Category FLIGHT and M ОТІOIM

The momentum of an aircraft or space­craft traveling in a straight line is called linear momentum. The momentum of something that spins is called angular momentum. An object’s total angular momentum stays the same if no other

forces act on it. This is also known as the law of conservation of angular momen­tum. It can be used to control the move­ment of a satellite in space.

The direction in which a satellite points is known as its attitude. Devices called momentum wheels, or reaction wheels, are often used to control a satel­lite’s attitude. Inside the satellite, three wheels at right angles to each other are spun by motors. If a wheel is made to spin in one direction, then—to conserve angular momentum—the spacecraft must spin in the opposite direction.

The advantage of using momentum wheels to turn a satellite is that they use no fuel. If rocket thrusters or gas jets were used for attitude control, the satel­lite could only be controlled for as long as the fuel or gas lasted. After that time, the satellite would be out of control.

Momentum wheels should keep working as long as the satellite because they are powered by electricity generated from sunlight by solar panels.

Momentum wheels can turn a satel­lite to point at a precise part of the sky. The Hubble Space Telescope can take several hours to make an image of a very distant star or galaxy. It must point in exactly the same direction, without wavering, to capture a sharp image while it orbits Earth. The telescope uses momentum wheels to achieve this. Most Earth-orbiting satellites use momentum wheels to keep their antennae, solar panels, cameras, and other instruments pointing in the right direction.

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SEE ALSO:

• Force • Gravity • Hubble Space

Telescope • Laws of Motion • Rocket

• Satellite •Speed

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The only manned spacecraft that have navigated through space and into an orbit other than Earth’s were those of the Apollo mission, when nine space­craft went into orbit around the Moon. The Apollo program also landed twelve astronauts on the Moon between 1969 and 1972. On the way to the Moon, the spacecraft’s inertial guidance system figured out its position by sensing changes in its speed and direction. The position was double-checked by taking sightings of stars using a sextant and telescope. Thirty-seven stars were used as guides to navigate the spacecraft. The sextant, telescope, inertial guidance sys­tem, and guidance com­puter provided Apollo’s primary guidance, naviga­tion, and control system (PGNCS). The astronauts called it “Pings.”

О Timing and precision are crucial to space navigation. Scientists successfully launched the probe Deep Impact to intercept the comet Tempel 1 in 2005. The probe released an impactor to create a crater, so releasing debris to gain informa­tion about the comet’s interior.

The most precise Apollo navigation system was not in the spacecraft at all. It was on Earth. The huge radio dishes of NASA’s Deep Space Network were trained on the spacecraft and relayed communications between Apollo mis­sions and Earth. Tracking a spacecraft using these dishes showed exactly where it was. This information was sent up to the spacecraft and used to correct any errors in its own guidance system. If the spacecraft lost radio contact with Earth, its own guidance system would be cor­rect and could guide it until contact with Earth was restored. Some losses of con­tact were expected. While the spacecraft

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SEE ALSO:

• Bird • Communication • Global

Positioning System • Radar

• Space Probe

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was in orbit around the Moon, it lost touch with Earth every time it disap­peared around the far side of the Moon. NASA still uses the dishes of the Deep Space Network in California, Spain, and Australia to communicate with space probes in all parts of the solar system.

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AN EXPENSIVE ERROR

When navigation goes wrong, the results can be catastrophic. In 1998 NASA sent a probe named the Mars Climate Orbiter to Mars. It was sup­posed to orbit Mars, but instead it entered the planet’s atmosphere and burned up. An inquiry found that some of the navigation data was calculated using U. S. standard units (feet/pounds/seconds), and this became mixed up with data calcu­lated in metric units (meters/ kilograms/seconds). The navigational error resulted in the loss of the multimillion-dollar spacecraft.

