Rockets. Jet propulsion

Let's consider several examples confirming the validity of the law of conservation of momentum.

Surely many of you have observed how an air-inflated balloon, if you untie the thread that tightens its hole.

This phenomenon can be explained using the law of conservation of momentum.

While the hole in the ball is closed, the ball with the compressed air inside it is at rest, and its momentum is zero.

When the hole is open, it comes out with quite high speed a jet bursts out compressed air. Moving air has some momentum directed in the direction of its movement.

According to the law of conservation of momentum operating in nature, the total momentum of a system consisting of two bodies - a ball and the air in it - must remain the same as it was before the outflow of air, i.e., equal to zero. Therefore, the ball begins to move in the direction opposite to the air stream at such a speed that its impulse is equal in magnitude to the impulse air jet. The impulse vectors of the ball and air are directed in opposite directions. As a result, the total momentum of the interacting bodies remains equal to zero.

The motion of a ball is an example of jet motion. Jet propulsion occurs due to the fact that some part of it is separated from the body and moves, as a result of which the body itself acquires an oppositely directed impulse.

The rotation of a device called a Segner wheel is based on the principle of reactive propulsion (Fig. 46). Water flowing from a conical-shaped vessel through a curved tube connected to it rotates the vessel in the direction opposite speed water in streams. Consequently, the reactive effect is exerted not only by a gas stream, but also by a liquid stream.

Rice. 46. ​​Demonstration of jet propulsion using a Segner wheel

Jet propulsion is also used by some living creatures, such as octopuses, squids, cuttlefish and others. cephalopods(Fig. 47). They move by sucking in and then forcefully pushing water out of themselves. There is even a species of squid that, with the help of its “jet engines,” can not only swim in water, but also on a short time fly out of it in order to quickly overtake prey or escape from enemies.

Rice. 47. Cephalopods use jet propulsion to move: a - cuttlefish; b - squid; c - octopus

You know that the principle of jet propulsion is widely practical use in aviation and astronautics. IN outer space there is no medium with which the body could interact and thereby change the direction and magnitude of its speed. Therefore for space flights only reactive ones can be used aircrafts, i.e. rockets.

Launch of a launch vehicle with the Soyuz spacecraft

Let's consider the question of the design and launch of so-called launch vehicles, i.e. rockets designed to launch artificial Earth satellites, spacecraft, automatic interplanetary stations and other payloads into space.

Any rocket, regardless of its design, always has a shell and fuel with an oxidizer. Figure 48 shows a cross-section of the rocket. We see that the rocket shell includes a payload (in this case it is spacecraft 1), an instrument compartment 2 and an engine (combustion chamber 6, pumps 5, etc.).

Rice. 48. Rocket diagram

The main mass of the rocket is fuel 4 with oxidizer 3 (the oxidizer is needed to maintain fuel combustion, since there is no oxygen in space).

Fuel and oxidizer are supplied to the combustion chamber using pumps. Fuel, when burned, turns into gas high temperature And high pressure, which rushes outward in a powerful jet through the bell special form, called nozzle 7. The purpose of the nozzle is to increase the speed of the jet.

What is the purpose of increasing the exit velocity of the gas stream? The fact is that the speed of the rocket depends on this speed. This can be shown using the law of conservation of momentum.

Since before the launch the momentum of the rocket was equal to zero, then, according to the law of conservation, the total impulse of the moving shell and the gas ejected from it should also be equal to zero. It follows that the impulse of the shell and the impulse of the gas jet directed opposite to it must be equal in magnitude. This means that the faster the gas escapes from the nozzle, the greater the speed of the rocket shell will be.

In addition to the speed of gas outflow, there are other factors on which the speed of the rocket depends.

We looked at the device and principle of operation single stage rocket, where the stage means the part that contains the fuel and oxidizer tanks and the engine. In space flight practice, multi-stage rockets are usually used, which develop much higher speeds and are designed for longer flights than single-stage ones.

Figure 49 shows a diagram of a three-stage rocket. After the fuel and oxidizer of the first stage are completely consumed, this stage is automatically discarded and the second stage engine takes over.

