China is another step closer towards its ambition to establish a permanent human presence in earth orbit, with the country’s second Space Laboratory module Tiangong 2 lofted by a CZ-2F (Long March 2F) launch vehicle on Thursday 15 September. The rocket carrying the space module lifted off at 22:04 CST (14:04 UTC) from Pad 921 at the Jiuquan Satellite Launch Centre in northwest China, following a two-month launch campaign that began in early July.
Designed and built by China Academy of Space Technology (CAST), the Tiangong 2 mission aims to achieve three key objectives: to demonstrate medium-term orbital living (2 men for 30 days), to demonstrate in-orbit cargo and propellant resupply, and to carry out large-scale space science and applications experiments. The entire Tiangong 2 programme consists of four orbital launch missions: the test launch of the CZ-7 (Long March 7) launch vehicle in June 2016, the launch of the Tiangong 2 module in September 2016, the Shenzhou 11 crewed flight mission in October 2016, and the Tianzhou 1 cargo resupply test mission in early 2017.
Preparation for the Launch
The launch of Tiangong 2 came almost exactly five years after the launch of the first Space Laboratory module Tiangong 1 in September 2011, though the airframe of a second module was constructed the same time as Tiangong 1 to serve as a backup. Following the successful launch of Tiangong 1, the backup vehicle was developed into an improved Space Laboratory module for the follow-up missions. To ensure the ‘new’ space module was still qualified for orbital flight, engineers of CAST carefully assessed the conditions of nearly 300 components and parts on the space module, either putting them through a life-extension process or replacing those that no longer met the requirements.
The launch campaign began with the arrival of the Tiangong 2 module at the launch centre on 9 July, after a two-day train journey from its fabrication facility at the Beijing Space City where it was stored for the past five years. Two CZ-2F launch vehicles for the Tiangong 2 and Shenzhou 11 missions were delivered to the launch centre by railway in early August.
Upon its approval, the Tiangong 2 module was examined and assembled inside the Spacecraft Non-Hazard Operation Building, before being moved to the Spacecraft Hazard Operation Building for the irreversible process of fuelling with liquid propellants and loading of pressurised gases. The completed spacecraft inside its payload fairing was rolled to the Launch Vehicle Vertical Processing Building, where the CZ-2F launch vehicle was assembled. The spacecraft inside the payload fairing was then hoisted onto the top of the launch vehicle for payload integration.
On 9 September, the CZ-2F/Tiangong 2 launch vehicle stack sitting vertically atop the Mobile Launcher Platform (MLP) was rolled out to Pad 921 situated 1.5 km away from the processing building. Once the launch vehicle stack arrived at the pad, the rotating platforms of the umbilical tower were swung to ‘embrace’ the vehicle to allow the final checkout procedure to be conducted. After the mission managers gave a Go to the launch, fuelling of the CZ-2F launch vehicle with liquid propellants began in the morning of 14 September, with the launch window set at 22:04 CST (14:04 UTC) on 15 September.
Tiangong 2 and its predecessor Tiangong 1 have been referred to by official Chinese writings as “Space Laboratories” – temporarily-manned, single-module, Earth-orbiting spacecraft that can be visited and tended by astronauts on a short-term basis and fly in an automated mode between visits. A Space Laboratory module is comparable in size to an individual module of a larger permanent space station such as Mir and the International Space Station, but is fitted with self-contained life-support, propulsion, and flight control systems for autonomous flight.
The China Manned Space Programme (Project 921) has planned to launch a number of 8,000 kg-class Space Laboratory modules during the second phase of the programme to develop all the the techniques and technologies required for building a ‘real’ permanent space station. These missions would help Chinese space engineers to gain valuable insights into the difficulties facing orbital flight and living for a prolonged period of time, and validate the design of the various systems required for the future space station. Originally a total of 3 missions were planned but they have been subsequently reduced to 2 missions as the programme evolved.
The Tiangong 2 module is almost identical to its predecessor in size and appearance, consisting of two cylinder-shaped sections: a habitable Experiment Compartment serving as the main living quarters and laboratory for the crew; and an inhabitable Service Compartment that houses propulsion, power, life support, and communications systems. A pair of solar wings each with 4 solar panels are attached to the Service Compartment. Visiting astronauts can enter the Experimen Compartment via the hatch on the docking port at the front end of the space module. There are two windows on either side the Experimental Compartment for observation of the outside.