Military pilots fly with the U. S. Air Force, U. S. Navy, U. S. Marine Corps, U. S. Army, U. S. Coast Guard, and the Air National Guard. In the U. S. Air Force, the Air Education and Training Command (AETC) is based at Randolph Air Force Base near San Antonio, Texas. Personnel hoping to become pilots receive up to 25 hours of initial flight training from civilian instructors. Selected candidates move on to further training by military instructors. Student pilots learn to fly on fairly slow training airplanes, such as the turboprop T-6II, moving up to the twin-jet T-37, and then to a supersonic jet such as the T-38.

All students learn basic flight skills. Then they are selected for one of sever­al advanced training tracks, depending on the type of aircraft they will fly. Helicopter pilots, for example, receive
special training, on the UH-1 Huey heli­copter. Student airlift (transport) and tanker pilots train on the T-1A Jayhawk, and others fly the T-44 to learn how to pilot a multi-engine, turboprop airplane such as the C-130 Hercules.

Pilots complete their training at U. S. Air Force bases around the country. For example, fighter pilots qualifying from the T-38 course at Randolph Air Force Base go on to fly the F-15 Eagle at Tyndall Air Force Base, Florida, or the F-16 Fighting Falcon at Luke Air Force Base, Arizona. On completion of their military service, many pilots continue to fly as civilian pilots.

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SEE ALSO:

• Barnstorming • Bleriot, Louis

• Coleman, Bessie • Curtiss, Glenn

• Earhart, Amelia • Lindbergh,

Charles • Night Witches

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Many home security systems have motion detectors that sense when someone is moving around in a room. Some of these detectors work by picking up the heat of the person’s body, but others use radar. They flood the room with microwaves that bounce back to the detector. If someone enters the room, the steady pattern of reflec­tions is disturbed, and an alarm sounds.

Some of the cameras used to monitor the speed of vehicles on highways work by radar. The radar system measures the speed of the vehicles. If a vehicle is moving faster than the speed limit, the radar system triggers a camera that photographs the car, includ­ing its license plate, so it can be traced to the speeding owner.

Archaeologists use a variety of methods to map structures under­ground and ground-penetrating radar is one of these methods. Radar can probe down to 33 feet (10 meters) deep and show buried features of ancient buildings such as walls and floors.

Science satellites carry out a range of tasks to observe objects and phenomena in space. They have transformed many scientists’ views of the universe. Science satellites’ instruments often measure radiation in various forms. IRAS (Infrared Astronomical Satellite) was launched in 1983. In its ten-month lifespan, it discov­ered 20,000 galaxies (including a new kind called a starburst galaxy), 130,000 stars, and a comet. In 1999, the Space Shuttle Columbia launched an X-ray observatory named Chandra. It has an unusual elliptical orbit that brings it to

SPACE JUNK

Old satellites and leftover pieces of satellite launch vehicles end up as trash drifting in space. Space junk is heaviest at a height of around 530 miles (850 kilometers), where most satellites orbit. After a half-century of space launches, there is now a lot of space junk. Scientists have recorded at least 11,000 objects larger than 4 inches (10 cen­timeters) in diameter. The junk includes used-up rocket stages, tools lost by astronauts, and lumps of solidified fuel. Space junk is a potential hazard, since a lump of fist-size debris, traveling at more than 21,000 miles per hour (33,800 kilometers per hour), can make a serious hole in an expensive spacecraft.

within 6,000 miles (9,650 kilometers) of Earth and then swings out to 86,000 miles (about 138,400 kilometers)-about a third of the way to the Moon. Each orbit takes 64 hours. Being so far out means Chandra keeps clear of the belts of charged particles that surround Earth and so provides astronomers with longer peri­ods of clear observation time.

SEE ALSO:

• Gravity • Rocket • Space Probe

• Space Race

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Aeronautical research has pro­gressed amazingly fast. Only seventy-three years after the Wright brothers’ historic first powered flight, Concorde passengers were relaxing in air-conditioned luxury, flying at twice the speed of sound nearly 11 miles (18 kilometers) above Earth.

There are now military air­craft that can fly without a pilot. Some of them are flown by a pilot in a cockpit on the ground, linked to the aircraft by radio. The latest unmanned air vehicles (UAVs), or drones, are able to fly themselves and carry out missions on their own without a person in control.

