Category FLIGHT and M ОТІOIM

When it reaches its destination, a probe may stay in orbit, radioing data and images back to Earth, or it may attempt to land a capsule on the surface. Landing on a planet many millions of miles away, under remote control, is always a challenge. The speeds of approach can be enormous. In 2003, the Galileo probe accelerated to 108,000 miles per hour (173,780 kilometers per hour) as it dived toward Jupiter.

Once on the target planet, a lander can use remote-controlled arms and scoops to collect samples of rock and soil. Its instruments analyze the samples and the gases in the atmosphere and measure temperature, pressure, and radi­ation levels. A few probes have released a small rover to explore the areas farther from the lander.

Some probes are sent to collect mate­rial and return it to Earth at the end of their mission. A reentry capsule drops down through Earth’s atmosphere, by parachute, for recovery on the ground or in the air using the “air snatch” tech­nique, by which an airplane scoops up the capsule before it hits the ground.

After a trouble-free start, the Space Shuttle program was severely affected by the losses of two spacecraft, Challenger and Columbia, which caused the deaths of fourteen astronauts. On January 28, 1986, Challenger blew up

just seconds after takeoff from Kennedy Space Center and on February 1, 2003, Columbia was destroyed 16 minutes before it was due to land back on Earth at Kennedy Space Center. On its twenty-eighth mission (STS-107), Columbia disintegrated at a height of about 40 miles (64 kilometers).

After both tragedies, NASA suspended planned Space Shuttle flights while experts investigated the causes of the accidents. On both occasions, after modifications to the remaining fleet, the Space Shuttles went back into space. After the 2003 disaster, flights resumed in July 2005, with the launch of Discovery on mission STS-114. This mission experienced a new scare, when onboard cameras showed a section of foam breaking off during launch (the problem that had caused the destruction of Columbia). During its orbital mission, when Discovery docked with the International Space Station, two astro­nauts made a spacewalk to check for damage-the first time astronauts had worked beneath the craft in space.

Although the Space Shuttles are fly­ing again, the two accidents damaged NASA’s high reputation for “safety first” engineering. Critics of the Space Shuttle complain of the cost. At times, the pro­gram has consumed 30 percent of NASA’s budget, and overall it has cost more than $150 billion. Some people argue that the Space Shuttles were sim­
ply overworked, launching all kinds of payloads, including commercial and military satellites. The size of payloads was reduced after the 1986 Challenger accident. Heavy-lift rockets, such as Delta IV and Ariane, now provide an alternative to the Space Shuttle for satellite launches but not for Space Station visits. The Shuttles are due to be retired in 2010 and replaced by NASA’s new Orion spacecraft.

SEE ALSO:

• Astronaut • Challenger and

Columbia • Future of Spaceflight

• NASA • Spaceflight

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The B-2 Spirit bomber, which first flew in 1989 after years of secret develop­ment, is much larger and heavier than the F-117. It was built by Northrop – Grumman, a manufacturer that pio­neered flying wing designs in the 1940s.

In the 1980s, Northrop-Grumman tested a stealth plane, code-named Tacit Blue, which has been described as “an upside – down bathtub with wings.” The B-2 was more elegant, shaped like a flying arrowhead. Computer-aided design gave the B-2 similar radar-baffling character­istics to the F-117. The B-2 carries a crew of two and has a range of 6,000 miles (9,650 kilometers). It relies on its stealthy approach to outwit defenses because it is relatively slow, flying at around 500 miles per hour (800 kilome­ters per hour). The B-2’s uses are very specialized, and the plane is expensive to produce. Like the F-117, the B-2 has been built in small numbers—there are only about twenty B-2s in existence.

Other modern warplanes, such as the F-22 Raptor, the F-35 Lightning II (Joint Strike Fighter), and the European Typhoon have stealth characteristics, but their shapes are more conventional
than that of the F-117 and the B-2. A key element in their design is that no feature (such as an engine outlet or weapons bay) gives off more than the minimum radar reflection. Stealth fea­tures must be balanced against other elements of the design, such as the high speed that is essential for a fighter plane.

akeoff and landing are the maneu­vers that aircraft or spacecraft per­form to launch into flight and then return to the ground. They are potentially the most hazardous part of a journey in the air or in space.

