Atlas IIAWhether powered by liquids or solids, rocket systems are propelled by gas pressure resulting from fuel combustion. The force driving them forward is called thrust.

If the rocket system—the liquid or solid booster plus payload—is to move upward against gravity, the thrust must be greater than gravity’s counteracting downward pull.

If you hold a one-pound ball in the palm of your extended hand, you are exerting one pound of thrust. In doing so you are burning fuel and oxygen—the foods you eat and the air you breathe—to exert the energy to cancel the gravitational pull on the ball. The ball is not going up. If you let it go, the ball will be pulled to the ground by gravity.

Now bring your hand up quickly, so the ball leaves your palm and flies upward a bit farther before falling back to the ground.

You have done something more than merely cancel gravity for a moment. You have imparted thrust of more than one pound to the ball.

The Atlas was the first U.S. intercontinental ballistic missile. It was also used extensively as a space booster. Fully fueled, the Atlas rocket weighed about 260,000 pounds (1,160,000 newton). Its engines developed a thrust at liftoff of more than 360,000 pounds of force (1,600,000 newton), considerably more than its total initial weight.

The ratio between these two values is called the thrust-to-weight ratio of the booster. At liftoff, the Atlas IIA thrust-to-weight ratio is 1.2, enough to accelerate a payload to speeds of 8 kilometers per second (18,000 miles per hour), sending the Atlas IIA to targets or orbits many thousands of miles away. As propellants burn, the thrust-to-weight ratio increases, and the booster continues to accelerate at higher rates.

There is a direct analogy here to the experiment with the one-pound ball mentioned above. Your hand and arm are, in effect, the Atlas rocket with its engines and tanks of fuel and liquid oxygen. The ball is the payload.

The upward swing of your hand duplicates the powered-flight phase of an Atlas launch. The instant you stop your hand is similar to the instant of engine burnout or shutdown on the Atlas, when the rocket’s fuel is exhausted or the engines are stopped by plan.

From this instant, both the ball and the payload begin decelerating, slowing down in their ascent. Gravity pulls each back toward Earth.

How have we put vehicles and people into orbit around Earth, landed scientific payloads and humans on the moon, sent spacecraft to land on Mars with an automated laboratory, and launched other space probes to help unravel the mysteries of our planetary neighbors?

By exerting more force, or thrust, to a given payload, it will achieve a much higher velocity before thrust termination. It will also go much farther from Earth before gravity succeeds in pulling it back. Velocity, therefore, is a critical factor.

The velocities needed for specific space missions were calculated long before spaceflight was possible. To put an object into orbit around Earth, for instance, a velocity of at least 8 kilometers per second (18,000 miles per hour) must be achieved, depending on the precise orbit desired. This is called orbital velocity.

Orbital velocity is attained when the vehicle is moving in the right direction, fast enough to miss Earth as it falls back. The actual resulting course keeps it in space. In effect, it is falling continually around Earth.

To break away from Earth’s gravity for distant space missions, a velocity greater than 11 kilometers per second (25,000 miles per hour) is required. This is called escape velocity.

Gravity's Pull

Gravity's Pull

During the early years of spaceflight, remarkable achievements were made in space, even though the rockets available had less thrust than users of space might have liked. How? The payloads were kept as small and as light as possible. America’s first artificial satellite, Explorer 1, weighed only 14 kilograms, or 30.8 pounds.

Bigger and more powerful rockets are available today. The Delta IV Heavy generates 8.7 million newton (2 million pounds) of thrust at liftoff. NASA’s Saturn V generated 33 million newton (7.5 million pounds); and the space shuttle has about 28 million newton (6.25 million pounds) of thrust.

Sometimes, however, small is still beautiful. Today new designs for microsatellites, nanosatellites, and picosatellites are paving the way for new space applications of very small but capable spacecraft that are easy to launch. The picosatellite built by The Aerospace Corporation weighs only 0.275 kilograms (about half a pound) and is the smallest spacecraft to be launched and successfully communicated with while in orbit. Perhaps someday swarms of tiny spacecraft will perform the same functions carried out by heavier space vehicles today.