Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 7:15 pm
by dmtrask
There is a great explanation by Geoffrey Forden in his blog. His explanation is copied below but you will have to go to his site to see the pictures and "cartoons" he uses to illustrate his points.
Shocking Good Fun
posted Friday February 19, 2010 under missiles by geoffrey_forden
The Atlas V that carried the Solar Dynamics Observatory into orbit on 11 Feb. 2010 created shockwaves that rippled through a cloud layer. I’ve counted 11 distinct shockwaves, marked by arrows in this image.
Ok, this has little if anything to do with security or arms control (well, perhaps a little) but it is such fun that I couldn’t resist writing about it. And it is certainly very educational. There is a very fun video of the Solar Dynamics Observatory launched on February 11, 2010. In that video (and in the image above) you see the SDO’s Atlas V launch vehicle passing through a cloud layer, with shockwaves radiating out. Of course, the shockwaves are generated by the missile and the cloud layer is only providing a way of seeing them. In fact, what is visible is not a single shockwave radiating through the cloud layer but rather multiple shockwaves passing through the layer. As these shockwaves “follow” the missile, each point of intersection with the cloud layer moves outward. I’ve tried to indicate this with the cartoon below:
Shockwaves are formed at “discontinuities” along a rocket’s airframe. They radiate energy away from the point on the missile where they are created. As the missile moves along, this constant creation of shocks form what appears to be a continuous “cone” that trails along the rocket. The angle the cone makes with the rocket is therefore related to the rocket’s speed. Of course, the more discontinuities there are on the airframe, the more shockwaves are formed and the more energy is radiated away. ...
I’ve been able to make a rough correspondence between the number of discontinuities on the Atlas V and the number of shockwaves visible in the cloud layer. (See the image at the top of this post as well as the images to the left.) Of course, not all discontinuities make shockwaves that are visible around the entire vehicle. For instance, the vehicle’s airframe shields a whole hemisphere from a shockwave caused by a pipe sticking out on one side. But this correspondence is close enough to really illustrate this physical process.
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Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 9:31 pm
by JohnD
DID ANYONE HEAR ME?
JohnD wrote:Are such ripples so unusual?
Certainly after a rocket launch, but ripples in the atmosphere are there all the time, we just don't see them until some effect shows them up. This site http://www.w7ftt.net/sundog1.html shows parallel ripples that are otherwise very similar.
Does the APOD pic demand a supersonic explanation? A thermocline can have a distinct border, with different densities on either side. Just like a water/air interface, some solid object splashing through will cause ripples to spread, just as shown in the APOD.
John
Sorry to shout, but I'd like to know if I'm right, completely wrong, or have a point.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 9:35 pm
by Chris Peterson
JohnD wrote:DID ANYONE HEAR ME?
JohnD wrote:Are such ripples so unusual?
Certainly after a rocket launch, but ripples in the atmosphere are there all the time, we just don't see them until some effect shows them up. This site http://www.w7ftt.net/sundog1.html shows parallel ripples that are otherwise very similar.
Does the APOD pic demand a supersonic explanation? A thermocline can have a distinct border, with different densities on either side. Just like a water/air interface, some solid object splashing through will cause ripples to spread, just as shown in the APOD.
John
Sorry to shout, but I'd like to know if I'm right, completely wrong, or have a point.
I think you are wrong. The ripples in the video are obviously associated with the rocket passing near the cloud, and are an acoustic event of some sort, not related to the sort of slow ripples sometimes seen in clouds.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 9:48 pm
by Thomas A. Fine
dmtrask wrote:There is a great explanation by Geoffrey Forden in his blog. His explanation is copied below but you will have to go to his site to see the pictures and "cartoons" he uses to illustrate his points.
Shocking Good Fun
posted Friday February 19, 2010 under missiles by geoffrey_forden
Shockwaves are formed at “discontinuities” along a rocket’s airframe. ...
I’ve been able to make a rough correspondence between the number of discontinuities on the Atlas V and the number of shockwaves visible in the cloud layer.
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I'm pretty sure this person is wrong, simply based on geometry. The rocket is moving at the speed of sound, and shock waves are moving at the speed of sound. So if these are separate shock waves from different parts of the rocket, then the spread across all the shock waves can be no bigger than the rocket. In fact it might be smaller if we accept the idea that we are seeing only a horizontal component of sound waves moving at an angle through a cloud layer (and I'm not saying I accept that either).
