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Bullets and missilesby?Chris Woodford.?Last updated: September 3, 2018.Has any other single invention changed history quite so much as explosives? As the power behind bombs and missiles, chemical explosives have made possible most of the great wars of the last 1000 years or so, altering the course of history time and time again. Before the invention of gunpowder, the first chemical explosive, people had to fight their enemies hand-to-hand on the battlefield with crude weapons like swords and spears. Today, you don't even have to be able to see your enemy—let alone touch him: it's easy to drop bombs from?airplanes, shoot them from?submarines, or launch them on rockets from one side of the Earth to the other. But even though modern missiles are incredibly sophisticated, the basic science and technology behind them is pretty much the same as it was 1000 years ago!Photo: A bullet firing from a handgun looks almost like a rocket launch—and works in very much the same way. Picture by Tech. Sgt. Larry E. Reid Jr. courtesy of?US Air Force.How guns fire bulletsBullets and missiles come in all shapes and sizes. At 21.8 meters (71 ft) long, one of the world's biggest intercontinental ballistic missiles, the US Air Force LGM-118A Peacekeeper, is three times the length of a station wagon (estate car)! But it works pretty much the same way as a handgun bullet the size of your pinkie.What's inside a bullet cartridge?When people talk about a "bullet" in everyday language, they often mean a?cartridge, which is a three-part vehicle with the actual bullet mounted on the very end. The cartridge is the thing you load into a rifle; the bullet is the part of a cartridge that fires out the end. Cartridges are a bit like fireworks and they are arranged in three sections: the primer, the propellant, and the bullet proper. At the back, the?primer?(or?percussion cap) is like the fuse of a?firework: a small fire that starts a bigger one. The next section of the cartridge, effectively the bullet's "main engine," is a chemical explosive called a?propellant. Its job is to power the bullet down the gun and through the air to the target. The front part of the cartridge is the actual bullet: a tapering metal cylinder that hits the target at high speed. It tapers to a point mainly to reduce air resistance, so it goes faster and further, but also to help it penetrate metal, flesh, or whatever else the target may be made from (it must penetrate the target before it can do damage).Artwork: The three main parts of a cartridge. 1) The primer "launches" the bullet by igniting the propellant. 2) The propellant accelerates the bullet down the gun. 3) And the bullet proper (the red and yellow bit at the end) is the part that exits the gun, flies through the air, and does the damage. This one has a complete outer casing known as a?full-metal jacket, which means it can be fired faster and further, but it retains its shape on impact. Bullets with a softer point spread out on impact and do more damage, but don't travel as fast or far.What happens when you fire?Bullet cartridges are designed to be (relatively) safe until the moment when you fire them. When you pull the trigger of a gun, a?spring?mechanism hammers a metal firing pin into the back end of the cartridge, igniting the small explosive charge in the primer. The primer then ignites the propellant—the main explosive that occupies about two thirds of a typical cartridge's volume. As the propellant chemicals burn, they generate lots of gas very quickly. The sudden, high pressure of the gas splits the bullet from the end of the cartridge, forcing it down the gun barrel at extremely high speed (300 m/s or 1000 ft/s is typical in a handgun). It's only the bullet that fires from the gun; the rest of the cartridge stays where it is. It has to be ejected after firing (sometimes manually, sometimes automatically) to make way for the next cartridge—and the next shot.Photo: Launch of a Peacekeeper missile by Don Sutherland, courtesy of Defense Imagery.The propellant chemicals in a handgun cartridge are not designed to explode suddenly, all at once: that would blow the whole gun open and very likely kill the person firing it. Instead, they are supposed to start burning relatively slowly, through a process called?deflagration, so the cartridge moves off smoothly down the gun. They burn faster as the bullet accelerates down the barrel, giving it a maximum "kicking" force just as it comes out of the end. As the cartridge emerges, the whole gun recoils (leaps backward) because of a basic law of physics called "action and reaction" (or Newton's third?law of motion). When the gas from the explosion shoots the bullet forwards with force, the whole gun jolts backwards with an equal force in the opposite direction.The explosion that fires a bullet happens in the confined space of the gun barrel. As the bullet flies out of the gun, the pressure of the explosion is suddenly released. That's what makes a gun go BANG! It's a bit like uncorking a bottle of wine at much higher speed and pressure. Some bullets also make noise because they go so quickly. The fastest bullets travel at around 3000 km/h (over 1800 mph) —about three times the speed of sound. Like a supersonic (faster-than-sound) jet fighter, these bullets make shock waves as they roar through the air.How bullets travelPhoto: Unlike a conventional weapon, this 75mm recoilless rifle doesn't jerk back when fired. It's open at the back so the explosive blast escapes from the rear of the gun, eliminating the usual recoil. You can clearly see the heat of the explosive charge exploding from the front and the blast simultaneously shooting out from the rear. The gunner barely moves at all. By Blake R. Waltman, courtesy of US Army.Gun barrels have spiraling grooves cut into them that make bullets spin around very fast as they emerge. A spinning bullet is like a gyroscope: a sort of "stubborn" spinning wheel that always tries to keep turning the same way. If you try to tilt a gyroscope while it's spinning, it will try to resist whatever force you apply and, if you let go, it will soon tilt back the other way. This is why, when things are spinning, they are very hard to deflect from their path. We call this idea gyroscopic inertia or stability. A bullet behaves in exactly the same way: once it's spinning, it follows a straighter path as it goes through the air, so it's harder to deflect and much more likely to reach its target.We think of bullets flying in perfectly straight lines—but nothing could be further from the truth. Several different forces act on a bullet as it goes through the air. Over very short distances, bullets? ................
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