Spaceflight Now

Hearing focuses on debris, breakup mechanics
Posted: March 18, 2003

A NASA flight director told the Columbia Accident Investigation Board Monday he was amazed the doomed spacecraft was able to continue flying in relatively normal fashion for nearly 10 minutes while shedding multiple pieces of flaming debris as a plume of superheated air burned its way into the stricken ship's left wing.

  Doug White and Paul Hill
With the orbiter's ground track as a backdrop, United Space Alliance's Doug White and NASA flight director Paul Hill describe Columbia's last minutes as revealed by telemetry and video. Photo: Steven Young/Spaceflight Now.
The shuttle's flight computers faced an impossible task. But in silent testimonial to the foresight and cleverness of their programmers, the computers methodically adjusted the positions of wing flaps, or elevons, to offset steadily increasing levels of aerodynamic drag acting to pull the ship's nose to one side.

At 8:52:05 a.m. on Feb. 1, one minute and 12 seconds after Columbia entered the region of peak heating off the coast of California, analysis of telemetry from the doomed craft shows the first indications of the computer system reacting to counteract an unusual yaw to one side by adjusting the shuttle's aileron trim. Fifty-six seconds later, the computers began counteracting an additional rolling motion, or moment.

The space shuttle is not equipped with ailerons like a typical airplane. Instead, each wing has two large flaps, or elevons, one inboard and one outboard, that can be positioned independently to produce the same effect as ailerons.

"The aileron is a theoretical difference between the elevon position on one side of the vehicle and the elevon position on the other side of the vehicle," said Robert "Doug" White, director of operational requirements for United Space Alliance, the shuttle prime contractor. "By adjusting the elevons ... you can create an aileron effect. So that aileron effect was keeping the vehicle flying the way we wanted it to. So as the forces began to change on the vehicle, the trim changed and we saw that in the data."

At 8:58:03 a.m., the telemetry shows a sharp increase in the movement of the shuttle's elevons to keep the ship pointed in the right direction.

"The vehicle was in control and was responding to commands up to that point and after that point something changed, apparently," White told the CAIB. "It still continued to be in control and still continued to respond to commands, but the rates and the amount of muscle it needed to continue flying the vehicle the way it should be flown was continuing to increase. Something definitely happened at that point, again we don't know what, but something definitely happened at that point to cause the flight control system to need more muscle and start having to fight harder to control the vehicle."

Board member Sheila Widnall, an aerodynamicist, observed: "That really coincides with a rather sharp increase in the rate of rise of dynamic pressure."

"Yes," said White, "it does."

What might have happened at that moment is not yet known. But Columbia was in distress almost from the moment it began experiencing the effects of atmospheric heating high above the Pacific Ocean approaching the California coast. Video shot by amateurs, several of them technically accomplished amateur astronomers, show multiple pieces of debris falling away from Columbia beginning at 8:53:44 a.m., roughly three minutes after entering the region of peak heating. It is likely debris was shed even earlier than that, out of view of the cameras.

Investigators believe a breach at or near the leading edge of the left wing allowed a jet of super-heated air to burn its way into the unpressurized structure. The latest thinking, NASA sources say, is that the plume entered the wing around the position of reinforced carbon carbon panel No. 6, one of 22 such protective carbon composite panels making up the leading edge of the wing. This region of the wing was hit by debris falling off the shuttle's external tank 81 seconds after liftoff. The RCC panel or its mounting hardware may have been damaged enough by the debris strike to cause a breach during entry. It's also possible damage to a carrier panel just behind the lower edge of the panel provided an entry point for hot air as Columbia descended.

NASA is having tests conducted at Wright Patterson Air Force Base in part to determine if debris seen floating away from Columbia the day after launch could have come from this area. While engineers are still working to pin down the exact breach location, the above scenario provides the best match yet with telemetry and the known aerodynamic forces acting on Columbia as it returned to Earth.

Once inside the wing, the plume of super-heated air appears to have burned through electrical cables routed along the outside of the left main landing gear wheel well just a foot or so behind the RCC panel, ultimately eating its way into the well itself where it triggered additional sensor failures and elevated temperatures. The plume may have jetted out of the wing through gaps near the corners of the heat-softened aluminum landing gear door.

Whatever the exact sequence of events, the left wing ultimately failed as the shuttle yawed out of control around 9 a.m.

Flight director Paul Hill, who has been coordinating NASA's efforts to integrate amateur video into the agency's second-by-second entry timeline, said imagery of Columbia's catastrophic plunge across the southwest holds no clues as to exactly what was falling away from Columbia or even how large the pieces, whatever they were, might have been. Only relative sizes can be inferred.

"I am confident that many, if not all, of the things we label as 'debris shedding events' are, in fact, some object coming off of the orbiter," Hill said. "Can I tell you is it golf ball sized, or is it the size of this sheet of paper? I can't.

"It very well could be something as small as a marble in most of those videos and the ones we think are so significant and have gotten us so excited, those things could be golf ball sized. We really don't know. We know relative sizes, we know relative motions, but we don't know specifically what they are.

"But we are very confident, based on the way they behave after they separate from the orbiter, that they are, in fact, separate ballistic objects or objects that have mass in almost all cases. In the case of some of these flares, they could be something different like combustion products."

A major flaring event occurred around 8:54:33 a.m., just prior to the separation of an object known as debris 6. Other flares were observed later in the sequence.

"It is possible it is something that burned and came off the vehicle," Hill said of debris 6 and its associated flare. "It is what you would expect to see if we were to, say, vent a fluid or if we were to burn something and as we gave off combustion products - significant combustion products, not something on the order of, say, one of our reaction control jets - but if we were actually burning something substantial and as we put that out in the plasma wake, you would expect, because that would have relatively no mass, certainly when compared to an object, that those combustion products would immediately go essentially static compared to the orbiter or compared to what we consider normal ballistic behavior for an object that has significant mass.

