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When the Powers of the Air are chained to my chair,

Is the million-colored how; The sphere-fire ahove its soft colors wove, While the moist Earth was laughing helow.

Percy Bysshe Shelley, from The Cloud

IT should not be surprising that chemical rockets derive their thrust from chemical reactions. The reaction between a rocket fuel and an oxidizer (a chemical compound containing oxygen) releases a tremendous amount of energy. The energy is released as new chemical bonds are formed in the "burning" process. Channeling the energetic reaction products outward from the vehicle using a directional nozzle produces thrust. The hot gas goes one way, the rocket ship the other, and momentum is conserved. Chemical rockets are propelled by reactions of either liquid or solid fuel combinations. Solid fuels are generally easier to store for long periods of time; liquid propellants are typically more energetic.

payload

oxidizer tank fuel tank

exhaust

FIGURE 3.1 A chemical bipropellant rocket.

The chemical reactions that drive a rocket are called "exothermic," meaning that these reactions release more chemical energy than they require. Most chemical rockets (as shown in Figure 3.1) utilize bipropellants—a fuel and an oxidizer. In the liquid engines of the space shuttle, the fuel is liquid hydrogen and the oxidizing agent is liquid oxygen.

Some chemical fuel combinations, such as hydrogen and oxygen, react explosively on contact. Others must be preheated to react.

Rocket designers spend a great deal of time optimizing such elements as valves, nozzles, and pumps. But barring breakthroughs in the field of exotic chemical fuels, the space shuttle main engines' 4.55-kilometer-per-second exhaust velocity is close to the theoretical maximum. It cannot get much better than that.

Although rockets were first applied to space travel during the 1940s and 1950s, the history of the Rocket Principle is a good deal more venerable. Hero of Alexandria may have been the first rocketeer in the first century AD when his "aeropile" used jets of steam to rotate a suspended structure.

However, almost a millennium would pass until the first rocket-propelled devices, the "fire arrows'' of the Chinese inventor Feng Jishen, would streak through Earth's lower atmosphere. These hollow bamboo tubes had holes at one end and were filled with gun powder. With a bamboo stick attached for stability, the early fire arrows could climb into the sky, producing the ancestor of modern firework displays.

According to legend, the first attempt at manned rocket flight may long predate the twentieth century. Before the year 1500, a Chinese mandarin named Wan Hu, perhaps world weary or a bit mad, directed his retainers to attach a large number of fire arrows to a chair, which was also equipped with a few kites to, perhaps, aid his atmospheric cruise. Wan Hu sat in the chair, which had been dragged outside. His retainers lit the fuse and probably ran as fast as possible. This intrepid (or foolhardy) pioneer disappeared in the ensuing explosion. Perhaps Wan Hu lived long enough to view the moist, blue Earth below his spinning rocket chair; but he more likely died instantly.

Early rockets were not capable of precision flight but they were certainly impressive. Their earliest application in Asian warfare probably was as terror weapons to demoralize opposing armies with loud (and unpredictable) explosions.

When Marco Polo reinvigorated east-west trading contacts at the dawn of the Renaissance, one concept that moved west was the Rocket Principle. Although used in western firework displays, the rocket does not surface as an effective European weapon until the eighteenth century.

This delay was most likely due to the difficulty in controlling the rocket's post-launch trajectory. Perhaps the most successful military rocketeer of that era was Sir William Congreve, a British artillery officer. His specially constructed "rocket ships'' were used as an effective siege weapon during the Napoleanic wars. One has been immortalized in American patriotic music by the phrase "by the rockets red glare.''

A number of non-military rocket applications were also experimented with during the eighteenth century. One was an effort by Prussian and British rocketeers to develop a line-carrying rocket that, after launch from a lighthouse, could become entangled in the sails of a foundering ship, allowing crew and passengers to scamble to land.

With all the nineteenth-century experimentation, it is surprising that early science-fiction writers chose huge guns rather than rockets to propel their spacecraft through interplanetary space. But the first person to seriously consider the rocket for space propulsion was a visionary scientist rather than a fiction author.

Konstantin Tsiolkovsky (1857-1935) was born in Czarist Russia and lived to see his theoretical rocket researches widely acknowledged. In his spare time, this mathematics teacher composed treatises that dealt with weightlessness, the problems of maintaining closed ecological systems in space, the capabilities of various chemical rocket fuels, and the concept of staged rockets. He successfully identified liquid oxygen and hydrogen, the fuel of the American space shuttle's liquid rockets, as an extremely energetic propellant for chemical rockets. Tsiolkovsky was perhaps the first to realize that high exhaust velocity was the key to rocket-propelled interplanetary travel.

Although Tsiolkovsky was a theoretician who never actually constructed a rocket, his work inspired further research. One of his successors, the Romanian Hermann Oberth (1894-1992), published best-selling books on the potential of the rocket that led to the formation of German amateur rocket societies. The experiments of these groups, principally the "Verein für Raumschiffarht,'' were later incorporated into Nazi-sponsored research leading to the development of the V2. Although principally applied as a terror weapon capable of bombarding London from launching pads hundreds of kilometers away, the V2 was the first terrestrial vehicle that rose above the Earth's atmosphere.

Robert Goddard (1882-1945)—an American contemporary of Tsiolk-ovsky and Oberth—was a physics professor with a strong interest in experimentation. He constructed a number of liquid-fueled rockets and launched them from sites in Massachusetts and New Mexico. He was granted more than 200 patents for his rocket research and described much of it in his 1919 publication A Method of Reaching Extreme Altitudes. Goddard's innovations included the application of gyroscopes to rocket guidance, use of vanes in the exhaust stream for steering, and valves to control propellant flow.

Oberth and Tsiolkovsky were lionized by the Russian and German public and political authorities. Goddard, on the other hand, was ridiculed by the American press and his experimental launches were banned in Massachusetts.

After the Second World War, the victorious powers realized that an ultimate weapon might be a ballistic missile carrying a nuclear device as its payload. Thus began the "space race,'' as the USA and the USSR sought to improve missile reliability, range, and payload capacity and to develop miniaturized electronics for guidance computers and nuclear payloads. Most American and Russian space launches prior to the Apollo Moon-landing program utilized converted or uprated war rockets.

Utilizing these craft as they competed for world technological dominance, the space powers chalked up an impressive list of space accomplishments in the first decade of the Space Age. Early satellites orbited the Earth at a variety of altitudes, where they discovered such aspects of the terrestrial environment as Earth's Van Allen radiation belts. During the first few years of the Space Age, crude robots impacted the Moon, followed by others that orbited and touched down on the lunar surface. The exploration of the solar system moved into high gear as probes journeyed to Earth's neighboring worlds Venus and Mars.

Although it is largely taken for granted today, the commercial space infrastructure is another accomplishment of pioneering space missions. Before 1970, weather, communication, and Earth-viewing satellites had made their appearance.

But the public's attention, of course, was concentrated upon the human drama of the Moon Race between the USA and the USSR. Those old enough to witness it will never forget how they held their breath as early astronauts and cosmonauts flew ballistic trajectories to the fringe of space, orbited the Earth, and began to reach for the Moon. Along the way, they learned how to rendezvous and dock in space using rockets, to ship components up from Earth to be used in the construction of large space stations, and ultimately to develop a permanent human presence above the Earth's atmosphere.

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