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#1
10-26-2004, 07:33 PM
 Incubus Guest Join Date: Apr 2002
Why is 50% of a bombs weight in the casing?

I have heard that 50% of the weight of most gravity bombs (the ones dropped from planes) are in the casing and non-explosive parts. Why is so much of the weight going to the casing? Wouldn't this mean that a 1000 kg bomb only has 500 kg of explosives? Why not design a bomb where as much % of the weight goes towards the explosives inside, to maximize the boom for the buck?
#2
10-26-2004, 07:37 PM
 paperbackwriter Straight Dope Science Advisory Board Join Date: Apr 2001 Location: Connecticut Posts: 1,698
If it's a GP or AP (General Purpose or Anti-Personell), it's not the boom that does most of the damage, it's the shrapnel. These are the most common types of dumb bombs, so that may be the answer.
#3
10-26-2004, 07:40 PM
 jayjay Guest Join Date: Nov 2000
Quote:
 Originally Posted by Incubus I have heard that 50% of the weight of most gravity bombs (the ones dropped from planes) are in the casing and non-explosive parts. Why is so much of the weight going to the casing? Wouldn't this mean that a 1000 kg bomb only has 500 kg of explosives? Why not design a bomb where as much % of the weight goes towards the explosives inside, to maximize the boom for the buck?
Possibly because the thinner the casing, the less resistance it has to the pressure of the explosion and therefore the lower the pressure when the explosion does break the casing open, thus causing less damage?
#4
10-26-2004, 07:44 PM
 Squink Guest Join Date: Oct 2000
The density of high explosives is ~1.8 g/cc. The density of iron is ~7.86 g/cc.
#5
10-26-2004, 09:22 PM
 Smeghead Guest Join Date: Apr 2000
Quote:
 Originally Posted by paperbackwriter If it's a GP or AP (General Purpose or Anti-Personell), it's not the boom that does most of the damage, it's the shrapnel. These are the most common types of dumb bombs, so that may be the answer.
Exactly. You want just enough explosive to generate lots and lots of shrapnel. The shrapnel come from the bomb casing.
#6
10-26-2004, 09:27 PM
 mks57 Guest Join Date: Dec 2003
Balloons

A general purpose bomb behaves like a balloon when it detonates. You want the internal pressure to be as high as possible when the casing bursts.
#7
10-26-2004, 09:34 PM
 KP Guest Join Date: Sep 1999
Quote:
 Originally Posted by jayjay Possibly because the thinner the casing, the less resistance it has to the pressure of the explosion and therefore the lower the pressure when the explosion does break the casing open, thus causing less damage?
Though this is definitely part of the answer, I don't think it is the major reason. Though my knowledge is a bit out of date, the bomb casings I knew were basically cast iron, rahter than highly engineered or harder materials able to contain higher pressures with less weight.

In the extreme case, fairly "inert" bombs, which have no explosives, have seen wide use in combat. They can neutralize or heavily damage a target with minimal collateral damage. Many modern bombs and most air-air missiles have surprisingly small explosive charges. "bunker busters" and armor piercing artillery and firearm rounds are also among the many specialty ammunion types where mas and velocity are valued over explosive power.

For antipersonnel use, especially in non-urban settings, I've always felt that hundreds of cast iron darts (bomblets) launched as a cluster bomb with "smart bomb" electronics would've been a much-overlooked "economy option" for most of the conflicts of the 20th century, and are even more attractive as a specialty weapon today (if reducing cost were a major consideration, rather than increasing profit for military contractors). The relatively ineffectual ones used in Vietnam were, IMHO, too large (2ft/61cm in length), considering that their terminal velocity when dropped from B-52s wasn't much less than a .45 pistol round at combat range.

Consider that a 1" (25.4mm) long .45 (11.5mm) pistol round has admirable lethality and an even more desirable capacity for injury (a casualty places a heavier burden on the enemy than a fatality). WWII flechette artillery rounds showed an impressive ability to penetrate several floors of "normal" (civillian vs. purpose-built hardened military) buildings. Consider how useful this capability might be when rooting out a third-world "unconventional warfare" cell or headquarter amid a dense population of innocent civilians

While a bomblet/flechette the size/mass of a .45 round is too small, 2ft is clearly too far up the "size vs. number" curve to be effective. Imagine the effect of 500 lbs (i.e. thousands) of finned flechetes in a 100 m^2 target region. It'd have much the same effect as a 2-sec burst from the guns of a A-10, but with a bomber's high-altitutde standoff and wide targeting range, and a lot less tech and risk of personnel)

