All of the expensive technology that goes into a fighter jet,
helicopter or bomber
wouldn't be much use on the battlefield without any
ordnance. While they're not as expensive or complex as
the military vehicles that carry them, guns, missiles and
bombs are the end technology that finally gets the job done in
combat. And most of today's missiles and bombs are pretty
impressive aircraft in their own right. Smart weapons
don't just sail through the air; they actually find their own
way to the target.
In this edition of HowStuffWorks,
we'll look at one of the oldest and most successful smart
weapons in the U.S. arsenal, the legendary AIM-9 Sidewinder
missile. As we'll see, the small and simple Sidewinder is
a highly effective combination of electronics and explosive
power, brought together with incredible technical ingenuity.
The Sidewinder AIM-9 (air
intercept missile 9) is classified as a short-range,
air-to-air missile. Simply put, its job is to launch from an
airborne aircraft and "kill" an enemy aircraft (damage it to
the point that it goes down). Missiles like the Sidewinder are
called smart weapons because they have built-in seeking
systems that let them home in on a target.
Photo courtesy U.S.
is a short-range missile for air-to-air
The technology of smart weapons really got going in the
decade following World War II. Most early guided weapon
prototypes were built around radar
technology, which proved to be expensive and problematic.
These missiles had their own radar sensors, but obviously
could not carry their own radar transmitters. For the guidance
system to lock on an enemy plane, some remote radar system had
to "illuminate" the target by bouncing radar beams off of it.
In most cases, this meant the pilot had to keep the aircraft
in a vulnerable position after firing in order to keep a radar
lock on the enemy until the missile could find it.
Additionally, the radar equipment in the missile was large and
expensive, which made for a high-cost, bulky weapon. Most of
these missiles had something around a 90 percent failure rate
(nine shots out of 10 missed their targets).
In 1947, a Naval physicist named Bill McLean took it upon
himself to build a better system -- a missile that would seek
out the heat from an enemy aircraft's engine system.
Since the missile would home in on the target's own emitted
energy, rather than reflected radio energy, the pilot could
"fire and forget" -- that is, he could launch the missile and
get clear. In place of the bulky radar equipment, the missile
would use a relatively small heat-sensing photovoltaic
cell to "see" the target. This meant it could be built
much smaller than the current radar prototypes, and at a much
Officially, the Navy had no interest in non-radar guidance
systems, but at the China Lake, California, Naval Ordnance
Test Station (NOTS) where McLean was employed, researchers
had enough freedom to pursue unconventional projects. Under
the guise of missile fuze
development, McLean and his colleagues worked out the design
of the first Sidewinder prototypes. Six years later, in
September 1953, the missile had its first successful test run.
Since that time, the Sidewinder has taken a number of
different forms, each model adding new technology and
capabilities (check out this
page for details on the specific models). While today's semiconductor
guidance systems are a lot more advanced than the vacuum tubes
on the original designs, the overall operation is pretty
close. In the next couple of sections, we'll examine the
current Sidewinder model, the AIM-9M, and also take a peek at
its upcoming replacement, the AIM-9X.
- Length: 9 feet, 5 inches (~2.9 m)
- Diameter: 5 inches (~13 cm)
- Weight: 188 pounds (~85 kg)
- Finspan: 2 feet, 3/4 of an inch (~63 cm)
- Cost: $84,000
- Top Speed: Mach 2.5
- Range: 18 miles (~29 km)
- Manufacturer: Raytheon
The Overall System
As we saw in the last
section, the central idea of the Sidewinder system is to home
in on the heat, or infrared energy, from an
enemy aircraft (from the engine exhaust or from the hot
fuselage itself). Essentially, the missile's job is to keep
flying toward the infrared energy until it reaches the target.
Then the missile blows up, destroying the enemy aircraft.