Rice. 49. Diagram of a three-stage rocket

Decrease total mass rocket by discarding an already unnecessary stage allows you to save fuel and oxidizer and increase the speed of the rocket. Then the second stage is discarded in the same way.

If return spaceship is not planned to land on Earth or land on any other planet, the third stage, like the first two, is used to increase the speed of the rocket. If the ship must land, then it is used to slow down the ship before landing. In this case, the rocket is turned 180° so that the nozzle is in front. Then the gas escaping from the rocket gives it an impulse directed against the speed of its movement, which leads to a decrease in speed and makes it possible to land.

Konstantin Eduardovich Tsiolkovsky (1857-1935)
Russian scientist and inventor in the field of aerodynamics, rocket dynamics, aircraft and airship theory. Founder of theoretical cosmonautics

The idea of ​​using rockets for space flights was put forward at the beginning of the 20th century. Russian scientist and inventor Konstantin Eduardovich Tsiolkovsky. Tsiolkovsky developed the theory of rocket motion, derived a formula for calculating their speed, and was the first to propose the use of multi-stage rockets.

Half a century later, Tsiolkovsky’s idea was developed and implemented by Soviet scientists under the leadership of Sergei Pavlovich Korolev.

Sergei Pavlovich Korolev (1907-1966)
Soviet scientist, designer of rocket and space systems. Founder of practical astronautics

Questions

1. Based on the law of conservation of momentum, explain why a balloon moves opposite to the stream of compressed air leaving it.
2. Give examples of the reactive motion of bodies.
3. What is the purpose of rockets? Tell us about the structure and principle of operation of the rocket.
4. What determines the speed of a rocket?
5. What is the advantage multistage rockets before single-stage ones?
6. How is a spacecraft landed?

Exercise 21

1. From a boat moving at a speed of 2 m/s, a person throws an oar of mass 5 kg with horizontal speed 8 m/s is opposite to the movement of the boat. At what speed did the boat begin to move after the throw, if its mass together with the person is 200 kg?
2. What speed will the rocket model get if the mass of its shell is 300 g, the mass of gunpowder in it is 100 g, and gases escape from the nozzle at a speed of 100 m/s? (Consider the gas outflow from the nozzle to be instantaneous.)
3. On what equipment and how is the experiment shown in Figure 50 carried out? Which physical phenomenon in this case, it is demonstrated what it is and what physical law underlies this phenomenon?

   Note: the rubber tube was positioned vertically until water began to flow through it.

4. Perform the experiment shown in Figure 50. When the rubber tube deviates from the vertical as much as possible, stop pouring water into the funnel. While the water remaining in the tube flows out, observe how it changes: a) the flight distance of the water in the stream (relative to the hole in the glass tube); b) position of the rubber tube. Explain both changes.

Rice. 50

Let's consider several examples confirming the validity of the law of conservation of momentum.

Surely many of you have observed how a balloon inflated with air begins to move if you untie the thread that tightens its hole.

This phenomenon can be explained using the law of conservation of momentum.

While the hole in the ball is closed, the ball with the compressed air inside it is at rest, and its momentum is zero.

When the hole is open, a stream of compressed air escapes from it at a fairly high speed. Moving air has some momentum directed in the direction of its movement.

According to the law of conservation of momentum operating in nature, the total momentum of a system consisting of two bodies - a ball and the air in it - should remain the same as it was before the outflow of air, i.e. equal to zero. Therefore, the ball begins to move in the direction opposite to the air stream at such a speed that its momentum is equal in magnitude to the impulse of the air stream. The impulse vectors of the ball and air are directed in opposite directions. As a result, the total momentum of the interacting bodies remains equal to zero.

The motion of a ball is an example of jet motion. Reactive movement occurs due to the fact that some part of it is separated from the body and moves, as a result of which the body itself acquires an oppositely directed impulse.

The rotation of a device called a Segner wheel is based on the principle of reactive propulsion (Fig.). Water flowing from a conical-shaped vessel through a curved tube connected to it rotates the vessel in the direction opposite to the speed of the water in the streams. Consequently, the reactive effect is exerted not only by a gas stream, but also by a liquid stream.

