Sir Isaac Newton

What is known about rocketry today can be traced back to the time of Sir Isaac Newton (1642 - 1727).  Newton described the motion of objects falling to the Earth in his book Philosophiae Naturalis Principia Mathematica where he outlined three laws of motion. Although Newton was merely describing principles of nature, Newton’s Laws apply to the physics of rocketry.  His laws are simple statements regarding the physics governing motion and can be used to provide precise explanations of rocket flight.

Newton’s First Law of Motion

Objects at rest will remain at rest and objects in motion will remain in motion in a straight line unless acted upon by an unbalanced force. 

During a model rocket flight, forces become balanced and unbalanced all the time. A rocket on the launch pad is in a state of rest.  This is called inertia. A rocket is balanced all the time because the surface of the pad pushes the rocket up while the force of gravity tries to pull it down.  An unbalanced force must be exerted for a rocket to lift-off from the launch pad. A rocket blasting off the launch pad changes from a state of rest to a state of motion. It will keep moving in a straight line at the same speed unless it is acted upon by an unbalanced force (gravity and drag).  There are four forces (drag, gravity, thrust, lift) that act on all objects that travel through the air. Drag and gravity are the two unbalanced forces that act on a model rocket. Drag is the resistance or frictional force between the surface of a moving object and air.  Drag increases with speed. Gravity is the force pulling an object back to the surface of the Earth.  The amount of this force is proportional to the mass of the object.  When the rocket lifts off the launch pad it is guided by the launch rod in a straight line upward. The unbalanced forces (drag and gravity) cause it to arch and fall to the ground.

Newton’s Second Law of Motion

Force is equal to mass times acceleration. (F = ma)

The amount of thrust (force produced by a rocket engine) will be determined by the mass of gases created and how fast the gas escapes the rocket. The greater the rate at which the rocket fuel is burned and the faster the velocity of the escaping gas, the greater the thrust of the rocket engine. 

Newton’s Third Law of Motion

For every action there is an equal and opposite reaction.

With rockets, the action is the expelling of gas out of the engine. The reaction is the movement of the rocket in the opposite direction.  The rocket is pushed by the escaping gases produced by the chemical reaction of fuel and oxidizer combining in the combustion chamber. 

Center of Gravity / Center of Pressure

The basic principle is that the center of gravity must be ahead of the center of pressure for the rocket to be stable. The center of gravity (CG) is the point at which the mass of the rocket is balanced because the weight forward from this point is equal to the weight to the rear of this point. (Think of this as balancing a pencil on your finger. The pencil will balance when there is an equal amount of mass on both sides of your finger.)  The center of pressure (CP) is the point on the rocket at which half of the aerodynamic surface area is located forward and half to the rear.

Fins make the rocket fly straight. A rocket without fins will tumble around its CG (also called the balance point) when flying through the air like a balloon that is inflated and then let go. The balloon will fly erratically because it is uncontrolled. With fins, a rocket has more surface area behind the CG than in front. When the rocket is flying through the air, the air has more surface area to push against behind the balance point than in front because of the greater surface area provided by the fins. Therefore, the rocket tends to stabilize itself. The rocket will rotate until the nose is pointing forward in the air and the fins are pointing backward. 

If you point a rocket straight up, the CP should be below the CG.  This will allow the rocket to have a stable flight.  You can accomplish this by adding fins (as noted above) or by adding mass to the front of the rocket. meaning if the rocket is pointed upward, the CP should be below the CG. Neutral stability is when the CG and CP are at the same point and the rocket’s flight will be random.  If the CP is in front of the CG (in other words, the surface area is greater towards the front or top of the rocket), then the flight will be unstable.