Tianzhou 4 cargo launch to begin busy space station year for China

by Adrian Beil

China has launched the Tianzhou 4 cargo resupply mission on Monday, May 9 at 17:56 UTC (13:56 EDT) to the Tiangong space station. The Tianzhou spacecraft flew on a Chang Zheng 7 (Long March 7) rocket from the Wenchang Space Launch Site in Hainan, China, to an approximate 389.5 x 395 km orbit with an inclination of 41.58 degrees.

The Tiangong space station is currently unoccupied, with the Shenzhou 13 mission undocking after its six-month stay on April 15. A new crew of three will follow Tianzhou 4 to the station in early June, with the final two module additions to the station planned for later this year.

Tianzhou 4

In preparation for the mission, the Tianzhou 2 craft was undocked from the forward docking port of Tiangong’s single module, Tianhe, on March 27. The Tianzhou 3 cargo craft then relocated to the forward port to open the aft port for Tianzhou 4.

Tianzhou itself is derived from the Tiangong 1 space station, which was the first Chinese space station. The Tianzhou crafts feature a service module with flight control, propulsion, and steering, and a 15m³ cargo module for payload. It is equipped with two solar panels for power.

At 17.9 m in length, 4.2 m in maximum diameter, and an on-orbit mass of up to 13,500 kg, each Tainzhou can bring up to 6.5 tons of cargo to Tiangong. It can also transfer fuel to the space station for the occasionally needed orbit reboosts and maneuvering.

A Shenzhou crew vehicle prepares to dock with a soon-to-be-expanding Tiangong space station. (Credit: Mack Crawford for NSF)

All Tianzhou launches have taken place on the Chang Zheng 7 rocket (known as the Long March 7 outside of China) from Wenchang’s Launch Complex-2.

Onboard Tianzhou 4 is all of the necessary supporting hardware, crew supplies, experiments, and propellant to enable operations of the station over the next few months as the Shenzhou 14 crew performs their upcoming mission.

The Shenzhou 14 three-person crew is set to launch no earlier than June 5 for a primary mission of not only science operations but also preparing the station for and installing the upcoming two science modules.

Those modules are Wentian and Mengtian, which are set to launch on July 24 and in October, respectively.

The Wentian module (“Quest for the Heavens”) and Mengtian (“Dreaming of the Heavens”) will both launch on the Chang Zheng 5B rocket and will provide a platform to conduct the scientific experiments planned for the station.

Overall, Tianzhou 4’s launch brings an end to the “key technology verification phase” of the modular Tiangong space station.

After this, controllers and teams will enter the “on-orbit assembly and construction phase,” which will include the installation of the science modules.

It also kicks off a busy year for Tiangong in general. In addition to the four flights in Tianzhou 4, Shenzhou 14, and Wentian and Mengtian, an additional cargo flight with Tainzhou 5 is planned for early November just before the Shenzhou-15 crew launch — which will see six taikonauts living in and working together on Tiangong for 5-10 days.

The first module of Tiangong (“Celestial Palace”) was launched in April 2021 to improve Chinese research capabilities in zero gravity as well as enable more international cooperation.

To date, only the first module, Tianhe, has been launched and presently resides in a roughly 376 x 392 km orbit inclined 41.58° degrees to the equator. After final assembly, Tiangong will have a mass of 100 tons — or 1/5 that of the International Space Station.

Chinese officials plan to operate the station for 10 years, with a possible extension of five more years after that. Presently, over 1,000 experiments are tentatively approved by the China Manned Space Agency, including biotechnology, material science, and fundamental physics.

However, another feature of Tiangong will be its co-orbiting Xuntian Chinese Survey Space Telescope (CSST). This telescope is planned to launch in 2023 and will operate close to the space station, allowing for periodic docking for maintenance. It will be able to see up to 40 percent of the sky using a 2.5 gigapixel camera.

Artist’s impression of Xuntian telescope. (Credit: CSNA)

Rocket and launch site

Tianzhou 4’s launch vehicle was the Chang Zheng 7. One of the newer launch vehicles of the Chang Zheng family, it launches from the Wenchang Space Launch Site.

Wenchang first opened in 2016 as a modern spaceport used to operate the new generation of Chinese launchers, such as Chang Zheng-5, Chang Zheng-7, and Chang Zheng-8. It permits launches out over the open ocean to protect populated areas.

The Chang Zheng 7 is one of the heavy lifters for China, with a payload capacity to low Earth orbit of 13,500 kg, to geostationary transfer orbit of 7,000 kg, and through trans-Lunar injection of 5,000 kg.

At liftoff, the rocket stands 53.1 meters tall with a diameter of 3.35 meters and features four side-mounted boosters that are 2.25 meters in diameter. Its mass is 597,000 kg at launch.

For first stage flight, six YF-100 engines use a combination of RP-1 kerosene and liquid oxygen to each produce 1,200 kN of thrust with a specific impulse at sea level of 300 seconds.

Chang Zheng 7 with Tianzhou 4 rolls out to the launch pad at Wenchang. (Credit: China Science, state-affiliated media)

Four engines are located on the boosters (one on each) and two are located on the first stage for a total liftoff thrust of 7,200 kN. This is comparable to the Falcon 9 which has 7,600 kN of thrust at liftoff.

On top of that, the second stage features four YF-115 engines with a combined thrust of 706 kN. They use the same fuel as the first stage and perform at a specific impulse of 341.5 seconds in a vacuum. This engine is also used on the Chang Zheng-6 rocket.

So far, only one launch of the Chang Zheng-7 family has suffered a failure during the XJY-6-01 mission in March 2020. While no official reason for the failure has been released, it is suspected that one of the four side boosters was the reason for the loss of the mission.

Overall, Tianzhou 4 was the 15th launch for China this year and the second from Wenchang.

(Lead image: A Chang Zheng 7 lifts Tianzhou 3 toward orbit)

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