Airplane Takeoff

Whether an airplane is a small, single­engine light aircraft or the biggest air­liner, the process of taking off is much the same. The crew goes through a checklist to ensure that all switches and other controls are in the right positions for takeoff. Flaps and leading-edge slats are deployed to produce more lift for takeoff. The aircraft taxis out to the end
of the runway. When it is cleared for takeoff, the engine power levers are pushed forward, the brakes are released, and the takeoff run begins.

A typical takeoff speed for a small plane is about 65 miles per hour (105 kilometers per hour). A large airliner takes off at about 140-190 miles per hour (225-305 kilometers per hour). The actual speed depends on the aircraft type, its weight, and the weather condi­tions. As an airliner accelerates along a runway, it reaches a speed called V1. Beyond this point, there is not enough runway left for the plane to stop safely,
so it must take off. The aircraft then reaches VR-the speed at which the pilot raises the plane’s nose and takes off. The plane lifts off and continues to acceler­ate. The next significant speed is V2, the minimum speed the plane must reach to climb away from the ground safely. The normal climb-out speed is a little higher than V2.

Gliders have no power of their own to get off the ground, so they are usual­ly towed into the air. One end of a cable is attached to the glider’s nose. The other end is attached to an airplane, and the glider is towed until its wings generate enough lift to fly. The pilot drops the cable by pulling a lever in the cockpit, and the glider soars away.

wind tunnel is a piece of test equipment used by designers, scientists, and engineers to study the effects of air flowing around aircraft, rockets, missiles, automobiles, and even buildings. Every modern aircraft makes its first flight in a wind tunnel. Instead of the aircraft moving through the air, the aicraft is held still and the air moves around it.

Wind tunnel tests help designers and engineers to find problems with a design and to test solutions without risking a pilot in test flights and without the expense of building a full-size aircraft.

Most wind tunnels are not big enough to hold an entire aircraft. A small and very accurately built model of an air­craft may be used, or just part of an aircraft. Wind tunnels vary greatly in size and airspeed, but they have the same basic parts.

The United States entered World War I in April 1917 with hardly any airplanes. U. S. pilots, some of whom trained in Europe, flew mostly French planes, although DH-4s were built in the United States. Flying schools were set up to train American pilots, and in February 1918 the 95 th Pursuit Squadron was the first U. S. Army fighter squadron to arrive in France. The 94th Pursuit Squadron scored the Americans’ first victories, in April 1918, when two of its pilots flying French Nieuport 28 fighters shot down two German planes. The 96 th Aero Squadron, formed in France in May 1918, was the first U. S. bomber squadron.

Air commanders such as Britain’s Hugh Trenchard wanted to use air power independently of the armies and navies, but they were restricted by lack of air­planes and by orders to support Allied army offensives. Trenchard got his wish in April 1918 with the formation of the British Royal Air Force (RAF). By the end of the war, the RAF had 22,000 air­craft and had destroyed more than 8,000 enemy airplanes and airships.

Even at this late stage of the war, most air battles were small. Just two British 0/400 bombers, for example, flew to attack the Badische industrial plant in Germany in August 1918. On a rare occasion, large numbers of airplanes were used together. In September 1918, U. S. Brigadier General Billy Mitchell massed 1,500 Allied aircraft for an attack on German positions during the Battle of Saint Mihiel in France.

When the space age began in the 1950s, scientists were eager to expand their knowledge of the worlds beyond Earth, previously seen only through tele­scopes. After the launch of the first satellites by the United States and Soviet Union in 1957 and 1958, the world waited expectantly for the first

rocket shot at the Moon. This came in January 1959, when the Soviet probe Luna 1 flew within 3,700 miles (5,920 kilometers) of the Moon. Two months later, the United States sent its probe Pioneer 4 to fly by the Moon. In September 1959, the Soviet Luna 2 probe crashed onto the Moon. Luna 3 flew around the Moon in October 1959 and took photographs of the far side, never before seen from Earth.

peed is the rate of an object’s motion usually expressed as the distance traveled per unit of time, such as miles per hour or meters per sec­ond. Aircraft fly at a wide range of speeds, from an airship gliding slowly through the air to a fighter plane streak­ing across the sky faster than the speed of sound. Spacecraft travel even faster.

