When you think about Space, one tends to visualize a giant vacuum devoid of any measurable form of air. You will be correct, but this inevitably raises the question of how rockets are able to maneuver and accelerate in Space.
Rockets are able to accelerate in Space through Newton’s Third Law of Motion by generating and pushing hot gases at high velocities through the back of the vehicle’s nozzle, which produces thrust that propels the rocket forward. Increasing its thrust allows a rocket to accelerate in Space.
Whenever objects on or near the Earth’s surface move around, it is not that difficult to understand their movement or provide a logical explanation of how they can move, slow down, or accelerate.
For example, whenever one observes an automobile traveling on the road, one can understand that the engine provides power that drives the wheels, and it’s the friction caused between the rotating wheels and road surface that allows it to move forward.
A propeller-driven aircraft’s ability to fly and stay in the air is also relatively easily understood when the forward motion of the engine’s thrust and the lift provided by the wings, which allows the plane to stay in the air, is explained.
However, it can be difficult to grasp a rocket’s ability to maneuver in Space where there is no surface to provide friction for movement or any air to push against. Fortunately, this puzzle was solved in the 1600s when Sir Isaac Newton published his Three Laws Of Motion.
As the following section illustrates, these laws apply to the motion of all objects in the known universe but are especially applicable to a spacecraft’s ability to move and accelerate in the vacuum of Space.
How Newton’s Three Laws Of Motion Apply To A Rocket’s Movement In Space
In 1685, Isaac Newton published his Three Laws Of Motion in a publication called “Philosophiæ Naturalis Principia Mathematica.” These laws primarily describe the state of an object’s motion and its relationship with the external forces acting upon it.
In summary, these laws state that:
- An object will remain in a state of rest or a uniform motion in a straight line unless acted upon by an external force.
- The rate of change in the motion of an object acted upon is proportional and in the direction of the force acting upon it.
- For every action, there is an equal and opposite reaction.
All three principles can directly be applied to a rocket’s motion and explain how it can accelerate in Space. However, it is Newton’s Third Law Of Motion that is specifically applicable to a spacecraft’s ability to maneuver and accelerate in Space.
By describing how each law works in practice, it will help to explain how a spacecraft utilize them for maneuvering in a vacuum:
1) An Object Will Remain In A State Of Rest Or A Uniform Motion In A Straight Line Unless Acted Upon By An External Force
This law simply means that if no external forces are present, a stationary object will remain stationary, and an object in motion will continue in the same direction and speed it is traveling in. In the world we live in, however, there are always external forces at play.
For example, a tennis ball lying motionless on the ground has two forces acting upon it. The Earth’s gravity pulls it down, while the surface it is lying on pushes it up. (This is, in fact, an example of Newton’s third law, but more on that later.)
However, Newton’s first law can still be seen in practice here. The ball will remain motionless on the ground until an external force, like a shoe kicking it, or the wind blowing it, causes it to start moving.
A rocket’s movement in Space is the perfect example of an object in motion. If no external gravitational forces were present, a spacecraft in motion would continue to travel at a constant velocity in a straight line indefinitely.
This illustrates Newton’s first law as external forces like air resistance, which would slow the craft down in the atmosphere, and the Earth’s gravity, which would pull it to the surface, are not present.
2) The Rate Of Change In The Motion Of An Object Acted Upon Is Proportional And In The Direction Of The Force Acting Upon It
Newton’s second law describes how an external force influences the motion of an object. Whether an object is stationary or in motion, the strength and direction of an external force will determine the resulting change in speed and deflection of the object.
For example, as the diagram above illustrates, if an aircraft is traveling in an easterly direction encounters a crosswind traveling in a south-westerly direction, it will cause the craft to slow down and be deflected in a southerly direction.
It also explains how a rocket is able to reach orbital velocity. After the launch vehicle’s first stage accelerates the craft into the upper atmosphere, the second stage’s thrust adds to the existing velocity, allowing the rocket to increase its acceleration into orbit.
(Learn more about how exactly a rocket works and the different components that make up a launch vehicle in this article.)
3) For Every Action, There Is An Equal And Opposite Reaction
This is the law most applicable to rockets and their ability to fly and maneuver in both the atmosphere and in Space. Essentially, it means that when two forces interact with each other, one force’s reaction is equal but in the opposite direction of the force applied to it.
A rocket typically works by combining its fuel with an oxidizer in the rocket’s combustion chamber, where it is burned to produce hot gases, which are pushed out the vehicle’s nozzle at supersonic speeds. This, in turn, propels the rocket forward.
In terms of Newton’s Third Law, the first force that comes into play is the thrust created by the rocket’s engine as it pushes the hot gases out the rear through the nozzle. The thrust from the gases creates the second force, which pushes the vehicle forward as a reaction.
It is this principle that also gives a rocket engine the unique ability to operate in the vacuum of Space, unlike aircraft engines used within Earth’s atmosphere. A conventional aircraft requires two conditions to function and stay airborne, which a rocket doesn’t:
- An atmosphere
Oxygen is needed by aircraft engines for burning fuel to function and rotate their turbines/propellers to push air to the back, allowing an aircraft to move forward. Rockets don’t need external oxygen, as they carry their oxygen internally in pressurized tanks.
An atmosphere is necessary for aircraft to stay airborne since the air allows enough lift to be created beneath its wings, which, combined with a high enough horizontal speed, keeps the vehicle in the air.
However, there is no air present in space for a craft to push against to move forward. Since rockets use Newton’s Third Law Of Motion, they don’t need air to maneuver since the gases that are pushed out the back of the rocket thrusters propel the vehicle forward.
It is important to note that this article focused on the mechanisms & principles that propel a spacecraft in a specific direction. To perform maneuvers like changing its orientation or rotation, it uses a Reaction Control System, which one can learn more about in this article.
As this article explained in detail, rockets are able to accelerate, slow down and perform complex maneuvers in Space because, like all other objects, they have to adhere to the laws of physics. In particular, the principles laid out by Newton’s three Laws Of Motion.
Although rockets are some of the best examples that can be used to illustrate Sir Isaac Newton’s Laws Of Motion, these laws apply to all objects in the known universe. Anyone reading this article right now is experiencing at least one of these laws at this very moment.
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