Hi all,

I was wondering: given normal glass windows, what is the minimum explosive power required, from a few miles distance (say 5), to incur broken glass? Or does this depend on the creation of a shock wave?

For instance, could the usage of air to air missiles over a populated area break windows?

Thanks,
M74

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Mythbusters did an episode where they tried to break windows with sonic booms, including having a Blue Angels F/A-18 fly over a specially constructed building at very low altitude. They didn't have much luck. That's a shock wave phenomenon. It may have to do with the overall pressure intensity of the shock.

They also tried to paint the interior of a building by setting off explosives surrounded with paint. Again, the glass survived. It's tougher than you might think.

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As Trebuchet said, it may have to do with the overall pressure intensity of the shock. But it also may have just as much to do with the resonant frequency of the glass and the shock wave. I don't really know.

And as Trebuchet said, glass is quite a bit stronger than most people think. In fact, when it first cools and hardens it is even stronger than steel, which many people don't know.

Eric

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It's an overpressure effect. You need an overpressure of about 10% of an atmosphere to break normal windows. The overpressure declines roughly as the cube of the distance from the explosion.
There are certainly equations out there to relate the overpressure at a given distance to the energy of the explosion, which seems to be what the OP needs: quarry workers who set off explosions regularly use such equations to be sure that they're not going to break windows in nearby buildings. But the notes I'm working from relate to human injuries from blast, and they don't get involved in back-calculating explosive energy.

Grant Hutchison

It's off-the-cuff reckoning but I was going to mention something about resonance and the like. We had a lot of glass break in our downtown area a couple (or three-ish) years ago due to sustained high winds, gusting to more than 80 mph. And that wasn't just any old residential glass. At least some of it was on earthquake-rated commericial buildings.

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I remember when I was in Army at Ft Sill, OK which is Artillery Center of world (literally). Around '85ish, iirr... Every now and then, windows were broken from 'booms' in the town of Lawton which sits next to Sill. I can say firsthand that pressures can carry a LONG ways given the right conditions. I was outside at a friends house just yesterday about 15 miles or so from the eastern 'edge' of Ft Sill, and there was some serious thumping of the ground happening (training of the gun-bunnies, per se) and we could feel the pressures within the house. I even hear (and feel to a degree) occasional pressures/sounds here in Marlow which is about a 30 minute drive (minimum).

5 mile distance for an artillery size 'pop' is nothing for a window, IMO. I have even had a house window bust/crack from an F-4 overflight in Austin a 'few' years ago (serious, I was home at the time, though Air Force denied it, of course!) So I would bet heavily that a sizable 'missile motor' could produce a large enough noise (being ~pressure, that is)

Also, if a window 'rattles' in the frame, it'll break *much* easier/sooner than a firmly-held window, in my experience as well, LOL... The type glass plays a role as well as thickness, etc... No simple answer other than it can if all is 'just right' for the glass used. And if there is a tiny defect/scratch which will focus the 'transference' of energy, so to speak. Like a glasscutter uses to cut thick glass cleanly - small nick and glass breaks with a very,very small force Make sense?

HTH,
Alex

Everyone's answers were very helpful, thanks. I'm definitely one of those people who didn't realize glass can be stronger than steel when it's first hardened.


Alex, thanks for the detailed response. That's pretty remarkable that you could feel the vibrations caused by those guns from 15 miles away!

Do you have any idea how high up the F-4 was during that overflight? Did the AF ever pay for your window (i'm guessing, no)?

Thanks all,
M74

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A few decades back, an Air Force pilot slipped the surly bonds of sound while doing a flyby over the Academy and busted quite a few windows.

It's entirely doable.

I've viewed F-4 nose camera footage (taken by #2) of an F-4 booming a shack in the middle of the desert. Forget the windows - he leveled the shack.

Ah, that's very interesting. Thanks for the great anecdote.

M74

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During 'Nam ground attack pilots who expended their munitions but still had men on the ground in deadly danger would sometimes put their aircraft into "off label" usage.

The most desperate I've seen footage of was F-4 pilots doing nose high "tail walks" in afterburner along an enemy trench line on a ridge top.

You just don't see that nowadays anymore.

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I am not sure how high the Phantom was, but it must've been less than ~ one thousand feet or so, iirr. I was not too far from Bergstrom AFB (now a civilian airport), and it was not rare to have an occasional hot-shot going a bit too low/slow and blasting the engine to do whatever was needed to get into the 'pattern' (or out of). Just after one crashed just shy of the highway, it seemed to mellow a bit, though, LOL... And no one I knew that had windows busted ever got reimbursed.

Alex

Howdy Alex, nice meeting you. Welcome to the board.

Once upon a time I was a flightdeck sailor in a Tomcat outfit. And you just haven't seen an airshow until you've seen one thrown by carrier pilots out beyond the reach of FAA regulations. I tell you true. Ever see a A-6 drop a full load of thousand pounders just outside of probable fragmentation range? That'll get the guests sitting back in their seats.

