Sonic Boom, Red Bull Stratos (possible?)

[A.T.]

It would been interesting if they had actually succeeded in reaching Mach 1 with this monstrosity.

What I was wondering about is the fact, that the navy planned this for carriers. Wouldn't the supersonic tips running in humid air produce similar clouds to those below, but right in front of the cockpit?

https://www.youtube.com/watch?v=gWGLAAYdbbc

 

[Zentrails]

What I was wondering about is the fact, that the navy planned this for carriers. Wouldn't the supersonic tips running in humid air produce similar clouds to those below, but right in front of the cockpit?

https://www.youtube.com/watch?v=gWGLAAYdbbc

Run that weird propeller plane on liquid oxygen and you'd have one heckuva cool submarine. LOL

I've overstayed my welcome on the supersonic propeller issue. LOL

You did remind me of my old navy days when I was on a destroyer (USS McCaffery DD860) that attacked positions close to shore in N. Vietnam.

You could feel the shock wave coming out of our 5" 38 cal guns if you were on deck.
It was like getting slapped on every inch of your body at the same time.
I don't remember hearing any noise at all.

Made me remember when we had a B-52 bombing run (3 planes IIRC) that landed close to our ship. You could feel that, too. Distinctly unpleasant and disorienting.


http://www.usscochrane.org/index.ph...randon-bay&catid=52:other-resources&Itemid=97

Here's my sonic boom related question:
After the B-52 raid about half of the bombs were purposely timed to go off in such a manner as to create as large a single combined shock wave as possible to maximize destructive power. They also timed the other half to do the same a minute or so later (I forget the exact time lag) to make another combined shock wave that would hit structures already weakened by the first combined shock wave.

Is there a term for this combined shock wave that is analogous to "sonic boom?"

 

[boneh3ad]

thanks for the very good summary on shock waves.

I don't quite get how "Back to shock waves: shocks arise any time the flow needs to make an abrupt change of some sort, typically in pressure" applies to supersonic planes flying at a constant velocity. Are you saying that there is a continuous abrupt change in pressure?

There wouldn't necessarily be the need for some abrupt change in pressure in that case, but there absolutely is a need for abrupt changes in direction of the flow as it encounters the leading surfaces of the plane. Any object moving through a gas at supersonic speeds will produce a combination of oblique and bow shocks.

The point is that at supersonic speeds the sound generated by the plane is much louder than at subsonic speeds and the change in intensity is not linear. It's sound coming from a supersonic source that makes a sonic boom, it really doesn't matter how that sound is produced, N-wave, shock wave, pressure front, whatever you want to call it. I like to simplify it into a "black box" that makes sound. Otherwise the physics goes above my pay grade.

I agree we're just arguing semantics.
IMO, sonic booms are not shock waves, but they are usually, but not always, generated by shock waves as you define them.

That isn't really true though. The sonic boom is absolutely, unequivocally tied to the presence of a shock wave. When an object like a plane moves through the air, it has to move the air molecules out of the way. This movement of air away from the plane takes the form of what are essentially continuously generated pressure waves which propagate through the air at the speed of sound. This is why, in a subsonic flow, you see the air start moving out of the way of the body before the body actually reaches it. The pressure waves given off travel forward to deflect the air before the object actually reaches that particular parcel of air.

When this object is moving faster than the speed of sound, however, the pressure waves emanating from the body cannot propagate upstream of the object since they can only move at the speed of sound but the object is moving faster than the speed of sound. The leading front of these waves then end up "piling up" and forming what we call a shock wave. That is also why a supersonic flow must turn abruptly when encountering an object as opposed to smoothly as it would in a subsonic flow. It physically cannot see any information about the presence of that object until it is already at the object or at the location of that oblique shock wave front emanating from it.

Now, imagine the effect of different sizes and shapes of objects moving through the air. A large body obviously has to displace more air to move through it, so it is going to be putting a lot more sound energy in the form of those pressure waves into the air to do it. Similarly, a very blunt object is going to have to turn the flow out of the way much faster, so the shock formed is going to tend to be a lot stronger.

Shock waves, as I already mentioned, have the effect of enacting near-discontinuous changes in the flow (near because it actually is continuous but on the order of 6 to 10 mean-free-paths of air molecules, so perhaps 1 micron maximum at sea level). In particular, they cause the pressure to increase sharply over very small distances, and therefore in a very short time in terms of that wave front passing by your ear.

