Atlas V's Sonic Boom Made Visible By Sundog 99
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."
Re:Wow (Score:5, Informative)
Indeed, and apart from that, the linked video is really, really cool.
Ripples in the sky FTW.
Re:In 3D (Score:5, Informative)
The shockwave is cone shaped rather than spherical for fast moving objects such as a rocket, I believe.
Very very cool though.
Re:Wow (Score:5, Informative)
To skip the first ~2minutes and cut to the... ripples: http://www.youtube.com/watch?v=SsDEfu8s1Lw#t=1m51s [youtube.com].
And for even more karma whoring: "A sun dog is a prismatic bright spot in the sky caused by sun shining through ice crystals. The Atlas V rocket exceeded the speed of sound in this layer of ice crystals, making the shock wave visible from the ground."
So I guess the normal compression wave by a sonic boom is not enough to alter the way light goes through it (think flickering air when looking across a heated highway), but these ice crystals do the trick. Right?
Re:In 3D (Score:3, Informative)
The 2D aspect was a result of the shape of the clouds that formed the sundog, not the shape of the sonic wave. Sonic waves are indeed conical.
Re:Not a sonic boom (Score:5, Informative)
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.
Re:In 3D (Score:2, Informative)
A shock is indeed spherical right at Mach 1 (which certainly is not "stationary" LOL). As the Mach number increases further, it becomes a cone. The cone half-angle decreases with Mach number and starts at 90 degrees at Mach 1.
That's not a Sun Dog... (Score:3, Informative)
Re:Not a sonic boom (Score:4, Informative)
Exactly [wikipedia.org].
The higher the speed, the more the shock waves become compressed into a series of cones stacked inside each other [google.ca] 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.
Re:Just "waves?" Motorized cam; music choice (Score:3, Informative)
That sounded like the music from the awards ceremony at the end, I thought.
Second POV (Score:5, Informative)
Closer to the pad, and less shaky:
http://www.youtube.com/watch?v=q9S0z1ofcIc [youtube.com]
(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)
Re:Just "waves?" Motorized cam; music choice (Score:2, Informative)
Film is still used, excellent resolution, expose now process later. it will always beat digital in rapid fire situations.
Sonic boom or not? Math (Score:5, Informative)
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 [umd.edu] 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 [nasa.gov] 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.
Just acoustic pressure waves (Score:3, Informative)
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 [youtube.com]
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.
Re:Sonic boom or not? Math (Score:2, Informative)