‘Unusual’ Flame Believed Cause of Shuttle Explosion : NASA Photographs Reveal ‘Plume’ on Right Booster Seconds Before Blast

National Aeronautics and Space Administration photographs released Saturday depict an “unusual plume” flaring from the side of the space shuttle Challenger’s right solid rocket booster 13 seconds before the spacecraft exploded.

The plume appears to shoot from a seam in the booster rocket and onto the skin of the large, expendable fuel tank attached to the spacecraft during liftoff.

NASA sources explained that it is believed the 6,000-degree heat from the plume most likely caused liquid hydrogen inside the fuel tank to expand. When the tank could no longer contain the expanding fuel, it exploded.

Worst Disaster

All seven crew members were killed Tuesday in the worst disaster in the history of American spaceflight.

Release of the photographs and a companion video film from the same angle represented the first public indication from NASA of what direction an internal investigation into the tragedy has taken.

The new pictures also served to support the theory of many outside experts about what caused the Challenger to explode 73 seconds into its flight.

NASA spokesman Hugh Harris nevertheless cautioned against drawing final conclusions from the evidence.

“The cause is still unknown,” Harris said, “and neither the (internal investigative) board nor NASA will speculate as to the cause or effects of this observation.”

In a statement accompanying the photographs, NASA said only that members of its interim investigative board--composed mainly of its own ranking administrators--”observed what appears to be . . . an unusual plume in the lower part of the right (solid rocket booster)”

However, with the photographs indicating that escaping combustion from the solid booster rocket triggered the explosion, the next investigative step will likely be to determine what split the booster’s wall.

Design Questions

In addition, the photos raise questions about the design of the shuttle’s elaborate computerized sensing system, which apparently failed to provide a warning to the crew that an event with such catastrophic potential was developing.

The color, computer-enhanced pictures may be some help in determining what caused the side-split in the booster. The frame in which the plume is first clearly visible was taken 59.82 seconds after launch--a critical phase in which the spacecraft and rockets begin to undergo extreme aerodynamic pressure as they head through the sound barrier.

Gary Flandrow of the Georgia Institute of Technology noted in a telephone interview Saturday that excessive vibration of the booster rocket could have contributed to a structural failure. Such vibrations are common to solid rocket motors, said Flandrow, who has been a consultant to the manufacturer of the boosters, Morton Thiokol Corp.

Cold temperatures at the time of ignition would tend to make the vibrations stronger, he said.

The temperature at the Kennedy Space Center had been below freezing for 10 hours the night before the Challenger launch.

The twin, 150-foot booster rockets are constructed out of 11 hollow steel cylinders approximately 12 feet in diameter and 13 feet long. They are bolted together to form four segments. The four segments are bolted together to form the complete rocket, and the outside joints are sealed with an epoxy resin.

The NASA pictures appeared to show the plume escaping through the first seal on the lower right booster. This seam also is where brackets attach the booster to the fuel tank, which could contribute to stress.

In addition to the conjecture that the flare overheated the tank, a NASA source said it is also possible that the flame could have eaten through the one-inch-thick skin of the fuel tank with equally devastating results.

Explosive Device

There was discussion among the board, a source said, about whether the fire, which appeared to eat forward to the front of the external tank, triggered an explosive device that is supposed to be detonated by remote control should the shuttle careen off course toward a populated area.

However, the source said, board members have concluded that, once the chain reaction of explosion, rupture and fire reached that point, the spacecraft already would be doomed.

The photographs were taken by cameras at NASA Tracking Station No. 10 at Playa Linda Beach north of the Kennedy Space Center, one of dozens of cameras that track each mission.

NASA spokesmen declined to say whether any of the data broadcast by the shuttle in its brief flight collaborates the evidence in the photographs. In the aftermath of the explosion, flight controllers at Mission Control in Houston said such data indicated nothing but a perfectly normal flight.

A report in the New York Times Saturday said that more detailed studies of the data by NASA investigators found that one of the boosters had suffered a sudden drop in propulsive power about 10 seconds before the blast. The report also said that, when the booster lost power, the shuttle’s five engines--the three liquid-fueled engines at the back of the orbiter and each of the two solid-fuel engines--immediately swiveled their nozzles to one side to keep the spaceship flying straight.

The report appeared to be consistent with the possibility of a structural failure that would allow rocket exhaust to escape from the booster’s combustion chamber.

Search Continues

Meanwhile, more than 30 military vessels and planes pressed on with their bleak task of finding and recovering debris from the Challenger. Beneath the waters, a submersible camera hunted unsuccessfully for a large object that NASA officials had said could be the pressurized compartment where the crew rode.

A computer-enhanced analysis of another videotape of the flight by United Press International, however, showed that Challenger’s cabin section was ripped apart by a smaller blast an instant after the main explosion, making it appear unlikely the section will be found intact.

NASA officials disclosed that one piece of wreckage already had provided a small but intriguing piece of the sad puzzle of what went on in the final moments of the flight.

Examination of a nose cone from one of the Challenger’s twin solid booster rockets showed that its three separation motors had not been activated, indicating that Commander Francis R. (Dick) Scobee was unaware of the pending grave danger. Had he been, he would likely have reacted to blow away the solid rocket boosters.

But the revelation that the separation devices had not been fired presented clear evidence that this did not occur and made it seem more likely the crew never knew what hit them.

Currents continued to push wreckage from the shuttle north, and more ships and airplanes were brought into the effort to recover debris. The biggest find on Saturday occurred 100 miles offshore of Savannah, Ga., where an orange-colored conical object was spotted.

Times Science Writer Thomas H. Maugh II contributed to this story from Los Angeles.