What this means is that the sonic boom is not some aggregate of all the sound generated by the plane and its engines and its moving parts and the flow over the plane; instead, it is the sound generated by the sharp pressure changes associated with shock waves. Most planes have two such shocks, one at the tip and one at the tail where the flow must straighted back out when it leaves the plane's body. This would result in two booms if the shocks are strong enough or you are close enough to the plane, e.g. the re-entering space shuttle. Most planes would only give off a single boom to the casual observer since they are so far away.

I will quibble a little bit with your claim that "the most common reason for their formation is when an angular object moves through a gas" (what do you mean by "angular" object?)

The most common source of shock waves, IMO, is in ultrasonic units, which I use professionally to disperse carbon nanotubes in my research at the University of Pennsylvania physics dept. (thank goodness IED's aren't around as much as a few years ago)

Ironic isn't it? Sound waves (albeit at 50 KHz in my units - bats can probably hear it so technically I can call it sound LOL) can generate shock waves.

By an angular or oblique object, I mean any object whose surface is not either parallel or normal to the incoming free stream. This absolutely is the most common source of shock waves - at least on Earth. Most likely the actual most common source of shock waves would be supernovae or the ejecta from black holes or stars.

I think maybe what is going on is that you are interchanging the concept of a shock wave and a sound wave, which are related terms but not technically the same. A shock wave is, in essence, a very strong shock wave. What you are describing with ultrasonic perturbers are ultrasonic sound waves. Shocks, in comparison, are the result of many sound waves coalescing to form near-discontinuous changes in flow properties.

In my research group we routinely measure sound waves on the order of 250 kHz, but they are just that, sound waves. Measuring them is quite a task, though.

In fact you can feel the shock waves in your joints, not a very pleasant feeling, but a nice treatment for my Carpal's Tunnel pain (my sonicators have very large baths and are pretty powerful). Works quite well, my research group also does medical research so we'll probably publish these results before too long, assuming no one reads this thread and beats us to the punch. LOL

They need not be shock waves in order to travel through your bones to your joins. Simple sound waves can do that just fine.

Hopefully they get the SCRAM jet engines, or something like that, working soon so we don't have to use compressors anymore for supersonic engines (and yes I know that you need to reach high Mach numbers for SCRAM engines to work). Am I correct in remembering that SCRAM jets actually require turbulent supersonic air flow for them to work?

We already have compressor-less jet engineers in use in ramjets. Those utilize normal shocks or a combination of oblique and normal shocks at the entrance to compress the air without requiring a mechanical compressor stage. The engine is designed such that compression results in subsonic flow through the combustor, which is then expanded again through the nozzle to provide thrust. Scramjets (supersonic combustion ramjets) are a variant of ramjet where the initial compression is done such that the flow remains supersonic throughout, which leads to supersonic flow in the combustor. The advantage is that it is much more efficient at high Mach numbers than ramjets (Mach 5 and above, roughly).

The reason for preferring turbulent flow is that when the flow is laminar, the only real mixing that occurs is through molecular diffusion, which is very, very slow. If the flow is laminar through the combustor and you try to inject fuel, you won't have a good fuel-air mix in the combustor since the diffusion is very slow and the air moves very quickly through the engine and the engine will flame out or unstart. Turbulent flows, however, exhibit a great deal of mixing due to the eddies that form at a variety of different scales. This means that if you inject fuel into the flow, you stand a much better chance of having a good fuel-air mixture to ignite. It still doesn't always work, however, which is one of the main reasons we still don't have any scramjet that has advanced past the research and testing phase.

Turbulent flow is absolutely terrible for most other purposes though. In particular, it leads to a lot more drag and a lot more heat transfer as compared to a laminar boundary layer. Because of this, hypersonic testbed aircraft like the X-43 were designed to maximize laminar flow on most surfaces, which isn't all that difficult on a shape such as the X-43 (in stark contrast to a conventional airliner). They needed turbulent flow going into the engine though, so they had to put very large boundary layer trips on the underside of the aircraft to try and trip the flow as well as design the contour in such a way to destabilize both the second mode instability and the Görtler instability as much as possible in hopes of reaching turbulence before fuel injection. It was a moderate success.

