Connection to Newton's 3 laws

Newtons 1^st law

Newtons 1^st law is that an object will remain at rest or will keep moving unless acted upon by an external force also known as inertia. Inertia is an objects ability to resist change. The main factor is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object maintains a constant velocity. This is relevant in rocket propulsion as before firing, the rocket has the airspeed of zero but when the engine fires thrust increases from zero and the weight slightly decreases as the fuel burns. Eventually when the thrust is greater than the weight the rocket accelerates upwards and the velocity increases. Ultimately newtons first law is relates to rocket propulsion because the rocket will not move until acted upon by an external force and inertia is the ability of the rocket to resist change.

Newtons 2^nd law

Newtons second law is force = mass x acceleration (F=M x A). Several forces act upon a rocket. Launching a rocket involves a propulsive force. The rocket can be made to move more rapidly in two ways. They are either increasing the propulsive force and/or decreasing the rocket’s mass. Acceleration is how quickly a rocket increases speed off the launch pad. Rocket acceleration equals the force applied, divided by the mass of the rocket. As the force is increases the acceleration increases. And similarly as mass increases the acceleration decreases. When a rocket is being launched (thrust) the acceleration rate should be maximized. Propulsive force must be maximized because of a= f x m. The force that makes the rocket change its speed by slowing it down is drag. Drag increases the rate at which the rocket slows down. Once the thrust force has stopped applying, the speed at which the rocket is moving can be kept fairly constant by minimizing the drag force.

Newtons 3^rd law

Newton’s third law is most common among rocket propulsion. Newton’s third law is ‘for every action or force there is an equal and opposite reaction.’ Rockets use the third la in their launch as the exhaust flow is pushed backwards the engine is propelled forward.  The rocket continues to accelerate because the use of propellant drives the mass of the rocket down, while the thrust of the engine continues the same. Various propulsion systems develop thrust in different ways, but all thrust is generated through some form of application of Newton's third law of motion.

all images ownership of http://www.nasa.gov/