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by Morris Jones
Sydney, Australia (SPX) Mar 03, 2014
In 2015, China is expected to launch its next space laboratory. Tiangong 2 will follow on from the Tiangong 1 module, which was launched in 2011 and is still in orbit at the time of writing. Tiangong 1 received two crews of astronauts and carried out China's first space dockings. It is a small, roughly cylindrical module with a crew cabin and a service module featuring solar panels. Although Tiangong 1 is officially designated as a "space laboratory", it is really a small space station.
The launch of Tiangong 2 has been expected for a long time, but space analysts are puzzled by the nature of this spacecraft. Originally, China planned to launch three Tiangong modules, and Tiangong 2 was expected to be a marginally improved version of the Tiangong 1 spacecraft. Later, China seemed to drop plans for three Tiangongs and launch just two. We wondered how this would affect the design of the next module to be launched. A whirlwind of rumours, speculation and conflicting reports circulated. China seemed determined to allow the confusion to flourish.
Recent snippets of information from China have helped to clear up some of the confusion, but have still not given us a totally clear picture of the next Tiangong.
China has essentially confirmed plans that a cargo spacecraft will dock with Tiangong 2. This is consistent with the long-term goal of the Tiangong program: To verify the hardware and technology required to build a large space station.
China has given a fair amount of information on the cargo spacecraft over the years. It's essentially based on the design of the Tiangong 1 module, but it's longer, larger and heavier. The cargo spacecraft, dubbed "Tianzhou" or "Heavenly Vessel", will also be manufactured in three variations: Totally pressurized, a combined pressurized and unpressurized version, and a totally unpressurized version.
This will allow different combinations of cargo to be delivered on different missions. Food and crew supplies will go inside the pressurized sections. Gear to be bolted to the outside of the station will travel in the unpressurized sections. Right now, it is not clear which version will be used on the mission to Tiangong 2. Tianzhou is also designed to be launched by the Long March 7 launch vehicle, which is currently undergoing development. Long March 7 has more power than the Long March 2F/G vehicle used to launch Tiangong 1.
It's nice to get so much information on the Tianzhou cargo spacecraft. Ironically, we probably know more certain facts about the cargo ship than the Tiangong 2 spacecraft itself!
We can still practice some deductive reasoning. In order to receive a docking from this cargo vessel, Tiangong 2 has to be a lot better than the small Tiangong 1 spacecraft. This analyst previously favoured the theory that Tiangong 2 would be a huge module, and could be a prototype for the core module of the Chinese Space Station. We still can't rule this out, but there is another possibility. Tiangong 2 could itself be nearly identical to the Tianzhou cargo spacecraft that will dock with it.
This would be a logical progression. A Tianzhou-based laboratory would be a major improvement on Tiangong 1, but it would still not stretch China's capabilities too far, too soon. It could also be practical for another reason.
A "huge" Tiangong would require the use of the Long March 5 rocket, which is also under production. Long March 5 has experienced a few technical problems, and may not be considered trustworthy enough for this mission. Although the smaller Long March 7 rocket is also untested, China may feel that it has a better chance of being ready to support the mission. Thus, it makes sense to match the cart to the horse.
The weird uncertainty of the nature of Tiangong 2 could be tied into the progress of these two new rockets. China could have considered a "medium" Tiangong launched by Long March 7 in parallel with plans for a "huge" Tiangong on Long March 5. Perhaps this competition has not been entirely resolved.
China plans to send at least one crew of astronauts to Tiangong 2. Trying to do this in concert with a cargo ship docking would be complicated. If Tiangong 2 features multiple docking ports (as a "huge" Tiangong probably would) then the cargo ship could be docked at the same time as a crew-carrying Shenzhou spacecraft. It is not clear if this will be the case. It could still be possible to fit a multiple docking adaptor to a "medium" Tiangong but we don't know if this will be done. Thus, the only choice would be to ensure that the cargo vehicle and the Shenzhou spacecraft both visit at different times.
This analyst favours the idea that the first ship launched to a "medium" Tiangong 2 will be a Shenzhou spacecraft with a crew of three astronauts. They will probably live aboard Tiangong 2 for roughly a month, then come home. This will allow them to inhabit the laboratory when it's still fairly "fresh" and its docking systems have not been over-used.
Months will pass, then a Tianzhou cargo ship will be launched to Tiangong 2. It will perform experimental dockings and undockings, then dock with Tiangong 2 for an extended period. The two spacecraft will be essentially mirror images of each other, producing a small but symmetrical space complex. Then China will leave the modules to orbit for at least two years while they perform engineering tests.
There are holes in the plot. What about cargo transfer on this mission? That probably won't happen. In any case, there's not much point if there is no crew aboard to use it. China has also given hints about refuelling tasks by the cargo spacecraft. Will these be performed with Tiangong 2?
This analyst is starting to reduce the odds of this happening. It largely depends on how the refuelling system works. Many analysts have probably expected that the Tianzhou spacecraft would use an automatic refuelling system like the Russian Progress cargo spacecraft. Progress connects hoses to a space station and pumps fuel and oxidizer directly into the station's tanks.
If Tianzhou can do this, then a refuelling experiment is highly probable. However, there could be a simpler alternative. An unpressurized Tianzhou could carry propellant tanks and simply bolt them to the exterior of the space station. This could be less complex than making plumbing links between two spacecraft and pumping volatile liquids. The propellant tanks could even be complete propulsion modules, with small thrusters already attached.
In such a case, the refuelling operation would be even more straightforward for China's future space station. But it would rule out any chance of this happening with Tiangong 2. Even though this technique is straightforward, an astronaut would need to unload the propellant module from the cargo spacecraft using a robot arm. No crew, and probably no robot arm, means no chance of this type of operation in the near future.
We know a bit more about the Tiangong program, but we still don't really know what the next module will look like. China needs to do a lot more to fill in the blanks.
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.
China National Space Administration
The Chinese Space Program - News, Policy and Technology
China News from SinoDaily.com
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