What Is Thrust? How Rockets Defy Gravity in Physics
Rocket science is the primary branch of aerospace engineering, which is the science of building, or designing rockets.
All the rockets follow Newton’s three laws of motion.
According to Newton’s first law, a rocket will stay on the launch pad until a force blast it off. Once in space, a rocket will proceed to move unless retrorockets are fired to slow down the rocket.
Newton’s Second Law : Force on a body equals the product of its mass multiplied by acceleration.
F = m * a
So, the main forces acting on a rocket in flight are the weight of the rocket, thrust of the rocket engines, and drag.
Now, as the rocket moves through the air, during its flight, it undergoes a few operations that its weight gets greatly reduced, therefore; it achieves a greater acceleration.
Newton’s Third Law of Motion: This law states that it is necessary to keep the rocket moving so that it ejects a high amount of gas at high speeds.
A rocket can lift off from a launchpad only when it ousts gas at high speeds from its engine. The rocket thrusts against the gas, and the gas, in turn, propels the rocket.
What causes Thrust in the Rocket?
Thrust is a force with which the rocket moves upwards. It is given by,
F = - u dm/dt
In space, rocket engines are ordinarily called reaction engines because the law of reaction crusades the spacecraft to move in a direction opposite to that of the engine’s thrust plume.
The negative sign in the formula indicates that thrust on the rocket is in a direction opposite to the direction of escaping gases.
How does a Rocket Work?
The term rocket science is often used to describe a concept that is quite difficult to comprehend.
Let’s understand the technology behind its working in a simple yet scientific manner.
To eject the high-speed mass from the rocket, a liquid fuel oxidizer mixture is burnt in the rocket combustion chamber, the combustion chamber also helps fuel and oxidizer to mix them efficiently.
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A high-speed jet is passed through a rocket nozzle, the function of the nozzle is to increase the exhaust velocity even further thus increasing the rocket’s thrust.
These types of nozzles are called converging-diverging nozzles.
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So, the subsonic flow gets converted to supersonic flow with the help of such a nozzle.
The liquid fuel before entering the combustion chamber travels around the nozzle body.
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This helps to reduce the nozzle’s cover temperature and also results in some energy savings.
To pump fuel and oxidizer at an adequate flow rate, two pumps are used. They are:
Fuel Pump
Oxidizer Pump
They both are connected with the same shaft.
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The pump-turbine is referred to as a turbopump. A gas generator produces hot gas which will turn the turbine. A bypass jet of fuel and an oxidizer are fed into the gas generator for combustion; exhaust from the turbine is mixed with the main rocket exhaust. This unit of the rocket is called the rocket engine.
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Here, the rocket engine is peculiarly the liquid propellant rocket engine.
The fuel and oxidizer required for rocket engine are started to two large tanks as shown below:
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During the liftoff, the thrust generated by the main engine may not be adequate.
Generally, a few solid propellant strap boosters are used to assist the liftoff.
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Solid propellant strap boosters
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Inner view of the solid propellant rocket
The rocket starts with zero speed at the ground.
However, it must accelerate at the final speed of 28000 Kmph to achieve orbit successfully.
The solid propellant strap boosters are burned off very quickly.
So, to reduce the weight of the rocket, they are abandoned after the burn-off.
This process is known as rocket staging.
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Rocket staging
When the main engine is burned off, it is also abandoned.
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The main engine abandoned
The next engine takes over the charge. In this way, the rocket’s weight is greatly reduced.
So, by the relation: F = m * a
Mass is reduced, therefore, it achieves greater acceleration.
Finally, after a few stages of operation, the payload is put into the desired orbit.
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The rocket staging up to five have been successfully tested.
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Payload set in its orbit
So how is the rocket able to maneuver its destination?
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So, there is a modern car technique named cabled thrust also called the Gimbaled Thrust.
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Diagram: Gimbaled thrust
Here, the rocket nozzle is tilted with high precision devices.
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Diagram: Gimbaled angle
Any deviation in the normal angle will produce a torque which in turn will make the rocket’s body rotate.
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Therefore, after achieving enough turn, the gimbal angle is set to 0.
What Rocket Scientists do?
