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03. Physics of Rocketry

Diagram of Newton’s Third Law | Courtesy of NASA.By Gus Posey, The Museum of Flight

The rockets of early science fiction, often depicted as gleaming tubes set atop massive, curved fins, convey the enormous power necessary to escape gravity, and suggest an intelligent and capable society whose technology allows limitless exploration in all directions. It would be easy to imagine that rockets themselves emerged from the furious technological and social explosion that followed World War II, another gleaming metal marvel of the 1950s. In actual fact, humans have been launching rockets for hundreds of years.

Although there may be earlier examples (probably based on simple fireworks), the Chinese record using arrows equipped with gunpowder-filled rockets to repel Mongol invaders in A.D. 1232, during the battle of Kai-Fung-Fu. The rockets were neither accurate nor particularly lethal, but to the attackers, the increased range of the Chinese arrows and their capacity for starting fires behind the Mongol lines represented an alarming development in weapons technology. This is perhaps demonstrated most clearly by the Mongols’ use of powder-filled rockets during their attack on Baghdad in A.D. 1258.

For the next several hundred years, rocket technology progressed slowly, benefiting from minor advances in the production of more powerful gunpowder and the ingenuity of inventors around the world. In 1420, for example, Joanes de Fontana published a design for a rocket-powered torpedo designed to set ships on fire. Still, the rockets themselves were largely unchanged from the simple powder-filled tubes the Chinese had introduced.

It was not until 1687 that the principles that make rockets work were scientifically defined. In his “Mathematical Principles of Natural Philosophy,” Sir Isaac Newton writes about the motions and actions of bodies. Among many other things, Newton described his Third Law of Motion. Put simply, the law stated that for every action, there is an equal and opposite reaction. This is the fundamental idea behind rockets and other reaction engines.

At its simplest, a rocket engine is a tube that is working to contain a controlled explosion, releasing some quantity of energy in one direction to cause an action that demands an equal and opposite reaction. The ejection of mass in one direction, resulting in movement in the opposite direction, can be demonstrated in a variety of ways. The water flowing out of a hose creates a certain amount of pressure in the opposite direction, a familiar sensation for the amateur gardener. Increasing the pressure of the stream creates a stronger force, as does a hose with a larger volume. For every molecule of water ejected out of the hose, there has to be an opposite and equal reaction. For the line of firefighters straining against a writhing fire hose, this relationship is apparent. Because rocket engines are ejecting large amounts of mass at tremendous speeds, the spacecraft they propel move very quickly.

Despite the increase in scientific understanding of rocketry, the actual construction of rockets was still very similar to the processes perfected centuries earlier: filling large tubes with combinations of gunpowder and other solid propellants. Even as these new rockets found their uses (a Congreve rocket was shown to be capable of transporting a line to a stranded ship over 1,000 feet away), some consideration was being given to more challenging applications. On the heels of such science fiction as Edward Everett Hale’s “The Brick Moon” and Jules Verne’s classic “From the Earth to the Moon,” scientists like Konstantin Tsiolkovsky were discovering that exploring space with rockets would require a change in technology and a shift to liquid propellants that could be turned on and off. In 1903, Tsiolkovsky published a paper suggesting that the use of engines that relied on liquid propellants would dramatically increase the range of rocket-powered spacecraft.

Twenty-three years later, Robert Goddard launched the first liquid-propellant rocket, powered by a combination of liquid oxygen and gasoline. The flight lasted only 2.5 seconds, allowing the rocket to climb to a seemingly unimpressive 41 feet. Nevertheless, this quick flight marked the beginning of the modern rocket era, defining the boundary between the simpler solid-propellant rockets and their more complex liquid-propellant descendants.

By the end of World War II, German rocketry had advanced with surprising speed and had produced the infamous V-2. A marvel of engineering, the V-2 was a supersonic rocket capable of traveling to the edge of space while carrying a one-ton warhead. Although the design was intended to deliver deadly payloads to European targets, the lifting capability was impossible to ignore, especially for those German scientists whose dreams of rocketry had more to do with exploration than annihilation. As the Nazi leadership came unraveled near the end of the war, the team responsible for the V-2’s construction decided to surrender to the American troops. Over the next two decades, the addition of the German scientists to the American rocket program would provide the leadership and technical insight that would allow the construction of the greatest rocket ever launched from Earth: the Saturn V.

Although earlier designs for staged rockets existed, the Saturn V combined the idea of multiple rocket motors running in sequence with the advantages provided by liquid propellants. At 363 feet tall, the Saturn V towered over other rockets of the era, challenged only by the Russian N1, a slightly shorter rocket that was never launched successfully, despite four attempts. With 7.5 million pounds of thrust, the Saturn V was capable of lifting 260,145 pounds (118,000 kilograms) to Low Earth Orbit, and 103,617 pounds (47,000 kilograms) into Lunar Orbit.

Today, rocket technology continues to advance with developments in fields such as nuclear propulsion, electric rockets, and even exotic science fiction staples such as the ion drive, as demonstrated by the autonomous spacecraft Deep Space 1. The efficiency of rocket motors will only continue to increase. The trip to the moon took the Apollo astronauts three days; NASA’s New Horizon spacecraft made the same trip in nine hours. Despite these changes, the principles of rocketry remain unchanged. Newton’s Third Law of Motion continues to define the basic transfer of energy that makes rockets possible, whether the rocket in question is soaring above the treetops or skimming across the rings of an alien world.

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Physics of Rocketry - September 18, 20071.64 MB