The birth of Jade Rabbit

2014-06-20By Yu Fei

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Yutu

When Chinese space technologists began drawing the blueprint for China’s first moon rover, Yutu, or Jade Rabbit, some of their ideas were straight out of science fiction.

Would it be a four-wheeled, six-wheeled or eight-wheeled rover with rectangular, triangular or trapezoid shaped solar panels?

What emerged was the condensed wisdom and diligence of hundreds of researchers who spent a decade in its design and production. 

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Some of the creative ideas about Yutu are straight out of science fiction.   China Features

Conception

In July 2004, Jia Yang, a researcher with the China Academy of Space Technology (CAST), drew three overlapping circles, representing spacecraft, vehicle and robot.

“The moon rover concept was at the intersection of the three circles. The biggest difference between a moon rover and other spacecraft was that it would interact with the moon’s surface,” Jia recalls.

At the start of China’s Chang’e lunar exploration program in 2004, more than a dozen research institutes and universities vied to take part in the development of the lunar rover. They had many designs. Some moved fast; some had deft robotic arms; some could overcome big obstacles.

Eventually, more than 100 institutes and research agencies joined the project and the lunar rover was a combination of technologies from various sectors, says Jia, who became deputy chief designer of the Chang'e-3 lunar probe in 2008.

“We studied many foreign documents to understand the basic research. But when we met difficulties, we could only rely on ourselves,” Jia says.

According to fellow deputy chief designer Zhang Yuhua, of Shanghai Academy of Spaceflight Technology, no cooperation was carried out between China and the United States in lunar exploration.

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Researchers do experiments for Yutu.   China Features

Function over looks

On the initial blueprint, the body of the moon rover was a cube-like box. Some critics likened it to an ice-cream vendor’s trolley.

The designers created many new shapes, but chose the most feasible in terms of technology.

“Since we had many restrictions, such as the weight, we kept the design simple to meet all the functional demands,” says Jia. “Maybe some thought the moon rover was not pretty, but it was perfect in our eyes because it met all the technological requirements.”

The designs at different stages reflect the evolution. The rover’s body transformed from a box to a complicated shape enveloped by cooling panels; the wheels were connected in different ways; the formerly foldable solar panels on each side became one foldable, one fixed; the antenna and the pole holding the cameras were combined.

Getting about

Experts and designers pondered every conceivable question. One question was if the rover turns rolled over, could it right itself?

“After many trials, our conclusion was no, it couldn’t. If the delicate equipment on the rover, as well as the solar panels, were covered with lunar soil, they could not work. So we could only consider how to keep the rover from overturning,” says Jia.

Sometimes researchers argued fiercely. After they decided on a six-wheel rover, they debated for months on two plans for connecting the three wheels on each side.

Plan A had only a strong forward movement, while computer simulations showed plan B had a better ability to reverse. Researchers imagined a situation where the rover crossed a small obstacle and then faced an insurmountable obstacle. Then it must retreat. If they chose plan A, the rover could have problems. Finally, plan B won.

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Yutu works on the moon.   Xinhua/China Features 

Slimming

Long before the launch of the lunar probe, Jade Rabbit’s weight troubled its developers.

Designing the prototype, the researchers were required to keep its weight within 120 kilograms. The moon rovers of the former Soviet Union were more than 700 kilograms, while those of the United States were more than 200 kilograms.

The weight restriction was a result of China’s rocket capacity and industrial capabilities.

“At the beginning, many veteran specialists thought it was impossible to make such a light rover. Even if we made it, it could be unreliable,” Zhang Yuhua says.

“We tried everything we could think of to reduce the rover’s weight,” Jia says.

For instance, the antenna and the pole holding the cameras were combined. Changing from two foldable solar panels to one fixed and one foldable took off 500 grams.

Researchers replaced several instruments with an integrated electronic device, taking off 20 kilograms.

Even the amount of solder and the number of screws was discussed and tested

Little by little, Yutu’s weight fell from 200 kilograms to the final 136 kilograms.

Experience gained from reducing the weight, such as the multi-functional design of the solar panel and the integrated electronic technology, could be adapted for future spacecraft, says Jia.

Surviving a moon night

One night on the moon is about 14 days on the earth, during which the temperature falls below minus 180 degrees centigrade. The storage battery could provide heat for Yutu during the moon night, but it was abandoned because it was too heavy.

Researcher Shao Xingguo came up with an idea of making use of the gravity of the moon to develop a two-phase fluid circuit. The circuit is like a heating pipeline to convey the heat emitted from the radioisotope heater units outside the rover body to warm the instruments inside.

According to initial plan, Yutu had to find a suitable position before the coming of the moon night, so the sunlight could reawaken it a fortnight later.

However, after Yutu landed on the moon, it couldn’t find such a position, so it used its trump card – rolling some of its wheels to dig a little pit to adjust its angle.

The solution was inspired by solving another problem.

The U.S. Mars rover, Opportunity, was trapped in the soft sands on Mars in 2005. Engineers on earth struggled for a long time to get it out.

“It was a warning for us. Being trapped in the lunar soil was possible. So we conducted trials. We buried one third or half of the wheels in soil to test how to get it out by rolling the wheels and adjusting its posture,” says Jia.

“And then we thought the posture adjustment method could be used to prepare for the moon night.”

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Researchers go to a desert, China's most moon-like place,  in northwest China's Gansu Province to test Yutu.          China Features

New frontiers

About half of the more than 100 times lunar exploration missions by all countries have succeeded.

“The rover was not as complicated as an airplane or aircraft carrier. But the severe and unpredictable environment on the moon and the fact that the rover could not be repaired after its launch increased the difficulty and risk of the mission,” Jia says

To ensure the success and safety of China’s space missions, the use of new products and new technologies on each spacecraft is usually limited to less than 30 percent, but they exceeded 80 percent on the Chang’e-3 lunar probe.

Many veteran space specialists worried about the success of the mission, and researchers conducted strict tests on Yutu before its journey to the moon.

They built a lab covering 800 square meters within CAST to simulate the lunar environment. A 50-cm deep floor of volcanic ash similar to the lunar soil was dotted with rocks and craters.

Here they tested Yutu’s ability to move, turn, climb, cross obstacles, take pictures, send messages and detect soil content.

“Although we did our utmost, residual risks remained. The pressure was huge,” Jia says.

Even though China lags far behind the United States and Russia in the space sector, it boasts a large number of young talents in the field.

The average age of the developers of the Chang’e-3 lunar probe was in the early 30s. When Sun Zezhou was appointed chief designer, he was 37, the youngest chief designer in China’s space sector.

Sun Jiadong, chief designer of China’s lunar exploration project, says these young men will bring China’s space industry into a golden age in 20 years.

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