Spaceflight Now

Columbia could not be saved, NASA study shows
Posted: April 30, 2003

As NASA and independent investigators close in on the root cause of the Columbia disaster, one question lingers in the minds of many armchair analysts: What, if anything, could have been done to save the crew if engineers had known early on that the orbiter had a non-survivable breach in the leading edge of its left wing?

The answer, according to a detailed NASA analysis obtained by CBS News, is that Columbia was doomed from the moment the wing was damaged, most likely during ascent, and that nothing could have been done to reduce the stress of re-entry enough to save the ship and its seven astronauts.

Not that NASA wouldn't have tried. But given the severity of the leading edge breach investigators now know was present at the start of Columbia's descent, there simply were no Apollo 13-class engineering rabbits to pull out of the hat.

"I have wracked my brains over this," LeRoy Cain, the entry flight director for mission STS-107, said in an interview. "There just was no way we were getting that vehicle back. If we'd gone and taken some pictures and done whatever else anybody could think of, it wouldn't have changed the outcome for Columbia."

NASA's oldest space shuttle was launched Jan. 16 on a planned 16-day science mission. Eighty one seconds after liftoff, a large piece of foam insulation broke away from the ship's external fuel tank and hit the left wing's leading edge at some 450 mph.

While an independent investigation into the disaster is not yet complete, many engineers believe the foam impact, possibly in combination with other factors, damaged one of the wing's reinforced carbon carbon composite panels, providing a path for super-heated air to enter the wing during re-entry Feb. 1.

In the wake of the resulting catastrophe, Cain was charged with carrying out an "entry options" review to determine what might have been possible to reduce re-entry temperature extremes, or loads. The study will be presented to shuttle program management next week.

As it turned out, the only viable options involved lowering the shuttle's orbit before beginning the descent and drastically reducing the ship's weight by as much as 15 tons.

Diverting Columbia to the international space station was never an option because the two spacecraft were in different orbital planes and the shuttle did not carry nearly enough fuel to make such a rendezvous. Cain's review did not address the possibility of launching an emergency shuttle rescue mission. But engineers say they do not believe it would have been possible to get the next shuttle in the launch sequence - Atlantis - into orbit before Columbia's crew ran out of carbon dioxide-scrubbing lithium hydroxide.

That said, three options were evaluated in Cain's review but the best results were achieved in the third, most extreme scenario, one that assumed the astronauts dumped everything possible overboard to reduce the shuttle's weight to an absolute minimum while keeping barely enough fuel and other supplies on board to ensure a survivable landing.

It is not a scenario flight controllers would ever actually implement in its entirety. It is fraught with extreme risk and major unknowns, risks that might well outweigh the threat posed by a damaged thermal protection system. But the goal of the entry options review was to assess what might be possible, in theory, regardless of likely operational constraints.

Scenario 3 assumed the astronauts, staging at least two emergency spacewalks, could dump 31,321 pounds of equipment and supplies overboard, including Columbia's pressurized Spacehab research module (18,071 pounds), a pallet of experiments in the cargo bay known as Freestar (4,428 pounds) and unneeded crew equipment (4,663 pounds). Another 4,159 pounds of consumables - propellants, hydrogen, oxygen, water, hydraulic power system fuel - also would have to be dumped or used up.

The scenario requires numerous flight rule violations and would leave the shuttle at "absolute minimums in critical systems" with no deorbit waveoff opportunities and only a minimal ability to cope with additional failures. But it did reduce the maximum temperatures associated with re-entry.

"When we messed around enough with the weight, we started to see some reductions, getting some thermal relief in a generic sense, which makes sense," Cain said. "If we bring the altitude down before we deorbit, we're reducing the energy, if we get a bunch of weight off then we're reducing the energy. All that weight translates into energy dissipation we have to do during entry.

"So we did that. And really, what we did here is we tried to bound the problem and that's important to understand. We did it not because we said this is realistically things we can go do, this is throwing everything in the kitty, pulling out all the stops, doing things we really wouldn't really be able to accomplish all of and adding it all up and seeing what it does for you."