What I was wondering about is the fact, that the navy planned this for carriers. Wouldn't the supersonic tips running in humid air produce similar clouds to those below, but right in front of the cockpit?

Those vapor cones are not necessarily directly related to shock waves or supersonic flow. Their exact nature is actually not quite understood, but there is nothing in the physics of a supersonic flow that would point to those being a result. The prevailing idea right now is that it is related to the Prandtl-Glauert singularity, which tents to take the changes in pressure predicted by incompressible theory and amplify them, meaning much lower pressures in regions of expansion and therefore lower temperatures and vapor condensation.

The bigger issue would be the visible schlieren as a result in the density gradients generated by the shock waves. Those would throw off any aiming you tried to do I would think.

 

[Zentrails]

There wouldn't necessarily be the need for some abrupt change in pressure in that case, but there absolutely is a need for abrupt changes in direction of the flow as it encounters the leading surfaces of the plane. Any object moving through a gas at supersonic speeds will produce a combination of oblique and bow shocks.
That isn't really true though. The sonic boom is absolutely, unequivocally tied to the presence of a shock wave. When an object like a plane moves through the air, it has to move the air molecules out of the way. This movement of air away from the plane takes the form of what are essentially continuously generated pressure waves which propagate through the air at the speed of sound. This is why, in a subsonic flow, you see the air start moving out of the way of the body before the body actually reaches it. The pressure waves given off travel forward to deflect the air before the object actually reaches that particular parcel of air.

When this object is moving faster than the speed of sound, however, the pressure waves emanating from the body cannot propagate upstream of the object since they can only move at the speed of sound but the object is moving faster than the speed of sound. The leading front of these waves then end up "piling up" and forming what we call a shock wave. That is also why a supersonic flow must turn abruptly when encountering an object as opposed to smoothly as it would in a subsonic flow. It physically cannot see any information about the presence of that object until it is already at the object or at the location of that oblique shock wave front emanating from it.

Now, imagine the effect of different sizes and shapes of objects moving through the air. A large body obviously has to displace more air to move through it, so it is going to be putting a lot more sound energy in the form of those pressure waves into the air to do it. Similarly, a very blunt object is going to have to turn the flow out of the way much faster, so the shock formed is going to tend to be a lot stronger.

Shock waves, as I already mentioned, have the effect of enacting near-discontinuous changes in the flow (near because it actually is continuous but on the order of 6 to 10 mean-free-paths of air molecules, so perhaps 1 micron maximum at sea level). In particular, they cause the pressure to increase sharply over very small distances, and therefore in a very short time in terms of that wave front passing by your ear.

What this means is that the sonic boom is not some aggregate of all the sound generated by the plane and its engines and its moving parts and the flow over the plane; instead, it is the sound generated by the sharp pressure changes associated with shock waves. Most planes have two such shocks, one at the tip and one at the tail where the flow must straighted back out when it leaves the plane's body. This would result in two booms if the shocks are strong enough or you are close enough to the plane, e.g. the re-entering space shuttle. Most planes would only give off a single boom to the casual observer since they are so far away.
By an angular or oblique object, I mean any object whose surface is not either parallel or normal to the incoming free stream. This absolutely is the most common source of shock waves - at least on Earth. Most likely the actual most common source of shock waves would be supernovae or the ejecta from black holes or stars.

I think maybe what is going on is that you are interchanging the concept of a shock wave and a sound wave, which are related terms but not technically the same. A shock wave is, in essence, a very strong shock wave. What you are describing with ultrasonic perturbers are ultrasonic sound waves. Shocks, in comparison, are the result of many sound waves coalescing to form near-discontinuous changes in flow properties.

In my research group we routinely measure sound waves on the order of 250 kHz, but they are just that, sound waves. Measuring them is quite a task, though.
They need not be shock waves in order to travel through your bones to your joins. Simple sound waves can do that just fine.
We already have compressor-less jet engineers in use in ramjets. Those utilize normal shocks or a combination of oblique and normal shocks at the entrance to compress the air without requiring a mechanical compressor stage. The engine is designed such that compression results in subsonic flow through the combustor, which is then expanded again through the nozzle to provide thrust. Scramjets (supersonic combustion ramjets) are a variant of ramjet where the initial compression is done such that the flow remains supersonic throughout, which leads to supersonic flow in the combustor. The advantage is that it is much more efficient at high Mach numbers than ramjets (Mach 5 and above, roughly).

