Consider a machine gun mounted on a lightweight cart. If the gun is fired, the bullets go in one direction while the cart recoils in the other.
The magnitude of the momentum of the bullets equals the momentum of the cart but the directions are opposite. Thus, one momentum is positive and one is negative, making the total change their sum zero.
Although things are now moving, the total momentum of the gun-cart system has not changed. In a similar manner, a rocket moves in space because the gases are given momentum as they are expelled by the rocket engine.
Consider the rocket resting in space. There is no momentum in the system. Next, the engine ignites. As the exhaust gases go in one direction, the rocket goes in the other to keep the total momentum of the system constant. This momentum change of the gases gives the rocket the "push" to go forward. We call this push, the thrust of the rocket, i. This thrust depends upon the speed of the exhaust gases and the mass of gas being expelled each second, sometimes called the burn rate in pounds of fuel per second.
On Earth, air tends to inhibit the exhaust gases getting out of the engine. This reduces the thrust. Rockets and engines in space behave according to Isaac Newton's third law of motion: Every action produces an equal and opposite reaction.
When a rocket shoots fuel out one end, this propels the rocket forward — no air is required. NASA says this principle is easy to observe on Earth. If you stand on a skateboard and throw a bowling ball forward, that force will push you and the skateboard back. In other words, when one object exerts a force on a second object, that second object exerts a force on the first object that is equal in magnitude, but opposite in direction.
So, when a rocket violently pushes gas out of its nozzles, that same gas, a plasma composed of a myriad of tiny atoms accelerated at very high speed, pushes in unison on the rocket, propelling it forward. The more massive the object you throw, the more the boat will accelerate in the other direction and the faster it will move. What about a rocket manoeuvring in the vacuum of space? Remember Sandra Bullock in Gravity, using a fire-extinguisher to propel herself from one space station to another?
In the case of chemical propulsion, propellant is burned in a combustion chamber that produces very hot, high-pressure combustion products. These combustion products are accelerated through a convergent-divergent nozzle bell shape , which raises the gas velocity to the speed of sound at the throat point of minimum cross-sectional area and then further accelerates the flow beyond the speed of sound in the divergent section of the nozzle.
This velocity of the combustion products, combined with their mass, is the momentum which defines the reaction force. This can also be achieved with electric propulsion, which emits charged particles at much higher velocity but with much lower mass.
In addition, there is a pressure force acting on the surface of the divergent nozzle section, which is dependent upon the difference between the ambient pressure and the pressure at the nozzle exit plane. Phil Gadsby, propulsion engineer, Dawn Aerospace.
Reaction force is exactly how they work. It throws part of its mass one way the burning fuel which was fully inside the rocket and it goes the other way. On average, the original mass is still moving at the same rate conservation of momentum. On Earth, things can push against other objects using friction what the tyres of a vehicle do to make it move, or the wings of a plane do against the air to help it turn which you cannot do in space, hence the pointlessness of X-wings in Star Wars films that appear to make the spacecraft bank and turn, when they would do nothing of the sort.
A real spacecraft would have to push fuel fast to the right to start to move to the left, but the way it turns looks completely different to the way an aircraft turns in the atmosphere. A large fraction of the rocket engines in use today are chemical rockets; that is, they obtain the energy needed to generate thrust by chemical reactions to create a hot gas that is expanded to produce thrust. A significant limitation of chemical propulsion is that it has a relatively low specific impulse, which is the ratio of the thrust produced to the mass of propellant needed at a certain rate of flow.
Historically, these propellants have not been applied beyond upper stages. Furthermore, numerous concepts for advanced propulsion technologies, such as electric propulsion, are commonly used for station keeping on commercial communications satellites and for prime propulsion on some scientific space missions because they have significantly higher values of specific impulse.
However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.
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