The space module is 10.4 m in length and 3.35 m in diameter, with an orbital mass of 8.6 t and a habitable internal volume of 14 cubic metres (2.0 x 1.8 x 4.0 m). The module is connected to the visiting Shenzhou and Tianzhou spacecraft via an androgynous docking mechanism developed by Shanghai Academy of Spaceflight Technology (SAST). The system is believed to have derived from the Russian APAS-75, consisting of a docking port, radio beacons, transponders, communication antenna, UHF radar, laser rangefinder, and electro-optical tracking system. The inside diameter of the docking port is about 0.8 m.
Despite being same in size and mass to its predecessor, Tiangong 2 features a number of improvements in its equipment and capabilities. The most significant new feature of the space module is its capability to be resupplied by a visiting Tianzhou cargo vehicle, allowing the transfer of both ‘dry’ and ‘wet’ cargoes between the two spacecraft vehicles. An orbital refuelling experiment package was carried aboard the CZ-7 (Y1) launch vehicle that was launched from the Wenchang Space Launch Centre in June this year to demonstrate the transfer of liquid propellant in zero-gravity conditions.
Tiangong 2 is also fitted with a robotic arm developed by CAST. Similar in function to the Mobile Servicing System (MSS) equipped by the International Space Station, the 10 m-long robotic arm is designed to help the assembly and maintenance of the space station, move equipment and supplies around the station, and support astronauts in EVA.
Like its sister spacecraft, Tiangong 2 carries 60 man-days worth of food and suppliers, allowing a two-man crew to live on the station for a maximum of 30 days. The living conditions on the station are said to be better than the previous Space Laboratory module, and the new laboratory also carries more scientific experiment and applications payloads. The sole expedition mission to Tiangong 2 is expected to be launched in October aboard the Shenzhou 11 crew vehicle. The cargo resupply capability means that the space module can be resupplied to extend its occupation time far beyond the 30-day limit, but no further crewed mission has been planned for Tiangong 2, as the China Manned Space Programme is eager to move into the next phase of beginning to construct the actual permanent space station.
Tiangong 2 Mission Profile
The CZ-2F (T2) launch vehicle inserted the Tiangong 2 module into an initial 200 x 350 km parking orbit with an inclination of 42°. The space module then uses its own propulsion to manoeuvre to a 380 km near-circular orbit. Over the next few weeks, the ground mission control will carry out various checks remotely to ensure that the space module is readied for the docking operation.
CZ-2F/Tiangong 2 flight sequence:
T-3 hours....Final launch countdown began T-40 min.....Launch vehicle internal power switched on T-30 min.....Rotating platforms on the umbilical tower retrieved T-15 min.....Launch time inputted into the launch vehicle’s flight control computer; Telemetry system began recording T-5 min......Automated launch sequence initiated T-3 secs.....Main engine and boosters ignition T+0 sec......Launch vehicle lift-off T+12 secs....Launch vehicle made roll programme to fly downrange T+155 secs...Jettison of strap-on boosters T+159 secs...Main engine cut-off T+159.5 secs.First-stage separation and second-stage ignition T+210 secs...Payload fairing jettison T+463 secs...Second-stage main engine cut-off T+582 secs...Second-stage swivelling motors cut-off T+585 secs...Second-stage separation; Payload orbital insertion
Next the Shenzhou 11 mission will be launched about a month after the Tiangong 2 launch, in mid-October. The vehicle will carry a crew of two astronauts, both male, for a 33-day orbital flight mission, including 30 days onboard the Space Laboratory module and 3 days for transits between the station and Earth. During this time, the crew will also deploy a Nano-satellite named Banxing 2 (BX-2), which is attached to the external of the crew compartment.
The next visit is scheduled for the first half of 2017, when the Tianzhou 1 cargo vehicle will be launched atop a CZ-7 launch vehicle from the Wenchang Space Launch Centre on Hainan Island, for a cargo resupply demonstration mission.
No further mission has been planned for Tiangong 2, though the space module may follow suit to its predecessor to remain flying autonomously in orbit for another few years before deorbiting. This will allow the Chinese mission control to gain experience in long-term spacecraft orbital maintenance and management.