Rocket Propulsion: Definition & Principles

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  • 0:04 Understanding Rocket…
  • 0:55 Types of Rocket Propulsion
  • 2:01 Principles of Propulsion
  • 5:04 Lesson Summary
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Lesson Transcript
Instructor: Lori Jones

Lori has a degree from Stanford, was Principal of a K-12 private school that she started, has a Master's degree, and taught at the high school level.

Rockets and traveling to space has long been a fascinating subject for young and seasoned minds alike. In this lesson, you'll get an overview of rocket propulsion and learn some of its principles, concepts, and terms.

Understanding Rocket Propulsion

The loud rumble of the engines. The sparks shooting from boosters. The steam rising from the ground as a structure weighing in at approximately 2,000 tons lifts off and escapes our atmosphere to enter the vastness of space!

Many of us have been awed by a rocket or a space shuttle launch, but we don't often think about the process and technology required. We may have heard the term 'rocket propulsion,' but we might not quite understand everything that's involved in getting a rocket or shuttle through the earth's atmosphere and into space.

When rockets leave the launch pad, they need a lot of power to lift them off the ground. Accomplishing this feat, however, isn't easy when you consider that rockets can weigh upwards of 4.4 million pounds.

Because of this massive weight, rockets have to rely on a powerful propulsion system. Rocket propulsion is the process that uses force to move a rocket off the ground and into the atmosphere.

Types of Rocket Propulsion

For propulsion to work, rockets need a solid or liquid propellant, which consists of a fuel source and an oxidizer, the source of oxygen that's needed to burn the fuel.

There are two types of rocket propulsion that will launch a rocket off the ground:

  1. solid propulsion: the fuel and oxidizer are combined as a solid
  2. liquid propulsion: the fuel and oxidizer are stored separately, then later combined and ignited

Solid propulsion rockets, primarily used as boosters for launch, tend to be safer, more reliable, lighter, and lower in cost. However, once ignited, the engine cannot be shut down or restarted, and the amount of power cannot be controlled.

Liquid propulsion rockets are more complicated and costly, but are also more controllable, can reach greater velocities, and can be both shut down and restarted.

In NASA's shuttle program, both types of engines have been used during a launch: the engines of the space shuttle used liquid fuel, and the two solid rocket boosters (SRBs) used solid fuel. Once the solid fuel of the SRBs was exhausted, these boosters were separated from the shuttle, and later recovered for future use.

Principles of Propulsion

Here are the different principles of rocket propulsion, given one at a time.

1. Combustion

The presence of the fuel and oxidizer alone, however, isn't enough to launch a rocket. A chemical reaction (the burning of the fuel) must take place in a controlled environment. This is called combustion.

This environment is found in the three main components of a rocket engine: the combustion chamber, the throat, and the nozzle.

2. Thrust

The propellant moves through these three components, burns, and converts to gas. This rapid transformation produces thrust, which is the mechanical force or push that moves a rocket off the ground and/or through the air.

In thrust, we see Newton's third law of motion in action: ''Every action has an equal but opposite reaction.'' Gasses rapidly leaving the engine push downward (action), which then forces the rocket upward and off the ground (reaction).

Because rockets vary in size, weight and type of propellant, the thrust required also varies. The heavier the rocket is overall, the more thrust is required to lift it off the ground. Ironically enough, most of the weight of a rocket at launch is taken up by the fuel.

To further complicate matters, as the rocket ascends, the solid and/or liquid propellant continues to burn, which reduces the weight of the rocket with every passing second. The thrust must therefore be adjusted until the rocket leaves the atmosphere. This is done through liquid propulsion, as the thrust of solid propulsion rockets cannot be controlled.

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