Aeronautical research con­tinues in all developed countries today. The National Aeronautics and Space Administration (NASA) is known as the agency that oversees U. S. space explo­ration, but it is also a world leader in aeronautical design. Large aircraft manu­facturers and universities also carry out research in all aspects of aeronautics and aeronautical engineering.

The advantage of an air-cushion vehicle over conventional craft is that it can travel over water faster than most ships.

О u. s. Marines load a Humvee onto a Landing Craft Air Cushioned (LCAC) during a 2006 exercise in North Carolina. Two huge propellers are visible at the rear. The skirt will inflate with a cushion of air supplied by four fans when the craft leaves the shore.

Amphibious ACVs have the added advantage of being able to travel over­land, too, and they can do so faster than most trucks or military vehicles. Amphibious ACVs can cross deserts, swamps, lakes, or ice with equal ease.

At first, ACVs seemed to offer enor­mous potential for public transportation and military use. Problems in their use reduced their commercial value, how­ever. The airscrews were too noisy for ACVs to move around cities. At sea, ACVs traveling fast over the ocean put out a lot of spray, and the salt spray damaged the gas turbine engines. The engines also used a lot of fuel, making ACVs expensive to operate. While com­fortable for passengers in calm water, even big ACVs could not cope well with rough seas. These disadvantages caused the early optimism about them to fade.

The military in Britain and the United States experimented with ACVs as amphibious assault and patrol craft. The U. S. Marine Corps and U. S. Navy use an ACV designed in the 1980s as a landing craft (a vessel used for taking troops and equipment to shore). The Landing Craft Air Cushioned (LCAC) is carried inside a large naval ship. Offloaded from the ship, the LCAC can move inshore and up a beach to land troops and supplies. The LCAC has four engines (two for propulsion, two for lift) and four fans; its top speed is about 45 miles per hour (72 kilometers per hour).

While some ACVs are used in public and private transportation, the ACV has not yet developed into the widespread system that its inventors expected. The air-cushion principle has been tried in other forms, however. High-speed hovertrains have been tested for railroad use. Enthusiasts and model makers enjoy building small ACVs as a hobby.

RAM WINGS

An interesting vehicle that uses the ground-effect principle, rather like flying boats did in the 1920s, is the ram-winged craft. It looks like an airplane, but it never takes off. Instead of flying, it skims over the surface. The Japanese and the Russians have built ram-winged machines, which are good at travel­ing over lakes and icy terrain.

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SEE ALSO:

• Aircraft, Military • Flying Boat and Seaplane • Lift and Drag • Pressure

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The 1940s and 1950s were decades of more experimentation, as designers tried out new wing shapes and jet engines. The 1947 Northrop YB-49 had no fuse­lage or tail plane. It was just a curved wing, with engines, fuel tanks, and crew compartment inside the flying wing. This airplane looked so unusual that some people claimed they had spotted a

PIGGYBACK PLANES

Several designers experimented with the idea of one aircraft carrying another. In the 1930s, engineers tried out the idea with a small seaplane fixed on top of a flying boat. The fly­ing boat took off before releasing its passenger craft to fly on alone. The idea was to extend the seaplane’s range for mail flights by saving fuel on takeoff. A similar idea was tried in 1948 by the U. S. Air Force. A tiny "parasite" fighter was hooked to the underside of a bomber. The McDonnell XF-85 Goblin fighter managed the risky maneuvers of launching and rehooking onto the bomber several times before the project was canceled. NASA revived the piggyback idea in the 1970s for ferrying Space Shuttles across the country on the back of a Boeing 747.

UFO or flying saucer. The Northrop designers also tested a smaller flying wing, the XP-79B. It was supposed to destroy enemy bombers by slicing through their tails. It flew once, for 15 minutes, in 1945; the pilot reported it was uncontrollable.

Other oddities included Convair’s F2- Y1 Sea Dart (1953), the only jet fighter with water skis, which enabled it to take off and land on water. Unfortunately, it needed almost a mile of water to take off. Hiller’s Pawnee of 1955—officially named the Experimental Ducted Fan Observation Platform—looked like a table lifted by air jets. A soldier could ride on top of it and fly around the battlefield. It never caught on.

