Hunt for Fatal Flaw Cuts Path From Utah to Florida

When Challenger exploded 73 seconds after liftoff Tuesday, the proximate cause was a massive malfunction of one of the two solid rockets designed to boost the shuttle to an altitude where its own engines could complete the task of lifting it into orbit.

The ultimate cause of the disaster, however, lies elsewhere--somewhere along a 2,500-mile path linking the missile’s manufacturer in Utah to workers in Florida who assembled the largest solid-fuel rocket flown by man. Precisely where the fault occurred may not be known for quite some time, as William R. Graham, acting NASA administrator, suggested Sunday.

The solid rocket boosters--generally known by the acronym SRBs--are an outgrowth of the rockets originally designed for the Minuteman and Poseidon C3 missiles. NASA chose the solid fuel technology for its shuttle boosters because it was thought they would be much safer than liquid fuel rockets.

Each SRB begins life as 11 identical casings made of a high-performance steel. Each casing is approximately 12 feet in diameter, 13 feet long and less than half an inch thick. They have a flange at each end that is part of a so-called “clevis” joint, used for bolting the sections together.

The casings are reusable. When an SRB is recovered after a launch, it is disassembled into the component casings, which are cleaned and used for the manufacture of new SRBs. Some of the casings used on the Challenger’s SRBs had already flown on two or three missions.

These casings are one source of speculation concerning the Challenger explosion because other high-performance steel alloys have been found to become brittle when exposed to heat. Such embrittlement is thought to be the cause, for example, of a jet engine failure that led to the crash of a British Airtours Boeing 737 in Manchester, England, in August, 1985.

The Federal Aviation Administration has subsequently required that most such jet engines be examined periodically to check for embrittlement. The metal in those engines is exposed to heat for much longer periods than the SRB casings, but the casings are exposed to higher temperatures than the jet engines.

The initial stages of manufacture of the SRBs take place at the Thiokol Chemical Corp. division of Morton Thiokol Corp. at its plant in Brigham City, Utah.

Bolted Together

There the 11 segments are bolted together to form four larger segments. This is accomplished by slipping a U-shaped piece, the clevis, over the flanges on two adjacent pieces and joining them together with 177 bolts.

The inside of the casings are then coated with a thin insulating lining composed of asbestos and silica held together by a polymer.

A precisely shaped core is then inserted into the middle of the casing and the space between the casing and the core is filled with the fuel, which solidifies in place much like Jell-O solidifies in a mold. The core is then removed to form a combustion chamber.

The fuel itself is a finely ground aluminum powder that burns at a very high temperature, about 5,900 degrees Fahrenheit. It is combined with a much larger quantity of ammonium chlorate, which breaks down under heating to produce the oxygen that allows the aluminum to burn. Both are dispersed in a polymer that solidifies to a texture much like that of the eraser on a pencil.

If this polymer does not “cure” properly, the fuel might contain an anomalous region that would burn more quickly than the rest of the fuel. That could permit the exhaust to burn through the steel casing. Such a burn-through might have caused the “unusual plume” emanating from the Challenger’s right-side SRB.

Quality Control

A poor “cure” should be caught by quality control testing at Thiokol. A similar failure in the curing of a polymer that coats the inside of the SRB nozzle was not detected, however, and nearly caused a burn-through of the nozzle on Challenger’s third mission.

Near Fill-Joint Seam

The segments are then shipped by rail to the Kennedy Space Center in Florida. There the flanges of the four main SRB segments are bolted together and the clevis joint is covered with a fiberglass tape and an epoxy resin for protection and additional strength. This site is called a fill-joint seam. It is at or near the fill-joint seam that the plume was observed.

Significantly, no cement or filler is used to join the fuel segments together. This had been a source of concern to the original designers of the shuttle, who feared that a gap might be left between the segments during assembly. Such a gap would allow rocket exhaust to eat through to the steel casing and burn a hole through it.

