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Star Talk: Rockets make combustion possible in vacuum

Sunday, October 28, 2012
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When we lost Neil Armstrong this summer, not only did we lose the first man on the moon, but we also lost a man who was a daring test pilot. Armstrong was among the group of mid-20th century test pilots to fly vehicles such as the X-1 and X-15 rocket planes that touched the edge of space.

A rocket is the only vehicle that can fly in space. Scientists say that a rocket can fly because of the action-reaction principle. This is well known as Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction. Today, we call forces — pushes and pulls — “actions” and “reactions.”

Vehicles that move through the atmosphere, such as cars, jets and planes, all depend on the oxygen in the air to support combustion in the engine. The ignition of the fuel with the oxygen moves the engine’s pistons. This motion is then converted into the rotational motion of a car’s wheels, the rotation of the propeller for an airplane and the turning of the turbine blades in a jet.

When a car’s tires push back against the road (the action), the road pushes against the tires of the car (the reaction), making the car move forward. For a plane, the propeller or turbine pushes the air backward and the air reacts by pushing back against the plane, propelling it ahead.

However, in space, there is no air and therefore no oxygen to allow combustion of fuel. A rocket supports combustion by bringing along its own oxygen. The most efficient way to carry the most oxygen is to cool it to cryogenic temperatures so that it liquefies and then bring along the liquid oxygen under pressure so it does not evaporate quickly.

In a rocket engine, the liquid oxygen is combined with the fuel, an igniter sparks and the mixture explodes. This combustion process produces hot gases that escape through the rocket engine’s nozzle to push the rocket forward. The faster these combustion gases can be ejected or the more of these gases that can be expelled (or both), the greater the rocket thrust.

Rockets in history

It is disputed exactly when the rocket was invented but it is generally recognized that rockets were used by the Chinese in the 13th century, probably first as fireworks and then for military applications. These first rockets had long poles attached to stabilize their flight.

The technology spread when Genghis Khan invaded eastern Europe in the 13th century using rockets acquired from the Chinese. Then the Ottomans used rockets to take Constantinople in the 15th century, no doubt having been influenced by the Mongol invasions.

A 17th century artillery manual contained information about the construction and properties of rockets, and during the 19th century the British took an interest in rocketry. Englishman William Congreve developed the rocket further by using stabilizing fins and a conical nose to improve its flying characteristics.

The British used these rockets in the War of 1812 during the assault on Fort McHenry. This attack inspired Francis Scott Key to describe “the rockets’ red glare” in the “Star-Spangled Banner.” It also spread rocket interest in the western world.

At the beginning of the 20th century, Konstantin Tsiolkovsky, a Russian scientist, pioneered astronautic designs for rockets with steering thrusters and multistage boosters. While Tsiolkovsky was a theorist, American Robert Goddard was building the world’s first liquid-fueled rocket in Massachusetts. He had his first successful launch on March 16, 1926, in Auburn, Mass. Soon after, in Germany, Wernher von Braun and his mentor, Hermann Oberth, were developing military rockets that could fly hundreds of miles.

As World War II neared its end, Von Braun surrendered to the Americans and came to the United States with other rocket scientists. Here, they developed rockets such as the Jupiter that launched America’s first space satellite, the Redstone that launched America’s first man in space and the Saturn V that took men to the moon.

Now the world’s most successful private space launch company, SpaceX, based in California, has made its second flight to the space station with its new rocket design. Called the Falcon-9, it is a two-stage rocket with nine engines in its first stage and one in its second stage. It is designed for low Earth orbit, in particular, to provide transport of materials and personnel to the space station.

November Sky

As we start our last week of daylight saving time, of the five planets that are visible to the unaided eye, one appears too close to the sun to be seen, two are very low in the southwest and two others are high above the horizon before sunrise.

Ringed Saturn is lost in the radiance of the sun right now, but by mid-November it will be far enough west of the rising sun to be visible above the eastern glare of dawn. As November continues, Saturn will rise above the sunrise glow.

In the evening sky, Mercury is close to the southwestern horizon; to see it, you will need to use binoculars, have a clear view of the southwestern horizon and start searching about a half-hour after sunset.

A better view of the innermost planet will come during the last week in November when it is visible above the southeastern horizon about a half-hour before the morning sky starts to brighten.

Very bright Jupiter rises from the northeastern horizon before 8 p.m. this week and, because of its motion against the stars, before 6 p.m. after our first week of standard time. Jupiter is just about visible all night long and is found against the stars of Taurus. On Nov. 1 and again on Nov. 28, the moon will appear with Jupiter.

The spectacular brilliance of Venus is unmistakable in the predawn eastern sky. Its dazzling brightness will pair with vastly dimmer Saturn before sunrise on Nov. 26.

Thereafter, Venus will be positioned between Saturn and Mercury. Watch for the crescent moon near Venus on the morning of Nov. 11.

Richard Monda is an astronomer living in the Capital Region.

 
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