The solid rocket booster now suspected of playing a key role in the Challenger disaster has long been considered one of the most reliable -- if complex -- components in the shuttle system.

Its principal parts -- fuel and rocket motor -- are based on technology used for more than two decades in the American space arsenal.

The fuel, which has the consistency of a pencil eraser -- hence the name, "solid" -- is relatively safe to handle compared with the exotic and volatile gases and fuels used by orbiter vehicles.

The half-inch-thick steel casing on the solid rocket booster -- called an SRB by NASA -- makes it "strong as a bridge," compared with the relatively fragile external fuel tank and the winged orbiter, according to engineers here at the Kennedy Space Center.

Yet, National Aeronautics and Space Administration photos released Saturday night indicate that a major failure occurred in the casing of the right-hand SRB about one minute into the Challenger's ill-fated flight. Flame can be seen spurting through the outer skin of the rocket.

The SRB is made of 11 individual, reusable steel segments that are stacked and bolted together to assemble the two SRBs per shuttle flight.

So reliable is this design that NASA is counting on reloading and reusing the powerful rockets for up to 20 shuttle flights each before retiring them. And so crucial is flawless SRB performance to the shuttle launch that there is no fail-safe crew escape plan during the first two minutes, 5 seconds of flight during which the SRBs are firing.

"SRB failures are not things you really like to think about," said David Pendley, a former NASA safety engineer. "If you get a burnthrough, it would happen so quick it would be hard to respond to."

Although NASA acting administrator William Graham refused to speculate on the import of the tongue of flame shown in the photographs released Saturday night, it is widely believed here that review board investigators now think a white-hot jet of flame from the SRB burned a hole in the giant external tank carrying liquid hydrogen and liquid oxygen, causing it to explode.

The SRB's segmented construction, chosen by NASA because it makes the rockets reusable and therefore cheaper, is the center of attention. A spokesman for Morton Thiokol, the manufacturer, declined to comment on information released by NASA. "We intend to comply with NASA's request that they be allowed to release information about the situation," the spokesman said.

One SRB is fastened with explosive bolts to each side of the external tank, atop which rides the orbiter spacecraft. The SRBs are the shuttle's primary booster, providing 71 percent of the thrust needed to get the 4.6 million-pound shuttle system off the ground and headed toward orbit.

Standing 149 feet high and 12 feet in diameter, they weigh 1.3 million pounds apiece, and each packs 1.1 million pounds of fuel called PBAN. The fuel is composed of ammonium perchlorate as oxidizer, aluminum powder as fuel and iron oxide for a catalyst. The mixture is bound together by a rubberlike polymer.

PBAN propellant burns at 5,800 degrees Fahrenheit, four times the temperature needed to burn through the aluminum skin of the external tank if a jet of flame was directed toward the tank.

The dramatic NASA photos have centered attention on the way the SRBs are made. Unlike most solid fuel rockets, which are encased in a single strong steel casing, the SRBs are assembled in cylindrical sections that are piled atop each other and bolted together in a "stack" before launch.

There are four separate propulsion segments, each containing two sections of solid fuel. These main segments are fastened together with 177 steel pins, and flanges that cover the joint.

A guidance segment and a nose cone with three huge parachutes packed inside stand atop the rocket. The rear end contains a flare skirt to guard against rocket exhaust, and a nozzle.

Once ignited, the propellant cannot be extinguished. But when the "burn" ends, the explosive bolts are fired, and the rockets fall away from the shuttle, which continues riding the external tank into orbit.

Meanwhile, the SRBs pop their parachutes, and land in the Atlantic to be snared by special tugs and floated back to shore. They are disassembled, cleaned and shipped back to Morton Thiokol for refurbishing. New fuel segments and the restored casing sections are shipped to the space center here, restacked and mated to the next external tank and orbiter. The steel casings are considered interchangeable, with different segments stacked up for different flights.

A local newspaper reported today that last November, workers assembling the left-hand SRB on an earlier Challenger flight damaged a segment. According to Florida Today, a NASA inquiry board found that the workers involved in the accident had failed to follow proper assembly procedures, or to report the incident adequately.

NASA spokesmen today confirmed the incident, which did not involve the booster used last week by Challenger, but said they had not seen such an official report.

In the only previously documented serious incident involving an SRB, NASA reported a nozzle from one of the rockets had almost burned through during its launch phase. It caused no problems with the launch, however. NASA blamed a defect in the nozzle, and said corrective action has been taken.