Rocket scientists are aerospace engineers who are experts in the designing and manufacturing of spacecraft.
They diligently work with the principles of science and engineering to create vehicles that aviate within or above the Earth’s surface.
FAQs on Rocket Science: Key Concepts and Thrust Explained
1. What is the fundamental principle of rocket propulsion?
The fundamental principle of rocket propulsion is based on Newton's Third Law of Motion and the law of conservation of linear momentum. For every action, there is an equal and opposite reaction. A rocket expels hot gases downwards (the action), which creates an equal and opposite force, known as thrust, that pushes the rocket upwards (the reaction).
2. how does a rocket generate thrust in the vacuum of space where there is no air to push against?
A common misconception is that rockets need air to push against. however, a rocket's engine works by pushing against the propellant it expels, not against the external environment. According to the law of conservation of momentum, as the rocket ejects high-velocity exhaust gases in one direction, the rocket itself gains momentum in the opposite direction. This principle allows a rocket to accelerate effectively in the vacuum of space.
3. What is the role of a converging-diverging (de Laval) nozzle in a rocket engine?
A converging-diverging nozzle is a critical component that maximises a rocket's thrust. Its primary role is to accelerate the hot, high-pressure exhaust gases from the combustion chamber to supersonic speeds.
- The converging section compresses the subsonic gas, increasing its velocity.
- The diverging section then allows the gas to expand and accelerate to speeds greater than the speed of sound.
4. What are the main types of rocket propellants and how do they differ?
Rocket propellants are the chemical mixtures burned to produce thrust. The main types are:
- Liquid Propellants: These involve a liquid fuel and a liquid oxidiser stored in separate tanks and pumped into a combustion chamber. They offer high performance and can be throttled, shut down, and restarted, providing precise control. Examples include Liquid Oxygen (LOX) and Kerosene (RP-1).
- Solid Propellants: here, the fuel and oxidiser are mixed together into a solid block. They are simpler, more stable, and provide high thrust quickly. however, once ignited, they cannot be controlled or shut down.
5. Why are multi-stage rockets necessary for launching satellites into high orbits?
Multi-stage rockets are used because of the limitations described by the Tsiolkovsky Rocket Equation. To achieve the high velocity needed for orbit, a rocket must shed mass as it ascends. By jettisoning a stage after its fuel is consumed, the rocket becomes lighter. This allows the remaining stages to accelerate the payload more efficiently, achieving a higher final velocity than a single-stage rocket with the same amount of fuel could.
6. What is the Tsiolkovsky Rocket Equation and what is its significance?
The Tsiolkovsky Rocket Equation is a cornerstone of rocket science, given by Δv = vₑ ln(m₀ / mᵣ). It calculates the maximum change in velocity (Δv) a rocket can achieve. Its significance lies in explaining the key factors for rocket performance:
- vₑ (Exhaust Velocity): The speed of the gases leaving the engine. higher exhaust velocity leads to a greater change in velocity.
- ln(m₀ / mᵣ) (Mass Ratio): The natural logarithm of the initial mass (m₀, with fuel) divided by the final or residual mass (mᵣ, without fuel). A higher mass ratio (more fuel relative to the rocket's structure) is crucial for achieving high speeds.
7. how does the law of conservation of momentum apply to rocket propulsion?
The law of conservation of momentum states that the total momentum of a closed system remains constant. A rocket and its unburnt fuel at rest on the launchpad have zero total momentum. When the engine ignites and ejects exhaust gases with a certain momentum in the downward direction, the rocket must gain an equal amount of momentum in the upward direction to keep the total momentum of the system (rocket + gases) at zero. This upward momentum gain is what causes the rocket to accelerate.
8. What is the difference between thrust and specific impulse (Isp) in rocketry?
While both relate to rocket performance, they measure different things:
- Thrust is the raw force that pushes the rocket forward, measured in Newtons (N). It determines how quickly a rocket can accelerate and overcome gravity. high thrust is needed for liftoff.
- Specific Impulse (Isp) is a measure of the engine's efficiency. It describes how much thrust is produced per unit of propellant consumed per second. A high Isp means the engine can generate thrust for a longer duration with the same amount of fuel, making it ideal for long-duration manoeuvres in space.