In short strokes, the idea is to kill the target, not to make the biggest possible boom. Brute explosive force dissipates by the inverse square law (overpressure is proportional to the square of the radius from impact), and it's quite possible to improve on that. Flying shrapnel, for example, keeps the energy at lethal concentrations at larger distances than pure blast, even if you double the explosive. Admittedly it's a bit hit-or-miss, but blast will kill a man many times over,in most of it's effective radius; "killing (or wounding) once" over a larger range is more desirable.
#8
10-26-2004, 09:58 PM
 Boyo Jim Member Join Date: Nov 2001 Location: Madison, WI Posts: 29,510
A bomb has lots to do besides explode.

Bombs designed for different purposes may have even less explosive in relation to total weight. For instance, the Bunker Buster 4000 pounds total weight, but the explosive is just over 600 pounds.

The only reference I can think of at the other end of the explosive/casing spectrum is a fictional work by Tom Clancy, A Clear and Present Danger. One of the weapons described in it was a bomb witj a cellulose casing, supposedly to reduce the radar signature of the bomb. The idea was to allow stealth aircraft to carry bombs on external racks with the bombs themselves becoming trackable radar blips. I don't know whether there is a real-world program to match this techno-thriller subplot or not.

For all kinds of bomb specs, check out here
#9
10-27-2004, 02:30 AM
 David Simmons Charter Member Join Date: Nov 2001 Posts: 12,684
Quote:
 Originally Posted by jayjay Possibly because the thinner the casing, the less resistance it has to the pressure of the explosion and therefore the lower the pressure when the explosion does break the casing open, thus causing less damage?
In addition, you want the bomb casing to survive hitting the ground from high altitude in a condition to contain the explosion until high pressure has been built up. The high pressure results in a detonation in which the burning rate exceeds the speed of sound in the medium so that a shock wave results and the blast effect is as high as you can get.
#10
10-27-2004, 05:43 AM
 DWMarch Guest Join Date: Sep 2003
I'd also like to chime in and mention Tom Clancy's Clear and Present Danger. As mentioned upthread, a cellulose case is used with ~500kg of Octol (the same stuff that went missing in Iraq!). It's enough to flatten a big house but one of the military guys in the book does make a comment along the lines of "what's the point of a bomb that just pisses the other guy off?" So presumably, the weight of a traditional bombcase is meant to be used as shrapnel.
#11
10-27-2004, 11:38 AM
 paperbackwriter Straight Dope Science Advisory Board Join Date: Apr 2001 Location: Connecticut Posts: 1,698
Except that Clear and Present Danger is fiction. In reality, such a weapon wouldn't be very useful, and it would still require some stonger components (plastic, graphite, aluminum, something) in order to get to it's target.

Leaving aside deep penetrators, runway bombs, and other special purpose weapons, bombs really have two lethal effects: projectile wounds from shrapnel and blast overpressure. The fictional cellulose-cased bomb would maximize the second, while having little or none of the first. The problem with this type of design is that the first is much more effective.
#12
10-27-2004, 02:10 PM
 Roches Guest Join Date: Aug 2003
A cellulose bomb casing also might not have the necessary strength to break through a building's structure intact. It might, because of the sheer amount of kinetic energy it'd have, but it might also tend to break apart when it hit the target. Presumably bombs have to be fused to explode when they undergo an impact that is not sufficient to break them apart, so a cellulose bomb could have a very delicate fuse, but that would limit its usefulness. Blast overpressure (the main way that a bomb without a steel casing would inflict damage) is much more destructive inside a confined space than in the open air. (For example, certain grenades are designed so that they will cause severe damage if thrown into a bunker but will just cause a small explosion with little fragmentation if they are thrown in the open.) A cellulose bomb may explode harmlessly outside a reinforced structure rather than penetrating and destroying it. It might still have certain applications where blast overpressure is needed, but those applications could be handled by other types of munitions (such as a fuel/air explosive).