Much experimenting went into mak­ing planes that would not need long runways at airfields. In 1954 Convair tested the turbo-prop XFY-1, which was nicknamed “Pogo.” This plane rested on its tail, facing straight up, for takeoff. The tail-sitting design was tried again in the Ryan X-13 Vertijet of 1955. The British went for a more conventional, horizontal position in the Short SC-1
and Hawker P-1127. These experiments in the 1960s led to the production of the Harrier V/STOL (vertical/short takeoff and landing) jet fighter.

Another strange shape was the 1980s Sikorsky X-Wing. This took off like a helicopter, but its X-shaped rotors func­tioned as fixed wings, making the X – Wing capable of faster forward flight.

ll large cities and many smaller ones have an airport, a place where passenger and cargo air­craft can land and take off. Airports are important transportation hubs. Civil air­ports are run as businesses, although they are often owned by governments, especially the larger facilities. There are also military airfields, which function as airports for armies, navies, and air forces.

Types of Airports

In the United States, the Federal Aviation Administration (FAA) classifies airports into two main categories: com­mercial service and general aviation. A commercial service airport handles air­liners on all routes: short commuter
trips, internal regional flights, national flights across the United States, and international flights to and from other countries. Commercial service airports also operate as cargo airports, where air­freight comes in and out.

A commercial service airport per­forms two main functions. First, it must make sure that airplanes land and take off safely. Second, it needs to handle passengers and cargo smoothly and rap­idly. Airports have facilities to process passengers and baggage through ticket­ing, check-in, security, and departure and landing procedures. They also have areas for freight handling and storage.

General aviation airports have either no scheduled passenger flights or small numbers of them. They handle all other kinds of airplanes-business, charter, and privately owned-except military flights. (Military airplanes usually use their own airfields.) A small general avi­ation airport handles light aircraft, such as single-engine private planes, while a larger one can also manage executive jets. General transportation airports, the biggest kind of general aviation airport, can handle a large airliner.

Most large U. S. airports are owned by city, county, or state governments or public corporations. Small airports are often privately owned. While most large airports are miles away from downtown, small airports in city centers can handle

V/STOL (short for vertical/short takeoff and landing) airplanes and helicopters. A landing area built specially for heli­copters is sometimes called a heliport.

ltitude means the height above a certain surface or level. Air temperature and air pressure fall with increasing altitude. In aviation especially, altitude is measured from the ground at sea level.

Why Altitude Is Important

Altitude is important in aviation for sev­eral reasons. Aircraft need to fly at a minimum altitude that enables them to safely clear obstacles on the ground. Every aircraft also has a “flight ceiling,” or maximum altitude at which it may fly. This ceiling is determined by the
aircraft’s capabilities and by whether or not it has a pressurized cabin.

Large airliners, such as the Boeing 747 jumbo jet, cruise at altitudes from about 28,000 feet (8,530 meters) to

41,0 feet (12,500 meters). The big jets fly at standard altitudes called flight levels. The last two zeroes of a flight level are usually left out, so an altitude of 37,000 feet (11,280 meters) is known as Flight Level 370 to a pilot.

An important reason to measure alti­tude is to avoid collision with another aircraft. Aircraft must maintain vertical distance from each other to prevent accidents when they are flying in the same area. For airplanes traveling below

29.0 feet (8,840 meters), the standard vertical separation is 1,000 feet (305 meters). Aircraft above 29,000 feet (8,840 meters) maintain a vertical dis­tance of either 1,000 feet (305 meters) or

2.0 feet (610 meters).

To stay safe from collision, it is essential that all pilots are measuring altitude compared to the same level. Otherwise, two planes flying at the same altitude-but one flying over a hill while the other is over lower ground-would register different altitudes but, in reality, be in danger of colliding. To avoid this, aircraft in flight measure their altitude compared to a reference point called mean sea level (MSL). Charts and maps used by pilots show the heights of mountains and high ground as heights above mean sea level.