Liquid rocket engine

Liquid rocket engine - a motor fuel for which are liquefied gases and chemical liquids. Depending on the number of components of LRE are divided into one-, two- and three-component.

A brief history of the development

            For the first time the use of liquefied hydrogen and oxygen as fuel for rockets was proposed by K.E. Tsiolkovsky in 1903 year. The first prototype of the LRE was created by American Robert Howard in 1926 year. Subsequently, such developments were carried out in the USSR, USA, Germany. The greatest success was achieved by German scientists: Thiel, Walter, von Braun. During World War II, they created a whole line of LRE for military purposes. It is believed that create the Reich "V-2" before, they would have won the war. Subsequently, the Cold War and the arms race became a catalyst to accelerate the development of LRE with a view to their application in the space program. With the help of RD-108, the first artificial Earth satellites were put into orbit.

Today LRE used in space programs and heavy Missiles.

Scope

As mentioned above, the rocket engine is used mainly as a motor spacecraft and launch vehicle. The main advantages of the LRE is:

• the highest specific impulse in the classroom;
• the ability to perform a full stop and restart, paired with traction control, gives increased maneuverability;
• significantly less weight in comparison with the fuel chamber with the solid propellant.

Among the shortcomings of the LRE:

• more complicated and expensive device;
• increased requirements for safe transport;
• in a state of weightlessness, it is necessary to use additional engines for the deposition of fuel.

However, the main disadvantage of LRE is a limit of fuel energy capacity, which limits the development of space with them up to a distance of Venus and Mars.

Design and function

The operating principle of a rocket engine, but it is achieved by using various schemes devices. Fuel and oxidizer enter by means of pumps from tanks at different mixing head is injected into the combustion chamber and mix. After a fire under the pressure inside the fuel energy is converted into kinetic and flows through a nozzle, creating a jet thrust.

The fuel system includes fuel tanks, pipes and pumps to the turbine for injecting fuel from the tank into the pipe and the valve-regulator.

Fuel feed pump creates high pressure in the chamber and, consequently, a greater expansion of the working fluid, by which the maximum specific impulse.

Nozzle head - unit injector for injecting fuel into the combustion chamber components. The main requirement to the nozzle - quality and speed of mixing the fuel supply to the combustion chamber.

Cooling system

Although the share of heat transfer from the structure during combustion is insignificant, the problem of cooling is urgent due to the high combustion temperature (> 3000 K) and threatens with thermal destruction of the engine. There are several types of cooling of the chamber walls:

• Regenerative cooling is based on the creation of cavities in the walls of the chamber, through which the oxidant without fuel, cooling the chamber wall and the heat with the coolant (fuel) back into the chamber.

• Boundary layer - is created from the fuel vapor layer of gas from the chamber walls. This effect is achieved by installing on the periphery of the head nozzles feeding fuel only. Thus the fuel mixture lacks oxidizer and combustion near the wall is not as intense as in the center of the chamber. The temperature of the boundary layer isolates the high temperatures in the center of the chamber from the combustion chamber walls.

• The ablative method of cooling a liquid-propellant rocket engine is carried out by applying a special heat-shielding coating to the chamber walls and nozzles. At high temperatures, the coating changes from a solid to a gaseous state, absorbing a large proportion of the heat. This method of cooling a liquid propellant rocket engine was used in the Apollo lunar program.

Starting the LRE very critical operation in terms of explosive failures in its implementation. There hypergolic components with which there is no difficulty, but when using an external initiator to ignite the ideal consistency needed to supply it with fuel components. The accumulation of unburned fuel in the chamber is devastating explosive power, and promises serious consequences.

The launch of large liquid-propellant rocket engines takes place in several steps with the subsequent reaching the maximum power, while the small engines start with an instant output at one hundred percent power. 

The automatic control system of liquid propellant rocket engines is characterized by safe engine start and exit to the main mode, control of stable operation, thrust adjustment according to the flight plan, adjustment of consumables, shutdown when entering a given trajectory. Due to the moments that cannot be calculated, the liquid-propellant engine is equipped with a guaranteed supply of fuel so that the rocket can enter a given orbit in case of deviations in the program.  

Fuel components and their choice in the design process are decisive in the design of a liquid propellant rocket engine. Based on this, the conditions of storage, transportation and production technology are determined. The most important indicator of the combination of components is the specific impulse, which determines the distribution of the percentage of fuel and cargo mass. The dimensions and mass of the rocket are calculated using the Tsiolkovsky formula. In addition to the specific impulse, the density affects the size of tanks with fuel components, the boiling point can limit the operating conditions of missiles, chemical aggressiveness is inherent in all oxidizers and, if the rules for operating the tanks are not followed, it can cause a tank fire, the toxicity of some fuel compounds can cause serious harm to the atmosphere and the environment ... Therefore, fluorine, although it is a better oxidizing agent than oxygen, is not used due to its toxicity.

Single-component liquid-propellant rocket engines use a liquid as fuel, which, interacting with a catalyst, decomposes with the release of hot gas. The main advantage of single-component rocket engines is their simplicity of design, and although the specific impulse of such engines is small, they are ideally suited as low-thrust engines for orientation and stabilization of spacecraft. These engines use a positive displacement fuel supply system and, due to the low process temperature, do not need a cooling system. Single-component engines also include gas jet engines, which are used in conditions of inadmissibility of thermal and chemical fumes.

In the early 70-ies US and the Soviet Union developed the three-component liquid rocket engines that would use hydrogen as fuel and hydrocarbon fuel. Thus, the engine would operate on kerosene and oxygen at start-up and switched to the liquid hydrogen and oxygen at high altitude. An example of the three-LRE Russia has a RD-701.

Missile control was first used in missiles "Fau-2» using graphite gasdynamic rudders, but this reduced engine power, and are used in modern missiles rotary chamber attached to the housing by hinges creating flexibility in one or two planes. Furthermore PTZ cameras are used as control motors which are fixed the nozzles in the opposite direction and, when appropriate control device in the space.

A closed-cycle LRE is an engine, one of the components of which is gasified when burned at a low temperature with a small part of the other component, the resulting gas acts as the working fluid of the turbine, and then is fed into the combustion chamber, where it burns with the remaining fuel components and creates jet thrust. The main disadvantage of this scheme is the complexity of the design, but at the same time the specific impulse increases. 

The prospect of increasing the power of liquid rocket engines

In the Russian school of the creators of the LRE, headed by Academician Glushko for a long time, they strive to maximize the use of fuel energy and, as a result, the maximum possible specific impulse. Since the maximum specific impulse can only be obtained by increasing the expansion of the combustion products in the nozzle, all developments are conducted in search of the perfect fuel mixture.