Calling All Rocket Scientists

August 15, 2007 11:27 AM

Is it the impact that causes the missile/torpedo/what-have-you to explode? Or some sort of 'on-board' electronic targeting system that tells it to detonate?

Like, could a missile slam into the earth but not detonate because of some electronic failure?

Tried doing a search of this on The Google but it did nothing. This specific, anyway.

Thanks.

posted by ryecatcher to technology (13 comments total)

Depends on the weapon. Some are impact, some are proximity, some are timed or work by distance flown, some are "explode when you reach this coordinate". Some are probably all of the above.

...and yes, a missile can hit the ground without exploding. Unexploded weaponry is a giant pain in the ass; it happens all the time.
posted by aramaic at 11:33 AM on August 15, 2007

It depends. For example, IIRC, the atomic bomb dropped at Nagasaki was detonated once radar told it that it was a certain height above the ground. Some armor piercing shells had fuzes which would detonate the thing a certain time after impact, so give it time to go through armor. The Patriot missile, (again IIRC), calculates a point at which it is a certain distance from the target, the explodes.

During WWII, US Naval torpedoes had all sorts of trouble exploding because the contact detonator would fail, and the magnetic detonators were miscalibrated. Etc.
posted by Comrade_robot at 11:41 AM on August 15, 2007

Even weapons intended to be detonated by impact will typically have a mechanical detonator. The high explosives used in munitions is deliberately highly stable and they engineered so they dont explode due to unintended impacts. You dont want a bomb laden plane that has a takeoff problem to blow a huge crater in your runway.
posted by Good Brain at 11:48 AM on August 15, 2007

I've simplified things enormously, of course. Wikipedia has a brief discussion on fusing, and looking into UXO (UneXploded Ordnance) online will tell you more than you probably ever wanted to know.

Missiles, and their related weapons, are quite complex. This means there are lots of things that can go wrong, and designers typically try to have things blow up only in very particular circumstances (eg: setting weapons to arm after they've been flown/fallen a certain distance; if the arming mechanism gets stuck, the fuse isn't active, and the now-dud weapon smacks into whatever it was shot at or dropped on).

Perhaps 30% of "bomblets" don't go off -- this is one of the main reasons cluster bombs are controversial.
posted by aramaic at 11:48 AM on August 15, 2007

Ah, a vaguely-related-to-my-career AskMeFi question! Hurrah! :)

There are numerous types of fuse for just about every type of military application. The type of fuse depends on when and where you want the explosive to actually detonate. "Well, when you hit the target, surely" is the intuitive but often incorrect answer.

Air-to-air and surface-to-air missiles work by filling a volume of space with fast moving fragments of metal. Aircraft are surprisingly fragile, and if the target is inside this volume, you have a hit. Similarly, most Artillery shells are designed to do the same, often exploding a distance above the ground to maximise effect. These are blast-fragmentation warheads, and tend to work by proximity (VT) fuses using radar or laser to sense when they are a pre-set distance from their target (e.g. the earth, a plane, etc) then detonate.

Tanks, on the other hand, are very tough targets to kill. Anti-tank missiles tend to use shaped-charge warheads, which need to detonate a specific distance from the surface of the armour for maximum effect. For this reason, many HEAT (high-explosive anti-tank) rounds use a standoff physical switch (see the "stick" on the front of the round here).

Torpedoes are often multi-fused depending on the selected target. WWII saw the introduction of magnetic fusing, which allowed a torpedo to be fired at a given depth below a target ship, which would detect the influence of a large mass of steel (the ship's hull) on the earth's magnetic field, and trigger the explosive. This would cause a shockwave that would break the ship's keel ("spine"), which is the most effective way of destroying a surface ship.

Unexploded munitions are possible, but less likely with electronic-fused weapons compared to mechanical fused weapons - cluster bombs have upwards of 500 mini-bomblets, usually mechanically fused, and a 1% failure rate - considered very, very good - will still leave 5+ unexploded warheads in the target area - see here and here.
posted by Nice Guy Mike at 11:53 AM on August 15, 2007 [3 favorites]

The fuzes get even more complicated -- redundant detonation systems, fail-safes, centrifugal force mechanisms, adjustable time delays. The explosive train (detonation sequence) within a missile's fuze usually has at least three elements: detonator, booster, main. Generally they go from small quantities of sensitive materials in the detonator, to slightly larger quantities of less sensitive materials in the booster, to the large quantities of relatively insensitive material in the main charge. This is for safety reasons, usually.

