, the CZ-5B core re-entered on 9 May — with any debris that survived falling into the Indian Ocean. 

With the smaller CZ-7 rocket being used for Tianzhou 2’s launch, only the smaller second stage and payload will reach orbit.

The rocket was rolled out to Wenchang’s Pad 201 on Sunday, 16 May in preparation for launch. Pad 201 is one of two launch pads at Wenchang and is shared by the Chang Zheng 7 and Chang Zheng 8 rockets. The larger Chang Zheng 5 vehicles fly from Pad 101, situated 650 meters to the southwest.

Both launch pads use vertical integration away from the pad itself. Two vertical assembly buildings are located approximately 2.4 kilometers to the north. The CZ-7 was assembled atop a mobile launch platform, which was then moved into position at the launch pad along rails. An umbilical tower mounted on the launch platform hosts the cables and lines that will be used to power, fuel, and prepare the rocket for launch, while a larger service tower at the launch pad contains work platforms to provide access for final inspection and pre-launch preparations.

The Tianzhou 2 launch is scheduled to be China’s second in the space of 13 hours, following a Chang Zheng 4B launch from the Jiuquan Satellite Launch Center at 04:03 UTC on Wednesday, 19 May that deployed the Haiyang-2D ocean research satellite. China’s next launch will occur in about a week’s time, with a Chang Zheng 3B due to lift off from Xichang with the Fengyun-4B weather satellite. The next Chang Zheng 7 launch is expected in September with Tianzhou 3.

(Lead image: Chang Zheng during roll out on Sunday to the launch pad for the Tianzhou 2 mission. Credit: CASC)

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