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Friday, February 07, 2003

By Dawn's Early Light


So I have 20, 30 pages maybe of notes, and a fairly complete page with graphics, etc. on the Columbia accident, explaining it from the point of view of an aeronautical engineer. Surprisingly enough, the news media, and NASA, have done a reasonably good job of keeping the investigation open, honest, and informative for the general public. The entire NASA team is doing a great job. So my mother of all posts is not going to happen.
CNN, in the first day or two of the investigation, decided that the ET's insulation breakaway just had to be the cause, since they had video of it. The likelyhood of 2.67 pounds of foam insulation damaging the orbiter enough to cause a breakup and reentry is a tad remote. Anyone who has taken the Kennedy Space Center tour and seen a demo of the Shuttle's tiles should know that. CNN continues to astound me with their lack of responsibility, as in the past few days NASA has said publically that the chances of the insulation issue being the root cause of the problem has been tossed out, yet CNN keeps bringing it back.
Give it up, Ted.

Now, why would little old me have a lot to say about what happened to the shuttle?
I may have been in this silly computer business for a while, but it's not the path in life I originally chose. When I was in college at the wonderful University of Maryland, I wasn't studying Computer Science, but aeronautical engineering. Specifically, hypersonic aerothermodynamics.
Yeah, try saying that 5 times fast. I dare ya!
Hypersonic aerothermodynamics focuses on the flight environment at speeds above about Mach 5 (5 times the speed of sound). That's literally faster than a speeding bullet. The fastest operational airplane in the world (retired now, unfortunately), the SR-71 family, flew at a max speed of Mach 3.4. The experimental X-15 flew up to Mach 6.7 during it's test program in the 1960s (though it only did so once, and none of it's flights spent more than a few minutes or seconds at hypersonic speeds). We don't know much at all about flying faster than Mach 5. The total flight time of all aircraft above Mach 5 would amount to less than an hour. The shuttle is the only aircraft that flies at those speeds right now - there are no X-planes flying in the hypersonic regime, no military flights, nothing.
During the 1980s and early 1990s, there were the beginnings of a renaissance in hypersonic flight. President Reagan announced the X-30 program in 1986, designed to create a family of aircraft that would not only provide cheap access to space, but fast "Orient Express" airliners that would cross the globe in minutes rather than hours. New York to Tokyo in 2 hours, etc. Over the next few years billions of dollars were spent pursuing this vision with not a whole lot to show for it. The X-30 was cancelled not long after the overall configuration was finally approved in 1994. With the death of the X-30 came a decline in interest in hypersonic flight, and here we are today still with no hypersonic flight test data to speak of. And this cycle of spending billions before we get an airframe has been happening since the 1950s.

Why is this important? When the shuttle broke up, it was travelling at 207,000 feet and Mach 18.3. At that speed, more than 12,000 mile an hour, you could circle the entire planet in about 2 hours. Los Angeles to New York would only take minutes. That's how fast they were going. Fast enough that even that high up in the atmosphere, on the edge of space where the air is super thin, the friction of the air moving over the shuttle's wing created a 2,000 degree plasma around the shuttle. The shuttle is moving so fast that the force of the air around it might as well be harder than diamond. In that environment, most metals melt instantly, if they can survive the strange chemical effects of oxygen and ozone molecules bumping into them at extreme speeds. In that environment, the smallest defect will become a catastrophic structural failure. If, say, a landing gear door is not sealed properly, the forces acting on the orbiter will tear it off and start a chain reaction, taking the rest of the wing structure with it before crushing the rest of the orbiter.

The last seconds of the Columbia's flight were a battle. The flight control computers were trying to correct for an increase in drag on the left side of the orbiter (which would pull the nose to the right). The computers tried to correct using the flight control surfaces on the wings and tail of the orbiter, but could not generate the force necessary to combat the increasing drag (most likely increasing as more of the left side orbiter structure disintegrated). As a last resort, the computers even tried to use the Reaction Control System thrusters in the right side nose of the orbiter to compensate (the RCS thrusters are rockets, normally used in space). At that point the computers lost the battle for control of the orbiter and seconds later the vehicle broke up, most likely from aerodynamic stress.

Photos of the orbiter over the southwestern United States taken by the [Starfire Optical Range] telescope were released today by NASA, and at first glance they seem to show something on the left side leading edge of the orbiter coming apart.

Space.com:[Piece of Columbia Wing Found, Photos of Re-Entry Examined]
Spaceflightnow.com/AvWeek:[Air Force imagery confirms Columbia wing damaged]
Washingtonpost.com:[NASA Examines Air Force Photos of Shuttle]

Starfire is actually one of the most advanced optical facilities in the world, being part of missile defence research since at least the 1980s. Starfire's telescope use [adaptive optics]- flexible, computer controlled mirrors- to correct for atmospheric distortion as it looks up into the sky (ie it removes the twinkling from the stars). Since the photo of Columbia was taken not of a star in the night sky but an aircraft crossing the morning sky at 18 times the speed of sound, it will take the experts to determine what, exactly, those photos show. If it is damage to the leading edge of the orbiter, all the other pieces of telemetry would make sense, from the loss of temperature and pressure sensors in the landing gear bay to the increase in drag on that side of the orbiter.

Last Saturday, the day of the accident, Chelle asked a very good question.
"Was it preventable?"
While we still don't know all the details of the accident, there are some things that are certain already. The orbiter was lost because of structural failure due to extreme aerodynamic loads. We also know that the flight control system not only knew there was a problem, but fought to correct it for several minutes before it completely lost the struggle and the orbiter broke up. Wether or not this is a software "defect" might be open to debate. Modern fly-by-wire flight control systems such as that on the F-16 Falcon would have been able to maintain control in a subsonic or supersonic flight environment. There are flight control systems that are even designed to adapt to large portions of the aircraft coming off or being inoperable. If we had more data from a dedicated hypersonic flight test program, such as any one of the dozen or so cancelled in the last decade alone, the shuttle's software probably could have handled the changing flight conditions (ie increased drag on the left side) better. The software could easily have been updated to handle a loss of control authority at high speed. It may even have been smart enough to not try to conpensate for the increase in drag, as the induced rolling moment and yawing that the computer's actions generated are probably what caused the sudden structural failure of the orbiter.

It's unfortunate that the [Shuttle Infrared Leeside Temperature Sensing] cameras mounted in Columbia's tail were removed during it's last overhaul. During past flights these cameras have imaged the reentry of Columbia and the effects of heating on the thermal protection system - and specifically the left wing of the orbiter. If this camera had been in place, we would have full imagery of whatever happened to the left wing of Columbia.

2/07/2003 04:41:00 PM ] [  0 comments  ]
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