I used to be an ordnance engineer, and I've studied many different fuze systems. Here's some info for you: link.
posted by yesster at 11:55 AM on August 15, 2007 [1 favorite]

Another link - sensor fuzed weapons -- the one featured is from Textron.
posted by yesster at 12:00 PM on August 15, 2007

I love this site.

Thank you all so much for the help.
posted by ryecatcher at 12:26 PM on August 15, 2007

Having several fragments of a shell embedded within me and loads of war experiences around them, I can definitely tell you that the lower-level munitions fired onto (my city of) Sarajevo had a pretty low failure rate and only exploded on impact. I don't know how you'd classify these things - certainly not "torpedos" - fired from what we civilians called "rocket launchers" (in my language, more or less) and capable of demolishing part of a house or destroying a car or killing a dozen or so people if the people were crowded together closely enough. A few times I encountered unexploded shells and it was always a crazy and bewildering experience since, as civilians, we had no idea what to do with them or whether they were safe or not or what the impact would be if they suddenly exploded without the velocity of flight.

For what it's worth, they still find unexploded shells all over the place - Bosnia, London, Japan - every now and then. I just saw a Romanian movie, "California Dreamin'" in which the plot is driven by an unexploded bomb in Romania from WWII which explodes about 60 years later.
posted by Dee Xtrovert at 12:52 PM on August 15, 2007 [2 favorites]

Jennet Conant's Tuxedo Park recounts the development of radar proximity fuses in great detail, including why it was such a big deal.
posted by djb at 3:09 PM on August 15, 2007

Specifically regarding (naval) torpedoes, there have been many designs which do not impact-detonate.

Shipbuilders realized pretty quickly after WWI that torpedoes were serious business, and began building layered "belts" just below the waterline. This was a fairly common feature on WWII battleships (and possibly some earlier ones). Many different anti-torpedo schemes were tried, including liquid- and air-filled voids in addition to conventional armor.

In response, weapons engineers developed torpedoes that detonate beneath ships, breaking them rather than trying to punch a hole in them. They initially managed this by using magnetic detonators and depth-control mechanisms. The torpedo would 'swim' at a preset depth (using a pressure-sensing system) in the direction it had been launched until a magnetic sensor triggered the warhead, hopefully right under the enemy ship's keel. The development of reliable depth-control and magnetic fusing mechanisms was not trivial, and at times plagued with problems. (The Germans and the British both abandoned magnetic-detonating torpedoes in WWII in favor of contact detonation because of reliability concerns. In fact, 30s-era contact-detonating torpedos were used up until the 1980s, and are probably still in service.)

More modern torpedoes (like the Mk 48 ADCAP and Spearfish) use either wire or autonomous sonar guidance and can detonate either on impact or at a distance from the target. (Apparently, the Spearfish's contact-detonation mode is designed to allow its shaped-charge warhead punch a hole in well-armored submarines like the Russian Alfa.) Early-version Mk 48s used magnetic detonation, but it's not clear whether current ones still are, or use sonar to detonate.
posted by Kadin2048 at 3:18 PM on August 15, 2007

Fuse vs. Fuze. I had no idea.
posted by Chris4d at 4:25 PM on August 15, 2007

Bunker Buster bombs detonate well after they make contact:

From wikipedia:

The traditional fuze is the same as a classic armor-piercing bomb: a combination of timer and a sturdy dynamic propellor on the rear of the bomb. The fuze is armed when the bomb is released, and detonates when the propellor stops turning and the timer has expired.

Modern bunker busters may use the traditional fuze, but some also include a microphone and microcontroller. The microphones listens, and the microcontroller counts floors until the bomb busts through the desired numbers of floors.
posted by jpdoane at 9:54 PM on August 15, 2007