Spaceflight Now

CAIB accepts, agrees with NASA failure scenario
Posted: May 6, 2003

For the first time, the Columbia Accident Investigation Board has endorsed a detailed failure scenario developed by NASA and contractor engineers that traces the shuttle's destruction to a breach in the ship's left wing at or near leading edge panels 8 and 9. That scenario, first reported by CBS News on April 20, assumes Columbia began its ill-fated Feb. 1 descent to Earth with a breach in the leading edge and that hot gas ate its way into the interior of the wing less than eight minutes after the orbiter fell into the discernible atmosphere 76 miles above the Pacific Ocean.

How large the initial breach might have been is not yet known. But the day after Columbia's launching, military radars tracked an object separating from the orbiter. Investigators believe the "flight day 2 mystery object" left an opening of some sort that provided a direct path for hot air to enter a cavity behind the U-shaped reinforced carbon carbon panels making up the leading edge of the left wing. More than two dozen shuttle components have been tested to find the best match with the radar data and today, experts testified before the CAIB that only two candidates are still on the table (a third has yet to be tested).

One is a large section of an RCC panel measuring up to 120 square inches. The other is a T-seal, one of 22 such fasteners used to lock the RCC panels together. Under the scenario developed by NASA and the CAIB, the mystery object could be either the T-seal between RCC panels 8 and 9 or a large section of the lower portions of the RCC panels themselves.

"From all the testing and analysis we've done, we feel RCC T-seals as a class cannot be excluded and RCC, what we call acreage, or pieces of the panel, cannot be excluded," said Steve Rickman, chief of the thermal design branch at the Johnson Space Center. "But there is another point to be made there, that the panel acreage itself would have to be on the order of 0.33 inches thick for it to have the correct ballistics. ... It turns out that on the lower panel acreage in the panel 8-to-9 region you do have RCC panel acreage that is of this thickness."

The loss of a T-seal would have left a slot-like gap between panels 8 and 9 just a few inches across as opposed to the much larger hole represented by the loss of a section of RCC panel measuring between 90 and 120 square inches. Engineers currently are trying to determine how big the actual breach must have been to permit the entrance of enough heat to explain the timing of sensor readings and dozens of subsequent sensor failures.

"My understanding is that NASA is, in fact, doing specific analyses for those different shapes, sort of two-dimensional analysis," said board member Sheila Widnall. "My understanding is the hole sizes they've been using to date are quite a bit smaller than the RCC panel that was suggested in the Wright Patterson radar tests. So I think more analysis is clearly required."

Board chairman Harold Gehman said "this is one of the key areas we're going to continue to focus on."

"The way I like to describe it is that the breach that was there at the time of entry has to be big enough to cause the heat scenario that we saw but it also has to be small enough to permit the orbiter to get all the way to Texas. Keeping in mind that we've got some bounds in there and we've got a very, very rich timeline, I believe we've got a good chance of achieving the analysis it's going to take to be more specific about this. We're talking about weeks of work here. It's just plain hard work."

But it is critical work and it will play a major role in how NASA and CAIB investigators design upcoming tests at the Southwest Research Institute in San Antonia, Texas, to fire external tank foam insulation into leading edge components in a bid to simulate what went wrong in the first place.

As is well known by now, just 82 seconds after liftoff Jan. 16, a large piece of foam broke away from Columbia's external fuel tank. The debris originated at or near the left bipod ramp, an aerodynamically shaped area of foam just in front of a strut that helps hold the nose of the shuttle to the tank. At least three distinct pieces can be seen falling away from the bipod area in enhanced footage from ground cameras. But only one - the largest - actually hit the left wing. Experts testified today the large piece measured 24 inches by 15 inches with an uncertainty of 3 inches. The thickness of the debris has not been determined but it is believed to be relatively thin. The impact velocity was somewhere between 610 feet per second and 840 feet per second, or between 416 mph and 573 mph.

The debris hit the left wing in a "footprint" centered on the lower side of RCC panels 6 through 9. It's possible the impact damaged the T-seal between panels 8 and 9 - or cracked one of the RCC panels - and that the damaged component finally shook free the day after launch, leaving an opening into the cavity behind the leading edge. Investigators have not yet come up with a credible mechanism to explain how the damaged component was able to remain in place for a full day and then separated on flight day two.

But for their part, Gehman said the board has deliberately excluded the foam impact as a direct cause of the disaster in its working hypothesis of what went wrong.

"We were careful not to say the foam knocked a hole in the leading edge of the orbiter because we can't prove it," he said. "Now that's not to say we don't believe that's what happened, but we were careful here to base our working scenario on agreed facts and right now, we aren't willing to make that kind of a statement."