Another reason why bombs are made with steel or cast iron casings is that the explosives inside are equally destructive if the bomb explodes accidentally as if it explodes at a target. If a plane carrying bombs with metal casings is involved in a fire, there's time to put the fire out before the bombs explode. Cellulose bombs would have to be stored and loaded very carefully because a fire would probably set off the bombs and destroy the storage facility, plane, or crew. Some explosives do not explode in fires (they need explosive shockwaves to set them off) but it's still possible that they could explode.
#13
10-27-2004, 02:38 PM
 Rodd Hill Guest Join Date: Apr 1999
Just to be a nit-picky weasel:

the difference between shrapnel and shell/bomb splinters.
#14
10-27-2004, 02:50 PM
 Airman Doors, USAF Charter Member Join Date: Feb 2001 Posts: 12,903
Actually, in most cases it's more than 50%. I was looking up the MK-82 500 pound bomb today to answer another question and I found out that the explosive comprises only 192 out of those 500 pounds.
#15
10-27-2004, 04:43 PM
 Loach Guest Join Date: Apr 2003
Quote:
 In reality, such a weapon wouldn't be very useful, and it would still require some stonger components (plastic, graphite, aluminum, something) in order to get to it's target. Leaving aside deep penetrators, runway bombs, and other special purpose weapons, bombs really have two lethal effects: projectile wounds from shrapnel and blast overpressure. The fictional cellulose-cased bomb would maximize the second, while having little or none of the first. The problem with this type of design is that the first is much more effective.
I do agree with most of your points but don't quite understand what I underlined above. Seems to me they would have no trouble getting to their target. I have handled plenty of M1 tank ammo. 120mm main gun rounds have a cellulose casing. They are strong and durable and I see no reason a bomb made out of that material would have any problem getting to the target.
#16
10-27-2004, 04:54 PM
 ElectricZ Guest Join Date: Feb 2004
IIRC, the cellulose bomb in Clear and Present Danger was chosen because they didn't want to leave shrapnel evidence at the target site. They were trying to start a fight between drug lords, so they dropped the laser-guided cellulose-cased bomb onto a truck parked outside a cartel meeting, giving the impression that a rival criminal was trying to take out the others.

Fake, but it's the stuff technothrillers are made of!

EZ
#17
10-27-2004, 05:08 PM
 Valgard Member Join Date: Feb 2003 Location: SF Bay Area Posts: 4,158
In many cases a bomb is designed to penetrate into the target before detonating. The casing has to be strong enough to allow this to happen. If your bomb disintegrates on the surface of the target, even if the detonator survives and the bomb explodes, it'll have far less effect than if the bomb stayed in one piece and exploded deep inside the target.

So that's why you need an extremely heavy and structurally sound casing.

IIRC the fuses have delays after impact of a few miliseconds, that's all the time it takes for the bomb to penetrate deeply enough.

The first generation "bunker buster" bombs from Desert Storm were built out of howitzer barrels as that was the only thing onhand that was (a) suitable for a bomb and (b) able to survive the designed impact.
#18
10-27-2004, 05:11 PM
 Airman Doors, USAF Charter Member Join Date: Feb 2001 Posts: 12,903
If you read the book you'd see that it was designed for the developing low-observable (aka Stealth) bomber, because it would generate fewer radar returns as an external store. The movie didn't refer to that, so you're left with the impression that it's a fantasy weapon. It is, of course, but at least in the plot of the book it makes sense.

The book is way, way, way better than the movie. Actually, I can't think of a case where this isn't so.
#19
10-27-2004, 09:25 PM
 paperbackwriter Straight Dope Science Advisory Board Join Date: Apr 2001 Location: Connecticut Posts: 1,698
Quote:
 Originally Posted by Loach I do agree with most of your points but don't quite understand what I underlined above. Seems to me they would have no trouble getting to their target. I have handled plenty of M1 tank ammo. 120mm main gun rounds have a cellulose casing. They are strong and durable and I see no reason a bomb made out of that material would have any problem getting to the target.
Strong and durable in handling, transportation, storage no problem. I just don't know that it'd be durable enough for the aerodynamic stresses hanging off the outside of a jet, or for delivery. I don't work for Picatinny Arsenal, so I could easily be wrong on this point.

Another advantage is that these casings burn up on firing, saving you from getting stacked knee-deep in empties.
#20
10-27-2004, 09:54 PM
 Loach Guest Join Date: Apr 2003
Quote:
 Originally Posted by paperbackwriter Strong and durable in handling, transportation, storage no problem. I just don't know that it'd be durable enough for the aerodynamic stresses hanging off the outside of a jet, or for delivery. I don't work for Picatinny Arsenal, so I could easily be wrong on this point. Another advantage is that these casings burn up on firing, saving you from getting stacked knee-deep in empties.
If it is made out of the same substance that 120mm sabot and HEAT rounds are made of then it would be plenty durable to be strapped to the outside of a jet. I don't know what would happen on impact.