Rice. Demonstration of jet propulsion using a Segner wheel

Jet propulsion is also used by some living creatures for their movement, such as octopuses, squids, cuttlefish and other cephalopods (Fig.). They move by sucking in and then forcefully pushing water out of themselves. There is even a species of squid that, with the help of its “jet engines,” can not only swim in the water, but also fly out of it for a short time in order to quickly overtake prey or escape from enemies.

Rice. Cephalopods use jet propulsion for their movement: a - cuttlefish; b - squid; c - octopus

You know that the principle of jet propulsion has wide practical application in aviation and astronautics. There is no medium in outer space with which a body could interact and thereby change the direction and magnitude of its speed. Therefore, only jet aircraft, i.e. rockets, can be used for space flights.

Launch of a launch vehicle with the Soyuz spacecraft

Let's consider the question of the design and launch of so-called launch vehicles, i.e. rockets designed to launch artificial Earth satellites, spacecraft, automatic interplanetary stations and other payloads into space.

Any rocket, regardless of its design, always has a shell and fuel with an oxidizer. The figure shows a cross-section of a rocket. We see that the rocket shell includes a payload (in this case it is spacecraft 1), an instrument compartment 2 and an engine (combustion chamber 6, pumps 5, etc.).

Rice. Rocket diagram

The main mass of the rocket is fuel 4 with oxidizer 3 (the oxidizer is needed to maintain fuel combustion, since there is no oxygen in space).

Fuel and oxidizer are supplied to the combustion chamber using pumps. The fuel, when burned, turns into a gas of high temperature and high pressure, which rushes out in a powerful jet through a specially shaped socket called nozzle 7. The purpose of the nozzle is to increase the speed of the jet.

What is the purpose of increasing the exit velocity of the gas stream? The fact is that the speed of the rocket depends on this speed. This can be shown using the law of conservation of momentum.

Since before the launch the momentum of the rocket was equal to zero, then, according to the law of conservation, the total impulse of the moving shell and the gas ejected from it should also be equal to zero. It follows that the impulse of the shell and the impulse of the gas jet directed opposite to it must be equal in magnitude. This means that the faster the gas escapes from the nozzle, the greater the speed of the rocket shell will be.

In addition to the speed of gas outflow, there are other factors on which the speed of the rocket depends.

We examined the design and principle of operation of a single-stage rocket, where a stage means the part that contains tanks with fuel and oxidizer and the engine. In space flight practice, multi-stage rockets are usually used, which develop much higher speeds and are designed for longer flights than single-stage ones.

The figure shows a diagram of a three-stage rocket. After the fuel and oxidizer of the first stage are completely consumed, this stage is automatically discarded and the second stage engine takes over.

Rice. Diagram of a three-stage rocket

Reducing the overall mass of the rocket by discarding an already unnecessary stage saves fuel and oxidizer and increases the rocket's speed. Then the second stage is discarded in the same way.

If the spacecraft is not planned to return to Earth or land on any other planet, then the third stage, like the first two, is used to increase the speed of the rocket. If the ship must land, then it is used to slow down the ship before landing. In this case, the rocket is turned 180° so that the nozzle is in front. Then the gas escaping from the rocket gives it an impulse directed against the speed of its movement, which leads to a decrease in speed and makes it possible to land.

Konstantin Eduardovich Tsiolkovsky (1857-1935)
Russian scientist and inventor in the field of aerodynamics, rocket dynamics, aircraft and airship theory. Founder of theoretical cosmonautics

The idea of ​​using rockets for space flights was put forward at the beginning of the 20th century. Russian scientist and inventor Konstantin Eduardovich Tsiolkovsky. Tsiolkovsky developed the theory of rocket motion, derived a formula for calculating their speed, and was the first to propose the use of multi-stage rockets.

Half a century later, Tsiolkovsky’s idea was developed and implemented by Soviet scientists under the leadership of Sergei Pavlovich Korolev.

Sergei Pavlovich Korolev (1907-1966)
Soviet scientist, designer of rocket and space systems. Founder of practical astronautics

Homework.

Task 1. Answer the questions.