The reason for preferring turbulent flow is that when the flow is laminar, the only real mixing that occurs is through molecular diffusion, which is very, very slow. If the flow is laminar through the combustor and you try to inject fuel, you won't have a good fuel-air mix in the combustor since the diffusion is very slow and the air moves very quickly through the engine and the engine will flame out or unstart. Turbulent flows, however, exhibit a great deal of mixing due to the eddies that form at a variety of different scales. This means that if you inject fuel into the flow, you stand a much better chance of having a good fuel-air mixture to ignite. It still doesn't always work, however, which is one of the main reasons we still don't have any scramjet that has advanced past the research and testing phase.

Turbulent flow is absolutely terrible for most other purposes though. In particular, it leads to a lot more drag and a lot more heat transfer as compared to a laminar boundary layer. Because of this, hypersonic testbed aircraft like the X-43 were designed to maximize laminar flow on most surfaces, which isn't all that difficult on a shape such as the X-43 (in stark contrast to a conventional airliner). They needed turbulent flow going into the engine though, so they had to put very large boundary layer trips on the underside of the aircraft to try and trip the flow as well as design the contour in such a way to destabilize both the second mode instability and the Görtler instability as much as possible in hopes of reaching turbulence before fuel injection. It was a moderate success.
Those vapor cones are not necessarily directly related to shock waves or supersonic flow. Their exact nature is actually not quite understood, but there is nothing in the physics of a supersonic flow that would point to those being a result. The prevailing idea right now is that it is related to the Prandtl-Glauert singularity, which tents to take the changes in pressure predicted by incompressible theory and amplify them, meaning much lower pressures in regions of expansion and therefore lower temperatures and vapor condensation.

The bigger issue would be the visible schlieren as a result in the density gradients generated by the shock waves. Those would throw off any aiming you tried to do I would think.

Thanks, all that makes sense.

I would just suggest that a sonic boom can have components to it that are not related to the shock waves, such as when a pilot turns on the afterburner, although that probably makes shock waves as well as a hell of a racket.

You could probably also tow a noise maker of some sort using a supersonic jet. That noise would certainly make a sonic boom, wouldn't it? I'm just saying that most sonic booms are indeed generated by shock waves, but there are a few rare cases where they are not.

Maybe we are just defining sonic boom differently. Maybe there is some other term for what I am talking about.

Ultrasonic cleaners supposedly work by creating exceedingly small areas of very high temperature, which definitely causes a shock wave due to the sudden change in pressure (mini explosions, if you will, sometimes they call them "rapidly collapsing bubbles" or "cavitation"). That's the mechanism by which they are effective in cleaning metals and that's also the reason they are very effective at separating carbon nanotubes from each other, which are extremely hard to disperse by any other method.

If that's wrong, then the company that custom makes my sonicators doesn't know what they are talking about. I remember long ago as a sonar operator how easy it was to detect Russian submarines (and surface ships) because their screws had such bad "cavitation" due to poor machining. Made kinda of a "chug chug chug" sound. Maybe cavitation is not the right word.

http://www.zenith-ultrasonics.com/what_is_ultrasonic_cavitation.htm

I'm certain that I can feel that when I put my had in our sonicators, but ONLY in joints where there is synovial fluid. It's kind of an unpleasant prickly feeling deep in the joint itself. I feel nothing on the skin or bones. I haven't tried my foot yet, maybe I should do that to see if the sensation is the same. My feet hurt lately anyway, I'm getting old as dirt. LOL

I know that shock waves are different than sound waves. The best analogy I can think of is: a shock wave is similar to a Tsunami, while a sound wave is similar to the ripple pattern you get by dropping BBs into water at a steady rate.

"A shock wave is, in essence, a very strong shock wave." unless of course if it is a weak one. LOL
Did you mean "a very strong sound wave?"

I like the Tsunami example because you can get all different shapes to the propagation, just like you can with shock waves, that are, like you said, highly dependent on the dimensions of the object causing the shock wave.

A good example would be the ancient gigantic mountain slide off the coast of Norway that propagated a somewhat close to a planar propagating Tsunami - It was so powerful that it disrupted the gulf stream "conveyor belt" system for nearly 50 years, causing weather changes that resulted in the complete drying up of the Nile river for decades and the collapse of the Old Kingdom of Egypt.