And they buzz the boat a lot closer than a thousand feet while breaking Mach during these shows. Painfully close, and I'm not talking about your ears. I'm talking about close enough that it stings your skin through your clothes and physcally pushes you back two feet or so.

And one time I had tried to warn a photogragher's mate who was an E-5 who hadn't been to sea before. He believed this was going to be like a land based airshow and what he thought was a "high speed" pass over land and civilians is nothing compared to a high speed pass at sea.

I had just come up the ladderway that brings you to the angle, where the waist catapults are, and saw one of my birds approaching so fast the ocean under him was "laying down" well beyond the width of his wingtips and that haze vortice was causing the aircraft to appear and disappear from view like a malfunctioning cloaking device.

Not the best time in the world to have a big camera with a foot long lens pressed to your face.

I saw all that at once and had enough time to tap the guy and tell him fast "You don't want that camera pressed against your face when that bird buzzes us!"

The officious prig sneered at me!

I just stepped back and let karma deal with it.

Let's see, Mr. Zambito was doing five knots above Mach 1, 100 feet off the ocean and 100 feet off the port side of the ship. The flightdeck is 85 feet up itself, placing us even closer.

That poor photog got completely beat up. The eye the camera was against was cut bone deep all the way around and that giant lens was snapped off at the base. And I had the good grace not to say "I told you so" and he had the grace to acknowledge I was right.

But that was the hardest I've been buzzed. During my first cruise, it was Mr Zambito again, the aircrews were buzzing us hard in practice for a show for the Air Marshall of Oman, (a pleasant man who reminded one of David Niven) and Mr Z almost brought an end to all the close flyby's when he buzzed us at just under Mach 2 less than five hundred feet up.

Impressive isn't the word for the physics and engineering displayed in that. Air is still a bit thick at that altitude. Transferred the energy really well.

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In your rush to call everyone "entrenched" or closed-minded or "limited" you fail to note that the "limit" here has a very natural boundary: that point at which the evidence stops. - JayUtah

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Depends on the missile. Some AIMs have 50 pound warheads others have 150 pound warheads. Then you have the different warhead types.

Blast fragmentation, expanding rod, "band saw", others.

I'll say heck yeah.

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Hi BigDon, thanks for your reply and for your account as well. I still don't think I could understand what that experience was like for you. Nothing compares to the first hand experience.

I'm studying in Mississippi and there's quite a bit of AF activity. I was amazed (stunned?) when some kind of cargo plane began making passes over campus. The way it maneuvered...it seemed impossible. And, it was loud. It made me think: if something like a cargo plane could be found unsettling, I can't begin to imagine what it would be like to experience a fighter jet - especially if it was gunning for me.

M74

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Did it look like this?

Hmm...here's something technical that might help: Nuclear Weapon Blast Effects (http://www.fas.org/nuke/intro/nuke/blast.htm).

It gives the overpressure--that is, the excess of pressure over ambient--needed to shatter glass windows as about 3.5 to 7 kilopascals. That sounds like a lot, but it's really only a few percent greater than standard atmospheric pressure. In short, your explosion needs to be either really close or really big--note that there's a typo in the table in that link, probably reversing 1 and 20 kiloton effects.

For a modern example, according to Wiki, the Pepcon explosion in 1991 (and you really must see the footage of that one) peaked at about 1 kiloton, and it damaged windows a good 11 kilometers away.

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That's comfortably close to the figure I gave in post #4. The fact that mine is a few kPa higher is probably because it comes from data relating to human blast-related injury. Injury from falling/flying broken glass becomes a significant risk only when the majority of windows are broken simultaneously.

Grant Hutchison

I just checked out the Pepcon explosion footage. It was insane. I watched the discovery channel version and another version not affiliated with the the discovery channel. I just have to say, what the heck is wrong with the people running the discovery channel? They purposely resynched the audio such that you hear the explosion at the same time you see it. Are they intentionally trying to confuse people?

That aside, it was eerie to see the shock wave travel across the ground in silence.

Also, thanks for the link Romanus.



@mugaliens

Yep, I'm pretty sure that was it. Quite an impressive machine.

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Maybe no comparison with Big Dave's stories, but I was next door to Filton aerodrome when the Last Concorde left. The plan was for it to take off and fly back over the runway in farewell. What it did do was to fly along the runway, at hedgetop height, with the after-burners on. That noise lasted a bit longer than a boom!
John

A-6 Intruders in loiter mode.

So bleepidy bleeping loud that they are agonizingly painful even with full flightdeck hearing protection. (Earplugs covered by "Mickey Mouse" ears)

And, pretty much against us anyway, you probably won't hear the jet that's gunning specifically for you. Not until after it passes, if you're still around to hear it.

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In your rush to call everyone "entrenched" or closed-minded or "limited" you fail to note that the "limit" here has a very natural boundary: that point at which the evidence stops. - JayUtah

Science fiction was never meant to be an educational tool. - Editor Amazing Tales

Different types of glass will yeild different results:
http://en.wikipedia.org/wiki/Bologna_bottle