CZ-5B Y3 and CZ-5B Y4 missions will launch the two space labs of the CSS between May to Sep. Wentian (WT) aka space lab I will be the first to be launched.

WT space lab is divided into the work segment, air lock segment, and resource segment. The work segment house multiple scienctific experiment racks, carrying scientific payload to carry out experiments in space by scientists. In addition it has three sleeping cabin and one toilet. In addition to the three sleeping cabin and one toilet in the Tianhe core module, CSS will have a total of six sleeping cabins and two toilets. In comparison, ISS has a total of eight sleeping cabins.

Three sleeping cabins and one toilet in the Tianhe core module.

Sleeping cabin with a window in Tianhe core module.

There're two purposes for the WT space lab to be equipped with additional sleeping cabin and toilet. The first purpose is to allow six crew to stay in the CSS during crew rotation. In 2022, SZ-15 will perform the first crew rotation with SZ-14 on CSS.

Note that the hatch door connecting the utility node to Tianhe core module and space lab are bi-directional air tight hatch, allowing either or both side of the hatch to be in vacuum state.

During the third EVA, Ye Guangfu lock the hatch door connecting Tianhe core module to utility node.

The second purpose of the extra cabins and toilet is for the WT space lab to serve as backup living quarter during emergency, when the Tianhe core module lost pressure and become vacuum due to accident such as hitting by space junks.

The emergency refuge onboard the CSS is the WT space lab.

After closing the hatch, the utility node will remain accessible to the WT space lab, while seal to Tianhe core module. It will allow astronauts access to the SZ spacecraft. The astronauts will be able to continue living in the CSS staying in the WT space lab.

If the Tianhe and SZ spacecraft are disabled, the astronauts will stay in WT space lab awaiting rescue.

Serving as refuge during emergency, the WT space lab is equipped with backup of all management system, maintenance system, recycle system, control system, and life support onboard Tianhe core module. It can take over the control of CSS after Tianhe core module is disabled.

Control moment gyroscopes onboard the Tianhe core module.

Work segment of the WT space lab, the redundant control moment gyroscopes are in the A section. B section is the sleeping cabins and toilet. D to F sections house scientific experiment racks.

Unlike the air lock for EVA on the utility node that open on top.

The WT space lab has a air lock segment open at the bottom facing the Earth. The purpose is the stay out of the sun light.

The air lock segment will serve as the backup of the EVA air lock on the utility node. If either air lock malfunction during EVA, the astronaut will enter from the other air lock.

A smaller 5 m robotic arm with 7 degree freedom that can handle 3 tons payload, is installed on the air lock segment of the WT space lab. Although it has smaller payload capacity, it has higher precision than the 10 m robotic arm on the Tianhe core module.

The smaller robotic arm can also walk around the CSS like the 10 m robotic arm.

The smaller 5 m robotic arm can combine with the 10 m robotic arm to extend the reach up to 15 m.

During the third EVA, a connector that allow both robotic arm to be combined has been installed on the 10 m robotic arm.

The smaller 5 m robotic arm can serve anywhere on the CSS, but its main purpose is to help perform exterior scientific experiments on the Mengtian (MT) space lab to be launched next.