The same wave hit the coast of Ireland, split in two, traveled inward and meet in the middle of Ireland, killing just about every living thing except maybe birds and cockroaches. Fascinating stuff.

Maybe the right way to think of sonic booms is that they are a shock wave's little sister.

If a shock wave is traveling faster than the speed of sound, clearly it is not sound.
If it is traveling at the speed of sound, I think it gets a little dicey calling it sound or not.
A philosopher would probably argue that if you can hear it, it's a sound.
different strokes...

"They need not be shock waves in order to travel through your bones to your joins. Simple sound waves can do that just fine."
Yes, I've done the tuning fork to the skull experiment.

Another fun one is: tie your index fingers to a metal coat hanger with string, stick your fingers in your ears plugging them up completely, then have someone bang on the hanger with a stick. Makes an amazing sound that is going right though bone.

 

[Zentrails]

In my research group we routinely measure sound waves on the order of 250 kHz, but they are just that, sound waves. Measuring them is quite a task, though.

How the heck do you do that? That's impressive. You have some kind of specialized piezolectric microphone?
I have EMIT tweeters in my Infinity speakers that I think go up to 100 KHz or maybe a little higher, something like that?
Custom made ribbon microphones?
electrostatic microphone with an extremely narrow gap?

"A shock wave is, in essence, a very strong shock wave." unless of course if it is a weak one. LOL
Did you mean "a very strong sound wave?"
If you add, "of very short duration," then I'll buy that.

I like your username, by the way, it was my nickname in high school. LOL

 

[boneh3ad]

Thanks, all that makes sense.

I would just suggest that a sonic boom can have components to it that are not related to the shock waves, such as when a pilot turns on the afterburner, although that probably makes shock waves as well as a hell of a racket.

You could probably also tow a noise maker of some sort using a supersonic jet. That noise would certainly make a sonic boom, wouldn't it? I'm just saying that most sonic booms are indeed generated by shock waves, but there are a few rare cases where they are not.

Maybe we are just defining sonic boom differently. Maybe there is some other term for what I am talking about.

You could probably get some really loud noise in those ways, but by the standard definition of a sonic boom, those would just be loud noises, not sonic booms.

Ultrasonic cleaners supposedly work by creating exceedingly small areas of very high temperature, which definitely causes a shock wave due to the sudden change in pressure (mini explosions, if you will, sometimes they call them "rapidly collapsing bubbles" or "cavitation"). That's the mechanism by which they are effective in cleaning metals and that's also the reason they are very effective at separating carbon nanotubes from each other, which are extremely hard to disperse by any other method.

If that's wrong, then the company that custom makes my sonicators doesn't know what they are talking about. I remember long ago as a sonar operator how easy it was to detect Russian submarines (and surface ships) because their screws had such bad "cavitation" due to poor machining. Made kinda of a "chug chug chug" sound. Maybe cavitation is not the right word.

http://www.zenith-ultrasonics.com/what_is_ultrasonic_cavitation.htm

For one, companies regularly use buzzwords of sorts to advertise their products. Regardless, cavitation, a well-studied phenomenon and the collapse of cavitation bubbles can definitely produce shock waves. That doesn't make the ultrasonic waves themselves shock waves. The waves themselves are simply high-frequency sound waves. The effect they have on the liquid, namely a combination of lowered pressure and raised temperature to unstable bubbles, which promptly implode, is what causes the shock waves.

Fun fact about your submarine example: when I was an undergraduate in a fluid mechanics lab, I had to do a lab report about cavitation. We had been studying it in class and the lab was partially designed to produce it and illustrate its effects. In the report, we had to answer a question about adverse effects of cavitation. The standard answer for the class was supposed to be how on valves and pumps, if you have cavitation, the shocks can be so powerful that they will erode the metal and it lessens the life of the equipment. I also made mention of the fact that cavitation on submarine screws can make a lot of noise that can be heard on passive sonar and used to determine the location, speed and type of submarine. The TA didn't buy that, even though it is 100% true.

