Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 1:16 PM GMT on January 30, 2013
There's been plenty of fodder for blogs this week (snow from a nuclear plant, severe weather), but I didn't want January to end without remembering the Challenger disaster on January 28, 1986 (video).
Given the historical significance of this tragedy and the role that vertical wind shear (the topic of yesterday's blog) played as a contributing factor, I believe that the Challenger disaster is a legitimate topic to address here. At the very least, I'm hoping to convey to you that vertical wind shear has more than one application in the real world, particularly as it relates to aviation.
The Challenger Disaster
On the morning of January 28, 1986, I remember gathering with colleagues in an office adjacent to the Weather Station here on Penn State's campus and watching the live broadcast of the liftoff (Christa McAuliffe was slated to be the first teacher in space, and there was great national interest in this shuttle mission). It was clearly one of those "where were you when" moments that stand out in my life.
The 12Z temperature and dew-point soundings from Cape Canaveral, Florida, on January 28, 1986 (the morning of the Challenger disaster). Note the vertical profile of winds on the right of the image. Courtesy of the University of Wyoming
Big Florida Chill
Shown above are the 12Z temperature and dew-point soundings at Cape Canaveral, Florida, on January 28, 1986, the morning of the disaster. At the time, the surface temperature was well below 0 degrees Celsius, indicative of the unseasonably cold air mass over the Southeast States on this date. Check out the 12Z surface analysis from the NOAA archive of Daily Weather Maps and note the sprawling high-pressure system centered over the eastern Gulf of Mexico (which promoted a clear sky and light winds). In his book, Challenger, A Major Malfunction, Malcolm McConnell reports a temperature of 24 degrees at 7 A.M. EST as the "coldest period of the long night." Icicles as long as 18 inches were observed on the shuttle's support structure.
The temperature at liftoff later that morning was only 36 degrees Fahrenheit, by far the coldest launch to date in the shuttle program. The 12Z reanalysis of 500-mb heights showed a full-latitude trough over the eastern United States, which was consistent with a cold snap over Florida.
Not surprisingly, the 12Z reanalysis of 850-mb temperatures indicated readings below 0 degrees Celsius over east-central Florida, which is quite cold compared to the 850-mb climatology for the date. Granted, this climatology from the Earth System Research Laboratory is based on data from 1968 to 1996 (some of these data came after the disaster), but the long-term average 850-mb temperatures shown on the ESRL reanalysis are close to what would have been considered "climatology" on January 28, 1986. At the very least, this climatology of 850-mb temperatures should give you a better sense for the severity of the cold snap.
I'll begin my story with the controversy leading up to the liftoff (read more). The debate about whether to postpone the launch due to cold weather focused pretty much on the "resiliency" of the rubber O-rings that sealed the joints of the solid-rocket boosters. Engineers at Morton Thiokol pointed out that the capability of O-rings to operate efficiently would be reduced at air temperatures lower than 53 degrees Fahrenheit, but their objections were overruled by Thiokol management. Just before liftoff, there was already gray smoke leaking through an O-ring on the right solid-rocket booster, indicating that the seal was not operating properly (photograph).
Vertical Wind Shear
Okay, let's get the the issue at hand...vertical wind shear. Revisit the skew-T above and note the strong winds at high altitudes shown on the right of the skew-T above. According to telemetry, Challenger disintegrated at about 46,000 feet, which is roughly 14,000 meters. If you compare this altitude to the 12Z raw rawinsonde data from the University of Wyoming, you get the sense that things went terribly wrong around 150 mb. Note the rather rapid change in wind speeds with altitude in the upper troposphere. Prior to liftoff, pilots conducting test flights reported some vertical wind shear earlier that morning, but it was within acceptable limits. Obviously, vertical wind shear must have dramatically increased after 12Z (conditions were considerably worse during liftoff). According to the President's Commission Report:
"At approximately 37 seconds, Challenger encountered the first of several high-altitude wind shear conditions, which lasted until about 64 seconds. The wind shear created forces on the vehicle with relatively large fluctuations. These were immediately sensed and countered by the guidance, navigation and control system.
The steering system (thrust vector control) of the Solid Rocket Booster responded to all commands and wind shear effects. The wind shear caused the steering system to be more active than on any previous flight."
You can read more about the investigation that followed the Challenger disaster in this NASA's report (pdf file).
A series of photographs showing flame emerging from the malfunctioning O-rings. Apparently, the O-rings that had leaked smoke on the launch pad before liftoff had formed a working seal after being heated, but the aerodynamic forces associated with strong vertical wind shear likely broke these seals, allowing flame to escape to the outer surfaces of the right solid-rocket booster and setting the stage for Challenger's destruction. Courtesy of NASA and aerospaceweb.org
I also recommend the very informative account at aerospaceweb.org. Below is a specific reference (from this Web site) that directly connects wind shear to the failure of the O-rings. The context of this excerpt is a follow-up to the cloud of gray smoke observed while Challenger was still on the launch pad (right before liftoff).
"Nevertheless, no further puffs of smoke were observed since the joint apparently sealed itself. This new seal was probably due to a combination of two factors. First, the O-rings were heated by the hot burning fuel within the booster which would've increased their temperature and resiliency. This behavior is actually common in many military missiles. Such missiles often generate large clouds of black smoke at ignition due to a temporary blow-by of their O-rings that is sealed as the rings heat up. Second, the solid rocket propellant contains particles of aluminum oxide that melt when heated, and the solidifying aluminum droplets probably sealed the gap. Indeed, laboratory tests have shown that damaged O-rings with notches cut into them can be sealed by such droplets.
The temporary seal apparently remained intact for nearly one minute into the flight since the chamber pressure within the right SRB remained normal. It is very possible that this seal could have been maintained indefinitely if not for the fourth and final factor that doomed the mission. At 56 seconds after launch, right around the time of max q, Challenger passed through the worst wind shear in the history of the Shuttle program. The wind loads on the vehicle caused the booster to flex and dislodged the aluminum oxide plug that had sealed the damaged O-rings. This event was marked by a reduction in chamber pressure and the appearance of a small flickering flame that emerged from the aft field joint at 58.788 seconds Mission Elapsed Time (revisit the series of photographs above)."
In short, aerodynamic forces generated by vertical wind shear broke the temporary O-ring seals. Clearly, vertical wind shear was a contributing factor to the Challenger disaster.
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