Clearly what we are seeing is much larger than the rocket, so these waves must have been generated at different times, all probably from the nose of the rocket.
tom
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Is there some misunderstanding about Max Q? This is maximum dynamic pressure on the rocket, and it occurs after the rocket is supersonic, not right when it crosses the sound barrier. These all look like photos of rockets crossing the sound barrier.
tom
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 11:28 pm
by Chris Peterson
Thomas A. Fine wrote:Is there some misunderstanding about Max Q? This is maximum dynamic pressure on the rocket, and it occurs after the rocket is supersonic, not right when it crosses the sound barrier.
And in the case of this launch, Max Q occurred about 15 seconds after breaking the sound barrier, well after the observed phenomenon.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 11:43 pm
by Katie
Hi
the picture looks like alpha particles (radioactive decay) being ejected. Which would coincide with the elements used in shuttle launches.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 11:45 pm
by bystander
Thomas A. Fine wrote:Is there some misunderstanding about Max Q? This is maximum dynamic pressure on the rocket, and it occurs after the rocket is supersonic, not right when it crosses the sound barrier. These all look like photos of rockets crossing the sound barrier.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Tue Feb 23, 2010 11:51 pm
by neufer
Thomas A. Fine wrote:
I'm pretty sure this person is wrong, simply based on geometry. The rocket is moving at the speed of sound, and shock waves are moving at the speed of sound. So if these are separate shock waves from different parts of the rocket, then the spread across all the shock waves can be no bigger than the rocket. In fact it might be smaller if we accept the idea that we are seeing only a horizontal component of sound waves moving at an angle through a cloud layer (and I'm not saying I accept that either). Clearly what we are seeing is much larger than the rocket, so these waves must have been generated at different times, all probably from the nose of the rocket.
Why all from the nose? Aren't the engines the noisiest part?
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 3:33 am
by ems57fcva
I don't think that we are dealing with a sonic boom with this phenomenon, or at least not with the waves that destroyed the sun dog. The ripples are appearing at nearly the same time and many of then are well away from the rocket. A sonic boom would clearly emmenate from the rocket, and move outwards from it. So my figuring is that these shock waves are a subsonic phonomenon. It seems that they are being emitted as the rocket is approaching the cloud layer. Obviously the rocket is nearing Mach 1, based on how quickly these spherical shock waves are intersecting the cloud layer, but I don't think that it was supersonic at that time.
I take as added evidence that that the rocket was subsonic the fact that additional shock waves appear a few seconds after the sub dog vanished and obviously behind the rocket. I assume that those are due to the shock created as the rocket went supersonic.
As for the nature of the waves: Maybe the cowling making the rocket bigger at the top than just below results in the atmosphere "flapping" against the neck of the rocket. So maybe these shocks have always been created, but only rarely has this been happening as one flew through a layer (or layers) of neatly ordered ice crystals. I assume that the ripples are the crytals being sent spinning or flapping, as the shock passed, after which the neat ordering had been destroyed.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 3:57 am
by Wailhound
Here's a great video of the launch.... you HAVE to check this out:
Click to play embedded YouTube video.
Note the speed... and note that the sky continues to ripple in a circular pattern, AFTER the rocket has passed....
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 4:24 am
by cwthorne
I think the rings are caused by the compression of the air as the rocket passes through it. When the rocked reaches the required speed and the appropriate altitude, as the air is compressed by the passing rocket, the moisture in the air is also compressed to the saturation point making it visible as a ring. As the sound wave expands or propagates, so do the rings, with the rings being visible during high pressure point in the sound wave and disappearing during the low pressure point in the sound wave. If sound velocity at the altitude where the rings appear is known, the frequency could be calculated by measuring the distance between rings. I also noticed some less distinct rings after the rocket had passed through the clouds. I'm thinking they were caused by the rocket engine. Another Chris.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 5:14 am
by ems57fcva
I advise poeple to not just look at Wailhound's link, but also to listen to it! You can hear a faint sound like a "pop-pop-pop" as the rocket is approaching the cloud layer. I'm sure that those pops are the shock waves being generated.
Those pops are not a sonic boom. I have heard (and felt) them. Sonic booms tend to be a single "boom" or "thud". Multiple shock waves from one object will pass you so quickly that they appear to be a single boom.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 5:23 am
by Chris Peterson
ems57fcva wrote:I advise poeple to not just look at Wailhound's link, but also to listen to it! You can hear a faint sound like a "pop-pop-pop" as the rocket is approaching the cloud layer. I'm sure that those pops are the shock waves being generated.
Keep in mind that at the distance of the rocket when the visual effect occurs, it is about 30 sound-seconds away. That is, any acoustic effects noted at the ground will occur about 30 seconds after any visual effects are seen.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 6:40 am
by bystander
If you listen closely, after the oohs and aahs and the laughter, you can hear the announcer say "The vehicle is now supersonic".