I'm certain that I can feel that when I put my had in our sonicators, but ONLY in joints where there is synovial fluid. It's kind of an unpleasant prickly feeling deep in the joint itself. I feel nothing on the skin or bones. I haven't tried my foot yet, maybe I should do that to see if the sensation is the same. My feet hurt lately anyway, I'm getting old as dirt. LOL

Right, because ultrasonic waves are typically so high in frequency and so low in amplitude that you wouldn't notice them in normal tissue. In your joints, however, they would potentially cause the same sort of cavitation effect that you would see in the sonicators you use. That almost sounds like it has the potential to damage your joints. That is actually, I believe, the reasoning behind using ultrasound to break up certain types of kidney stones.

"A shock wave is, in essence, a very strong shock wave." unless of course if it is a weak one. LOL
Did you mean "a very strong sound wave?"

Indeed. Typo.

I know that shock waves are different than sound waves. The best analogy I can think of is: a shock wave is similar to a Tsunami, while a sound wave is similar to the ripple pattern you get by dropping BBs into water at a steady rate.

I like the Tsunami example because you can get all different shapes to the propagation, just like you can with shock waves, that are, like you said, highly dependent on the dimensions of the object causing the shock wave.

In fact, tsunamis and other types of hydraulic jumps are very similar to shock waves in behavior, if not origin. They are very similar, at least, to two-dimensional shock waves. There are a few other notable difference, but the analogy is a pretty good one. Another really good one is that the wake coming off the front of a ship is analogous to the bow shock and oblique shock emanating from the front of a plane.

Maybe the right way to think of sonic booms is that they are a shock wave's little sister.

Not really. The standard definition of a sonic boom defines it as essentially being the sound resulting from a passing shock wave.

If a shock wave is traveling faster than the speed of sound, clearly it is not sound.
If it is traveling at the speed of sound, I think it gets a little dicey calling it sound or not.
A philosopher would probably argue that if you can hear it, it's a sound.
different strokes...

This is not true. They absolutely are sound. Take the image below for example. In it you have a sound generating device, call it a beeper, traveling through a medium at 1.4 times the speed of sound. To simplify things, it shows the beeper only emitting a sound wave once every fixed time period, call it [itex]\Delta t[/itex]. Those sound waves propagate out radially at the speed of sound in the given medium, [itex]a[/itex]. So, at a given time, the beeper emits a sound. This sound wave propagates only at the speed of sound. The next sound wave is emitted farther down and also propagates at the speed of sound. The thing is, the advancing fronts of the waves will all meet along a line at a given angle that depends on the Mach number of the moving object (or of air moving over an object). This is the shock front, and even though the individual sound waves are moving only at the speed of sound, the shock itself, being a bunch of coalesced sound waves, is moving faster than the speed of sound.

Sound waves from sound source moving at Mach 1.4. Source: Wikimedia Commons

How the heck do you do that? That's impressive. You have some kind of specialized piezolectric microphone?
I have EMIT tweeters in my Infinity speakers that I think go up to 100 KHz or maybe a little higher, something like that?
Custom made ribbon microphones?
electrostatic microphone with an extremely narrow gap?

We measure them in air flows, so what we do is use what is called a constant-temperature hot-wire anemometer. You take a very thin wire, 2.5 μm in diameter in our case, and you heat it up. When air blows over it, it will cool the wire down, so you hook it up to circuitry that maintains the temperature and reads out the voltage required to do so. This voltage can be correlated to the speed of the air going over the wire. You can then tune the heat transfer problem associated with the wire and the electrical problem associated with the circuitry to measure fluctuating velocities well into the hundreds of kHz range. The best we have gotten so far is around 260 kHz, though we just had our anemometers rebuilt to support a new probe design that should hopefully push this further.

We use this to measure high-frequency fluctuations in the boundary layer in hypersonic flows. The goal is to gain more insight into the physics of boundary-layer laminar-turbulent transition in high-speed flows, in our case at about Mach 6.

 

[Zentrails]

alas, no radar tracking for the Russian meteor.

http://news.silobreaker.com/all-rusian-radar-failed-to-track-meteor-5_2266616190333878274

But they are starting to find lots of meteorite fragments, which confirm that it was a mid-air explosion.

http://www.kdbc.com/news/russian-scientists-track-down-fragments-urals-meteor

 

[cjl]

But they are starting to find lots of meteorite fragments, which confirm that it was a mid-air explosion.

http://www.kdbc.com/news/russian-scientists-track-down-fragments-urals-meteor

They confirm that it broke up in midair. I'd hesitate to necessarily call that an explosion.