To do all of this, the Sidewinder needs nine major
- The rocket motor, which provides the thrust
to propel the missile through the air
- The rear stabilizing wings, which provide the
to keep the missile aloft
- The seeker, which sees the infrared light from
- The guidance control electronics, which process
the information from the seeker and calculate the proper
course for the missile
- The control actuation section, which adjusts
flight fins near the nose of the missile based on
instructions from the guidance electronics
- The flight fins themselves, which steer the
missiles through the air -- just like the flaps on an
airplane wing, the moving flight fins generate drag
(increase wind resistance) on one side of the missile,
causing it to turn in that direction.
- The warhead, the explosive device that actually
destroys the enemy aircraft
- A fuze system that sets the warhead off when the
missile reaches the target
- A battery to provide power to the onboard
To see how all these pieces work together, let's examine a
typical attack sequence. Before launching, the missile sits
under one of the aircraft's wings, mounted to a
launcher on the wing by several hangers. An
"umbilical cable" near the nose of the missile connects
the onboard electronic control system to the aircraft's
computer system. When the pilot gets the plane in position --
ideally, behind the enemy -- he or she activates the fire
control. The aircraft computer sends a command to the missile
control system to activate the Mk 36 rocket motor and
release the missile.
The rocket motor burns up solid propellant material
to generate a high-pressure gas that streams out the back of
the missile (the motor uses special low-smoke propellant
material to help hide the missile from the enemy). This
provides the initial thrust
necessary to get the missile off the launcher and push it
through the air at supersonic speeds (the current model flies
at about Mach 2.5). Once the propellant has burned up, the
missile glides the rest of the way to its target.
Photo courtesy U.S.
launches from an F/A-18
Each of the four rear wings, which provide the necessary
lift to keep the missile flying, is outfitted with a simple
stabilizing device called a rolleron. Basically, a
rolleron is a metal wheel with notches cut into it. As the
missile speeds through the air, the air current spins the
rolleron like a pinwheel.
If you've read How
Gyroscopes Work, you know that a spinning wheel resists
lateral forces acting on it. In this case, the gyroscopic
motion counteracts the missile's tendency to roll -- to
rotate about its central axis. The simple, cheap rollerons
steady the missile as it zips through the air, which keeps the
seeker assembly from spinning at top speed. This makes it a
lot easier to track the target, as we'll see in the next
Tracking the Target
seeker is something like the CCD in a
video camera. It has an array of sensors that generate an
electrical signal when exposed to the infrared light given off
by hot objects. Since it only sees things in terms of "very
hot" and "not very hot," the infrared system is much simpler
than a visible-light detection system (an ordinary video
camera). Additionally, infrared seekers don't need an
outside light source, so they work perfectly well night or
In the current Sidewinder models, the infrared sensor array
is coupled with a conical scanning system. The basic
idea of a conical scanning system is to continually move the
feed horn -- the assembly of lenses and mirrors that
directs light to the sensor -- around in a small circle (to
visualize this, imagine holding the eraser end of a pencil
steady in one hand while moving the pointed end around in a
circle). As a whole, the moving feedhorn scans a large section
of the sky. The guidance control system figures out where the
target is based on fluctuations in the detected infrared light
as the feed horn moves around the circle. If the target is to
the left of the missile, for example, the sensor will detect
greater infrared light when the feed horn is aimed to the left
than when it is aimed to the right.
The guidance control system's main goal is to keep the
infrared image of the enemy aircraft roughly centered so that
the missile nose continues to point toward the target. If the
infrared image moves off center, the control system sends a
signal to the servo assembly. The servo assembly
includes a gas generator that feeds high-pressure gas to
pneumatic pistons. The pistons are connected to rocker arms,
which move the fins back and forth. The command signal from
guidance control activates electric solenoids,
which open and close valves leading to these pistons in order
to tilt the fins from side to side.
Photo courtesy U.S. Department of
A Dutch air force
armament technician repairs the guidance system on an
To compensate for the target's own motion, the control
system uses a strategy called proportional navigation.