The study examined heating at three representative points on the orbiter leading edge and under belly. In an important caveat, all the scenarios assumed there was no damage to the shuttle's thermal protection system (TPS) because there are no computer models capable of accurately predicting how even minor damage might affect heating.

"One of the major, significant caveats to the whole thing is we did this for a nominal TPS," Cain said. "If I have exposed structure, then even getting rid of 32,000 pounds isn't going to save the day for me. It's just not, whether it's exposed structure on the lower surface or its on the wing leading edge.

"We don't know what we had (on STS-107), but we had somewhere from some amount of damage to exposed structure. We don't have a model to put in some amount of damage and then understand the resulting thermal protection."

That said, the scenario 3 results show:

  • A 7 percent reduction in the wing leading edge maximum temperature near reinforced carbon carbon panel No. 9;

  • A 24 percent reduction in heat load and a 34 percent reduction in heat rate for tiles just in front of the main landing gear doors;

  • A 29 percent reduction in heat load and a 56 percent reduction in heat rate for tiles aft of the main landing gear doors.

Heat rate is the maximum temperature a component might experience during entry while the heat load is a measure of the amount of time the heat is applied to the orbiter's structure. To see how this works, consider a shuttle making a shallower descent than normal. While that would reduce the maximum temperatures experienced by the orbiter, it would increase the peak load because heating would be stretched out over a longer period of time.

"The thermal tradeoff line that we have today is a balancing act of those two things for the various surfaces," Cain said. "It's a delicate balance and the shuttle TPS is so complex. It just boggles my mind. It's even more complex than I thought it was before STS-107, just in terms of the different kinds of TPS we have and everything."

In Columbia's case, of course, it was the leading edge that mattered. Cain said a 7 percent reduction in leading edge temperature would not have been enough to prevent disaster. Even if the crew oriented the shuttle so that the belly of the craft was in shadow for more than two full days prior to entry, "cold soaking" the thermal protection system and lowering its temperature by 65 degrees, the outcome would have been the same.

"On STS-107, wing temps may have increased as much as 700 degrees in 400 seconds post EI (entry interface)," the entry options review stated. "A 65-degree decrease in EI wing temp would have resulted in (about a) 37-second delay in onset of same max temps and heat load."

Cain said the temperature improvement, however minor, possibly could be useful "if you had something that wasn't totally nominal in terms of TPS but it was maybe minor damage."

"Let's say on the wing leading edge you had a crack, you didn't have a breach yet but somehow you'd compromised the integrity of that part of the structure," he said. "Then that 6 or 7 percent, it might be significant for that case. I don't know enough to say today one way or the other. I do know that with respect to the 107 case it definitely wasn't significant."

During Columbia's flight, engineers concluded the foam impact during launch was not a "safety of flight" issue. As such, NASA managers did not request any spy satellite imagery to provide additional insight. Whether that was a good decision or not, the management team had no conclusive evidence that Columbia faced a serious threat.

But to have accomplished the extreme measures listed in scenario 3, planning would have had to begin almost immediately, meaning NASA managers would have needed convincing proof a safety of flight issue did, in fact, exist, within a day or two of launch.

"Excessive risks associated with any of the three options (scenario 1, 2 or 3 or any other combination) would require that significant and convincing data exist proving that the orbiter could not survive entry," the entry options review noted.

The scenarios all assumed "certified" re-entry profiles, meaning "our models match what we really see in flight," Cain said. "'Uncertified' means we don't know, we haven't done it for sure in flight, we haven't analyzed it and we haven't simulated it."

He said he could not imagine a real-world scenario that would lead the flight control team to order an uncertified entry requiring, for example, a higher- or lower-than-normal angle of attack to reduce heating on a specific area.

"There are certainly things that I can dream up where if you had some situation in flight that really put us in a bad situation, we're going to brainstorm and think of anything we can and at some point you've got to pick an option and go with it because you're running out of time," Cain said. "But for this case, I can't really imagine (that scenario)."

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