Whatever caused the breach in the leading edge, the pathway for hot air was large enough to trigger a remarkably rapid series of events. The shuttle entered the atmosphere at 8:44:09 a.m. Less than eight minutes later, at 8:52:05 a.m., the shuttle began responding to unusual aerodynamic forces. At some point between that moment and 8:52:16 a.m. - 11 seconds later - the hot air had burned its way through the wing spar and into the wing's interior. One minute later, by 8:53:10 a.m., 120 sensors had dropped off line as the super-heated air burned through wiring inside the wing just behind the spar.

That data was recorded on board and not transmitted to the ground. The first realtime indication of a possible problem in the left wing came at just about that same moment - 8:53:10 a.m. - showing up on a computer display monitored by the mechanical systems officer in mission control. By that point, the wing was in severe distress and complete failure was just seven minutes away.

The upcoming tests at the Southwest Research Institute, scheduled to begin in early June, "will demonstrate to us whether or not we have a plausible scenario," Gehman said. "But it doesn't seem to me that it will prove anything one way or the other."

Gehman said the board hopes to begin writing its report later this month. No more hearings are currently scheduled for Houston but the board plans to relocate in Washington next month and one or more hearings may be held there to flesh out questions about NASA's management and operating philosophy. Gehman said it will not matter in the long run whether investigators ever conclusively link the foam impact with the leading edge breach.

"We can make fairly good case of what we think the return-to-flight criteria should be with or without any positive knowledge or positive proof that the foam caused the accident," he said. "Because we're going to make return-to-flight recommendations that are designed to enhance the safety of the orbiter in every way we find that it needs to be enhanced.

"Just fixing the foam alone won't do it. So I'm not the least bit concerned that our inability to make a positive statement with proof that the foam knocked a hole in the leading edge of the orbiter in any way slows us up or in any way restricts what we need to do in order to come up with a criteria for return to flight. Most of our work on return-to-flight issues has to do with the fact that the safety margins have been changed over the years and we're going to try to restore those safety margins back to at least as far as we're comfortable with."

Lest there be any doubt Gehman is serious about addressing a wide-range of issues in the board's final report, he stressed again today that "we're looking at this program and these shuttles in a very, very broad way. We have to, because we don't have a single point failure like the O-rings (that caused the Challenger disaster). And therefore we're going to come up with a broad range of recommendations, which taken together, we believe, will make the program safer. The fact that we don't have a single causal event doesn't bother me in the least. ... It may not be quite so easy to explain, but practice wise and function wise, it doesn't bother me in the least."

He also said the board plans to address the overall risk of flying the shuttle to stimulate public debate.

"The board is going to attempt to characterize the true risk in our own words," Gehman said. "Whether or not we put a number on that, the board hasn't decided. But we are going to attempt to describe for our constituents - the Congress, the administration, the astronauts and the people of the United States - what the risk is in this enterprise. It's not zero, it's not anywhere near zero.

"I don't know that the board would be interested in putting a number on it. ... Whether or not we pass judgment on any number that NASA uses remains to be seen. But we will attempt to characterize the risk in our own terms and if it differs from NASA's, so be it. But that will be one of our goals, to restate the risks in terms that there can be a good public policy debate on whether or not we should be doing this or not."

Readers are encouraged to review the April 20 CBS News status report below for details about NASA's failure scenario. Here is the text of a CAIB news release today summarizing the board's conclusions to date:

Columbia Accident Investigation Board Releases Working Scenario

Houston, Texas The Columbia Accident Investigation Board (CAIB) today released their working scenario.

After three months of intense investigation including thorough reviews of hardware forensic analysis, orbiter telemetry, Modular Auxiliary Data System (MADS) recorder measurements, general public still and video photography, hypersonic wind tunnel testing, and aerodynamic and thermal analysis, the Columbia Accident Investigation Board, with the assistance of the NASA Accident Investigation Team (NAIT), has reached the following preliminary conclusions.

Approximately 81 seconds after a 10:39 EST launch on January 16, 2003, post launch photographic analysis determined that foam from the External Tank (ET) left bipod ramp area impacted Columbia in the vicinity of the lower left wing RCC panels 5-9.

While Columbia was on orbit for 16 days, there was no indication of damage based on orbiter telemetry, crew downlinked video, still photography or crew reports.

An Air Force Space Command post flight evaluation of radar tracking data indicated an object in the vicinity of the orbiter on flight day two, remained on orbit for approximately two and a half days, then reentered the atmosphere. Radar testing and ballistics analysis of various thermal protection system items and thermal blankets, along with careful inspection of downlinked orbiter payload bay video, has been used in an attempt to identify that object. Testing and analysis to date have eliminated from consideration all but a piece of an RCC T-seal or RCC panel with a rib. However, there is no conclusive evidence that either of these items was the object that departed the orbiter.

The de-orbit burn and entry targeting were accomplished using well-established Mission Control Center procedures. There were no problems identified with this process.

