COUNTDOWN TO DISASTER : Challenger’s Last Flight : 2. FAILURE : ‘They Never Knew What Hit Them’

No one saw it, not the millions watching on television, the thousands around the Florida launch site nor the astronauts watching the sky turn from blue to black as they thundered toward orbit.

The first glimmer of unspeakable disaster. A wisp of orange flame against the gleaming white steel booster rocket.

Films would later show it clearly. At 58.32 seconds into the flight, just after Challenger rammed through the sound barrier at about 24,000 feet, a small flame cutting like a welder’s torch escaped the steel casing of its right-hand booster rocket and poured out into the near-vacuum on the edge of space.

It burned through on the side facing away from the Challenger and its enormous fuel tank and, aided by shock waves swirling around the base of the vehicle, erupted into an uncontrollable 5,900-degree torch licking around the bottom of the massive external fuel tank loaded with 500,000 gallons of liquid hydrogen and liquid oxygen. Neither Mission Control in Houston nor Dick Scobee’s crew had a hint that anything was amiss.

In earlier days, a shuttle commander might have known that disaster loomed. But confident of the shuttle’s reliability, NASA had removed much of the instrumentation that once relayed data on the performance of the solid boosters and the condition of the fuel tank.

But even as the assurances were exchanged between Challenger and Mission Control that the shuttle was operating at full throttle, the flame was spreading. Unbeknownst to Scobee or to controllers, the fire spurting through the booster wall caused a 4% drop in its thrust. Sensing the suddenly unbalanced flow of power beneath it, Challenger’s computer swiveled the engine nozzle of the left booster and the three main engines of the spacecraft to the right, keeping the stricken vehicle flying true.

The orange flame, growing larger all the time, cooked the skin of the fuel tank as it spread precariously close to a 17-inch oxygen line linking the tank to the orbiter’s engines. Reaching from the top of the tank to Challenger’s engine compartment, the flame also spread upward, flowing around the spaceship as it throttled back to full power.

Less than 14 seconds after flame escaped the booster, the great aluminum tank fueling the orbiter’s main engines was fatally damaged, with vapor streaming out of both the hydrogen and oxygen containers. Halfway up the tank’s side the flames grew into a small fireball, and beneath Challenger’s flat belly, not far from the crew compartment, there was an explosion sending yellow-orange flames over the sides of the spacecraft, expanding until Challenger was almost engulfed. Then in a flash, beginning with a shimmering halo at the top, the entire tank, still carrying tons of fuel, blew up in an enormous orange, blue and white shrapnel-filled fireball reminiscent of the burst of a nuclear weapon.

A day postponed for 25 years had come.

At an altitude of 50,000 feet, the orbiter the size of a DC-9 airliner was obliterated by the explosion, blown into countless pieces--its wings and tail ripped away, its fuselage apparently crushed like an egg, the fuel in its heavily loaded orbital maneuvering system adding to the explosion of the external tank.

Some who viewed slow-motion film from space agency cameras thought they saw the crew compartment thrown clear only to be destroyed by a secondary blast. The smaller flammable parts were consumed by the poisonous 10-mile-high funeral pyre. The insulating tiles of its skin, pieces of its skeleton, and the heavy structures of its engines and landing gear rained down on the Atlantic for an hour, according to the Air Force.

Scobee’s crew members, pinned to their seats by the acceleration and exhilarated by the thrust of the five mighty engines beneath them, had no warning. Said former Apollo astronaut Tom Stafford: “They never knew what hit them.”

Out of the center of the blast, the two solid boosters emerged intact and flew crazily upward, their engine nozzles automatically centered for straight flight after they were torn from the orbiter. The swelling fireball and the boosters’ contrails forming a snaky “Y” shape against the sky were the first evidence of the worst tragedy of the Space Age.

For a few fleeting moments, there was hope that Challenger itself would somehow fly out of the fireball and turn back for the Cape for an emergency landing, or, at least, an attempt to ditch in the ocean. At the launch site, a few reporters started to run for the landing strip several miles away but stopped, realizing Challenger was gone.

At the Mission Control Center in Houston, consoles froze. On each screen, an “S” for “static” formed, meaning no information was coming back from Mission 51L. As flight controllers stared at their consoles in stupefied horror, the range safety officer, fearful that the uncontrolled boosters might turn toward a populated area, sent a radio signal blowing them apart.

Still, even after the explosion on the fringe of space, mission narrator Steve Nesbitt continued briefly to intone Challenger’s speed and altitude from the control center. Then, for 40 seconds, there was silence, and finally a shaken confirmation of the obvious: “Flight controllers are looking very carefully at the situation. Obviously, a major malfunction.”

It was an understatement for the ages. The thunderclap over Florida turned the country into a family as it had not been since the political assassinations of the 1960s, setting off repercussions from the Pentagon to the board rooms of aerospace companies to grade schools. About 2.5 million children watched on special television hookups and teachers wondered whether a whole generation had been permanently scarred.