In case you are wondering, the cellulose casing of a 120mm round burns up when it is fired leaving only the aft-cap (the metal plate on the bottom containing the primer). It makes the inside of the turret a lot less messy after firing a few rounds. The 105mm gun in the old M1 leaves behind a two foot metal casing.
#21
10-27-2004, 09:56 PM
 Loach Guest Join Date: Apr 2003
Quote:
 Another advantage is that these casings burn up on firing, saving you from getting stacked knee-deep in empties.
Like you said
#22
10-27-2004, 10:10 PM
 Boyo Jim Member Join Date: Nov 2001 Location: Madison, WI Posts: 29,510
Quote:
 Originally Posted by Loach If it is made out of the same substance that 120mm sabot and HEAT rounds are made of then it would be plenty durable to be strapped to the outside of a jet. I don't know what would happen on impact. In case you are wondering, the cellulose casing of a 120mm round burns up when it is fired leaving only the aft-cap (the metal plate on the bottom containing the primer). It makes the inside of the turret a lot less messy after firing a few rounds. The 105mm gun in the old M1 leaves behind a two foot metal casing.
You may be right but I'm not following the logic here.

You say this cellulose shell casing is used in M1 tanks. So it's carried around and never subjected to more than 3 or 4 g's when the tank is maneuvering. And it's almost certainly not subjected to anything like the temperature and humidity variations it would find on a plane. And when the shell is fired, the casing burns off in microseconds from a single ignition source.

And this gives you confidence that it would make reasonable strap on weapon to a supersonic jet? Except for your issue with impact, of course.
#23
10-27-2004, 10:13 PM
 Danalan Guest Join Date: Jun 2000
Here is a bomb with no explosives -- urban combat requires interesting technology. Yep - laser guided concrete.
#24
10-27-2004, 10:32 PM
 Boyo Jim Member Join Date: Nov 2001 Location: Madison, WI Posts: 29,510
Quote:
 Originally Posted by Danalan Here is a bomb with no explosives -- urban combat requires interesting technology. Yep - laser guided concrete.

Here is a BBC link that doesn't.
I have to say I think this is a great idea. It might be a spinoff of some star wars concepts that imvolved placing objects in orbit that could be "dropped" onto ground targets and kill with kinetic energy.
#25
10-27-2004, 10:50 PM
 Loach Guest Join Date: Apr 2003
Quote:
 And this gives you confidence that it would make reasonable strap on weapon to a supersonic jet?
I have no scientific evidence, just my perception from handling many rounds. If you held one you would think it is made out of some sort of metal or hard plastic. I'm not sure what you think g-forces would do to a bomb. The laser seeker in bombheads are made out of glass and plastic. They hold up pretty well under g-forces. What do you expect heat and humidity to do? The highest temperature would be somewhere around 115f, the same temp that it will reach in a tank in the desert. The lowest temperature would be what? Much lower than a bad winter in Germany or Korea? The casings are coated and water proof. So yes I don't know what an impact from 20,000 feet might do but I'm pretty confident that it would be able to stand up to whatever an old dumb bomb, a laser smart bomb, an external fuel tank or an electronics warfare package can handle. Don't think of cellulose as paper, it actually acts more like a ceramic until expose to high heat in the gun barrel.
#26
10-28-2004, 12:11 AM
 mks57 Guest Join Date: Dec 2003
Safety

As previously mentioned by Roches, safety would be a major concern with a cellulose casing.

There is an excellent documentary named "Situation Critical" on the fire that occurred on board the carrier USS Forrestal in 1967. It is occasionally shown on the Discovery Channel. John McCain, future POW and Senator, was sitting in his aircraft on the flight deck when it was hit by an accidentally fired Zuni rocket. He escaped the aircraft but the fire rapidly spread across the flight deck. It includes footage of fire crews attempting to extinguish multiple fires on the flight deck while GP bombs and other ordnance are cooking off.
#27
10-28-2004, 04:48 AM
 Princhester Charter Member Join Date: Jan 2001 Location: Brisbane, Australia Posts: 10,858
Quote:
 Originally Posted by Airman Doors, USAF The book is way, way, way better than the movie. Actually, I can't think of a case where this isn't so.
Star Wars. Sorry. Carry on.

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