Development of Rockets

The invention of the rocket is generally ascribed to the Chinese, who as early as A.D. 1000 stuffed gunpowder into sections of bamboo tubing to make military weapons of considerable effectiveness. The 13th-century English monk Roger Bacon introduced to Europe an improved form of gunpowder, which enabled rockets to become incendiary projectiles with a relatively long range. Rockets subsequently became a common if unreliable weapon. Major progress in design resulted from the work of William Congreve, an English artillery expert, who built a 20-lb (9-kg) rocket capable of traveling up to 2 mi (3 km). In the late 19th cent., the Austrian physicist Ernst Mach gave serious theoretical consideration to supersonic speeds and predicted the shock wave that causes sonic boom.

The astronautical use of rockets was cogently argued in the beginning of the 20th cent. by the Russian Konstantin E. Tsiolkovsky, who is sometimes called the "father of astronautics." He pointed out that a rocket can operate in a vacuum and suggested that multistage liquid-fuel rockets could escape the earth's gravitation. The greatest name in American rocketry is Robert H. Goddard, whose pamphlet A Method for Reaching Extreme Altitudes anticipated nearly all modern developments. Goddard launched the first liquid-fuel rocket in 1926 and demonstrated that rockets could be used to carry scientific apparatus into the upper atmosphere. His work found its most receptive audience in Germany. During World War II, a German team under Wernher von Braun developed the V-2 rocket, which was the first long-range guided missile. The V-2 had a range greater than 200 mi (322 km) and reached velocities of 3,500 mi (5,600 km) per hr.

After the war, rocket research in the United States and the Soviet Union intensified, leading to the development first of intercontinental ballistic missiles and then of modern spacecraft. Important U.S. rockets have included the Redstone, Jupiter, Atlas, Titan, Agena, Centaur, and Saturn carriers. Saturn V, the largest rocket ever assembled, developed 7.5 million lb (3.4 million kg) of thrust. A three-stage rocket, it stood 300 ft (91 m) high exclusive of payload and with the Apollo delivered a payload of 44 tons to the moon. Rockets presently being used to launch manned and unmanned missions into space include the U.S. Athena 1 and 2, Taurus, Titan 2 and 4B, Delta 2, 3, and 4, Atlas 2 ,3, and 5, and STS or space shuttle; the Chinese Long March 2C, 2E, and 2F; the Russian Soyuz and Proton K and M; the Japanese H-2A; the European Space Agency's Ariane 5 series; the Indian PSLV (Polar Satellite Launch Vehicle); the Israeli Shavit 2; the Brazilian VSV-30; and the multinational, private Sea Launch Zenit-3SL, which uses a converted oil platform located some 1,400 mi (2,250 km) southeast of Hawaii.

See also space science.

rocket

Rocket, any vehicle propelled by ejection of the gases produced by combustion of self-contained propellants. Rockets are used in fireworks, as military weapons, and in scientific applications such as space exploration.

Rocket Propulsion

The force acting on a rocket, called its thrust, is equal to the mass ejected per second times the velocity of the expelled gases. This force can be understood in terms of Newton's third law of motion, which states that for every action there is an equal and opposite reaction. In the case of a rocket, the action is the backward-streaming flow of gas and the reaction is the forward motion of the rocket. Another way of understanding rocket propulsion is to realize that tremendous pressure is exerted on the walls of the combustion chamber except where the gas exits at the rear; the resulting unbalanced force on the front interior wall of the chamber pushes the rocket forward. A common misconception, before space exploration pointed up its obvious fallacy, holds that a rocket accelerates by pushing on the atmosphere behind it. Actually, a rocket operates more efficiently in outer space, since there is no atmospheric friction to impede its motion.

Rocket Propellants

The most vital component of any rocket is the propellant, which accounts for 90% to 95% of the rocket's total weight. A propellant consists of two elements, a fuel and an oxidant; engines that are based on the action-reaction principle and that use air instead of carrying their own oxidant are properly called jets. Propellants in use today include both liquefied gases, which are more powerful, and solid explosives, which are more reliable; the space shuttle's main engines use liquid propellant, while its boosters are solid-fuel rockets. The chemical energy of the propellants is released in the form of heat in the combustion chamber.

A typical liquid engine uses hydrogen as fuel and oxygen as oxidant; a typical solid propellant is nitroglycerine. In the liquid engine, the fuel and oxidant are stored separately at extremely low temperatures; in the solid engine, the fuel and oxidant are intimately mixed and loaded directly into the combustion chamber. A solid engine requires an ignition system, as does a liquid engine if the propellants do not ignite spontaneously on contact.