In Washington, the work of the government all but stopped. In Huntsville, Ala., where the Marshall Space Flight Center is located, small engineering companies closed for the day and drivers spontaneously began turning on their headlights. Outside Houston, astronauts’ families sought comfort in each other.

Across America, people sat awed and horror-stricken before television screens watching it over and over again. “This was a day we wish we could kick back forever,” said Sen. John Glenn (D-Ohio), America’s most durable space hero. But it was one he had expected.

The malfunction occurred in a booster, a relatively simple component of an excruciatingly complex system. The booster rockets help propel the shuttle to an altitude of 25 miles by burning a mixture of powdered aluminum mixed with ammonium perchlorate. Its direction is controlled by sending orders from the orbiter to swivel its exhaust nozzle to direct its thrust.

All of America’s modern intercontinental ballistic missiles use similar propellants. When the United States settled on its design for the shuttle, it chose to use solid boosters, in part because of their simplicity and reliability, and in part because they are far cheaper than liquid-fueled boosters.

Their drawback is that once ignited, there is no shutting them down. They cannot be throttled, as can the orbiter’s hydrogen-burning engines, they cannot be safely dropped. Indeed the only way their power is controlled at all is through the geometric design of the fuel core. In the lower segment, which burns during liftoff, the core is designed to maximize burning speed. A change in the pattern causes the solids to burn slower during the transition to supersonic speed, at the same time the orbiter engines are throttled back to 65% of their peak thrust.

Otherwise, the giant rockets behave like Roman candles, firing until they expend themselves. Once empty, the tubes are parachuted into the sea from an altitude of 35 miles so they can be recovered for reuse.

Thus, when Challenger’s crew lifted off the launch pad, they rode the back of a tiger. Even had they known of the fire beneath them, they could not have escaped.

Pressured to get every ounce of useful payload into the shuttle, and confident of solid fuel technology, NASA discarded escape rockets on the orbiter’s nose from the early shuttle designs, taking the chance that there would not be a catastrophic failure of a booster during the first 2:09 minutes of flight.

The boosters cannot be shed from the orbiter until they burn out. Thus, in all likelihood, the Jan. 28 mission was doomed the moment its two solid fuel boosters ignited, three seconds after the firing of the shuttle main engines.

By the time of the roll maneuver, which put the orbiter into its familiar upside-down attitude as it arched over the Atlantic, an errant flame may have already been searing its way through a crack in the propellant headed for the booster’s outer shell. Once it reached the tank wall, it was blocked by a layer of asbestos and silica insulation, separating the fuel from the half-inch-thick steel casing. Under a flame half the temperature of the sun, the casing could resist only momentarily.

A long-lensed NASA camera on Playa Linda beach, one of 50 tracking the vehicle in the launch area, took the first picture of the “unusual plume” on the outside of the booster.

To many who saw the picture days later, it appeared that the fire had broken through at a seam where the right booster’s lowermost segments were fitted together like links of sausages after the rockets were shipped to Florida.

The question of whether the flame burned through the casing or erupted from the seam, or “field joint,” presented several possibilities as to the ultimate cause of the tragedy.

A classical burn-through, some engineers said, could have been precipitated by a lump in the fuel mass, causing one area to burn abnormally and create a hot spot. Or it could have been caused by a crack if a segment had been damaged in handling. The seals themselves might have been defective.

Some engineers speculated that the frigid temperatures before the launch had caused the flammable putty sealing the joints to separate, increasing the chances for a breakout at the seam. Temperatures dropping to 27 degrees the night before launch had caused concern among NASA engineers, among solid fuel experts in Utah and at the offices of Rockwell International in Downey.

Icicles hanging from the superstructure around the launch pad during the early morning created a visceral uneasiness among some old-timers unaccustomed to seeing ice in Florida. “I didn’t want to see them launch in that cold weather,” one NASA employee said later. “I’d have rather waited. What the hell is one more day in the life of man anyway?”

On Thursday, NASA officials told President Reagan’s blue-ribbon commission investigating the tragedy that engineers from the space agency and the manufacturer of the booster rockets had discussed the temperatures before the launch and agreed to proceed with the countdown. “We had no concerns for performance safety of the flight articles, nor do I at this time,” said Arnold D. Aldrich, manager of NASA’s Space Transportation Systems Program.

Even before photographs showed the plume break through the side of the booster about 13 seconds before the explosion, speculation had centered on the booster. “When I saw it on television,” a Huntsville engineer recalled, “I said, ‘Burn-through.’ ”

Such immediate suspicion was not without basis. During a launch of Challenger in September, 1983, a solid booster came within about 14 seconds of a burn-through in its exhaust nozzle. Analyzing the episode later, Lt. Gen. James Abrahamson, then NASA’s associate administrator for manned spaceflight, said simply: “God smiled on us.”

But in the end, it was not the booster alone that blew the shuttle to smithereens, bringing the American space program to a standstill and ending indefinitely the ambitious notion of taking untrained civilians into orbit. The liquid hydrogen technology that made it possible for the United States to put men on the moon played the ultimate role.