1. Based on the law of conservation of momentum, explain why a balloon moves opposite to the stream of compressed air leaving it.
2. Give examples of the reactive motion of bodies.
3. What is the purpose of rockets? Tell us about the structure and principle of operation of the rocket.
4. What determines the speed of a rocket?
5. What is the advantage of multi-stage rockets over single-stage ones?
6. How is a spacecraft landed?

Task 2. Solve the puzzle.

The file “This is interesting!” is attached to the lesson. You can download the file at any time convenient for you.

Sources used: http://www.tepka.ru/fizika_9/21.html

Questions.

1. Based on the law of conservation of momentum, explain why a balloon moves in the opposite direction to the stream of compressed air coming out of it.

2. Give examples of the reactive motion of bodies.

In nature, an example is the reactive movement of plants: the ripened fruits of a crazy cucumber; and animals: squid, octopus, jellyfish, cuttlefish, etc. (animals move by throwing out the water they absorb). In technology, the simplest example of jet propulsion is segner wheel, more complex examples are: the movement of rockets (space, gunpowder, military), water vehicles with a jet engine (hydrocycles, boats, motor ships), air vehicles with air jet engine(jet aircraft).

3. What is the purpose of rockets?

Rockets are used in various fields of science and technology: in military affairs, in scientific research, in astronautics, in sports and entertainment.

4. Using Figure 45, list the main parts of any space rocket.

Spacecraft, instrument compartment, oxidizer tank, fuel tank, pumps, combustion chamber, nozzle.

5. Describe the principle of operation of a rocket.

In accordance with the law of conservation of momentum, a rocket flies due to the fact that gases with a certain momentum are pushed out of it at high speed, and the rocket is given an impulse of the same magnitude, but directed in the opposite direction. Gases are ejected through a nozzle in which the fuel burns, reaching high temperatures and pressures. The nozzle receives fuel and oxidizer, which are forced there by pumps.

6. What does the speed of a rocket depend on?

The speed of the rocket depends primarily on the speed of gas flow and the mass of the rocket. The rate of gas flow depends on the type of fuel and the type of oxidizer. The mass of the rocket depends, for example, on what speed they want to impart to it or on how far it should fly.

7. What is the advantage of multi-stage rockets over single-stage ones?

Multistage rockets are capable of reaching higher speeds and flying further than single-stage rockets.

8. How is a spacecraft landed?

The landing of the spacecraft is carried out in such a way that its speed decreases as it approaches the surface. This is achieved by using a braking system, which can be either parachute system braking or braking can be done using rocket engine, while the nozzle is directed downward (toward the Earth, Moon, etc.), due to which the speed is reduced.

Exercises.

1. From a boat moving at a speed of 2 m/s, a person throws an oar with a mass of 5 kg at a horizontal speed of 8 m/s opposite to the movement of the boat. At what speed did the boat begin to move after the throw, if its mass together with the mass of the person is 200 kg?

2. What speed will the rocket model get if the mass of its shell is 300 g, the mass of gunpowder in it is 100 g, and gases escape from the nozzle at a speed of 100 m/s? (Consider the gas outflow from the nozzle to be instantaneous).

3. On what equipment and how is the experiment shown in Figure 47 carried out? What physical phenomenon is being demonstrated in this case, what does it consist of, and what physical law underlies this phenomenon?
Note: the rubber tube was positioned vertically until water began to flow through it.

A funnel with a rubber tube attached to it from below with a curved nozzle at the end was attached to the tripod using a holder, and a tray was placed below. Then they began to pour water from the container from above into the funnel, while the water poured from the tube into the tray, and the tube itself shifted from a vertical position. This experiment illustrates reactive motion based on the law of conservation of momentum.

4. Perform the experiment shown in Figure 47. When the rubber tube deviates from the vertical as much as possible, stop pouring water into the funnel. While the water remaining in the tube flows out, observe how it changes: a) the flight distance of the water in the stream (relative to the hole in the glass tube); b) position of the rubber tube. Explain both changes.

a) the flight range of water in the stream will decrease; b) as water flows out, the tube will approach a horizontal position. These phenomena are due to the fact that the water pressure in the tube will decrease, and therefore the impulse with which the water is ejected.