The Principles of Rocket Flight

Space Shuttle Discovery lifts off from Kennedy Space Center LC-39A on STS 85. Image credit NASA

It’s no secret that rockets are incredibly complicated machines. Unlike airplanes, rockets don’t have the luxury of using wings to fly. Even the wings on the space shuttle were only used for gliding down to land on a runway. Rockets have to use sheer power to push themselves through the atmosphere against the powerful force of gravity. In order to get into orbit, a rocket must accelerate to over 17,400 mph, almost 10 times faster than a speeding bullet! Additionally, the dense atmosphere and gravity can slow a rocket down during ascent by anywhere from 2,900 to 4,000 mph meaning a rocket launching from earth requires that much more acceleration. So how does a rocket make its journey from the launchpad into space? This article will provide a brief overview of the four forces acting on a rocket: thrust, drag, weight, and lift (to an extent).

A rocket’s forward motion is caused by thrust, the force created by the rocket engines usually placed on the bottom of the rocket facing downwards. I have written another article on how rocket engines work (here) but in short, they create thrust by expelling gas at supersonic velocities through a nozzle that directs the gas in the direction opposite to the rocket’s travel. In other words, thrust can also be explained by Newton’s Third Law of Motion, which states that for every action there is an equal and opposite reaction. As the gas is expelled out in a downward direction, the rocket is propelled upwards by an equal and opposite force.

The Space Shuttle Main Engines ignite at the base of the shuttle creating thrust. Image credit Wikipedia

The force of drag acts in opposition to thrust when a rocket is ascending through the atmosphere. It is the force due to air resistance on the body of the rocket and only occurs when the rocket is in the atmosphere. Drag is calculated using a complex equation. The explanation of the drag equation is outside the scope of this article; however, it should be noted that drag depends on a number of variables including the density of the air, the velocity of the rocket, and the size and shape of the rocket. The important takeaways are that the denser the air, the greater the drag; drag increases exponentially as velocity increases; and that the more streamlined the body of the rocket is, the less drag it will produce. It is probably not natural for humans to think of air as being dense or having weight, but it does. Relatively speaking, the air close to sea level is quite dense and is quite a hindrance when launching a rocket.

As the density of the air decreases, the rocket’s exhaust plume expands creating a spectacular visual effect. Image credit SpaceX

Weight is the force due to the mass of the rocket and the force of gravity pulling it down. Like drag, weight hinders a rocket because gravity wants to pull the rocket back down to Earth. Weight is one of the most important factors considered when designing a rocket because the heavier a rocket is the more thrust it will need to fly. This is known as the thrust to weight ratio (TWR). A rocket’s TWR must be greater than one or the rocket will not have enough thrust to lift off. For example, fully fueled and on the launchpad, the Space Shuttle weighed around 4.5 million pounds. The combined thrust of the three RS-25 Space Shuttle Main Engines and the two massive solid rocket boosters was 6.9 million pounds, giving the rocket a TWR of about 1.5 (6.9/4.5).

At liftoff, the mighty Satun V moon rocket had a lower TWR than the Space Shuttle, meaning it ascended off the pad much slower. Image credit NASA

Lastly, we have lift. As I mentioned before, lift is not utilized on rockets to make the rocket go higher as it is in airplanes. Instead, lift is sometimes used to stabilize a rocket horizontally as it climbs through the atmosphere using small fins usually placed on the very bottom of the first stage of the rocket.

Why does a rocket become unstable on ascent? At launch, gravity is pulling the rocket towards the center of the earth, straight down back towards the launchpad and drag is pushing the rocket straight down as well. Counteracting these forces is the thrust of the engines, which is pushing the rocket straight up away from the launchpad. This means that thrust is pushing in the exact opposite direction of gravity and drag, which is a relatively stable arrangement. Once a rocket has ascended upwards for a period of time, it tips over to gain horizontal velocity, and gravity and drag no longer work in direct opposition to thrust. Consider a rocket tipped over 25 degrees. Gravity is still pulling it straight down but thrust is now pushing the rocket at a 25-degree angle relative to the ground. In this situation, the rocket not only has to continue pushing forward at that 25-degree angle, but it also has to counteract the force of gravity that is trying to pull the front of the rocket down. Like gravity, the air pressure is also pushing the rocket straight down, and it is no longer in direct opposition to thrust. Both these conditions make the rocket inherently unstable.

Fins can be used to control the airflow along the rocket to make sure it stays at a constant pitch, counteracting the forces of gravity and drag that are trying to de-stabilize the rocket. Additionally, fins can be used to counteract any kind of spinning that may occur with a rocket.

Almost all modern rockets that fly today don’t actually use fins for stability. They use something called thrust vector control (TVC), also known as engine gimbaling. The rocket engines actually move around to direct the thrust in slightly different directions. TVC controls the rocket as fins do and can keep it stable by controlling its pitch, yaw, and roll.

An example of rocket engine gimballing

Wrapping things up, remember that the three main forces acting on a rocket are thrust, weight, and drag, but that lift is sometimes used for stability purposes. Weight is brought on by the mass of the rocket being pulled down by gravity and drag is caused by air resistance. Thrust (hopefully) counteracts both weight and drag to push the rocket upwards and into space and onto orbit.