Posted on 02/04/2003 1:34:19 AM PST by bonesmccoy
In recent days the popular media has been focusing their attention on an impact event during the launch of STS-107. The impact of External Tank insulation and/or ice with the Orbiter during ascent was initially judged by NASA to be unlikely to cause loss of the vehicle. Obviously, loss of the integrity of the orbiter Thermal Protection System occured in some manner. When Freepers posted the reports of these impacts on the site, I initially discounted the hypothesis. Orbiters had sustained multiple impacts in the past. However, the size of the plume in the last photo gives me pause.
I'd like to offer to FR a few observations on the photos.
1. In this image an object approximately 2-3 feet appears to be between the orbiter and the ET.
2. In this image the object appears to have rotated relative to both the camera and the orbiter. The change in image luminosity could also be due to a change in reflected light from the object. Nevertheless, it suggests that the object is tumbling and nearing the orbiter's leading edge.
It occurs to me that one may be able to estimate the size of the object and make an educated guess regarding the possible mass of the object. Using the data in the video, one can calculate the relative velocity of the object to the orbiter wing. Creating a test scenario is then possible. One can manufacture a test article and fire ET insulation at the right velocity to evaluate impact damage on the test article.
OV-101's port wing could be used as a test stand with RCC and tile attached to mimic the OV-102 design.
The color of the object seems inconsistent with ET insulation. One can judge the ET color by looking at the ET in the still frame. The color of the object seems more consistent with ice or ice covered ET insulation. Even when accounting for variant color hue/saturation in the video, the object clearly has a different color characteristic from ET insulation. If it is ice laden insulation, the mass of the object would be significantly different from ET insulation alone. Since the velocity of the object is constant in a comparison equation, estimating the mass of the object becomes paramount to understanding the kinetic energy involved in the impact with the TPS.
3. In this image the debris impact creates a plume. My observation is that if the plume was composed primarily of ET insulation, the plume should have the color characteristics of ET insulation. This plume has a white color.
Unfortunately, ET insulation is orange/brown in color.
In addition, if the relative density of the ET insulation is known, one can quantify the colorimetric properties of the plume to disintegrating ET insulation upon impact.
Using the test article experiment model, engineers should fire at the same velocity an estimated mass of ET insulation (similar to the object seen in the still frame) at the test article. The plume should be measured colorimetrically. By comparing this experimental plume to the photographic evidence from the launch, one may be able to quantify the amount of ET insulation in the photograph above.
4. In this photo, the plume spreads from the aft of the orbiter's port wing. This plume does not appear to be the color of ET insulation. It appears to be white.
This white color could be the color of ice particles at high altitude.
On the other hand, the composition of TPS tiles under the orbiter wings is primarily a low-density silica.
In the photo above, you can see a cross section of orbiter TPS tile. The black color of the tile is merely a coating. The interior of the tile is a white, low-density, silica ceramic.
Ice is definitely a fickle thing. You may have come through a lot of visible moisture on a flight with nothing but a light dusting, then, as you come in from the outer marker, whammo--it builds up as if it's trying to convince you there's no tomorrow.
Comment was made herein that someone was surprised that a chuck of ice wouldn't break up in the slipstream, as it was not "aerodynamic." Though I'm surprised how far down the ET ice was forming (or perhaps traveling--channeled down grooves?), one has to note that roughly half the shape of ice would be as a result of a structure that was designed to be at least somewhat aerodynamic, such that, if it presents itself to the relative wind and behaves as something other than an irregular mass in low-velocity turbulence, it should surprise no one.
Fickle as ice seems to be, it forms in areas where moisture is generally visible. That is, when the air temp meets or falls below the dew point. If the visible moisture is ice, it usually will be resistant to sticking (melting sufficiently then refreezing), but not always. So-called supercooled droplets which have some, presumably environmental reason for not crystalizing (impurity, micro-turbulence, ?), may all-too-readily "decide" they have an affinity for some passing by structure (whose surface temperature also tends to be below freezing).
In my limited(thankfully) first-hand experience with such phenomena, those structures which pose the greatest divergent angle to the relative wind are most prone to build-up, apparently because water molecules "linger longer" and can contribute to uncohesive airflow (turbulence). This is to say that ice seems to form most readily on those wetted areas which directly impact the relative wind. Ice formation farther removed from airflow diversion is unusual in my understanding.
Supposing it's ice I saw in the clips under discussion, it seemd to me the ice departed the ET fairly near to the shuttle's nose. That's unexpected! I wonder if the surface air pressure on the ET in such a vicinity is reduced by the aeodynamics of the shuttle itself. Could the shuttle's aerodynamics "help lift" ice from the ET body, through what must be a really high-speed, largely cohesive relative wind. The camera angles I've seen (and what little I could discern) tempt me to conjecture that the "ice" came toward the shuttle at its steepest angle immediately after leaving the ET (though my eyes could have been fooled), then the angle thereof decreased as it neared the shuttle body, only to impact near the port leading edge. To maintain such a divergent angle across a Mach One relative wind bespeaks one astoundingly large mass "launched" in that direction. Did it slide off some structure as if it were a ski jumper? If those suppositions are at least partly true, it could be that the configuration of the two main bodies and aerodynamics in those areas actually increase the possibility of being struck by ice, should ice be present. (note to self: check correlation of increased problems with winter weather.)
WRT channeling of ice along the surface of the ET, I'm led to wonder whether differential surface temps on the ET contribute to an uneven ice buildup or detachment likelihood.
Because of the latitude and proximity to moisture-bearing weather, potentially ice-laden Florida clouds are taller in the atmosphere than they would be elsewhere, as the troposphere goes to its greatest heights nearer the equator. Thus, a shuttle's lingering through such potentially ice-laden environment is greater than elsewhere, and such circumstances may be outside the ken of those trained in higher-latitude environments.
Translation: don't launch that the shuttle in the winter when clouds of any significance lie in its flight path.
HF
Boy, in retrospect that's an unfortunate choice of words.
Is the ET insulation orangish all the way through, or like the shuttle tiles, is it white inside with only a colored outer coating?
I saw someone in the know from Florida say that it is actually a light tan. The surface gets a bit darker as it sits in the sunlight due to reflected light. This person said the color looks differnt depending on light intensity.
My own experiences say that the colors are in the light tan to chaulky reddish/white range.
-PJ
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