Nearly two months after its historical on-orbit refuelling operation, the Tianzhou 1 cargo vehicle undocked with the Tiangong 2 space laboratory module and began the autonomous flight phase of its six-month mission.
The cargo vehicle, which was lofted into orbit from Wenchang Space Launch Centre by a Long March 7 launch vehicle on 20 April, docked with Tiangong 2 on 22 April. Over the next five days, the two vehicles successfully carried out the first of the three planned refuelling demonstrations.
The two vehicles remained docked over the next two months to form an orbital complex. The second refuelling demonstration, which lasted for 48 hours, concluded at 18:28 CST (10:28 UTC) on 15 June.
On 19 June, Tianzhou 1 performed the second automated rendezvous docking. Under the command from the Beijing Aerospace Control Centre (BACC), Tianzhou 1 undocked with Tiangong 2 at 09:37 CST (01:37 UTC) and retreated to a hold point about 5,000 m behind Tiangong 2. The cargo vehicle remained at its position for 90 minutes while the ground control centre checked the status of its various sub-systems.
Once the ground control gave its go-ahead, Tianzhou 1 manoeuvred to the front of the space laboratory, with its docking port pointing backwards, while Tiangong 2 also performed a 180° turn to point its docking port forwards. Tianzhou 1 then approached Tiangong 2 from the front, before docking with the space laboratory once again. The rendezvous docking procedure was completed 14:55 CST (06:55 UTC).
This is the first Chinese orbital rendezvous using a -V Bar approach, where the ‘chaser’ vehicle closes in on the target vehicle from ahead. All previous Chinese rendezvous dockings used +V Bar approach, with the ‘chaser’ vehicle flying behind the target vehicle. The Chinese state media also revealed that the entire rendezvous docking procedure was performed under the control of Tianzhou 1’s mission computer with no ground intervention. The vehicle’s docking system is also an improved Version 2.0 design featuring 118 sensors and 5 computers.
The purpose of the second rendezvous docking was to demonstrate the advanced techniques required for the assembly of the future multi-module space station. Future spacecraft vehicles will need to have the capability to locate and dock on any free docking port on the space station, which may require +/-V Bar or even R-Bar or Z-Bar approaches.
At 09:47 CST (01:47 UTC) on 21 June, Tianzhou 1 undocked with Tiangong 2 under the ground command to begin its autonomous flight. The cargo vehicle first retreated to a 120 m hold point, before flying away and establishing its owner 3-axis stabilisation on a 390 km near-circular orbit. Over the next two months, the cargo vehicle will fly autonomously on orbit to carry out the various scientific experiments, including the release of a CubSate.
Towards the end of its mission, Tianzhou 1 will demonstrate a fast (6 hours) rendezvous docking procedure with Tiangong 2, followed by the third and final on-orbit refuelling demonstration.