Engine: The heart of a rocket

If the launch vehicle is the foundation of the development of the space industry and the "ladder" that lifts the spacecraft into space, then the rocket engine is the "heart" of the launch vehicle, generating a powerful force to propel the launch vehicle into space. To understand the launch vehicle, you must first understand the rocket engine.

Launch vehicle basics

It can be said that the basis of a launch vehicle is the rocket engine. The reason why there is a classification of rockets as a means of transportation is because the working principle of a rocket engine is different from that of other types of engines.

A rocket engine is a jet engine that completely utilizes its own energy and working fluid to generate high-temperature and high-pressure gas, which is then expanded and accelerated into a high-speed jet through a nozzle to obtain a reaction thrust. Its most notable feature is that it does not use air in the atmosphere and has the ability to work in both the atmosphere and in outer space.

In contrast, turbojet engines use air as a working fluid, carry their own fuel to burn with the inhaled air, and eject high-temperature combustion gas to generate thrust. The fuel carried by propeller engines is mainly used to generate mechanical energy and torque, driving the propeller to generate aerodynamic force in the air to move.

Gunpowder and Ancient Rocket Engines

Although modern aerospace originated in the West, and my country did not officially start its own aerospace industry until the 1950s, if we seriously consider the generation, the earliest rocket engine can be said to have originated in China. This is due to "gunpowder", one of the four great inventions of my country. The earliest gunpowder is also called black powder. The first book that records the formula of gunpowder in my country was written in the eighth or ninth century. Sulfur, saltpeter and charcoal are mixed together. This is the formula of gunpowder.

After being ignited by external energy, gunpowder can burn rapidly and regularly, producing a large amount of gas and heat. The original small solid gunpowder suddenly expands to several thousand times its volume. If the gunpowder is sealed in a container when it is ignited, it will explode; if the gas produced by the ignition of gunpowder can be guided to one direction and discharged in an orderly manner, it will become a rocket engine.

According to the "Song History·Military Records", in the third year of Kaibao in the Northern Song Dynasty, i.e. in 970 AD, Feng Jisheng, the military official, presented the method of making rockets to Emperor Taizu of Song, Zhao Kuangyin. This method is to tie a gunpowder cartridge to the front end of the arrow shaft, ignite it, and use the reaction force of the gas ejected backwards by the burning gunpowder to shoot the arrowhead. This is the earliest rocket in the world.

In general, the rocket engine in ancient times was just a byproduct of the invention of gunpowder and played only a very limited role. The specific impulse of ancient rockets made of black powder was generally around 50 to 150 seconds, while the liquid hydrogen and liquid oxygen rocket engines used in modern rockets can reach a specific impulse of 366 seconds at sea level. Even modern solid rocket engines have a specific impulse of more than 250 seconds. The improvement of rocket engine performance allows rockets to achieve a longer range with lighter mass.

The development of modern rocket engines

The fact that rocket engines have escaped from ignorance and gained scientific theoretical guidance is also attributed to the father of rockets, the famous Russian scientist Konstantin Tsiolkovsky. In the early 20th century, he theoretically proposed the rocket equation that consumes its own working fluid to achieve acceleration, laying the foundation for a clear definition of rocket engines. Through theoretical derivation, he proposed the concept of liquid rocket engines and proved that it is the most suitable power device for space flight. This scientific giant even proposed key design concepts for liquid rocket engines such as turbo pumps and combustion chamber regenerative cooling.

Rocket engines have entered the fast lane of development since then. Goddard of the United States invented the world's first liquid rocket engine and successfully conducted the world's first ground ignition test of gasoline/liquid oxygen rocket engine in 1923. And Oberth in Germany also presided over the design and manufacture of Europe's first liquid rocket engine in 1930, but its propellant used a combination of liquid oxygen and methane.

With the continuous advancement of technology, rocket engines have also developed into various types. According to the different energy types, rocket engines can be divided into chemical rocket engines, electric rocket engines, nuclear rocket engines, solar rocket engines, laser rocket engines, and the envisioned photon rocket engines.

At present, the most widely used are chemical rocket engines. The energy of chemical rocket engines comes from the chemical energy released by the chemical reaction and combustion of the propellant used. Chemical rocket engines can be divided into liquid rocket engines, solid rocket engines, hybrid rocket engines, gel rocket engines, paste rocket engines and other categories according to the physical form of the propellant used. Among them, the most common are liquid rocket engines and solid rocket engines.

Liquid rocket engine

Liquid rocket engines are currently widely used in the main power systems of various launch vehicles. The propellants they use include liquid fuel and oxidizer, which are stored in their own tanks and transported to the thrust chamber of the rocket engine by a dedicated delivery system during operation. Common propellant combinations for liquid rocket engines include liquid hydrogen/liquid oxygen, unsymmetrical dimethyl hydrazine/nitrogen tetroxide, kerosene/liquid oxygen, methane/liquid oxygen, etc.

Liquid rocket engines generally consist of a thrust chamber, a propellant supply system, and an engine control system.

The thrust chamber is composed of an injector, a combustion chamber, and a nozzle assembly. The propellant is injected into the combustion chamber through the injector, and after a series of processes such as atomization, evaporation, mixing, and combustion, combustion products are generated and a large amount of heat is released. At this time, the temperature in the combustion chamber can reach 2500-4100°C, and the pressure can even reach 30MPa. These high-temperature and high-pressure gases then expand through the nozzle and accelerate to a high speed of 1800-4300 meters per second and eject out of the engine. Since the temperature of these gases is about twice the melting point of steel, all surfaces on the liquid rocket engine that come into contact with the high-temperature combustion gas must be cooled or insulated.

The function of the propellant supply system is to deliver propellant to the combustion chamber according to the designed flow rate and pressure. There are two types of delivery methods: extrusion type and pump pressure type. Extrusion type supply system is generally used for low-thrust engines. High-thrust engines generally use pump pressure supply system, which uses gas turbine pump to increase the propellant pressure.

The function of the engine control system is to adjust and control operating parameters such as thrust size and propellant mixture ratio during the three stages of engine start-up, operation, and shutdown to ensure that it operates according to the predetermined program.

Solid rocket motor

Solid rocket engines are mainly used as engines for rockets, missiles and sounding rockets, as well as booster engines for spacecraft launches and aircraft takeoffs. They have the advantages of simple structure, high propellant density, and convenient and reliable daily use. The most prominent feature of solid rocket engines is that all of their propellants are stored in the form of grains in the engine combustion chamber. The grain contains all the chemical components required for complete combustion. Once ignited, the grain usually burns steadily at a predetermined rate on all surfaces exposed to high-temperature combustion gases until the propellant is completely consumed. By designing the shape of the grain, the rate at which the combustion products are produced can be controlled, thereby controlling the thrust of the solid rocket engine.

In addition to the propellant column and combustion chamber, the solid rocket engine also includes a nozzle assembly and an ignition device. The ignition device is usually composed of an electric ignition tube and a gunpowder box. After power is turned on, the electric heating wire ignites the black powder, which then ignites the propellant column. In addition to accelerating the expansion of the gas to generate thrust, the solid rocket engine nozzle is often equipped with a flexible component to control the gas injection angle together with the servo control system, thereby achieving control of the thrust direction.