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Curious Phenomena In Shuttle Launches
ugordan
post Jan 25 2006, 09:34 PM
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One thing has been bugging me for a while now. There's a curious feature that appears in many shuttle launch videos (well, not only shuttle videos, but they're most prominent there). I'm talking about the following optical effects:
Attached Image
Attached Image
Attached Image


Now, my sort-of-educated guess would be this is a shockwave related phenomenon. Once the launch vehicle becomes supersonic, it creates a shockwave that compresses air at its front. This changes the index of refraction if the shocking is strong enough.
The same thing happens during an atmospheric nuclear detonation where the shock can be seen "breaking" the traces of the smoke rockets behind the detonation (in case you ever wondered what the smoke rockets were for smile.gif)
Attached Image


I don't think it's related to optics in the tracker. Notice how the linear refraction is always perpendicular to the motion of the shuttle. What puzzles me is just how many of these linear features there are during the course of the liftoff. Furthermore, they seem to dance around each other exhibiting complex behavior one would not expect of a simple supersonic shock front...

Very interesting in any case, if anyone has more info about it, I'd be happy to hear it.
BTW, the launch video I took the screenshots from can be found at www.insideksc.com, specifically here.


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The Messenger
post Jan 25 2006, 10:07 PM
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I'm guessing that you are half right: That these are indeed pressure waves, but not from the shuttle flight surfaces - from fluctuations in the pressure waves caused by burning of the solid rocket propellant. Solid fuel burns quite chaotically -it's a rough ride. In addition, since they burn the entire length of the motor, there is a characteristic (very deep) resonant frequency. One of the reasons aluminum is used in solid rocket fuel is that the fluffly burn product - aluminum oxide - dampens the resonance which could otherwise build a pressure wave that blows off the nozzle or bursts the case.

The nozzles are also flexed-vectored during the launch, and this changes the direction and amplitude of pressure waves. Since your viewing angle is 'down wind' from the nozzle exhaust, the combination of both the natural variation in the burnrate, nozzle vectoring, and exhaust gas radiant heating of the surrounding air can all change the reflective index. This combination of factors is a likely cause of the visual artifacts you observe.

Edited to add: Looking more closely, it looks like the main liquid fuel is adding to the 'fire' too, so the expansion and radiant heat of all the exhaust gases is the most likely culprit.
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ugordan
post Jan 25 2006, 10:23 PM
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QUOTE (The Messenger @ Jan 25 2006, 11:07 PM)
Edited to add: Looking more closely, it looks like the main liquid fuel is adding to the 'fire' too, so the expansion and radiant heat of all the exhaust gases is the most likely culprit.
*

I have a problem with that reasoning since one would expect the "vibrations" to expand concentrically away from the engines, yet in the third picture it's clear it always seems to be a line, not a feature centered around the engines. In fact, that "line" part is most puzzling to me. The refractions don't seem to be curved at all. That IMHO implies the refracting is taking place much farther from the launch vehicle where the shockwave starts to look like a flat front.
It probably has to do with changing speed of sound and temperature gradients in the atmosphere as the vehicle rockets skyward.
I just can't believe the shockwaves are due to the rocket exhaust, that would imply vibrations so strong they would invariably rip through every launch vehicle in existence.


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dvandorn
post Jan 25 2006, 11:14 PM
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I've seen that artifact many times -- it even appeared on the same tracking camera during the New Horizons Atlas V launch.

There was once a long discussion of it on sci.space.shuttle, and it was decided that it must be a real image artifact that occurs between the rocket and the camera, and not a camera artifact. I'm still not completely convinced, though.

-the other Doug


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tasp
post Jan 26 2006, 02:27 AM
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The many protuberences of the shuttle (wings, OMS pods, rudder) all contribute to the multiple complex shock waves coming off the vehicle.



If I may be permitted (if this isn't too far off the topic), it seems about every launch I have watched closely, I will see a 'flare' or two in the shuttle main engine exhaust.

IIRC, they were even looked into during the Challenger accident investigation and were deemed 'acceptable' or some such thing.

But I have never seen anywhere, just what is it that causes them.

Tiny bits of turbine or bearings working there way through the plumbing? That sounds too scary to me to be correct.

It doesn't seem possible for there to be impurities in the fuel or oxidizer.

So does anyone know what makes the flares?

(incinerated pigeons caught in the exhaust? - that one is a joke)
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ugordan
post Jan 26 2006, 08:15 AM
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QUOTE (tasp @ Jan 26 2006, 03:27 AM)
So does anyone know what makes the flares?
*

I'm not sure what flares you have in mind, but I remember a recent study that announced that every shuttle launch brought a significant amount of water vapor over the south pole or something like that. I also clearly remember iron dust being mentioned as an additional exhaust product, apart from H2O.
Supposedly, the engine wears itself out ever so slowly, but enough to detect these trace amounts.
Is there a flare visible in the specific clip I linked above? If so, at what time?

While we're off-topic, there is a Proton launch (just about the only one I have in my collection) that also exhibited a flare at an early stage of liftoff. It gave out a red flash to an otherwise blue exhaust plume.

Also off-topic, notice how the bottom of the fuel tank insulation gradually gets charred. Is this due to infrared radiation from the engines or do some of the exhaust gases actually make it all the way up there and burn the foam?
The burn pattern looks pretty sharply defined so I'd bet on gas circulation. Which puzzles me - how can gas come back up when the whole vehicle is going supersonic and the air is dragging everything downwards? Must be one hell of a turbulence!