The activities of the work crew at the Kennedy Space Center also have been questioned. In November, NASA reported that one of the booster segments destined for the Challenger mission had been damaged in an “industrial accident.” Workers using a large crane to remove a 3.5-ton handling ring from one of the segments reported hearing a “sharp crack.”

The damaged segment was put into storage and replaced with a new one. A report of the investigation into the accident, however, concluded that the workers assembling the booster were performing tasks that they were not trained to do, that they handled critical components with cranes that often did not work properly, that they ignored procedures and that they frequently did not report problems as they occurred.

Low Temperatures

Speculation about the malfunction of the solid rocket boosters also has focused on the fact that immediately before the scheduled launch of Challenger, the space center suffered an unusual 10-hour period of below-freezing temperatures that delayed the planned liftoff by two hours.

Because of these temperatures, the solid fuel was much colder than normal at the time of ignition. Solid fuel rockets normally are subject to a great deal of vibration during firing, but the SRBs seemed to have an unusually severe vibration problem, said John Flandrow of the Georgia Institute of Technology, a former consultant to Morton Thiokol. The coldness of the fuel at the time of ignition, Flandrow told The Times, undoubtedly made the vibration much worse.

The Challenger explosion occurred during that part of the flight, about one minute after liftoff, when the craft is undergoing the maximum physical stress caused by the combination of the thrust from the engines and the resistance from dense air near the Earth’s surface. If, in fact, the SRBs were vibrating much more than normal, all components of the propulsion system would have been subjected to unanticipated--and potentially lethal--stress.

Presumably as a result of that stress, at approximately 58.32 seconds after liftoff--according to the pictures released by NASA Saturday--a small hole appeared in the side of Challenger’s right SRB, from which flames emerged. The flames seemed to be near the area where two segments had been joined together at Kennedy Space Center, but it is not clear if the rupture was at the fill-joint seam or simply close to it, NASA Administrator Graham said.

‘Appeared to Grow’

In subsequent pictures, according to Graham, “the flame appeared to grow and grow . . . until the explosion.” The flames, in fact, seemed to curl around the SRB until they could be seen in the original videotapes as a bright flash of light immediately preceding the explosion.

Press reports in the last two days have cited unofficial NASA sources as stating that the SRB registered a slight drop in thrust, about a 4% loss, a few seconds before the explosion. Graham, however, said that controllers in Houston had “detected nothing anomalous” in the SRB’s thrust and that it was within the “normal range of variation.”

The new pictures clearly show the leaking exhaust flames licking at the side of the external fuel tank. Even though the flame apparently did not eat through the tank’s skin immediately, it presumably heated the hydrogen fuel and caused pressures within the tank to increase sharply.

Sources at Rockwell International, the shuttle’s primary manufacturer, told The Times Sunday that a preliminary study of telemetry from the shuttle shows that this increased pressure popped pressure relief valves on both the hydrogen and oxygen chambers inside the large external tank, allowing some of the liquid to escape.

The original videotapes, in fact, seem to show some kind of fluid streaming backward along the fuel tank immediately before the explosion and feathery flames, which may have been burning hydrogen around the tail of the SRB.

Eventually, the exhaust presumably burned completely through the side of the fuel tank, igniting the hydrogen within. Another theory is that it may have ignited a self-destruct package that would be used to destroy the vehicle if it were heading toward a populated area. FOCUS OF THE SHUTTLE INVESTIGATION Newly released NASA photos show an “unusual plume” of flame flaring from Challenger’s right solid rocket booster just before the explosion. It is believed that the plume shot outward from the central core either through a seam between assembled booster sections (A) or through a crack in the booster hull (B). Once the flame shot through the booster hull, these are the possibilities under investigation: 1) The heat from the flame caused liquid hydrogen in the liquid fuel tank to expand to a point of explosion. 2) The flame burned through the skin of the fuel tank, igniting its contents. 3) the flame triggered a detonating device that was to be used only if the shuttle careened off course toward a populated area. 4) Excessive vibration--and cold temperatures at the time of launch--could have contributed to a structural failure of the booster rocket.