Audit Expert: Qian Hang
Aerospace science popularization expert of the First Academy of China Aerospace Science and Technology Corporation
Today is China's Space Day, which is a special day in the field of aerospace. On this memorable day, we should not only warmly celebrate the brilliant achievements of China's space industry, but also clear up a common misconception: aerospace is not exactly "the same thing". Although the terms "aviation" and "aerospace" are often used together, they have a clear demarcation in terms of technical fields and scope of applications. Today, let's dive into the differences between the two.
The dividing line between space and the earth – the Kármán line
There is no international consensus on the exact boundaries between aviation and space, but in order to establish a relatively common standard, the International Aeronautical Federation has introduced the concept of the "Kármán line".
Before understanding the Kármán line, we have to understand the atmosphere, which scientists divide into five layers, including:
Troposphere (0-10km)
Stratosphere (about 10-50km)
Middle layer (about 50-85km)
Thermal layer (about 80-800km)
Fugitive layer (800-2000 or 3000 km)
The flight altitude of the passenger plane we usually take is between 6800-8000 meters, near the junction of the troposphere and stratosphere, it can stay in the air without falling, mainly relying on the lift of the air, after the wing shape is carefully designed, as long as it reaches a certain speed, it can make the wing up and down to produce enough air pressure difference, and then produce the lifting force of the air, supporting the aircraft to fly. At the "Kármán Line", which is 100 km above the ground, the atmosphere is too thin for the aircraft to fly normally. Therefore, the Kármán line can be considered the dividing line between the atmosphere and space.
Kármán Line | China National Geographic
Qian Xuesen defined the scope of the three technical terms of "aviation, aerospace, and astronautics" as follows: Flight activities in the atmosphere are called "aviation"; flight activities outside the atmosphere and within the solar system are called "astronautics"; and activities that fly out of the solar system to the vast and boundless universe are called "astronautics."
In a nutshell, aviation specifically refers to the movement of aircraft within the Earth's atmosphere, whether manned or not. Spaceflight, on the other hand, refers to the voyage of a spacecraft out of the atmosphere and into space, also known as space flight or astronautics. With the desire and exploration of more space, our range of activities has expanded from land to ocean, from ocean to atmosphere, and finally to the universe. This process is a vivid illustration of the continuous expansion of the boundaries of human activity.
Flight environment and flight altitude
Aeronautical technology focuses on the research and development of military and civil aircraft, as well as the manufacture of air-breathing engines, while aerospace technology focuses on the development of unmanned and manned spacecraft, launch vehicles, and missile weapons. Aircraft and spacecraft are typical representatives of these two technologies, and the main differences between them are reflected in the flight environment and flight altitude.
C919 | People's Daily
Aircraft always fly in a dense atmosphere, and their operating altitude is limited. Currently, the maximum flight altitude of modern aircraft is about more than 30 kilometers above the ground. Even if the aircraft of the future increases its flight altitude, it will still not be able to break away from the dense atmosphere.
In contrast, spacecraft need to pass through a dense atmosphere, enter a near-vacuum space, and fly according to a pattern similar to that of a natural celestial body. The perigee altitude of the spacecraft is at least 100 km.
Long March-2D launch vehicle | People's Daily
It is also crucial for an operational spacecraft to study the space flight environment, which is closely related to its flight altitude from Earth.
Flight power plant and time limit for operation
Aircraft use air-breathing engines to provide thrust and absorb oxygen in the air as oxidants, and only carry incendiary agents; spacecraft use rocket engines to provide thrust when launching and operating, and carry both incendiary and oxidizing agents. The breathing engine cannot work without air, whereas the rocket engine without air has less resistance and greater effective thrust.
The flames spewing out of the engine | People's Daily
The breathing engine, including the incendiary tank, can be used multiple times with the aircraft, while the launch vehicle for launching spacecraft is currently mostly single-use. Although the Space Shuttle's solid booster can be reused 20 times after recycling, and its orbiter liquid rocket engine can be reused 50 times, it is still a small number of uses compared to the air-breathing engines used in aircraft.
The combustion agents used in air-breathing engines are only aviation gasoline and aviation kerosene, while the propellants used in rocket engines are varied, including liquid, solid, and solid-liquid types.
Aero Engines | Baidu Images
Whether it is a military or civilian aircraft, the maximum range is about 20,000 kilometers, and the longest flight time is no more than one day and night. The scope of its activities and working hours are limited, and it is mainly used for military and transport purposes. Although general-purpose light aircraft are widely used, the scope of each event is relatively small. And the spacecraft can last for a very long time in orbit.
Looking back on the development of China's space industry, from missile research and development to satellite launches, from the construction of the Beidou navigation system to the implementation of the lunar exploration project, each generation of astronauts has made a series of brilliant achievements that have attracted worldwide attention by virtue of their indomitable spirit and the wisdom of independent innovation. Today, space activities have played a pivotal role in China's economic construction and social development, and have injected a strong impetus into promoting the country's scientific and technological progress and industrial upgrading.