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by Morris Jones for SpaceDaily
Sydney, Australia (SPX) Nov 22, 2013
Things are going well for China's first lunar landing. The Chang'e-3 spacecraft and its rover are being processed at the launch site. So is the rocket to be used in this mission. With all preparations apparently nominal, we can probably expect liftoff in early December.
The Chang'e-3 spacecraft is larger and heavier than China's previous two lunar missions, which were both sent to simply orbit the Moon. Apart from the extra gear on this mission, Chang'e-3 must also carry a heavy fuel load to enable it to descend from lunar orbit and land safely.
All this extra mass means that the lighter members of the Long March 3 group of rockets, which were used for the first missions, don't pack enough force for this flight. China has now rolled out the most powerful member of its operational fleet of space launch vehicles. Enter the Long March 3B!
The Long March 3B has four liquid fuelled strap-on boosters, giving it greater performance than the Long March 3A, which has no strap-ons. This was the vehicle used to launch Chang'e-1 to lunar orbit. Chang'e-2 was an almost identical spacecraft launched by the Long March 3C, which, despite its name, is actually less powerful than the 3B.
This intermediate rocket has two strap-on boosters. The use of this upgraded rocket allowed Chang'e-2 to fly a different trajectory to the Moon. It also helped Chang'e-2 save enough fuel to allow it to fly on to the asteroid Toutatis after its lunar mission.
But even a standard Long March 3B isn't quite good enough for this mission. China has announced that there have been modifications and improvements to the rocket. We've heard remarks like this many times before major Chinese space launches, but the comments should not be dismissed. China is forever tweaking its rockets, mostly in small, incremental steps.
This is a hallmark of good engineering. Most of the changes are never precisely outlined, and some will be as trivial as changing a small internal battery or wiring system. Nevertheless, China has precisely announced one outcome. The modified Long March 3B for the Chang'e-3 mission can launch 30 kilograms more payload.
But what exactly does this mean? Basic rocket science notes that exactly how much payload a rocket can launch depends greatly on where it is going. The further out you go, the less you can carry. We could assume that the 30 kilogram boost is relative to an unmodified rocket on a similar trajectory. Otherwise the statement is somewhat ambiguous.
Does Chang'e-3 weigh too much for a standard Long March 3B rocket? Probably not. But as China has noted, the performance bonus does give mission controllers more flexibility in launch timings. Launches to the Moon depend on "windows", or times when the Moon is in the right position relative to the launch site for a fuel-efficient flight. A little extra performance from the rocket means that these "windows" are not so small. That could be the difference between launching or not launching if minor glitches occur in the final stages of the countdown.
How was the performance improvement achieved for the Long March 3B? There was probably nothing so exotic as tinkering with the engines or fuel tanks. This analyst suspects that much of the performance boost comes from deleting mass from the rocket itself. Some parts could have been made lighter or smaller.
Some redundant parts could have been deleted. Different materials could have been used in some areas. This would not affect the overall design or functionality of the launch vehicle.
The only real change that we will probably notice on the new Long March 3B will be the spiffy logo of the China Lunar Exploration Program (CLEP) on the payload fairing. Everything else will be buried deep inside.
If the launch is successful, the slightly tweaked Long March 3B will probably become the new "standard" version of this powerful rocket. China may even wish to add some more modifications, although it is questionable if any serious performance increases could be made without some major re-engineering.
Stay tuned for more flights of the Long March 3B. Although it does not fly as often as China's Long March 2 series rockets, the 3B will have another high-profile mission in the future. We can expect this rocket to launch the Chang'e-4 mission to the Moon in a few years. Like its predecessor, this second lunar lander will also require a big boost.
Dr Morris Jones is an Australian space analyst and writer. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will answer media inquiries.
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