"So it is reasonable to assume that something came off that was very light or that was some kind of combustion product, like potentially aluminum slag that also was burning as it came off the orbiter and then went stationary there in the wake and looked more persistent."

Of 16 documented debris shedding events, debris 6 and 14 were by far the brightest. Debris 14 was the brightest of all.

"Debris 6 you can see persists, depending on the video you look at, for between 6 and 12 seconds," Hill said. "Debris 14 we see persist for four-and-a-half to seven-and-a-half seconds. But debris 14 also is much, much brighter than any other object, including debris 6. How do you interpret that? We're not sure.

"We do think that relative brightness is an indicator of something that's larger and more massive," he said. "We think the amount of time that individual flares or the light around the debris persists is also indicative of the larger ballistic numbers, which tells you you're dealing with something that's probably larger and heavier. But that's as much as we know."

While the video does not yet shed light on the nature of the debris, it clearly documents a surprising number of events. Hill said engineers "continue to be shocked that we had debris coming off the orbiter as we crossed the California coast."

"We clearly had an external breach in the vehicle and had hot gas somewhere in the left wing for that significant period of time and the vehicle flew perfectly, no indication of what was going on in flight control and virtually no indication of what was going on in telemetry on the ground, other than we saw a few temperature, pressure indications that didn't make sense to us," he said. "But aside from that, the vehicle flew like a champ until right up until the breakup. So that did surprise us."

But Hill said he was not surprised search teams combing eastern Texas for debris have not recovered more wreckage.

"Are we surprised that we only have 15 to 20 percent by weight of the orbiter? I don't think so," he said. "I would think when you first see the debris count and you see how many individual pieces of debris (have been recovered), our first reaction was one of surprise, how could we have gotten that much of the orbiter down from 200,000 feet intact?

"I think you've all seen at KSC, what they have is a whole lot of little tiny pieces of what used to be an orbiter. And if you go look at it laying on the ground there in the high bay at KSC, you don't have a spacecraft laying there, you've got a whole lot of nothing. And I think that fits in with what our conventional wisdom was prior to this."

At Monday's hearing, the second held by the Columbia Accident Investigation Board, an expert in re-entry breakups said the shuttle's destruction was in keeping with the mechanisms known to be at work during more mundane satellite entries.

"The heating is like, in a sense, cooking an onion," said William Ailor, director of the Aerospace Corp.'s Center for Orbital and Reentry Debris Studies. "You basically start from the outside and then as you heat the pieces up to a point where the materials will fail, that'll expose some new materials, they'll go through the same process and the object can be broken apart."

Interestingly, Columbia's breakup, in general, mirrored the way much smaller, fragile satellites and rocket stages break apart when falling through the atmosphere.

"Basically, a typical way for things to break up when they re-enter is they will come down through the atmosphere for a certain amount of time, they look absolutely fine, we've seen videos of these things where they just look like spacecraft coming down. All of the sudden, they come apart and when they come apart, they just disintegrate.

"That altitude typically is around 42 nautical miles plus or minus a few nautical miles," he said. "But that's a pretty good guess. Just as a rule of thumb, it seems like a critical point for spacecraft re-entry and breakup is around 42 miles."

Unlike re-entering satellites, Columbia had a thermal protection system and wings to provide some amount of lift. Even so, the orbiter only made it to an altitude of around 38 miles before suddenly, and catastrophically, breaking up.

Ailor said the recovery of debris shed early in Columbia's descent likely will prove critical to determining the root cause of the disaster.

"The orbiter was controlled for a good period of the time and if evidence is found that could have occurred during that period and it indicates a particular flow pattern or something like that, I think that could be very useful," Ailor said. "I think the early debris would be very critical to an analysis like that."

Such debris will be difficult to find. As of now, nearly seven weeks after the disaster, no wreckage has been found west of Texas. But Hill's team has been able to determine the probable trajectories of several pieces of debris that fell off early. Using that data, researchers have found ground radar traces matching at least four debris shedding events, three of them in the debris 6 "footprint." Investigators are just now beginning to look for radar traces in the presumed debris 14 footprint.

"We think the brighter objects are more massive, are more significant, potentially higher ballistic numbers," Hill reiterated. "Certainly the (brightening associated with) the individual pieces of debris persists longer. We expect that those objects are more massive, higher ballistic number, because we think that the reason they persist longer is they are moving faster so they stay lit, they have their own plasma wake longer than, say, some lighter things, say an individual tile comes off versus some other heavier object.

"But I'll also say we cannot just look at these videos and determine what is it that's coming off the vehicle. Are we losing a tile here, are we losing some section of ... thermal blanket that's on part of the external surface of the vehicle? We can't tell that. And until this day, with the good data we have on the ballistic analysis and the footprints, we still cannot say exactly what it is we see coming off. We are making some judgments on which of them are more significant or more massive than the others (such as) debris 6 and debris 14."

Said Ailor: "The work that's going on relative to finding the debris is really an important part and that really has to be emphasized. That's going to be a key to solving this puzzle, I believe."

Hubble Calendar
NEW! This remarkable calendar features stunning images of planets, stars, gaseous nebulae, and galaxies captured by NASA's orbiting Hubble Space Telescope.

Hubble Posters
Stunning posters featuring images from the Hubble Space Telescope and world-renowned astrophotographer David Malin are now available from the Astronomy Now Store.

Earth Calendar
NEW! This amazing 2003 calendar features stunning images of mountain ranges, volcanoes, rivers, and oceans obtained from previous NASA space shuttle missions.