When the shuttle system was designed, its greatest technological leap forward, and the most difficult obstacle of all, was development of a new hydrogen-burning main engine that would continue to propel the orbiter after the solid boosters burned out and parachuted to the sea.

The highest-energy rocket propellant known to man, hydrogen is also one of the most temperamental because of the extremely low temperatures involved in handling it, and because it is necessary to use highly explosive liquid oxygen with it.

From the beginning, said a former NASA official at the Marshall Space Flight Center in Huntsville, the main engine was “the long pole in the tent.” Its small turbo-pumps generated thousands of horsepower. On ignition its turbines sped to thousands of revolutions per minute in a fraction of a second.

Until the shuttle, the United States had developed only two much-smaller hydrogen-burning engines. But with the shuttle, NASA and Rockwell International produced a new throttleable, hydrogen-burning rocket engine, twice as powerful as the one that fueled the upper stages of the Saturn 5 moon rocket. In early testing, pumps failed, blades broke, engines exploded on test stands and development fell far behind schedule. The engines also fell far short of NASA’s thrust requirements, but work continued to eventually bring it to 375,000 pounds of thrust and ultimately to 417,000.

The engines required a mammoth external tank bolted to the orbiter’s underbelly, the tank that erupted into the ghastly fireball that destroyed Challenger.

Until the final minutes before the spacecraft’s delayed liftoff, the hydrogen tank on the bottom and the oxygen tank at the top of the huge brown external tank were continually replenished to make sure they were full at liftoff.

Fabricated of aluminum alloys from stem to stern, the $20-million tank stays with the shuttle until it slips into orbit, then tumbles into the atmosphere, burning as it plunges toward the Indian Ocean. In an aging government plant outside New Orleans where workers have assembled moon rockets, PT boats, and Army tank engines, the fuel tanks are turned out faster than shuttles can use them, their massive skins rolled into shape like cigarettes.

After the early test flights by the shuttle Columbia, NASA felt it was safe to reduce the tank’s weight in order to get more payload into the orbiter. And so on its first launch three years ago, Challenger flew with a tank whose weight had been reduced by 10,000 pounds. Most of the weight loss was accomplished by “slimming down” structural components, but 600 pounds were saved by leaving off a coat of white paint, accounting for the tank’s distinctive brown color.

The scenario most thoroughly explaining how the super-cold fuel tank turned into a bomb holds that the path to destruction began with the searing flame licking on the tank, rapidly creating violent convection currents and mounting pressures within the hydrogen and oxygen spheres, blowing open pressure-relief valves.

Photographs taken of the stricken Challenger at 72:77 seconds indicate that the failure of the brown outer tank had begun--oxygen streaming from the top of the tank, hydrogen from the bottom.

Ignited by the fire from the leaking solid rocket, an explosion went off near the center of the tank a tenth of second later, the only thing in the deadly sequence that the astronauts might have heard or seen in the instant before their deaths.

This first burst sending flames over the fuselage apparently came when the spreading fire set off the range safety package--an explosive mounted on the tank to purposely destroy the vehicle in the event it strayed out of control toward a populated area during the first seconds of flight.

Ironically, the destruct package had been added to the tank in an effort to improve safety. When the vehicle was designed, plans called for destruct packages to be placed only on the boosters. Before Columbia made its first test flight, destruct packages were ordered placed on external tanks--just to be sure.

That decision, along with thousands of others, gave the shuttle a schizophrenic identity. As scientific, budgetary and political forces converged, compromise became the rule. And for the shuttle, such compromises were taking place even before the formal birth of the program.

59.82 seconds

1. An “unusual plume” has erupted from the lower section of a solid rocket booster.

72.85 seconds

2. The plume expands. Large flames dart underneath the Challenger, driven by shock waves or by the ignition of escaping hydrogen and oxygen gas.

72.95 seconds

3. Flames begin to engulf the Challenger. A hot spot appears underneath the forward section of the orbiter, possibly the result of detonation devices igniting on the external tank.

72.98 seconds

4. The vehicle explodes as hydrogen and oxygen mix in the nose of the external tank. Resulting fireball envelops shuttle.

POSSIBLE REASONS FOR BOOSTER FAILURE The booster rocket is powered by solid fuel designed to burn at an even rate along its entire length from the empty core outward to the casing. NASA officials are investigating the possibility that the fuel or the casing failed, producing an “unusual plume” of fire that eventually ignited the huge liquid fuel tank. These are the main theories that investigators are pursuing.

1) Freezing temperatures at the launch pad may have caused the rubber-like fuel to drop below allowable limits and cracks developed in the fuel. Cracks, in turn, may have created “hot spots” that burned through the casing.

2) Workers at Kennedy Space Center may have assembled the booster rockets improperly, leaving weak points at the seams or creating empty spaces allowing flames to explode outward toward the casing.

3) Workers at the Morton Thiokol plant in Utah may have improperly mixed the fuel, again possibly creating “hot spots”.

4) Resonant pressure waves inside the rocket may have jarred the solid fuel mixture, cracking the fuel during the flight.