Category FLIGHT and M ОТІOIM

The first wind tunnels were open-ended tubes through which air was blown. These were soon replaced by wind tunnels that blew the same air around in an endless loop. This layout is called a closed-circuit wind tunnel. The temperature, humidity (amount of moisture), and speed of the air can be better controlled in a closed – circuit tunnel.

Open wind tunnels still are used for some tests. If a working engine was being tested in a wind tunnel, a closed circuit tunnel would recirculate the engine exhaust gases, and that would affect the engine’s performance and thrust. In such cases, an open wind tun­nel is used. Fresh air is taken in, acceler­ated through the test section once, and then expelled along with the engine exhaust gases.

Wind tunnels also can be classi­fied according to the airspeed in their test section. There are subsonic tunnels, with airspeeds less than the speed of sound, and transonic tunnels, in which the air travels at about the speed of sound. Supersonic tunnels have airspeeds greater than the speed of sound, while hypersonic tunnels are for tests at more than five times the speed of sound.

In the summer of 1940, the German Luftwaffe launched an air assault on Britain. The German high command planned to destroy British air bases before an actual invasion or (they hoped) a British surrender. The plan failed, thanks to the resistance of RAF fighter pilots, a group that included Canadians, Australians, Poles, and American volunteers as well as British pilots. Many were just teenagers with only a few weeks’ training.

The Battle of Britain was the first Allied air victory of the war. The Allies owed their success to a new invention: a top-secret radar system that gave early warning of the approaching enemy bombers. The British also had two mod­ern fighter planes. The Hurricane, first flown in 1935 and designed by Sydney Camm (1893-1966), was used to attack German bombers, while the faster Spitfire tackled the German fighter escorts. The Spitfire (first flown in 1936) became one of the most famous air­planes of the war. Designed by Reginald J. Mitchell (1895-1937), it remained in service until the 1950s.

The Cassini spacecraft launched to Saturn in 1997 reached the planet in 2004. In January 2005, the spacecraft released the Huygens probe to explore Saturn’s moon, Titan. Parachutes slowed Huygens’s final descent, and its cameras began taking pictures of Titan’s surface from a height of 10 miles (16 kilome­ters). Finally, the probe landed on what looked like a shoreline-perhaps beside a lake of freezing liquid methane gas.

The Huygens probe continued to transmit data for 90 minutes, three times longer than scientists had hoped for. Signals from Huygens were transmitted to Cassini in orbit, and from Cassini back to Earth, where 45 minutes later they were picked up by large radio tele­scopes. The scientific instruments onboard the Huygens probe gave scien­tists much valuable data about Saturn’s large and distant moon.

Return to Mars

In 2004, NASA returned to Mars with twin robot rovers named Spirit and Opportunity. During the landing, each rover was protected inside a large airbag with a parachute attached. After impact, the ball bounced over the Martian sur­face until it came to a halt. Then the airbag deflated and opened to release the robot rover. The rovers landed at separate locations. One of the mission aims was to look for water-a discovery that would make future landings on Mars by astronauts a more realistic prospect. Landing during the Martian afternoon, with Earth in full view, meant that the landers could signal at once to the waiting scientists to let them know that the landing had been successful. The signals were sent to Earth by way of the Deep Space Network, a series of antennae in California, Spain, and Australia. Spirit and Opportunity were intended to work for about 90 days, but they were still busy two years after they landed. They found evidence that Mars was, in its past, apparently a watery planet.

putnik is the Soviet word for trav­eling companion, and Sputnik 1 was the first space traveler from Earth. The world’s first artificial satellite, it was launched by the Soviet Union on October 4, 1957.

International Geophysical Year

After World War II brought advances in rocket technology, interest in launching artificial satellites grew, in both the United States and the Soviet Union. The world’s science organizations designated 1957 to 1958 as International Geo­physical Year. Committees were formed to observe such phenomena as cosmic rays, gravity, and solar activity. It was hoped that the period also would see the launch of the first satellite.

The United States prepared two satel­lites, Explorer and the smaller Vanguard. No one was entirely sure that a satellite launch would work. For that reason, Vanguard was a tiny spacecraft, designed to test the theory that it was
possible to launch a satellite on top of a multistage rocket. Unlike the Soviets, the Americans had relatively small launch rockets. Also unlike the Soviets, they released details of their space program to the public.

