Let’s not lie, Chang’e 5’s foothold on the moon is mainly due to bees

On November 24, 2020, the much-anticipated Chang'e-5 entered space with the help of the Long March-5 carrier rocket, took 112 hours to enter the lunar orbit, and finally landed safely on the back of the moon. This feat opened a new chapter in my country's space exploration.

Did you know? When the Chang'e-5 probe landed on the lunar surface, although the reverse engine greatly reduced its descent speed, it still experienced about 4 times the gravity acceleration of the earth at the moment of landing. In other words, the overload borne by the landing support was equivalent to 4 times the weight of the probe on the ground. At the moment of landing, if the landing support broke due to being unable to withstand the overload, the Chang'e-5 probe and many high-precision scientific research detection equipment it carried would be damaged.

Therefore, how to make the landing bracket withstand higher overloads and protect the probe from damage is a major difficulty faced by engineers.

The lunar surface is uneven, so the probe's landing position, landing attitude, mass distribution, center of mass position, vibration characteristics during landing, and coordination of mechanism movement are all factors that need to be considered during design. In addition, since the impact force at the moment of landing will transmit highly destructive energy to the probe through the landing bracket, the landing bracket must not only be hard and stable, but also have the ability to efficiently absorb destructive energy.

Which material can be competent for this arduous task? After comprehensive screening, engineers finally adopted a protective material called honeycomb and installed it as a filling material in the hollow landing bracket of the Chang'e 5 probe.

Hey, honeycomb material? What does it have to do with bees' hives?

That’s right, the design inspiration of honeycomb materials comes from real honeycombs in nature!

Chang'e 5 uses a honeycomb buffer structure derived from nature. Image source: Homemade

1

Honeycomb material from nature

Bees are amazing social insects that often display intelligence comparable to that of higher animals. The hexagonal mosaic honeycomb that we often see is a unique secret (patentable) that the bee family has mastered after millions of years of evolution.

Bees are truly nature's skilled craftsmen. Image source: theconversation

Everyone must be familiar with the structure of a beehive: it is composed of many cells, some for breeding young bees, some for storing nectar... At the same time, the cross-section of each cell is almost a standard hexagon. This unique shape is so impressive that people will think of bees and their homes as long as they see a similar shape. Therefore, this plane structure composed of many hexagons is often called a "honeycomb structure".

Engineers and artists have long applied honeycomb materials in various industries. We can find hexagonal honeycomb elements in various buildings, sculptures, paintings and other works.

Beehive-style architecture, Image source: archcollege

Cameroon issued a honeycomb-style commemorative coin. Image source: powercoin

2

Honeycomb material contains mathematical beauty

Scientists are also very interested in honeycomb structures. Ancient Roman scholars such as Pharro and Ancient Greek scholars such as Popes have studied honeycombs. However, they did not give a definite explanation for why each honeycomb is hexagonal. Later scientists proposed a "honeycomb conjecture", believing that this honeycomb structure may have the best space utilization. But for a long time, this conjecture has not been confirmed.

In 1999, mathematician Thomas Hales used mathematical methods to prove the honeycomb conjecture: if a plane is to be divided into many areas of equal area, the required line perimeter is the smallest when regular hexagons are used. In other words, bees use this honeycomb design of tightly-packed regular hexagons to maximize space utilization while using the least amount of material. How clever!

Mathematician Thomas Hales, Image source: Wikipedia

3

Honeycomb material has high protection capability

The reason why honeycomb material was able to land on the moon with Chang'e 5 is that it also has great application value in the field of cushioning and protection.

If we place the honeycomb material between two very thin solid plates and glue them together firmly, we get a multi-layer composite plate called "honeycomb sandwich panel". This sandwich panel looks very thick, and the porous honeycomb material core occupies most of the volume, but its mass is very light. At the same time, the honeycomb sandwich panel also has high resistance to bending deformation and buffering protection, so it is very suitable for aircraft, rockets, satellites and other equipment that have very high requirements for weight, load-bearing efficiency and protection capabilities. In 1915, engineer Hugo Junkers first applied honeycomb sandwich panels in the structural design of aircraft, thus opening the era of honeycomb materials flying into the blue sky.

