The subject asked "ignition".

In a narrow sense, "ignition" is actually to ignite combustible materials and make them burn. We all know that combustion is a reflection of intense chemical heat release between combustibles and oxidants. Generally, we don't need to add oxidant to ignite some combustible materials on the earth, and we can usually directly consume oxygen in the air as the oxidant for combustion. This is what we do in our daily life.

But what if there is not enough oxidant in the air? For example, we need a higher temperature for metal welding, and then we usually use oxyacetylene welding.

In the process of oxyacetylene welding, the oxygen in the air is not used as the consumption source, and the oxygen content in the air is only 2 1%, which is not enough.

Oxyacetylene welding needs oxygen, so we often attach an oxygen cylinder to pour oxygen and acetylene into the welding torch.

At this time, strictly speaking, oxyacetylene welding can be used in underwater or vacuum environment.

In fact, this solves the problem of oxygen supply. So that combustion can be carried out in an environment without oxygen (air).

The same is true of rockets.

Early rockets stored oxidant (liquid oxygen, hydrogen peroxide, Figures 6 and 7) and fuel (Figure 4) respectively. In this way, the combustion of rocket engine is just like oxyacetylene welding, and it does not depend on the oxygen provided by the outside air.

Later we improved the complicated liquid rocket and got the solid rocket. At this time, fuel and oxidant are mixed into the solid rocket grain at the same time. As long as there is a little ignition planetesimals, it can ignite the whole grain and provide rocket thrust. This has nothing to do with whether there is air or not.

For example, the above picture shows an electronic ignition solid rocket device, which relies on the heat emitted between electrodes to ignite the solid engine of the rocket. People used to have all the fuels and oxidants, but in fact they just lack a little heat.

Electric ignition device is a kind of ignition of rocket engine, and there are actually chemical ignition devices. For example, unsymmetrical dimethylhydrazine is injected into nitrogen dioxide.

There is no need for ignition process, and the two liquids can start to burn as long as they contact, without the participation of outside air.

As for how to generate thrust without air, the thrust of a rocket does not depend on the reaction of air, when the rocket ejects gas at high speed. These gases have a mass of m and a velocity of v.

When the rocket forcibly ejects an airflow with a mass of m and a speed of v backward, according to Newton's third law, the rocket can also obtain corresponding reaction force and forward flight power (thrust).

Let me do a simple science popularization.

Aircraft in the atmosphere, the generation of thrust or tension depends on the role of airflow. After leaving the atmosphere and reaching outer space, there is no airflow, and the aerodynamic force is invalid, but what can still be effectively used is the conservation of momentum.

Everyone has seen the scene of firefighters spraying water with fire hydrants on TV. Because of the huge recoil of water, one can't support the huge reaction. This force is a momentum, not an aerodynamic force. There are also adventurers flying in the sky on the sprinklers on the water surface of the park, all of which are the same.

Rockets get soaring power by throwing substances down at high speed. Of course, the material spilled at high speed at this time is a lot of fuel. This is why the rocket is so big, because most of the space inside is used to load fuel. And it needs to use multi-stage rockets. After the fuel is used up, the empty shell is thrown away, reducing the load and improving the efficiency.

Chang 'e V was launched from the moon by a rocket, whose full name is rocket jet engine. Now advanced aircraft use jet engines. The biggest difference between the two is that the air jet engine itself only carries fuel and needs air for combustion.

Rocket jet engine carries both fuel and oxidant, so air jet engine can only work in air, while rocket jet engine can work in an environment without air.

The combustion chamber of a rocket can be understood as a container with an opening (nozzle) only at one end (rear). After ignition, fuel and oxidant burn in the combustion chamber to produce high-temperature and high-pressure gas, which expands rapidly and can only be ejected at the nozzle, and other directions are blocked by the combustion chamber, thus generating huge pressure. Wherein the pressure on the side wall of the combustion chamber is balanced and does not change the motion state of the rocket. However, in the flight direction of the rocket, only the bottom of the combustion chamber is under pressure, so the rocket is pushed to fly in the opposite direction to the injected gas.

It can be seen that the thrust of rocket flight comes from the gas produced by fuel combustion, which has nothing to do with the presence or absence of air outside. Not only does it not rely on the outside air to generate thrust, on the contrary, because there is no air, the rocket will not be resisted when flying, and it will be easier to launch.

You go to Baidu to study recoil, momentum, oxidant and reductant, and double the books on physics in junior high school and physics in senior high school.

In fact, when designing these detectors, people have long thought that they will encounter such a situation. Even if the probe in space needs to be put into orbit without the help of other objects, it can only change the orbit of the probe by burning the fuel it carries and injecting air.

The Chang 'e probe uses a rocket jet engine. Because it needs to be used in space, it must bring its own oxidant, so that even if there is no oxygen in space, it can be ignited and burned by its own oxidant for normal work.

Modern and advanced aircraft usually use jet engines. Because they only fly in the atmosphere, they can still use oxygen in the air to ignite, so the plane only needs to carry fuel.