Very little information had been released about Soviet space plans, although some details of space radio links had been made public, suggesting that the Soviets had a satellite program. This was the era of the Cold War, when the United States and the Soviet Union were building up their supplies of large rockets for military use as well as for scientific study. Rockets used as ballistic missiles could potentially deliver nuclear weapons over ranges of thousands of miles. Both sides kept their military developments secret, and the Soviet Union extended this secrecy to its devel­opment of space technology.

TECH’^TALK

SPUTNIK 1

The satellite Sputnik 1 was fairly sim­ple. It was an aluminum sphere pres­surized by nitrogen gas. Inside the sphere were batteries providing elec­trical power for two radio transmit­ters. Attached to the outside of the sphere were four whip-like radio antennae.

Launch date: October 4, 1957.

Size: 23 inches (58 centimeters) in diameter.

Weight: 183 pounds (83 kilgrams). Speed: About 18,000 miles per hour (28,960 kilometers per hour).

Orbital height: 143-584 miles (230-940 kilometers).

Orbital time: 96 minutes.

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During World War II, fighter pilots sometimes found their planes behaving oddly in a high-speed dive. The plane shook violently, and the controls seemed to become locked and ineffective. Some pilots were able to regain control. Others
were not so lucky, and they crashed. Some aircraft were shaken so violently that they broke up. There were more mysterious crashes when early jet planes were being tested at high speed. This gave rise to the idea of the sound barrier-an invisible wall that an aircraft had to break through.

The first plane to fly faster than sound in level flight was the Bell X-1 experimental rocket plane in 1947, with Captain Chuck Yeager at the controls. He experienced the same violent shaking and loss

О Originally designed for aerial combat, the F-100 Super Sabre was a supersonic fighter bomber. An F-100 is seen here firing rockets at a target in the Vietnam jungle below in 1967.

TEC

F-100D SUPER SABRE

Length: 49 feet (15 meters). Wingspan: 39 feet (12 meters). Engine: J57 turbojet.

Takeoff weight: 34,000 pounds (15,400 kilograms).

Top speed: 905 miles per hour (1,460 kilometers per hour). Range: 1,985 miles (3,190 kilometers).

Service ceiling: 48,000 feet (14,630 meters).

of control as earlier pilots. As the plane accelerated beyond the speed of sound, however, the shaking faded away and flight became smoother again.

Flying speeds increased rapidly in the years after Yeager’s flight. The F-100 Super Sabre was the first fighter to be designed from scratch for supersonic flight. It went supersonic during its first flight on May 25, 1953, and set a new world airspeed record of 755 miles per hour (1,215 kilometers per hour) five months later. The first Mach 2 flight was made in 1953 and the first Mach 3 flight was made in 1956.

elocity is speed in a particular direction. Velocity changes when speed or direction, or both, are altered. An airplane flying north at 500 miles per hour (805 kilometers per hour) has a speed of 500 miles per hour (805 kilometers per hour) and a velocity of 500 miles per hour (805 kilometers per hour) north. If two airplanes are flying in the same direction at the same speed, they have the same velocity. If the two planes are flying at the same speed but in different directions, they have differ­ent velocities.

Velocity and Change

The direction of velocity need not be a straight line. An object moving in a circle-such as an airplane turning or a spacecraft orbiting Earth-is said to have angular velocity. Anything that moves with a steady, unchanging veloc­ity is described as having a uniform velocity. According to Newton’s first law of motion, an object stays at rest or moves at a uniform velocity unless a force acts upon it.

When a spacecraft fires a rocket to go faster or change its orbit, its change in velocity is called delta-v. The Greek let­ter delta is often used by mathema­ticians and physicists to mean a change in something. In this case, it is the velocity (v) that changes.

Velocity also can be defined as the rate of change in displacement. Distance is the length of the path taken by a
vehicle. Distance is a number, so it is a scalar quantity. Displacement is the length of the straight line between the start point and end point of a vehicle’s journey. It is distance in a certain direc­tion, so it is a vector quantity. (A vector is something that has magnitude and direction.)

HYPERVELOCITY

An extremely high velocity, higher than about 6,700 miles per hour (about 11,000 kilometers per hour), is known as hypervelocity. Spacecraft returning to Earth enter the atmo­sphere at hypervelocity. According to Einstein’s theory of relativity, the highest velocity possible in the universe is the velocity of light. Light travels through space at a speed of

186,0 miles (about 300,000 kilo­meters) per second.

An airplane that takes off and flies 200 miles (320 kilometers) in a circle, landing back on the same runway where its journey began, travels a distance of 200 miles (320 kilometers), but its change in dis­placement is zero.