The basic idea of this approach is to over-compensate course
corrections. The control system evaluates how far off center
the target is, and adjusts its angle of flight
proportionally, based on a multiplier. If the
multiplier were 2, for example, and the missile were 10
degrees off course, the missile would change its flight
direction by 20 degrees. Then, a tenth of a second later it
would re-evaluate its heading, and adjust the fins again.
By over-correcting in this way, the control system
anticipates the path of the moving target, in much the
same way a quarterback throws a ball just ahead of a running
receiver, anticipating where the receiver will be when the
ball actually arrives.
In the next section, we'll find out what happens next, when
the missile reaches its target.
The Sidewinder isn't
designed to go off when it actually hits the target; it's
designed to go off when it gets very close to the
target. The missile control system uses an ingenious
optical target detector to figure out when it's within
The detector consists of eight laser-emitter diodes and
eight light-sensor diodes arranged around the outside of the
missile airframe, just behind the flight fins. When the
Sidewinder is in flight, the detector is constantly emitting
beams in a spoke pattern around the missile. If the
missile gets close enough to the target, the laser beams will
reflect off the aircraft body and bounce back to the sensor
diodes. The control system recognizes that the missile is
right next to the target and triggers the warhead.
Photo courtesy U.S.
An F-22 Raptor
fighter jet fires a
The current Sidewinder, as well as its replacement, the
AIM-9X, carries the 20-pound (9-kg) WDU-17/B warhead.
The WDU-17/B consists of a case assembly, a good amount of
PBXN-3 high explosive, booster plates, an initiator device and
nearly 200 titanium fragmentation rods. When the target
detector senses the enemy aircraft, it activates the fuze
mechanism, which sends an explosive charge through the
initiator (a train of low-explosive material) to the booster
plates. The explosive charge from the initiator ignites
low-explosive material in the booster plate channels, which
ignites explosive pellets surrounding the high-explosive
material. The pellets ignite the high explosive, causing it to
release a huge amount of hot gas in a short amount of time.
The powerful explosive force from this expanding gas blasts
the titanium rods outward, breaking them apart to form
thousands of metal pieces, all zipping through the air at top
speed. If the warhead goes off within range of the target, the
speeding titanium fragments will break apart the enemy
aircraft's fuselage. In some cases, the missile may go right
up the target's tailpipe, demolishing the aircraft from the
inside. The WDU-17/B is referred to as an annular blast
fragmentation warhead because the explosive force carries
the metal fragments outward in all directions, in an annular,
or ring-shaped, pattern.
Photo courtesy U.S
In addition to
fighter jets, you'll also find Sidewinders on attack
helicopters, like this AH-1W
While the end explosive effect of the new AIM-9X is pretty
much the same as the current AIM-9M, the new missile has a
couple of important modifications that improve its chances of
finding the target in the first place. An upgraded seeker
design expands the seeker system's view, so it can locate
targets well off boresight (in other words, targets
that aren't right in front of the plane that's launching the
A new thrust vectoring system gives the missile
greater agility, allowing it to make sharp turns in mid-air.
The basic idea is very simple: In addition to operating the
flight fins, the guidance control system controls small vanes
at the rear of the missile to divert the stream of hot gas
from the rocket motor. By shifting the direction of the
rocket's hot gas, the vanes use the thrust force to turn the
missile. For example, when the vanes direct the gas to the
right, the thrust pushes the back of the missile to the left,
and the front of the missile turns to the right. This allows
the missile to make very quick course adjustments to follow a
Photo courtesy U.S.
A Cobra attack
helicopter releases a flare salvo in training
exercises. Flares generate extreme heat away from the
aircraft to divert Sidewinders and other heat-seeking
These modifications, as well as an improved guidance system
and other technological additions, will update the Sidewinder
so it stays competitive with new aircraft, ordnance and
countermeasure technology. After half a century of active
service, the Sidewinder should remain one of the dominate
missile systems in the world for years to come.
For much more information about the Sidewinder and other
missile technologies, check out the links on the next page.
Lots More Information!
More Great Links