How Do Rocket Engines Work?

thrust, propellant systems enable exit from atmosphere

By Jonny Lupsha, Wondrium Staff Writer

Rockets differ fundamentally from planes and helicopters in terms of propulsion. Planes and helicopters rely on aerodynamic design for lift and the atmosphere for combustion. How do rockets fly?

Successful launch of a space rocket into outer space. Spaceship lift off into the starry sky.
Photo by Alones / Shutterstock

Airplanes depend on things like the shape of their chassis and wings to generate lift in order for them to leave the ground. The air they pass through while taxiing splits at the front of their wings, and due to their shape, air pressure is decreased on top of the wing and increased under it. This provides a downward force on the air and an upward force on the aircraft, enabling liftoff.

Rockets are a different story. The recent SpaceX launch of its rocket, Starship, began with several thrusters not firing before the rocket appeared to begin spinning. Finally, it exploded several minutes after launch, leaving many asking, “How do rockets fly?” In his video series The Science of Flight, Dr. James W. Gregory, Professor of Mechanical and Aerospace Engineering at The Ohio State University, outlines the non-exploding method of rocket flight.

How Do Rockets Fly in Space?

“Like a jet, the main force present in rockets is thrust, and that thrust is achieved by producing high pressure inside the combustion chamber and nozzle relative to the ambient pressure,” Dr. Gregory said. “The pressure difference creates a force that pushes the rocket along, and a result of the pressure difference is an acceleration of propellants.”

However, unlike a jet engine, rockets can operate outside the Earth’s atmosphere because they don’t depend on available oxygen for combustion purposes. Instead, rockets have their own oxidizers that aid in fuel combustion. In other words, the thrust produced by a rocket owes to the pressure difference between the combustion chamber—which is very high—and the ambient pressure, which is not. Put it all together and you have the acceleration of propellants mentioned earlier.

“The dominant factors that describe thrust production are the mass flow rate of the propellant expelled from the nozzle, and the exit velocity of those propellants,” Dr. Gregory said. “A rocket plume with high mass flow and high exit velocity will generate the highest thrust. It turns out that the same thrust equation we used for the jet engine is applicable here, which is an expression of Newton’s second law.”

How Do Rocket Propulsion Systems Work?

The two main types of rocket propellant systems are solid and liquid. Solid propellant systems feature pre-mixed fuel and oxidizer in solid form, which was used in the space shuttle’s twin solid rocket boosters. Solid propellant avoids the complexity of large, pressurized storage tanks for the fuel and oxidizer and is less volatile. However, once it’s lit, it can’t be stopped—plus, its specific impulse is less than that of a liquid propellant.

Specific impulse, Dr. Gregory said, comes from taking the thrust of a rocket and dividing it by the time rate of change of weight. For solid propellant, it’s between 200 and 300 seconds, which is less than ideal. So, what about liquid propellant?

“The main idea is to inject the propellants at very high pressure into a combustion chamber,” Dr. Gregory said. “One of the most common rocket fuel and oxidizer combinations is liquid hydrogen and liquid oxygen. The combination of these two produces a very energetic reaction that results in heat and water. The resulting specific impulse of this combination can be as high as 455 seconds for a vacuum.”

The downside of liquid propellant is that since the propellant must be stored in liquid form to keep the volume down, the process of fueling and onboard storage must be done at cryogenic temperatures—below 20 Kelvin for hydrogen and 135 Kelvin for oxygen.

The Science of Flight is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily