Bullets and missiles
Last updated: October 17, 2008.
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 crude 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: Oil painting of a sailor loading
ammunition, 1942,by McClelland Barclay (1891–1943), who
was killed onboard ship by a Japanese torpedo during his service as a
US war artist.
Picture courtesy of the US
Department of the Navy/Navy Historical Centre.
How guns fire bullets
Bullets 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 Airforce LGM-118A Peacekeeper, is three times the
length of a station wagon! But it works pretty much the same way as a
handgun bullet the size of your pinkie.
Photo: Launch of a Peacekeeper missile.
By Don Sutherland, courtesy of the US
Defense Visual Information Centre.
Bullets 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 bullet, effectively
its "main engine," is a chemical explosive called a propellant.
Its job is to power the bullet through the air from the gun to the
target. The front part of the bullet is a tapering metal cylinder that
hits the target at high speed. It tapers to a point to help it
penetrate through metal, flesh, or whatever else the target may be made
from.
Bullets 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 bullet, 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 bullet's volume. As the propellant
chemicals burn, they generate lots of gas very quickly. The gas shoots
from the back of the bullet, increasing the pressure behind it, and
forcing it down the gun barrel at extremely high speed (300 m/s or 1000
ft/s is typical in a handgun).
The propellant chemicals in a handgun bullet 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, so the bullet 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 bullet
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.
Photo: 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 the US Army.
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 travel
Gun barrels have spiralling 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.
Photo: US army gunner with a .50 caliber
machine gun in Iraq.
Picture by Sgt. Matthew Acosta, courtesy of the US Army.
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
do follow more or less a straight line. Over longer distances, they
follow a slight downward curve because gravity tugs them toward the
ground as they go along. Air resistance and the spinning, gyroscopic
motion of a bullet complicate things too. Usually, because of recoil,
the person firing wobbles the gun slightly when the bullet emerges.
When all these factors—the bullet's motion, gravity, air resistance,
recoil, and spinning—add together, they make a bullet follow a very
complicated corkscrew path as it flies through the air.
Why bullets do damage
When things move, they have momentum. The faster they move and the
heavier they are, the more momentum they have. A truck trundling along
slowly has a lot of momentum because it weighs so much. Even though
bullets are tiny, they have lots of momentum because they go so fast.
They also have huge amounts of kinetic energy,
which they get from the chemical energy of the burning propellant.
Bullets do damage when they transfer their energy to the things they
hit. The faster something loses its momentum, the more force it
produces. (One way to define force is as the rate at which an object's
momentum changes.) A rifle bullet coming to a stop in a tenth of a second
produces as much force as a heavy, slow moving truck coming to rest in
10 seconds. Imagine being hit by a truck—and you'll have some idea why
bullets do so much damage.
Photo: Left: Close-up of a bullet hole in the
fuselage of a US Air Force
plane, fired on while delivering relief supplies in Somalia.
Picture by TSgt. Val Gempis courtesy of US Air Force and
Defense Imagery.
Photo: Right: Now that's what I call kinetic energy!
This is what happens when you fire a 7g (0.25 oz) projectile at a velocity of
25,000 km/h (16,000 mph) into a cast aluminum block. This huge hole
has been made by something weighing about as much as an iron nail!
Even if something is that tiny, if it's travelling at very high speed it will
have enough kinetic energy to do a lot of damage.
Photo by R.D. Ward courtesy of the US
Defense Visual Information Center (DVIC).
How to protect yourself
There's no completely sure way of protecting yourself against the energy of a speeding bullet, though
protective materials certainly help.
You can find out more in our article about bulletproof glass.
Further reading
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