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Atlas V's Sonic Boom Made Visible By Sundog
Ross-Shire Geek writes "Atlas V lifted off on Feb 11 from Kennedy. As it goes supersonic through a sundog (aka parhelion) you can see (video link) wonderful visible ripples of the shock wave in the sky."
There is something incredibly belittling about trying to comprehend the vastness that vehicles like that help us explore.
I thought that bird was going to get skewered a few seconds into the launch...
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More impressively - the sonic boom "waves" effect is actually a three-D object - imagine if we could see it from multiple POVs, a spherical wave instead of 2D one :)
-- Sig down
Very appropriate, it seems to me, that a rocket carrying the Solar Dynamics Observatory should make pretty with a sundog.
They're bad luck for mariners. I respectfully disagree with you, sundog is a much better name.
Isn't that a type of Matrox Video Card?
A sonic boom looks like a single shock wave
Not necessarily - different edges on the craft will generate additional shock fronts. There are usually two main ones from nose and tail but also from fin tips and even antennae.
What the Saturn V encountered was an Iridescent Cloud
"The truth points to itself." - Kosh, Babylon5
Exactly.
The higher the speed, the more the shock waves become compressed into a series of cones stacked inside each other rather than the spheroids typical at slower speeds. Taken together, the passage of these shock waves through a plane perpendicular to the direction of travel would look a lot like circular ripples.
That sounded like the music from the awards ceremony at the end, I thought.
--
"I have also mastered pomposity, even if I do say so myself." -Kryten
Closer to the pad, and less shaky:
http://www.youtube.com/watch?v=q9S0z1ofcIc
(it has the voiceover from NASA TV, but doesn't have the launch clock visible ... it might've been a camera angle that they didn't use live, as I don't remember seeing this on TV)
Build it, and they will come^Hplain.
Those look like regular acoustic waves to me.
I don't doubt the the rocket can go faster than the speed of sound (which gets lower as you get further from the surface), but those waves distinctly lead the rocket's motion, which means that they are the product of acoustic perturbations moving at the speed of sound in the medium.
If the rocket were moving faster than the speed of sound in that medium, then we would see the usual 'shock cone', where those waves would appear an a fairly narrow cone around the rocket as it passed though - certainly not before.
I qualify 'medium' since it is possible that the rocket is moving faster than the speed of sound (in the air) but not faster than the medium that the sundog constitutes. Liquids, for example, have much higher speed-of-sounds and it is (conceptually, not physically) fairly simple for something to be moving faster than the speed of sound in air at sea level but not be anywhere close to the speed of sound in a liquid that it is travelling - and thus producing the regular u-c, u+c acoustic waves.
However, as I understand them, sundogs are collections of ice crystals and probably don't have a higher speed of sound than the air around them. But anyway, waves preceding the motion of a body in a medium are certainly not shockwaves - if we could visualize the waves any object makes in the air, you would see acoustic waves arising from the object's motion before and after it.
Still neat-looking, though.
Not going into the subject of "if you're going to whine about it, do it yourself" but if he hadn't been stationary, he might have missed recording the shockwave event, or recorded it so seeing it wasn't as impressive as we saw.
My kingdom for a donkey!
Once upon a time, men would have been sitting on top of that rocket. Men with no fear, who couldn't give a shit about politics. They just wanted the ultimate ride.
Every politician thinking about cancelling Ares needs to think about that. They wouldn't have cell phones and 3G internet and GPS if it weren't for the brave men and women who are, as Steve Buscemei so perfectly put it (minus the nuke):
"You know we're sitting on four million pounds of fuel, one nuclear weapon and a thing that has 270,000 moving parts built by the lowest bidder. Makes you feel good, doesn't it? "
I've looked at this a few dozen times. It's truly an amazing effect.
Being in the CGI effects business myself (and actually right in the middle of bidding some interesting atmospheric effects for a movie dogfight) I thought it was fake at first, mostly because I've never seen anything like this before. A spectacular display like this, I would have thought, would have been photographed many times if it was a normal occurrence. Some commenters have said that something similar happened on the Apollo XI launch, I haven't seen film that confirms that.
But no, it's clearly real. Many people saw it, several people filmed it.
What it is, is the shockwave moving through a thin layer of clouds and atmosphere. The shockwave disturbs that layer of clouds in some way (in the case of the sun dog, apparently disturbing the crystals orientation -- shockingly these sun dogs require the hexagonal crystals to be hanging more-or-less flat in the air) There are any number of films of airplanes flying above the speed of sound causing clouds to pop into existence and then disappear as the shockwave passes. Every nuclear bomb sequence has these kind of shock-induced clouds as well.
I suppose that clouds with exactly the characteristics to make this happen for rocket launches are rare, because I've watched film of hundreds of launches and never seen this. It always pays to look up!
I love Mondays. On a Monday, anything is possible.
Some debate here as to whether what we're seeing is a sonic boom, or just loud low-frequency sound waves. Let's do the math...
Basic question: is the rocket going at Mach 1 or greater when the phenomenon happens?
In the video, the launch happens at 0:38, and the ripples are seen at 1:53, 75 seconds later.
Here's a handy document showing the launch profile of an Atlas V. It doesn't show velocity vs time, but on page 19 there's an acceleration vs time graph for the Atlas V 401, the specific vehicle used in this launch. It shows the average thrust during the first 75 seconds is 1.4 +/- .05 g's (uncertain because I can't read the graph that accurately.)
Subtract out 1 g for gravity pulling the rocket down, to get a vehicle acceleration of 0.4 +/- 0.05 g, which over 75 seconds will lead to a final velocity of 294 +/- 36 m/s.
The speed of sound is 330 m/s. So at the time we see the ripples, the rocket is riiiiight about at the speed of sound, maybe a little over, maybe a little under, impossible to tell.
This transition to supersonic flow is often chaotic and irregular, which would explain the intense but complicated ripples seen. If the rocket was going at mach 2 or 3, we'd see a perfectly shaped set of concentric rings; if it was going at far less than mach 1, we'd see nothing at all.
Another video of the launch with clean audio. Rocket isn't supersonic until roughly 2 minutes after launch.
http://www.youtube.com/watch?v=TRFWq7gcj2E&feature=fvw
It didn't look like a shockwave to me from the start, as the name implies, it would be visible as a very sharp, immediate disturbance, not a bunch of ripples. Actually, would have been really cool if it *had* gone supersonic in that cloud layer.
It's at about 11 km above the ground after 75 s. At that height, the speed of sound is 295 m/s (WolframAlpha). So it is a sonic boom.
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