The Science Behind Launching a Rocket

No matter if you're setting off a model rocket in a field or launching a giant cargo rocket to Mars, the principles of how they work are the same.

During rocketry's early days, the flight of fire arrows and other rocket devices was all a matter of chance. Aside from flying back then, rockets often skittered around, shot sparks, and exploded in the air. It took centuries of trial and error until they became reliable. Real advancements in rocketry depended on a scientific and mathematical understanding of motion, which was born in the 17^th century due to the works of scientists such as Galileo and Isaac Newton.

For the most part, Newton's Laws of Motion were known intuitively by rocketeers. The laws relate force and direction from all forms of motion, and elevated rocketry to a science.

Image via nasa.gov.

Newton's first law, also referred to as Galileo's law of inertia (which means that all matter, because of its mass, resists changes in motion), as he discovered inertia, shows that an object at rest, such as a rocket on a launch pad, needs the exertion of an unbalanced force to cause it to lift off. To put it simply, the amount of force produced by the rocket engines has to be greater than the force of gravity holding it down. As long as the force of the engines continues, the rocket will accelerate. If the rocket runs of out of propellant, the forces once again become unbalanced, and gravity takes over and the rocket will to fall to Earth.

When a rocket reaches space, atmospheric drag is reduced greatly, sometimes eliminated. In the atmosphere, drag is an unbalancing force, but in space, that force is absent. Once the rocket escapes Earth's gravity at extreme speeds, inertia will cause it to continue on its way outward.

Often written as f = ma, Newton's second law relates force, acceleration, and mass. The implication of this law when it comes to rocketry is that the more propellant (m) you consume and the greater the acceleration (a) of the combustion products of the nozzle, the greater the thrust (f). This law applies to what is traveling out of the engine and not the mass of the rocket propellant inside the rocket to be consumed later on.

Most people are familiar with Newton's third law, which is the principle of action and reaction. When it comes to rockets, the action is the force made by the expulsion of gas, smoke, and flames from the nozzle end of a rocket engine. When a rocket launches, the combustion products from the burning propellants quickly accelerate out of the engine, causing the rocket to accelerate toward the sky. As Newton's first law states, the greater (or smaller) the mass, the greater (or smaller) the force required to move it.

Image via nasa.gov.

Obviously, blasting a rocket into space is much more complicated than Newton's Laws of Motion. Many other things come into play, but Newton's laws are really the beginning of it all.

For example, air pressure is very important while the rocket is in Earth's atmosphere. The internal pressure produced by the burning rocket propellants inside the rocket engine combustion chamber must be greater than the outside pressure in order to escape through the engine nozzle. The outside air is like a cork in the engine. As the rocket soars into the thinning atmosphere, the ambient pressure decreases and the engine thrust increases.

The changing mass of the rocket also plays an important role as it soars into space. As the rocket accelerates upward, it gains thrust due to the outside pressure changes. It also gets a boost because of its changing mass. As the combustion products leave the engine, the mass of the vehicle lightens. Its inertia, or resistance to change in motion, becomes less, resulting in upward acceleration for the rocket.

Though Newton's Laws of Motion explain the simple science behind successfully launching a rocket, whether it's into space or above the trees in your own backyard, the most important step is knowing how to apply those laws. For instance, you need to know how big the rocket must be, and how to make it go where it needs to go, as well as getting it to land safely back on Earth. It's certainly a complicated process, but without Newton's Laws of Motion, which seem extremely simple in comparison, the discoveries outside of our planet wouldn't be known today.

For more information on the science behind launching a rocket, download the research paper below.