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 12:47 pm
by DavidLeodis
It's a terrific and fascinating photo. It does a look a bit like the shockwaves form a view down a tunnel with a parallax effect
In the explanation the "sundog" in the sentence "a thin layer of ice crystals that were aligned to create the sundog" is a link to an image of a dog lying in the sunshine. The shadow of someone's arm looks like it could be holding a gun at the dog! I hope not!
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 1:19 pm
by Lucra
mark swain wrote:Air pressure density ripples, Changing refraction. Slightly . ?
Mark
Pressure density ripples are rapidly changing pressure variations. As a consequence the gas (air) is periodically heating en cooling with no chance to exchange heat with the surrounding gas (an adiabatic proces). Since temperature variations in a gas imply refraction variations the answer is yes.
Luc
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 2:26 pm
by cyburbva
Am I the only person to click on the penultimate mention of <sundog> in the text below the Feb 23 APOD photo? Don't miss it.
Someone has a great sense of humor.
I have a friend< Bob B, whom we called "Mr Kaboom" because of his expertise in underwater detonations. The vectors created by any disturbance in an atmosphere are not always predictable: is that correct. A bullet fired into a gelatinous substance varies according to velocity, exterior of the bullet (the missile) and the angle of the shooter.
Fascinating subject.
cyburbva
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 2:34 pm
by ems57fcva
Chris Peterson wrote:
ems57fcva wrote:I advise poeple to not just look at Wailhound's link, but also to listen to it! You can hear a faint sound like a "pop-pop-pop" as the rocket is approaching the cloud layer. I'm sure that those pops are the shock waves being generated.
Keep in mind that at the distance of the rocket when the visual effect occurs, it is about 30 sound-seconds away. That is, any acoustic effects noted at the ground will occur about 30 seconds after any visual effects are seen.
That is true, but time is also needed for the wavefronts to propagate, especially the ones fartherest from the rocket. Also keep in mind that the if my belief that the shocks are created when the rocket was subsonic is correct, then those wavefronts must have moved upwards to reach the cloud layer. So it is not impossible that 30+ seconds elapsed from the creation of a shock (which I am sure we heard) to that shock reaching the cloud layer.
Obviously someone needs to do a detailed study, but the situation is not as simple as your refutation makes it out to be.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 3:50 pm
by The Code
Lucra wrote:
mark swain wrote:Air pressure density ripples, Changing refraction. Slightly . ?
Mark
Pressure density ripples are rapidly changing pressure variations. As a consequence the gas (air) is periodically heating en cooling with no chance to exchange heat with the surrounding gas (an adiabatic proces). Since temperature variations in a gas imply refraction variations the answer is yes.
Luc
Thanks Luc
Gas under pressure, Acts like a liquid. Of course it does. Excellent.
Mark
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 4:32 pm
by neufer
Lucra wrote:
mark swain wrote:Air pressure density ripples, Changing refraction. Slightly . ?
Pressure density ripples are rapidly changing pressure variations. As a consequence the gas (air) is periodically heating en cooling with no chance to exchange heat with the surrounding gas (an adiabatic process). Since temperature variations in a gas imply refraction variations the answer is yes.
Sound in air is primarily pressure fluctuations due to (roughly) equal parts density & temperature fluctuations.
It is primarily the sonic density fluctuations that produce index of refraction fluctuations.
For rocket launch noise the resulting refraction fluctuations are roughly equivalent
to those in typical "mirages" caused by surface temperature gradient induced density gradients:
Sonic temperature fluctuations through clouds would happen too fast to modulate condensation/evaporation.
Sonic pressure fluctuations through clouds would probably be too weak to modulate
condensation/evaporation & happen too fast to coherently modulate ice particle orientation.
Re: APOD: Exceptional Rocket Waves Destroy Sun Dog (2010 Feb
Posted: Wed Feb 24, 2010 4:42 pm
by Chris Peterson
mark swain wrote:Gas under pressure, Acts like a liquid.
Under high enough pressure gas changes state, and becomes a liquid. But that is not happening here (the pressures are nowhere near high enough).
Gases are fluids, as are most common liquids, so there is a certain overlap of behaviors. But I wouldn't say that the gases we see in these videos are behaving like liquids. Any material has a refractive index, and in the case of gases it varies with density. So with proper illumination, it is possible to see sound waves (or any density variations) traveling in a gas. That's how Schlieren and shadowgraph images are made (such as those posted earlier showing shockwaves).
