Introduction

When we see a rocket launched into flight--whether it's the space shuttle leaving for orbit or a bottle rocket jetting out of the back yard--we have a least some intuitive understanding of what makes it go. Even a young child knows that it's the fire squirting out of the base.

That's not really rocket science. And it doesn't take a rocket scientist to realize that in order for the rocket to get off the ground, the thrust of the engine must be great enough to first overcome all the forces tending to hold the rocket down, then accelerate it from rest to a very high velocity.

The forces tending to hold the rocket back are simply the weight of the rocket together with the weight of the fuel, and the aerodynamic drag force which results from the motion of the rocket through the air.

With a little thought, you can probably see that these forces are not going to remain constant during the rocket flight.

• The fuel on board the rocket is going to burn, causing the total weight to grow ever smaller until all the fuel is used up.

• As you may remember from physics, the pull of gravity will decrease very slightly as the rocket moves away from the surface of the earth. However this effect will be much, much smaller than that caused by the loss of fuel mass.

• The drag force, that due to resistance from the atmosphere surrounding the vehicle, will be very small at first but will increase dramatically as the rocket gains speed.

The force which will cause the rocket to accelerate from rest and continue its flight will be the thrust provided by the rocket motor. In some rockets, this force will not be constant, but here we will assume that it is.