Riding the wind of rebirth

Luo Yujuan smiled and said: "The second favorable condition is that the canyon terrain of the launch base is good and the geological structure is solid, which is conducive to the overall layout of the launch site. In addition, the high altitude is conducive to the layout of ground launch facilities, technical equipment, tracking measurement, and communication networks, which can meet the construction of the launch site."

"As you know, the launch conditions of rockets are quite demanding. The ground structure needs to be very stable and it is best to have a wind and rain barrier around it. Liangshan Prefecture is located on a rocky base on the plateau, which is very solid. It is also located in a canyon, which is relatively hidden. Various equipment can be set up at different heights around it, which is very convenient for monitoring and tracking the initial status of the rocket launch."

This principle is not difficult to understand. Everyone nodded to show that they understood.

"The third factor is, of course, good weather. When you watch the live broadcast of a launch, you often hear the term 'launch window', which refers to the time period that is suitable for a rocket launch based on meteorological conditions."

"Liangshan Prefecture has an average annual temperature of 18°C, making it one of the regions with the least climate change in the country. It has days of sunshine and almost no fog. Such meteorological conditions allow the base to have a longer test cycle and more launch windows."

"In short, Liangshan Prefecture has a low latitude, high altitude, few clouds and fog, no pollution, high air transparency and a suitable climate. Such a place is not easy to find. Back then, our country's rocket experts traveled all over the country and finally found this 'treasure land' in the mountains north of Xichang. This is also our only aerospace base that can launch geosynchronous satellites."

"Sister, what is a synchronous satellite?"

"There are many kinds of satellites that people have made," Luo Yujuan explained. "These satellites will revolve around the Earth, but their angular velocities vary as their distances from the Earth change. Some of them rotate faster than the Earth, and when we look at them from Earth, we'll see they're heading west. Some of them rotate slower, and when we look at them from Earth, we'll see they're heading east."

"These satellites are attracted by the Earth's gravity, which points to the Earth's center. We call it centripetal force. When launching a satellite, the rocket gives the satellite a force to separate from the Earth. This force is away from the Earth's center. We call it centrifugal force. In different orbits, these two forces need to be balanced by the speed of orbiting the Earth."

Seeing that the kids were about to get confused, Zhou Zhi quickly joined in: "This is the principle behind throwing stones with braids when herding sheep."

The braid is a slingshot, made with two ropes tied to the two ends of a leather bag. Stones are placed in the bag, and the bag is swung up with the ropes, allowing the stones to whirl around the head. Then one of the ropes is released, and the stone will fly out at a very fast speed after it is freed from its restraints.

The children in Lianhe Township are all familiar with this thing, and many of them are experts in playing with it. Zhou Zhi asked everyone to recall the working principle of the braid, and then said: "The shorter the braid, the faster it needs to rotate, and the longer the braid, the slower it needs to rotate. In addition, the earth rotates itself. Therefore, if we want to stand on the ground and make the satellites look like they are running forward, we have to put them in a lower orbit. To make them look like they are running backwards, we have to put them in a higher orbit."

Now the kids all understood, and Zhou Zhi said to Luo Yujuan, "Teacher Luo, please continue."

"What the brother just said is very right," Luo Yujuan nodded and continued to introduce: "These places around the earth where satellites can be placed are called orbits. The ones close to the earth are called low-Earth orbits, and the ones far away from the earth are called far-Earth orbits. Satellites in low-Earth orbits run 'fast', and satellites in far-Earth orbits run 'slow'. If we use low-Earth orbit as the starting point and far-Earth orbit as the end point, and slowly move the satellite there, there will always be a point in the middle whose angular velocity is exactly the same as the earth's rotation speed, that is, it takes exactly one day for them to orbit the earth." "Looking from the ground, we can find that this satellite is completely stationary. We call such a satellite a geostationary satellite, and the orbit it is in is called a geosynchronous orbit."

"So a geostationary satellite is also called a geosynchronous satellite. Its characteristic is that it is completely aimed at a place on the earth and never moves."

"I understand!" Azi Ricong felt that he understood again: "The synchronous satellite is thrown over our heads and never moves, watching over our country!"

"Not necessarily." Luo Yujuan was happy: "We can also throw the satellite to other places, let it stay above other countries' heads and watch them. According to their uses, those satellites can become spy satellites."

"That makes sense..." Zhou Zhi now understood why aerospace technology is so important and why slightly more developed countries are working hard to develop this technology. This thing is just a cheating device!

However, because the technology is so complicated, only a few major countries have membership in the club.

"This synchronous orbit is about 3.08 kilometers above the equator, and the satellite's linear speed in orbit is about kilometers per second. The direction of rotation of its orbital plane is the same as the direction of the Earth's revolution, and the speed is also the same."

"But there is one thing to note. Our understanding just now is that the satellite is synchronized on the equatorial plane." Just when the child and the adults thought they understood, Luo Yujuan raised another question: "But China is not actually an equatorial country, so most of the synchronous satellites we need actually operate at a certain latitude."

"So the inclination of the synchronous satellites we launch must be greater than 90 degrees, forming a retrograde elliptical orbit with an altitude between several hundred kilometers and 6000 kilometers. The orbit with an inclination slightly greater than 90 degrees is close to the 'polar orbit', so that we can use the satellite to overlook the entire earth, including the polar regions."

"Launching a geosynchronous satellite requires high technology. Generally, a multi-stage rocket is used to send the satellite into a near-Earth circular orbit, which is called a parking orbit. When the satellite flies over the equator, the rocket is controlled to ignite again and accelerate for a short time, and the satellite will move in an elliptical orbit, also known as a transfer orbit. When the satellite flies to the apogee, it is ignited again to accelerate, and the satellite finally enters a geosynchronous orbit relative to the Earth."

"When launching a satellite, we need to pay most attention to the six elements of the orbit, that is, the orbit of the satellite is determined by six key points, namely the orbital inclination, the argument of perigee, the right ascension of the ascending node, the eccentricity, the semi-major axis of the orbit, and the time when the satellite passes the perigee."

"Do you still think it's easy to be a rocket expert?" Ah Zi grabbed Ah Zi Ricong's ear again, and said, "Why don't you continue to be so arrogant?" (End of this chapter)