This device (also known as the expanding-rod warhead, the annular blast warhead and the extending-rod warhead) is used on anti-aircraft weapons, for which it was designed. Using high explosives against aircraft is quite different than using them against ground targets, for several reasons:
  • They're moving faster
  • Fuzing is therefore more difficult
  • Your miss distance is likely greater
  • They're more fragile than most ground targets if actually hit
  • The shrapnel or other solid objects thrown by your warhead are almost always the avenue of damage. This is opposed to the fireball, which looks (to the plane) like some rather severe turbulence which might be strong enough to break it.
The first really effective anti-aircraft warheads (the flak shells of World War II) were not much more than bursting shells. The metal splinters of the shell casing, plus (sometimes) some added bits of metal inside the casing, would race out and create a temporary lethal area. The 'shell' of expanding fragments would swiftly slow to harmless velocities, which meant that if the explosion was too far in front of an aircraft, it would be ineffective; further, that miss distance did not need to be that great. Users compensated for this by simply lofting enormous quantities of shells into the air during attacks. (For more information on the use of flak, see triple-A, the Big Sky theory, and the countervailing Golden BB).

With missiles, the user typically only got one explosion, so it had better be on target! While most of the responsibility for this outcome lay with the guidance systems of the weapon, those could (until recently) only reach an approximate intercept. The warhead itself needed to 'reach out' those last few meters.

Two factors needed to be overcome. The first was that it was quite difficult to pack enough small splinters into a missile warhead to damage an aircraft that wasn't all that close. The second was the splinters would quickly disperse, so even if the warhead was detonated in front of the aircraft, this might not suffice.

This warhead is the solution. Rather than shrapnel, or individual particles of metal,the warhead contains a quantity of high explosive at its core (typically a rod along its long axis), and around that, a large number of steel rods (with their ends attached to each other in a particular pattern) are 'folded in' to the space. Upon detonation, the force of the explosion throws the rods outward. Since they are attached at the ends, they quickly erupt into a 'ring' or 'web' of steel. Even though this ring does not usually survive all that long, it survives long enough that rather than a dispersed cloud of fragments, the target is instead flying through a solid mass of metal (or has had the edge of it rip through the fuselage).

The greater mass hitting the aircraft, coupled with the tighter pattern, allow designers to construct much more lethal missiles for a given mean miss distance with less weight.

In recent years, there has been some success on the part of the major arms manufacturers towards moving to a 'hit-to-kill' weapon; the U.S. 'Patriot' missile (so reviled for its unfortunate misuse in the Gulf War) can be used in this mode. The weapon is agile and fast enough to actually hit a slow-moving aircraft target (slow compared to, say, a ballistic missile), although it has a blast warhead on it anyhow.

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