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helvick
post Jan 26 2006, 01:33 PM
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Just to clarify what I think is being discussed:

Shuttle Launch video (Real)

You can see two separate effects - from the onboard camera there are a number of "flare" transients visible in the exhaust. These do not appear to be shock wave effects to me but rather appear to be caused by inconsistencies in the fuel flow. It is also definitely possible that they are caused to some degree by throttle control on the main engines affecting the SRB exhaust dynamic. Then again they might just be simple vibration effects - these are definitely not imaging artificacts.

Later on after the shuttle has passed through a cloud layer a very distinct Prandtl-Glauert condensation cloud appears suddenly around the top of the shuttle - it appears as a sort of shroud and persists for quite a while. This is a fairly typical atmospheric transonic\supersonic shock wave effect. I suppose it might be possible for such a cloud to form near the leading edge of the exhaust and then cause the flare effect as it dissipates as they can form and dissipate very rapidly.
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tty
post Jan 26 2006, 02:03 PM
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QUOTE (ugordan @ Jan 26 2006, 10:15 AM)
Also off-topic, notice how the bottom of the fuel tank insulation gradually gets charred. Is this due to infrared radiation from the engines or do some of the exhaust gases actually make it all the way up there and burn the foam?
The burn pattern looks pretty sharply defined so I'd bet on gas circulation. Which puzzles me - how can gas come back up when the whole vehicle is going supersonic and the air is dragging everything downwards? Must be one hell of a turbulence!
*


There is certain to be an area of flow separation and turbulent flow behind the fuel tank and with this very complex geometry and pressure distribution I wouldn't be in the least surprised by some "wrong-way" flow. Stranger things happen. There have even been cases where dropped bombs simply refuse to leave and fly along with the aircraft until the speed drops below a certain point!

tty
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paxdan
post Jan 26 2006, 02:52 PM
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QUOTE (tty @ Jan 26 2006, 02:03 PM)
Stranger things happen. There have even been cases where dropped bombs simply refuse to leave and fly along with the aircraft until the speed drops below a certain point!
*


oh man that's priceless, i imagine that resulted in a few WTF moments. biggrin.gif
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GregM
post Jan 26 2006, 05:27 PM
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QUOTE (tty @ Jan 26 2006, 02:03 PM)
There is certain to be an area of flow separation and turbulent flow behind the fuel tank and with this very complex geometry and pressure distribution I wouldn't be in the least surprised by some "wrong-way" flow.
*



Wrong way flow? Were's a good example:

http://www.hq.nasa.gov/alsj/a11/ap11-KSC-69PC-413HR.jpg
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The Messenger
post Jan 26 2006, 05:28 PM
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QUOTE (ugordan @ Jan 25 2006, 03:23 PM)
I have a problem with that reasoning since one would expect the "vibrations" to expand concentrically away from the engines, yet in the third picture it's clear it always seems to be a line, not a feature centered around the engines. In fact, that "line" part is most puzzling to me. The refractions don't seem to be curved at all. That IMHO implies the refracting is taking place much farther from the launch vehicle where the shockwave starts to look like a flat front.
It probably has to do with changing speed of sound and temperature gradients in the atmosphere as the vehicle rockets skyward.
I just can't believe the shockwaves are due to the rocket exhaust, that would imply vibrations so strong they would invariably rip through every launch vehicle in existence.
*

They don't have to be shock waves - just enough variation in density to distort the image. Vectoring of the nozzle, and dynamic changes in the gas flow easily produce this type of effect. The linearity may be dictated by the displacement in the air stream caused by the leading shuttle surfaces. (Just a guess)

Turbulant flow definitely leads to heated backflow on the outside of the nozzles and the case. There was even one launch (non-shuttle) where a main engine shut down because the backflow overheated an internal sensor with an external vent - I don't recall exactly where or when.

It is not unusual for small - up to fist-sized chunks of propellent to break free from the main surface grain and burn down 'down stream' as glowing 'embers' in a solid rocket plume, and the nozzle pressure during a launch varies more than you would expect - I will try to find some real numbers.
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DDAVIS
post Jan 28 2006, 12:15 AM
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[quote=The Messenger,Jan 26 2006, 05:28 PM]
They don't have to be shock waves - just enough variation in density to distort the image.

I believe, based on repeated occurences seen over the years, we are seeing video artifacts caused by radar or other local electronic interference. They look too straight to be caused by the ascent itself.

Don
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DDAVIS
post Jan 28 2006, 12:17 AM
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message moved to new thread
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edstrick
post Jan 28 2006, 09:53 AM
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I've seen the straight line "oddities" in shuttle launch video, and I've noticed that they seem to sweep past the shuttle, remaining straight lines. The arm-waving speculation I've had for these features is that they're due to the line-of-sight from the tracking camera to the shuttle is crossing some overhead wires some tens of meters from the camera. Tracking cameras are telescopes, not mere lenses, and the wires themselves may be nearly invisible, but in the sunlight, might be heated enough to have hot air "envelopes" that could cause weak linear mirages, which is what the oddities strike me as resembling.
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djellison
post Jan 28 2006, 10:18 AM
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Personally, I think these are shock waves. Very similar features can be seen above Thrust SSC when it broke the sound barrier at Black Rock.

If you look carefully at some of the images of Thrust SSC - you can see some of the sort of thing - the slight 'double line' appears in the visualisation that appeared by kicking up dust

http://www.galleryoffluidmechanics.com/ss_cars/shockvsm.htm

And quite clearly here



Somehow - the shockwave(s) have a large refractive index, as you can see in the Thrust SSC image. Now - the same process would refract light from the exhaust plume around to make it look like little bright lines which is what we see.

Doug
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