To answer these questions ask yourself where do airplanes fly and where do spacecraft fly? You can see most airplanes which fly overhead, but spacecraft fly much higher. Airplanes fly within the earth's atmosphere, while spacecraft fly outside the earth's atmosphere in space.

Air and Spacecraft Fly At Different Altitudes Above The Earth

Why don't airplanes fly in space? There are different basic science laws which apply to aircraft which do not apply to spacecraft and vice versa. These science principles govern aircraft and spacecraft propulsion and control systems.

Let's look at how airplanes and rockets were developed.

Men and women have always dreamed about flying through the air and to the stars. Many different ideas were developed over hundreds of years. The first successful vehicles to carry people were balloons and blimps. These flying machines use gases which are lighter than air to float into the sky. The early aeronauts (people who fly and navigate balloons) soon learned how little control they had. Their balloon flight was at the mercy of the wind. Although blimps and dirigibles could be steered, these air vehicles are not able to carry many passengers or fly very fast.

Then in 1903, two bicycle mechanics named Wilbur and Orville Wright developed and flew the first powered airplane which carried a person who controlled the flight of the vehicle. It was called the Wright Flyer. After that people from many different countries developed airplanes and significant progress was made in the controllability (steering), speed and safety in aviation.

The shape of a wing on an airplane is essential in creating lift. Lift is created by the pressure differences between the top and bottom surfaces of an airplane wing. The airplane rises in the air.

A propeller on an airplane also has a wing shape. If a wing allows an airplane to rise in the air, which way does a wing-shaped propeller cause an airplane to move? A propeller pulls an airplane forward!

The propeller is attached to a motor which continuously turns and spins it. As it turns the propeller cuts through the air and thrust (forward motion) is created. Most aircraft use air-breathing engines. The engines use the oxygen in the air around it to ignite or burn the engine's fuel. The oxygen is referred to as an "oxidizer".

What happens in space? In space and in the upper atmosphere there is no air. A rocket must carry both its oxidizer and fuel. Lift can not be created in space with wings or a propeller. Rocket propulsion is based on Newton's Third Law which says "for every action there is an equal an opposite reaction".

Have you seen a rocket launch? Hot gases roar out of the rocket's tail end. Applying Newton's law: if the action is hot gas forced down towards the ground, the reaction is that the rocket will move in the opposite direction - Up!

In a "tug-of-war" the group pulling with the most force wins. Both teams use their combined strength to pull in opposite directions. The forces act in opposite directions. The upward force that a rocket experiences is called thrust. As the rocket rises through the air, its thrust must be greater than the other forces acting on the rocket, like its weight and the resistance of air against the rocket's surface (drag). If the thrust is not great enough then just like in the tug-of-war, the other forces may cause the rocket to move in a different direction.

Scientists and engineers express these forces using mathematical terms. Each term represents a physical characteristic. The forces are summed to determine the magnitude (size) and direction of the different forces on the object. If we ignore the effects of air resistance on the rocket, the thrust of a rocket is:

M' is the rate at which the gases are pushed (exhausted) out of the rocket. It is the rate at which the mass (amount of gases) leaves the rocket. V_exh is the velocity of those gases.

Here is an example. A fully fueled rocket weighs 50,000 lb. before it is launched. The rocket is launched and when all the fuel has burned, the rocket weighs 10,000 lb. That means it burned 40,000 lb. of fuel (50,000 - 10,000 = 40,000).

The exhaust velocity is 10,000 ft/sec and the rate the gases are exhausted are 10 slugs/sec. Assuming that the exhaust velocity and exhaust rate do not vary what would the thrust of the rocket be?

Thrust acts in the upward direction, while weight acts in a downward direction. When the rocket is full of fuel the total upward force would be the thrust minus the weight of the rocket: 100,000 - 50,000 = 50,000 lb. What would the net force be when the rocket has burned all its fuel? (You do the math on this one! Ans.: 90,000 lb.)