Chinese space authority announced on Thursday (27 April) that Tianzhou 1 cargo ship had successfully completed its first on-orbit propellant refuelling demonstration with the Tiangong 2 space laboratory module, marking a major milestone in the country’s 30-year quest to establish a permanent human presence in Earth Orbit.
The Tianzhou 1 cargo resupply ship was launched from the Wenchang Space Launch Centre at 19:41 CST (11:41 UTC) on Thursday 20 April. After entering an initial 200 x 380 km parking orbit, the cargo spacecraft, carrying nearly 6 tonnes of equipment and supplies including 2 tonnes of liquid propellants, performed a number of orbit elevation and refinement burns to meet the space laboratory module on a 400 km orbit. Tianzhou 1 then performed an automated rendezvous docking with the unoccupied Tiangong 2 module, and the two vehicle were fully docked at 12:23 CST (04:23 UTC) on Saturday 22 April.
Once having berthed at the space laboratory module, Tianzhou 1 switched off its docking system and propulsions. The spacecraft complex, under the control of Tiangong 2, then performed a 180° turn in azimuth direction, so that Tiangong 2 could resume its ‘normal’ flying position, with the docked Tianzhou 1 vehicle at front and the space laboratory’s engine thrusters towards back.
Tianzhou 1 carried over 100 bags of equipment and supplies, including a dummy EVA suit. However, the hatches on the docking port of both vehicles were not opened and no transfer of solid cargo was demonstrated, given that these tasks would need to be performed by a human crew. Instead, the cargo spacecraft was readied for the first demonstration of in-orbit propellant transfer.
The entire refuelling procedure took about five days to complete. Mission control first performed standard checkouts to ensure the integrity of the refuelling mechanism and detect any leakage. In order to create a difference in pressure between the supply and receiver tanks, the propellant tanks on Tianzhou 1 were maintained at a level of sufficient pressure, while receiver tanks on Tiangong 2 were maintained at a lower pressure to allow the transfer of liquid propellants. The propellants were transferred via the four fuelling nozzles (2 for fuel and 2 for oxidiser) on the docking port.
The transfer of propellants in low gravity condition is a highly complex and risky process, posing a number of technical challenges, including ensuring minimum of fluid loss in the propellant transfer line, difficulty in fluid management and vapour distributions in a tank in low gravity, the venting of large amount of vapour during the transfer of cryogenic liquid. To help understand the characteristics of liquid propellants in low gravity, a ‘glass box’ device containing liquid propellants was launched aboard the first Long March 7 test flight in June 2016, so that aerospace engineers could observe their behaviours in orbit remotely via a video link.
The first on-orbit refuelling demonstration was completed without a hitch at 19:07 CST (11:07 UTC) on Thursday 27 April. The mission has been hailed by Chinese media as a great accomplishment and major milestone, which marks the successful conclusion of the second phase of China’s human spaceflight programme (Project 921) and paves the way for the construction of a permanent space station on Low Earth Orbit (LEO) in the third and final phase of the programme by 2022.
According to its mission schedule, Tianzhou 1 will remain docked with Tiangong 2 over the next two months, with a second refuelling demonstration scheduled for late June, after which the cargo spacecraft will undock with the space laboratory module. Tianzhou 1 will then fully autonomously for at least three months, during which it will perform a number of sophisticated orbital manoeuvres and also carrying out scientific experiments with its onboard applications packages.
Then towards the end of its six-month mission, Tianzhou 1 will demonstrate a fast docking procedure with Tiangong 2, which will allow future crew and cargo spacecraft to dock with the orbital space station in 6 hours after launch. After this it will perform the third and final on-orbit refuelling demonstration. The cargo spacecraft will then undock with the space laboratory for the final time, before performing a controlled de-orbit to be burned up in the atmosphere.