Category FLIGHT and M ОТІOIM

Much of what scientists now know about the four “gas giant” planets-

Jupiter, Saturn, Uranus, and Neptune – came from NASA’s two Voyager probes, launched in 1977. Voyager 1 flew past Jupiter and Saturn before leaving the

DISTANT VOYAGERS

Voyager 1 and Voyager 2 each carry a gold disk showing the location of Earth within the Milky Way galaxy. The golden record also contain sounds and images chosen to portray the diversity of life on Earth. It is meant to communicate with any intelligent life-form that might col­lect one of the Voyagers. The Voyager spacecraft will take about 40,000 years to approach another star, however, and the probes are minute compared to the vastness of interstellar space. The chances of any alien life-form finding one of the probes is therefore remote.

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solar system. Voyager 2 journeyed on to visit Uranus in 1986 and Neptune in 1989. Jupiter has some of the wildest weather in the solar system, with winds up to 300 miles per hour (480 kilometers per hour). Jupiter also spins faster than any other planet. As a result, its day is less than 10 hours long.

In 1995 the Galileo probe orbited Jupiter and sent a small, cone-shaped lander plunging down into the atmos­phere through clouds of ammonia ice crystals. The probe survived for an hour, sampling the hostile atmosphere, until it was destroyed.

The Lockheed SR-71 Blackbird holds the official world airspeed record. On July 28, 1976, it flew 2,188 miles per hour (3,530 kilometers per hour) near Beale Air Force Base in California, with Eldon W. Joersz at the controls.

Other aircraft have gone faster than the Blackbird, but they do not qualify for the official world airspeed record because they cannot take off and land under their own power. In 1967, the X-15 rocket plane reached a top speed of 4,520 miles per hour (about 7,270 kilo­meters per hour), or Mach 6.7. It is the fastest manned aircraft that has ever flown, but it is launched in midair from beneath the wing of a B-52 bomber.

A spacecraft has to be boosted to a speed of about 17,500 miles per hour (28,000 kilometers per hour) to go into low Earth orbit. The highest speed ever attained by a manned space­craft is 24,791 miles per hour (39,900 kilometers per hour). The Apollo 10 spacecraft reached this speed during its return from the Moon in 1969. The fastest space probe, and also the fastest human – made object of any kind, was the Helios 2 solar space probe. It reached a speed of 157,000 miles per hour (252,700 kilometers per hour) in the 1970s.

SEE ALSO:

• Supersonic Flight • Velocity

The speed of sound in air depends on the temperature of the air. Sound travels faster through warm air and more slow­ly through cold air. The air high above the ground is very cold, so the speed of sound is lower there than at sea level.

As an aircraft climbs higher above the ground, the air gets colder. At a height of about 35,000 feet (about 10,700 meters), where airliners cruise, the air is as cold as -76°F (-60°C). At this temperature, sound travels through air at about 660 miles per hour (1,060 kilo­meters per hour). An airplane flying at, for example, 715 miles per hour (1,150 kilometers per hour) in warm air near

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OF SOUND IN AIR

Speed of Sound

660 mph (1,062 kph)

685 mph (1,102 kph) 714 mph (1,149 kph) 728 mph (1,171 kph) 741 mph (1,192 kph) 755 mph (1,215 kph) 762 mph (1,226 kph)

768 mph (1,236 kph) 775 mph (1,247 kph)

the ground is subsonic (below the speed of sound). The same plane flying at this speed at its cruising altitude would be supersonic. To avoid confusion, scien­tists invented Mach numbers.

An aircraft flying at the speed of sound flies at Mach 1, whatever its actu­al airspeed is. Mach 2 is twice the speed of sound, Mach 3 is three times the speed of sound, and so on. An aircraft’s Mach number is calculated by dividing its speed by the speed of sound in the air through which it is flying. An airliner such as the Boeing 777 flies at about Mach 0.84. Fighter planes such as the F-16 fly at up to Mach 2. A rare handful of manned self-launching aircraft, such as the Russian MiG-25R, can fly faster than Mach 3.

MACH 1 AT DIFFERENT ALTITUDES

hrust is the force that propels air­craft and spacecraft. Propellers, jet engines, and rockets all produce thrust. Thrust is one of the four forces that act on a powered aircraft. The other three are drag, lift, and weight. Thrust is opposed by drag, so it must overcome drag if an aircraft is to accelerate.

Thrust is most often generated by speeding up a gas. According to Newton’s third law of motion, accelerat­ing gas in one direction produces a reac­tion force in the opposite direction. The reaction force is thrust. The thrust pro­duced by a jet engine depends on the amount of gas accelerated and the increase in its speed. The more gas the engine accelerates, and the greater its acceleration, the greater the thrust.

Thrust, as a force, is measured in the same units as other forces: either pounds – force or newtons. A thrust of one pound is the same size of force as the down­ward force of the weight of one pound.

Imagine that you have a bag contain­ing a pound of sand

O Trent engines provide the thrust needed for the world’s largest airliner, the Airbus 380.

sitting on your hand. The downward force on your hand from the pound of sand is the same as a pound of thrust produced by a jet or rocket engine.

The most powerful jet engines fitted to an airliner are the General Electric GE90-115B engines that power the long – range Boeing 777-300ER. They regular­ly generate 115,300 pounds (510 kilo – newtons) of thrust, although they have produced as much as 127,900 pounds (570 kilonewtons) in tests.

When a propeller spins, its winglike blades produce lift, but instead of acting upward, the propeller acts forward. This forward-acting force is thrust. A spin­ning propeller lowers the air pressure in front of it and raises the air pressure behind it. The amount of thrust it produces depends on the size of the propeller and the pressure difference

it creates. The bigger the propeller and the bigger the pressure difference, the greater the thrust.

At takeoff, a rocket must generate enough thrust to overcome its weight and drag, both of which act downward. A rocket’s thrust depends on the speed of the jet of gas it produces and the rate at which mass is expelled from the rocket. The faster a rocket’s exhaust jet is, and the faster it burns its propellants, the more thrust it produces.

When the Space Shuttle lifts off, its three main engines burn propellants at the rate of 3,250 pounds (1,480 kilo­grams) per second and produce a total thrust of 1.2 million pounds (5,340 kilo – newtons). Their power is dwarfed by the two solid rocket boosters, which provide another 5.6 million pounds (25,100 kilo newtons) of thrust.

If an aircraft has a single engine, its thrust is directed along the vehicle’s centerline so that it flies straight. If it has more than one engine, they are arranged so that their thrust is balanced, or symmetrical. Unbalanced, or asymmetrical, thrust in a multiengine aircraft will make it yaw to one side.

Thrust may be made asymmetric pur­posely to help steer a vehicle. If an engine or just its exhaust nozzle is swiveled, its thrust is deflected, and the vehicle changes direction. This is called thrust vectoring. Rockets, airships, and

О The upward thrust that launches this Delta rocket is a reaction force produced by the down­ward jet of its exhaust gas.

some fighter planes use thrust vectoring.

Thrust also can be used as a brake to slow down a vehicle. When an airliner lands, a sudden roaring noise indicates that the crew has selected reverse thrust. The engine thrust is directed forward, and the aircraft slows down. Rockets also use reverse thrust for braking. Thrust in the opposite direction to the direction the rocket is flying makes it slow down.

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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.