At some time this year, China will launch the Tiangong 2 space laboratory. It will look much like its predecessor, Tiangong 1: A stubby cylindrical crew module with a smaller service module at its rear. Tiangong 2 is expected to feature several upgrades, including a regenerative life-support system.

Last year, this analyst concluded that the next Tiangong module will also be the last Tiangong. If China plans to launch the first module of the Chinese Space Station in 2018, there simply won't be enough time to squeeze in a third Tiangong and its associated missions. Thus, the flight of Tiangong 2 carries a degree of sentimentality for space analysts.

The second and last Tiangong could also bring the final flight of a special rocket. The Long March 2F/G made its debut with the launch of Tiangong 1. This vehicle seems to have been specially designed to support the launch of Tiangong modules.

It has not flown again since that module was launched. The rocket will be used again to launch Tiangong 2, which should have roughly the same mass as the first module. After that, its future is questionable. If China plans to launch no more Tiangong modules, we will probably never see this vehicle produced or flown again.

As its name implies, the Long March 2F/G is really a derivative of the Long March 2F rocket, which is used to launch China's astronaut-carrying Shenzhou spacecraft. The Long March 2F has flown ten times without a launch failure. It has carried astronauts for five of those launches. In turn, Long March 2F draws upon the legacy of previous Long March rockets in terms of its engines and other components.

The principal difference between the Shenzhou-launching Long March 2F and its 2F/G cousin is easy to spot. The 2F/G carries a very different payload fairing at its top. This accounts. for the larger dimensions of the Tiangong laboratory, which wouldn't fit inside the standard payload fairing for the 2F.

It also lacks an emergency escape system. With no astronauts on board, the escape rocket and stabilizer panels that help Shenzhou spacecraft to separate from their rocket in a launch failure are not needed. This simplifies the design and also reduces the weight of the rocket. That's critical. Tiangong modules weigh more than Shenzhou spacecraft, so this helps to keep the overall launch mass within performance limits.

It's also possible that there could be other minor differences between the 2F and the 2F/G that aren't as easy to spot. With no crew on board, some compromises in performance could be made. This could apply to vibrational and acoustic loads that could be experienced inside the payload fairing. The 2F/G could offer a rougher ride, but with nobody aboard, nobody cares.

It's understandable that China did not go to great lengths to design a special launch vehicle for Tiangong. The program was guaranteed to have a limited number of flights. It's possible that three launches of these small-sized Tiangong modules could have been made if one mission had failed.

This really seems unlikely now. Tiangong 1 performed very well, and we expect similar results from Tiangong 2. In the past, there was speculation that China intended to launch a Tiangong 3 module, but this was expected to be a "heavy" Tiangong that would have been too large for the Long March 2F/G.

With or without Tiangong 3, the total number of flights of the Long March 2F/G was always a balance between two or three missions, with the smart money always on merely two.

Thus, simply changing the payload fairing of an existing launch vehicle was a logical choice for Tiangong's launcher. It saves time and money. That being said, there has been a synergistic development of the Long March 2F/G and the Tiangong laboratories.

These modules are small, and their size has probably been at least partially dictated by the need to fit them aboard this rocket. The dimensions of Tiangong influence the design of the rocket, but the rocket itself also influences the design of Tiangong.

The standard Long March 2F has a bright future. It will continue to be the launch system for Shenzhou spacecraft for quite some time. It will launch crews to Tiangong 2 and to the Chinese Space Station. Having proved its worth with so many successful launches, China will be in no hurry to ditch something that works so well.

Its service record will be short, but it has been nice to see the Long March 2F/G in service. Without it, there would have been no Tiangong program, or the amazing crew expeditions that fly to Tiangong. Good luck for the final flight.

Dr Morris Jones is an Australian space analyst who has written for spacedaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will answer media inquiries.