Let’s Explore Rocket Propulsion

Even though the Chinese introduced rockets about 800 years ago, most of the important rocket development has taken place in the 20th Century. A rocket engine is the component that makes travel at top velocities possible and it is fueled through high-pressure movement of propellant material according to certain laws of physics – namely Isaac Newton’s Third Law of Motion. Rocket engines have structures that give them advantages over other types of engines and scientists are frequently applying technological improvements to them.

Simple Liquid Rocket Schematic

Background and Development

The guiding idea behind rocket engine movement is that the exertion of force must always be between two objects rather than onto a single object. Newton’s Third Law dictates that the force causing movement between these objects must be equal in strength in order for the desired speeds to be reached. Experiments with propulsion of objects according to this law date back to the first century BC; however, the full potential for its application to rocket engine technology was not fully understood until after the Industrial Revolution of the nineteenth century.

Beginning with the space race between the United States and Russia in the mid-twentieth century, scientists and engineers applied this law of physics to modern engine-building in earnest. Each nation had the goal of sending the fastest, lightest, strongest, and safest spacecraft into orbit. In the following decades, jet engines built according to these specifications soon began appearing in other crafts alongside the U.S. space shuttle – namely military aircraft and missiles.


The distinction between these types of engines and others is the structure of the internal combustion engine. The majority of widely used rocket engines fall into this category, though a smaller number have been built as non-combustible engines. In order for an internal combustion engine to operate, it needs to have a supply of fuel that is ignited by an oxidizing agent. When this occurs, the extreme pressure that results is forced through one or more nozzles. This pressure acts as the propelling agent that pushes the rocket forward at very high rates of velocity.


The pushing of a rocket forward and/or upward is known as thrust, and thrust is the by-product of exhaust created by the fuel supply. In most cases, this supply is from fossil fuels. The energy generated from the fuel’s heat creates quite high speeds as the exhaust is expelled from the rocket nozzles, pushing the rocket in the opposite direction as this exhaust. The flames that are visible from the back of a moving rocket are actually combusted fuel exhaust.

Modern rocket engines need both very high pressures and very hot temperatures to reach the desired speeds, particularly to create what is known as escape velocity in space travel. A rate of velocity higher than the pull of the earth’s gravitational atmosphere is required to reach orbit above the planet.

Chemical Reaction

Rockets with a chemical reactor engine are among the most frequently built and have been designed for fuel use efficiency according to basic thermodynamics. Fuel heated to extreme temperatures is done so through an exothermic reaction, simply meaning one from an external source. In order for this reaction to work correctly, a rocket needs to be fitted with a combustion chamber to allow for the oxidizer to reach the fuel in a controlled area. Other rocket engines without a combustion chamber can still be powered to the required levels using a separate heat exchanger. Use of these different chemical reaction systems often depends on the rocket’s intended purpose.

Additional and notable rocket propulsion systems under development and experimentation include:

  • Nuclear/Thermonuclear
  • Solar Propulsion
  • ION Propulsion
  • Plasma

Image Credit: NASA

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