Regimes of Flight

Aircraft speeds are divided into bands. From the slowest to the fastest, the bands are: low speed, medium speed, high speed, supersonic, and hypersonic. These speed bands also are called regimes of flight.

The low-speed regime includes light­weight craft, such as hang gliders and

airships, that fly up to 100 miles per hour (160 kilometers per hour). The medium-speed regime includes propeller planes flying up to about 350 miles per hour (560 kilometers per hour). High­speed aircraft fly up to about 700 miles per hour (1,100 kilometers per hour)— these aircraft are mostly jet airliners. Supersonic planes, such as the F-22 Raptor fighter, fly between Mach 1 (the speed of sound) and Mach 5 (five times the speed of sound). Hypersonic craft fly faster than five times the speed of sound. The Space Shuttle is an example of a hypersonic craft.

Stealth airplanes rely almost entirely on being nearly invisible to radar. This fact brings with it certain limitations that can put stealth aircraft at a disadvan­tage. First, neither the F-117 nor the B-2 carries defensive armament, so they cannot defend themselves. Second, most high-performance jet airplanes rely on engine afterburners to boost their speed in combat. Afterburning produces high­ly visible emissions of exhaust gases, however, which make the airplane more detectable. Stealth aircraft, therefore, lack afterburners and so do not have supersonic performance. This makes them vulnerable to faster fighters once they are detected. Third, the design of stealth aircraft may protect them from enemy missiles that use radar, but it offers no protection from other weapons. Fourth, the unorthodox shape and slow speed of stealth planes make them inferior to conventional fighters when engaged in aerial dogfights.

The stealth aircraft’s sophistication also can be a disadvantage. The elec­tronic fly-by-wire system required to keep a stealth plane like the F-117 flying adds to both the cost and weight of the aircraft. Advanced computers on the airplane are also a potential risk since electromagnetic equipment gives off radiation that can be detected by sensors on the ground, revealing the presence of the plane. All stealth airplanes need meticulous maintenance, since the air­craft skin must be kept flawless to pre­serve its radar anonymity. Even a scratch from a pebble thrown up during a landing, or weather damage to the paint, may increase the radar signature.

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SEE ALSO:

• Aerodynamics • Aircraft, Experimental • Aircraft Design

• Control System • Radar

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A pilot lands an airplane by first reduc­ing power so that the plane begins to slow down and lose altitude. Closer to the runway, the engines are throttled back to reduce the airspeed even more. To make up for the loss of lift, the plane’s nose is raised to increase the angle of attack of the wings. The flaps and slats used for takeoff are deployed again, and the landing gear is lowered.

Lights and radio beacons on the ground show the way to the runway. The plane also receives two radio signals from the runway: one is the localizer and the other is the glide slope. The localizer is transmitted along the run­way. It tells the crew whether the plane is flying to the left or right of the run­way or is flying down the centerline. The glide slope shows whether the plane is descending at the correct rate. Just after touchdown, spoilers spring up from the wings to reduce lift and keep the plane on the ground. Jet planes often use reverse thrust to help them slow down.

Helicopters

Helicopters do not need runways to land and take off. Like rockets and airships, they are capable of vertical takeoff and landing. This gives helicopters the abili­ty to land in and take off from small spaces or remote locations where there are no airports.

Helicopters and VTOL aircraft (verti­cal takeoff and landing aircraft other than helicopters) take off vertically by directing their engine power downward to thrust them upward. The helicopter pilot uses the collective pitch lever and the engine throttle to get the aircraft off the ground. The collective pitch control tilts the rotor blades, increasing their angle of attack and producing more lift. The throttle increases the engine speed, which makes the rotor spin faster. To land, these controls are used to reduce lift.

Airships

An airship is balanced so that it is neu­trally buoyant on the ground. Its weight is balanced by the lift produced by the lighter-than-air gas inside it. When it is ready for takeoff, an airship’s propellers and the elevators in its tail swivel up to drive the airship upward. Modern air­ships are filled with helium, and they

also have air-filled bags called ballonets inside them. As an airship climbs, the outside air pressure falls, and the gas inside the airship expands. Valves pop open and let air escape from the ballonets so the helium can expand safely.

To land, the propellers and elevators tilt down to drive the airship down. The outside air pressure rises and compresses the gas inside the airship. The airship’s engines force extra air into the ballonets to keep the airship fully inflated as it descends.