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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...
http://michaelsmith.id.au
The shockwave is cone shaped rather than spherical for fast moving objects such as a rocket, I believe.
Very very cool though.
wot no sig
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.
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.
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.
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.