Thrust is the mechanical force that propels an object forward through the air or space. It is the force generated by an engine or propulsion system designed to move the vehicle and overcome the opposing force of drag, which is the resistance from the surrounding fluid, like air or water. A propulsion system must be in physical contact with a working fluid, whether it is air, water, or exhaust gas, to produce this force.
The Core Principle of Thrust Generation
The fundamental physics of thrust generation is rooted in Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction. A propulsion system generates thrust by accelerating a mass of material in one direction. This action causes an equal and opposite reaction force to push the vehicle in the forward direction. This accelerated mass is often a gas, which is expelled at high speed to the rear of the engine.
A helpful way to visualize this action-reaction principle is by considering a simple, untied balloon filled with air. When the neck of the balloon is released, the compressed air rushes out in one direction, which is the action force. The escaping air then causes a reaction force, which is the thrust that pushes the balloon in the opposite direction. Rocket engines and jet engines operate on this same principle, only with much greater force and control.
The engine accelerates a working fluid, such as air or exhaust gas, in a controlled manner. Internal combustion of fuel within the engine provides the energy necessary to accelerate this mass. The resulting force pushes against the engine structure, pushing the entire vehicle forward. The magnitude of this thrust must be greater than the drag acting on the vehicle for it to accelerate forward.
Quantifying Thrust: Mass Flow and Velocity
The amount of thrust generated by an engine is directly determined by the characteristics of the mass being expelled. The magnitude of the propulsive force depends primarily on two interrelated factors: the mass flow rate and the exhaust velocity. Mass flow rate refers to the quantity of material, such as air or exhaust gas, that is being moved through the engine and expelled per unit of time.
The second factor is the exhaust velocity, which is how fast that mass is pushed out the back of the engine. More thrust is produced by accelerating a large mass to a moderate velocity or by accelerating a smaller mass to a high velocity. For example, a propeller engine moves a large amount of air at a low velocity, while a rocket engine expels a smaller mass of hot exhaust gas at a high velocity.
Increasing the density of the air entering an engine increases the mass flow rate and consequently the thrust. Higher air pressure results in denser air entering the engine, increasing thrust. Conversely, an increase in air temperature causes the air’s density to decrease, reducing the engine’s thrust output.
Thrust in Action: Different Propulsion Systems
Propulsion systems are categorized into air-breathing and non-air-breathing engines, based on how they acquire the necessary oxidizer for combustion. Air-breathing engines, which include turbofans, turbojets, and propeller systems, rely on the oxygen contained in the surrounding atmosphere to burn their fuel. These engines draw in vast quantities of air, compress it, mix it with fuel, ignite the mixture, and then expel the resulting hot, high-velocity gas to generate thrust.
Turbofan engines, commonly used on commercial airliners, are highly efficient. A large fan at the front pushes air around the engine core, generating a large proportion of the thrust. This bypass air is accelerated to a moderate speed, increasing the mass flow rate and improving efficiency. Propeller systems also move a high volume of air to create thrust mechanically, rather than through a combustion-driven jet of gas.
Non-air-breathing engines, most notably rockets, carry both their fuel and an oxidizer, such as liquid oxygen, onboard. This self-contained design means rockets do not depend on the atmosphere and can operate effectively in the vacuum of space. The combustion of the onboard propellants creates hot, high-pressure gases that are then accelerated through a nozzle to produce thrust.
Rockets must generate enough thrust to overcome the vehicle’s weight and accelerate it into orbit, which necessitates a very high exhaust velocity. Because they carry all their reaction mass, rockets are heavier than air-breathing engines. However, their ability to function in any environment makes them the only choice for space travel.