Its average speed is the distance traveled divided by the flight time, or 200 miles per hour (320 kilometers per hour). Its average velocity is its change in displacement divided by the flight time. As the plane’s change in displacement is zero, its average velocity for the whole flight is also zero.

Early wind tunnel tests sometimes produced results that did not agree with measurements taken from real aircraft in flight tests. Researchers found that more accurate results were often obtained by increasing the air pressure inside the wind tun­nel. If a model one-fifth the size of a real aircraft was tested, the results would be more accurate if the air pressure was increased fivefold. The first high-pressure wind tunnel was used at the Langley Research Center (now part of NASA) in 1923.

There also are refrigerated wind tunnels. The air is chilled to sub-zero temperatures to study the problem of icing, when ice builds up on an aircraft’s wings and air inlets.

The world’s biggest wind tunnel is at NASA’s Ames Research Center. Its test section is big enough for aircraft with a wingspan of 100 feet (30 meters). Air is blown through the tunnel by six fans, each as tall as a four-story building.

VERTICAL WIND TUNNELS

Vertical wind tunnels blow air straight upward. These wind tunnels are not used for aerodynamic testing. They are used by people who want to practice or just enjoy the sensation of skydiving without having to jump out of an aircraft. The skydiver is held aloft by the air blasting upward. Some vertical wind tunnels are completely enclosed and recirculate the same air. Others are outside, and the sky – diver floats in open air. The airspeed is usually adjustable, up to about 140 miles per hour (225 kilometers per hour), to match the ability of the skydiver.

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SEE ALSO:

• Aerodynamics • Aircraft Design

• Pressure • Speed • Wright, Orville and Wilbur

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The main weapons in air-to-air combat were machine guns and cannons. The Spitfire fighter was armed with eight wing-mounted machine guns; an

Me-109 had two machine guns and twin cannons; and a P-51 Mustang was equipped with six machine guns.

Fighter-bombers carried small bombs and rockets as well as guns. German fighters fired salvos of air-to-air rockets into formations of U. S. bombers. Allied fighter-bombers used rockets against ground targets, such as railroad trains and road convoys, during the 1944 battles in Normandy, France. Extra-fast airplanes such as the British Mosquito fighter-bomber flew without defensive armament, relying on speed to evade the enemy.

Such attacks highlighted a new threat to air travel and the need for tighter securi­ty at airports. The Hague Convention of 1970, an international agreement signed by more than 130 nations, was drawn up to combat skyjacking and stop terrorists from escaping to “friendly” countries.

From 1973, the Federal Aviation Administration in the United States has required all airlines to screen passengers and their baggage to prevent people from carrying weapons-or objects that might be used as weapons-onto flights. Airports tightened security procedures, especially at check-in and during bag­gage handling. Armed guards, called sky marshals, traveled on some flights.

When an airplane is hijacked, mili­tary craft may escort it to a landing field
agreed to by the authorities. After it has landed, troops and police will surround the hijacked aircraft while skilled nego­tiators try to talk the skyjackers into releasing the hostages and surrendering. Armed assault also may be used. In 1976, following the hijacking of an Air France flight, Israeli commandos flew in to attack the Palestinian hijackers, who were holed up at Entebbe Airport in Uganda, Africa. The commandos rescued more than 100 hostages. In most hijack­ing situations, however, airport and law enforcement agencies usually try to avoid a gun battle, which risks injuring or even killing innocent hostages.

Skyjacking incidents decreased in the United States during the 1980s and 1990s, but violent terrorist incidents continued to take place in other parts of the world. Some of these skyjackings resulted in airplanes crashing. In 1996, a stolen Ethiopian airliner crashed into the Indian Ocean. About fifty passengers

managed to survive, but the crash killed 125 of the people onboard.

NASA provides up-to-date information on its current space probe missions through its Web sites. The Jet Propulsion Laboratory provides updates on missions in progress and on future missions. NASA’s Web sites also display the latest photos taken by space probes.

Robot probes will continue to play an important part in twenty-first-century

space exploration. Some already have set out on their long journeys. The Messenger probe left Earth in 2004 and will arrive at Mercury in 2011. The New Horizons probe, launched in 2006, should reach Pluto in 2015.

Space probes continue their journeys into infinity, long after they have ceased to communicate with Earth. Scientists calculate that by the year 34,593, Pioneer 10 will have reached a star called Ross 248, 10.3 light years distant from Earth.

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