In fact, we can get from Newton's second law that under a certain thrust, the acceleration that an object can produce is inversely proportional to its mass, so of course, the smaller its mass, the better. For an aircraft, if it doesn't carry oxidant, it can reduce a lot of mass, and of course it also needs to use oxygen in the air.

But if you want to launch an aircraft into space, you need very high speed, and there will be dozens of Mach numbers at any time, so it is very difficult to ignite the engine in the air. Therefore, such an aircraft generally needs to bring its own oxidant, that is, it needs to use a rocket engine to work, and it needs to use a rocket engine when it reaches space.

In fact, the working principle of engines with fluid as the medium is basically the same, that is, according to the momentum theorem, these fluids are injected in the opposite direction that these machines need to move. Because these fluids have momentum, the premise is that these engines are needed to provide, that is, to provide a thrust. According to the principle of action and reaction, these ejected objects will give the engine a forward reaction, which will eventually form the thrust of these detectors. The fluid mentioned above, which can be gas or liquid, basically uses this principle.

In this way, in order to generate a very large thrust, the quality of these ejected gases is limited, and there is as much as the aircraft can. Once it's used up, it's gone

So be sure to make a fuss about speed. When these ejected fluids have a very high speed, they have a very large momentum, which of course can provide a very large thrust.

Then the fuel and oxidant of the rocket jet engine can be burned in the combustion chamber. In order to make these gases have a high injection speed, it is necessary to generate high temperature and high pressure in the combustion chamber to achieve this goal.

In addition, because the rocket jet engine does not need to get oxygen from the air, it only needs to leave a nozzle in the opposite direction of its movement, and the flight direction of the detector can be controlled by adjusting the direction of the nozzle.

To sum up, because Chang 'e uses a rocket jet engine, she will learn some oxidants, and even if there is no air in space, she can achieve the purpose of ignition. Its working principle is momentum theorem. When the gas with a certain momentum is injected backward, she can get the reverse thrust and push Chang 'e to fly.

Like automobile and aviation engines, (rocket) engines convert the chemical energy of fuel into mechanical energy and kinetic energy through combustion. But the difference is that cars and planes only need to carry combustible substances, such as gasoline, diesel oil and aviation kerosene. For rocket engines, in addition to combustible substances (liquid hydrogen, kerosene, unsymmetrical dimethylhydrazine, etc. ), but also need to carry oxidants (liquid oxygen, nitrogen tetroxide). This is because there is no oxygen outside the earth's atmosphere to support combustion; In addition, even if there is oxygen in the surrounding environment, because the rocket flies very fast, it can't get enough oxygen from the environment for a period of time to fully burn combustible materials.

Take the Long March 5 as an example. The Long March 5 has four boosters, each of which is equipped with two YF- 100 engines, and the propellant used is liquid oxygen+kerosene. Two YF-77s were installed in the first stage of the Long March 5 core, and the propellant was liquid oxygen+liquid hydrogen. The second stage of the core is equipped with two YF-75D engines, and the propellant is liquid oxygen+liquid hydrogen.

In addition to liquid oxygen+kerosene/liquid hydrogen, nitrogen tetroxide+unsymmetrical dimethylhydrazine is also a common propellant for rocket engines. For example, the propellants of booster, core I and core II in long march 3b are all nitrogen tetroxide+unsymmetrical dimethylhydrazine. However, these two substances are not very friendly to human beings. One becomes a strong acid when it meets water, and the other damages the liver and causes cancer.

65438+At 23: 00/0 on February 3rd, the 3000N engine of Chang 'e V lifter developed by the Sixth Institute of Aerospace Science and Technology Group began to work, and China realized the first take-off of an extraterrestrial body. Because of the large temperature difference between day and night on the surface of the moon and the complicated rendezvous and docking process, the fuel of the 3000N engine should be nitrogen tetroxide+unsymmetrical dimethylhydrazine (a wild guess).

The principle of rocket propulsion can be explained by the principle of conservation of physical momentum in high school, as shown in the figure below:

Generally speaking, the propellant in the rocket burns and expands, and the reaction products after combustion are ejected at high speed to generate thrust. This process can also be seen as the rocket pushing the combustion products backward at high speed, thus obtaining a forward thrust. It is worth noting that only by pushing out the combustion products can the reverse thrust be obtained. There is no need for an atmosphere (or any other medium). Therefore, under the condition that there is no air on the moon, the Chang 'e V ascender can also generate thrust through combustion and ignite smoothly.

This fuel is also called booster. It's mixed. Bring your own oxygen.

Lao tie, the content of physics in grade two, recoil

Fire, what is it? It is a phenomenon that one or several substances react violently to raise the temperature of the system to a certain extent, and oxygen is not necessary. Without air, the resistance is smaller, the friction resistance between ignition and lift-off is gone, and it can rise faster. The principle of thrust generation is momentum conservation. Has the subject never studied junior high school physics and senior high school physics?

Although there is no air in space, the engine on Chang 'e is equipped with fuel and accelerant. After ignition, the engine will inject gas backward, so that the engine will use the force opposite to the gas injection direction as the engine thrust. At this time, Chang 'e can take off without air.