On the morning of February 1, Columbia entered with unknown damage to an RCC panel or T-seal in the left wing RCC panel 5-9 area. Hardware forensic analysis and a review of MADS temperature and strain measurements on the left wing leading edge structure point to the RCC panel 8/9 area as the most likely area of damage. The forensic evidence indicated the RCC panel 8/9 area was subjected to extreme entry heating over a long period of time, leading to RCC rib erosion, severely slumped carrier panel tiles, and substantial metallic slag deposition on the RCC panels nearest the damaged area.

MADS data indicated the RCC cavity temperature and the temperature behind the wing leading edge spar began to rise at approximately 8:49:00 EST. This indicates that hot gas flowed into the RCC cavity between entry interface 8:44:09 EST and 8:49:00 EST. Three minutes later, at 8:52:00 EST, temperature and strain measurements indicated that hot gas penetrated the internal part of the wing via a breach in the wing leading edge spar. Immediately, the hot gas inside the wing began to heat wire bundles containing real-time telemetry and MADS data. A MADS upper left wing pressure measurement was the first to fail at approximately 8:52:16 EST. Over the next four minutes, 164 other measurements fail, with the last failure at approximately 8:56:24 EST. Most measurements failed very quickly, within the first two minutes of the breach.

Columbia's flight control system began to sense increased drag on the left wing due to the damage at 8:52:05 EST. The vehicle easily compensated for the initial aerodynamic disturbance. A significant change in the vehicle aerodynamics was observed at 8:54:20 EST, indicating a change in the damage to the left wing. At the same time several very bright debris events were seen in ground-based videos.

Soon after the hot gas entered the left wing multiple debris events were captured on video by observers on the ground. These video images begin at 8:53:46 EST (20 seconds after California coastal crossing) and end with Columbia's final break-up. The exact source of the debris may never be fully understood. However, upper wing skin and Thermal Protection System (TPS) parts are possible candidates. Damage to the internal aluminum wing structure was most probable during this timeframe as well. These debris events appeared to affect orbiter communication. There were 13 unexplained communication dropouts in this timeframe.

By 8:56:16 EST hot gas had penetrated the wheel well wall as indicated by an off-nominal rise in hydraulic line temperatures. Another significant change in Columbia's aerodynamics occurred at 8:58:09 EST, accompanied by several more debris events. The vehicle responded to this event with a sharp change in its aileron trim. Additionally, by 8:58:56 EST all left main gear tire pressure and temperature measurements were lost, indicating a rapid progression of damage inside the wheel well. A continual progression of left wing damage caused another abrupt change in the vehicle's aerodynamics at 8:59:29 EST. Columbia attempted to compensate by firing all four right yaw jets. By 8:59:32 EST the Mission Control Center had lost all telemetry data. MADS recorder data was lost at 9:00:14 EST. Based on video imagery, main vehicle aerodynamic break-up occurred at 9:00:23 EST.

Analysis and Testing Underway to Support the Working Scenario:
Although there is an abundance of existing evidence supporting the Working Scenario described above, the CAIB and NAIT have the following analyses and testing underway to refine the details of the scenario:

  1. Completion of RCC and tile impact testing at Southwest Research Institute.

  2. Aerothermal analyses to correlate off nominal heating trends in left fuselage sidewall and left Orbiter Maneuvering System (OMS) pod heating that were observed in the MADS data.

  3. Instrumentation wire burn-through arc jet tests and thermal analyses to support the timing of observed instrumentation failures.

  4. Instrumentation circuit analyses or testing to confirm the failure signatures observed in the data.

  5. Hypersonic wind tunnel testing and aerodynamic analyses to explain aerodynamic roll and yaw moments observed in flight data.

  6. Thermal analysis of RCC panel 9 clevis and spar temperature sensor responses to support or refute flight data.

  7. Gas flow and heat transfer calculations internal to the wing to support the MADS sensor readings in and around the wheel well.

  8. Arc jet testing and/or analysis of previous arc jet testing to determine feasibility of RCC erosion observed in several key pieces of RCC panel 8/9 debris.

  9. Continued forensic testing and analysis of significant recovered debris.

  10. ET dissection and cryopumping tests.

The CAIB has not reached any final conclusions and has not determined the cause of the loss of the shuttle and crew. The board's final report will be issued later this summer.

"By building a working hypothesis, it really enables us to focus the testing, it enables us to much more narrowly direct the analysis, it really produces a very synergistic effect on different disciplines," Gehman said.

"The real value of this is that it tells us what to do now and where to go next," he said. "The foam impact testing is very important to us, we have to continue the very, very hard work on the aerothermal analyses to correlate some of these events that we have really good data on but yet we can't quite fully understand everything that's happening, like how long does it take to burn through Kapton wiring, how long does it take to create a knife edge (burn pattern) in a piece of RCC that's a third of an inch thick, how long does it take to burn through aluminum skin and those kinds of things."

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