Honeycomb sandwich panel, Image source: see watermark (if infringement, please contact us to delete)

How do honeycomb materials protect rear equipment in dangerous environments such as explosions and high-speed collisions? We can observe this through a simple compression failure experiment.

Honeycomb materials are also called two-dimensional materials because they can be obtained by stretching the hexagonal mosaic structure in the two-dimensional plane in the vertical direction. It can be found that if the honeycomb material is squeezed in different directions, its failure mode must be different. In order to study the compressive failure of honeycomb materials in different compression directions, scientists generally define the X and Y directions in the two-dimensional plane as the in-plane direction, and the stretching direction (Z direction) as the out-of-plane direction.

Direction definition of honeycomb materials, Image source: Homemade

The following figure shows an out-of-plane compression test of a honeycomb material. Above it is a rigid pressure head and below it is an important device. Its task is to protect the device behind it as much as possible when the pressure head applies downward pressure.

After the compression test is started, the pressure head begins to move slowly downward and transmits the pressure to the rear through the honeycomb material. According to Newton's third law and the balance of forces, the pressure exerted by the pressure head on the honeycomb material is equal to the pressure exerted by the honeycomb material on the rear equipment. When the pressure exceeds the tolerance limit of the rear equipment, the equipment may be damaged.

Compression experiment in the out-of-plane direction, Image source: homemade

As the compression progresses, the pressure between the pressure head and the honeycomb material is indeed increasing rapidly, and it seems that it will soon exceed the endurance limit of the equipment behind it. But at this moment, the honeycomb material succumbs to the huge pressure in advance, and its side walls begin to bend and fold. This deformation of the side wall initially only exists in a local area, and then it will expand to a wider area as the pressure head continues to press down.

The law of pressure change, Image source: Homemade

Local bending and folding phenomenon of honeycomb materials. Image source: Reference [2]

Each time this local sidewall bending and folding failure occurs, a tiny gap will appear between the honeycomb material and the pressure head, just like a boxer's punch in the air, causing the pressure to drop instantly. The pressure will not resume rising until the pressure head continues to compress the honeycomb material.

During the entire compression process, a large number of sidewall bending and folding phenomena prevent the pressure from continuing to rise, and it always fluctuates around a constant value. Only when the honeycomb material is completely compacted into a pancake shape will the pressure continue to rise rapidly, posing a threat to the rear equipment.

The above is the case of out-of-plane compression. When the honeycomb material is subjected to in-plane compression, although the bending and folding rules of its side walls are different, the change rules of the pressure curve are still similar, and there is also a relatively obvious platform area. Before the pressure head completely compacts the honeycomb material, the pressure can never achieve an effective breakthrough, and naturally it cannot pose any threat to the protected object behind.

In the in-plane compression test of honeycomb materials, it can be seen that the side walls also bend and fold. Image source: Reference [3]

In short, honeycomb material is a self-sacrificing material. It converts destructive external energy into its own internal energy by undergoing a large-scale crushing destruction, thus ensuring the safety of its partners behind it. Therefore, honeycomb material was installed by engineers as a filling material in the landing bracket of Chang'e 5. Facts have proved that honeycomb material has indeed successfully completed its mission.

Scientists have also made innovative designs for the basic configurations of honeycomb materials, from common triangles, rectangles, hexagons to uncommon concave hexagons and Kagome types. These materials are collectively referred to as honeycomb materials, and each has many unique mechanical properties. This wonderful idea of ​​being good at learning and brave enough to break through and innovate can be said to be "the disciple surpasses the master"!

Various new honeycomb materials, Image source: homemade

References:

1.Hales TC. The Honeycomb Conjecture[J]. Discrete & Computational Geometry, 2001.

2. Qiao Jisen, Kong Haiyong, Miao Hongli, Li Ming. Mechanical response of gradient aluminum alloy honeycomb sandwich panel composites[J]. Journal of Plasticity Engineering, 2021, 28(03): 183-189.

3. Lorna J. Gibson, Michael F. Ashby; Liu Peisheng, translator. Structure and Properties of Porous Solids, 2nd edition [M]. Beijing: Tsinghua University Press, 2003.11.

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Tadpole Musical Notation original article, please indicate the source when reprinting

Editor/My Neighbor Totoro