The efficiency of a rocket engine is defined as the percentage of the propellant's chemical energy that is converted into kinetic energy of the vehicle. During the first few seconds after liftoff, a rocket is extremely inefficient, for at least two unavoidable reasons: High power consumption is required to overcome the inertia of the nearly motionless mass of the fully fueled rocket; and in the lower atmosphere, power is wasted overcoming air resistance. Once the rocket gains altitude, however, it becomes more efficient. as the trajectory, at first vertical, curves into a suborbital arc or into the desired orbit.

see: Development of Rockets 

Photochemistry

Photochemistry, study of chemical processes that are accompanied by or catalyzed by the emission or absorption of visible light or ultraviolet radiation. A molecule in its ground (unexcited) state can absorb a quantum of light energy, or photon, and go to a higher-energy state, or excited state (see quantum theory). Such a molecule is then much more reactive than a ground-state molecule and can undergo entirely different reactions than the more stable molecule, following several different reaction pathways. One possibility is that it can simply emit the absorbed light and fall back to the ground state. This process, called chemiluminescence, is illustrated by various glow-in-the-dark objects. Another possibility is for the molecule to take part in a photo-induced chemical reaction; it may break apart (photodissociate), rearrange, isomerize, dimerize, eliminate or add small molecules, or even transfer its energy to another molecule. Photochromic compounds—compounds that change color reversibly in going from the dark to the light—are generally compounds that are capable of reversible isomerization, or rearrangement. In the absence of light, the compound exists in its most stable form, which exhibits a particular color; in the presence of light, the compound goes to a less stable form, which exhibits a different color. After removal of the light, the compound will revert back to its original state. The best-known and most important photochemical reaction is photosynthesis, the complex, chlorophyll-catalyzed synthesis of sugars from carbon dioxide and water in the presence of light. Other extremely important and complex photochemical reactions take place in the eye. Photochemistry is indispensible to industries involved with dyes, photography, television, and many other applications of light and color.

Aerosol dispenser

Aerosol dispenser, device designed to produce a fine spray of liquid or solid particles that can be suspended in a gas such as the atmosphere. The dispenser commonly consists of a container that holds under pressure the substance to be dispersed (e.g., paints, insecticides, medications, and hair sprays) and a liquefied gas propellant. When a valve is released, the propellant forces the substance through an atomizer and out of the dispenser in the form of a fine spray. These devices are more properly termed spray dispensers rather than aerosol dispensers because the particles of the dispersed substance are usually larger than the particles of a true aerosol (see colloid), such as a fog or a smoke. Freon was the most common aerosol propellant, but its use has been banned because it is believed to contribute to destruction of the ozone layer of the stratosphere; common propellants now include propane, butane, and other hydrocarbons.

chronometer

Chronometer, mechanical timekeeping device of great accuracy, particularly one used for determining longitude (see latitude and longitude) at sea. Early weight- and pendulum-driven clocks were inaccurate because of friction and temperature changes and could not be used at sea because of the ship's motion. In 1735 John Harrison invented and constructed the first of four practical marine timekeepers. The modern marine chronometer is suspended to remain horizontal whatever the inclination of the ship and differs in parts of its mechanism from the ordinary watch. A chronometer may provide timekeeping accurate to within 0.1 second per day. See also Ferdinand Berthoud.

Electrocardiography

Electrocardiography (ĭlĕk'trōkärdēŏg`rəfē), science of recording and interpreting the electrical activity that precedes and is a measure of the action of heart muscles. Since 1887, when Augustus Waller demonstrated the possibility of measuring such action, physicians and physiologists have recorded it in order to study the heart's normal behavior and to provide a method for diagnosing abnormalities. Electrical current associated with contraction of the heart muscles passes through the various tissues and reaches the surface of the body. What is actually recorded is the change in electrical potential on the body surface. The first practical device for recording the activity of the heart was the string galvanometer developed by William Einthoven in 1903. In this device a fine quartz string is suspended vertically between the poles of a magnet. The string is deflected in response to changes in electrical potential and its movement can be optically enlarged and photographed, or, if an immediately visible record is desired, the string's movement can be recorded on a sheet of paper. A more sophisticated form of the electrocardiograph employs a vacuum-tube amplifier. The greatly amplified current from the body deflects a mirror galvanometer that causes a beam of light to move across a light-sensitive film. When an electrocardiograph is taken, electrodes (leads) are attached to the extremities and to the left chest. The recordings obtained in this manner are called electrocardiograms, or more simply EKG's or ECG's. A normal EKG shows a sequence of three waves arbitrarily labeled P, QRS, and T. The P wave is a small, low-amplitude wave produced by the excitation of the atria of the heart. It is followed by a resting interval that marks the passage of electrical impulses into the ventricles. Following this interval comes the QRS wave, a rapid, high-amplitude wave marking ventricular excitation, and then a slow-building T wave denoting ventricular recovery. Abnormalities may be noted from deviation in wave form, height, direction, or duration. The type of abnormal wave may sometimes indicate the type of heart disorder. Usually the physician must associate the EKG with other clinical observations to determine the cause of the abnormality.

nutation

Nutation, in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on the earth's equatorial bulge. However, the period of the motion is only 18.6 years, the same as that of the precession of the moon's nodes, as opposed to the nearly 26,000-year period of the precession of the equinoxes. Nutation was discovered by the English astronomer James Bradley in 1728 but was not explained until 20 years later.