A space shuttle is a relatively fragile compartment housing crew and cargo and perched precariously on a huge propulsion system that is essentially a controlled bomb.
The bomb is supposed to burn its fuel at a furious but controlled pace for 8 1/2 minutes, hurling the shuttle into orbit.
According to aerospace scientists outside the space agency, many of whom have spent the last two days analyzing Tuesday's explosion at a distance, the blast almost certainly resulted from a failure in one or more of many shuttle components that are supposed to control the rate at which the fuel burns.
Experts consulted by The Washington Post said they had viewed videotapes of the flight many times and concluded that something must have caused liquid hydrogen fuel to leak outside the controlling system and become ignited by heat from Challenger's fiery exhaust.
Such a leak could have occurred in several ways but, to understand them, it is necessary to know how the shuttle is put together.
Its plane-like main part, called the orbiter, carries the crew and cargo and has three of the five rocket engines used to climb into orbit but none of the fuel.
Fuel for those three comes from the biggest component of the launch vehicle, the huge external tank to which the orbiter is fastened by metal struts near the nose and at the base. Inside the external tank are two smaller tanks, one above the other.
The lower tank, 97 feet long and about 25 feet in diameter, contains 383,000 gallons of liquid hydrogen, which, like gasoline, burns when vaporized. But, because rapid burning requires enormous quantities of oxygen, an upper tank, also about 25 feet in diameter but only about 53 feet long, contains 143,000 gallons of liquid oxygen.
Both liquids flow out of the tanks through pipes that reach to the bottom of the external tank and then branch across, through the air, to the orbiter's engines.
Also fastened to the big tank are two pencil-shaped solid-fuel rockets, each with its own nozzle at the base. These operate independent of the liquid-fueled system.
At launch, all five engines burn as fast as they can. The solid rockets, once ignited, burn at maximum rate without further control. This is why they continued flying after the explosion, careering out of the fireball. Igniting the liquid-fueled engines, however, requires an elaborate control system in which independent 77,000-horsepower pumps deliver hydrogen fuel and oxygen to each engine's combustion chamber.
At full throttle, each engine consumes 16,000 gallons of fuel per minute. Expanding gases formed by the burning push on the combustion chamber walls with a pressure of 3,200 pounds per square inch. This is the force that, along with solid rockets' thrust, lifts the spacecraft and accelerates it into orbit.
"You've got to realize that, with all the tremendous forces operating during a launch, there are any number of places where something could go wrong," said Michael Micci, a professor of aerospace engineering at Pennsylvania State University. "One of the possible weak points, it seems to me, would be the point at which the external tank is connected to the orbiter."
Micci recalled that, shortly before the explosion, the shuttle's engines had been throttled down to ease passage through a brief period in which vibrational stresses of acceleration act most violently on the craft. Just as the shuttle was being throttled back to full power, the explosion occurred.
"It is possible that the throttle-up caused a pressure pulse that traveled up the fuel line to the external tank," Micci said. "If there was a weakness in the line or the tank, it could have ruptured."
If so, hydrogen could have leaked outside and been ignited by heat from the normally burning engines. Micci speculated that this may have caused the brief flame seen on videotapes just before the explosion. The small fire, he said, could have enlarged the rupture, spilling enough fuel to ignite the fireball.
Another speculation, offered by a former defense science official who has talked with National Aeronautics and Space Administration engineers, is that a fuel pump may have broken apart during the attempt to throttle up. If so, pieces might have been thrown sideways to puncture a hydrogen line.
Micci discounted the pump theory, saying that if one had broken up, fuel would have begun pouring through the engine and the huge explosion would have been seen to start there. Micci said that, as far as he could see from the videotapes, all three engines appeared to be firing normally when the explosion started.
Yet another possibility was offered by Herman Krier of the University of Illinois. At the time the accident happened, he said, Challenger was passing through a well-known period in which a peculiar effect of aerodynamics causes hot exhaust gases to billow upward and envelop the lower part of the vehicle, including the hydrogen tank.
"This is perfectly normal. We've seen it on practically every space flight," Krier said, "but what if there had been a defect in the insulation around the external tank?"
The big tank is coated with a one-inch thick layer of synthetic, non-inflammable foam to insulate the supercold liquids inside. If a piece of insulation had broken off, Krier said, the exhaust heat could have damaged the tank, possibly causing a rupture.
Technicians are believed to have performed the usual inspection of the insulation just before liftoff, but there has been speculation that chunks of ice on the shuttle, formed during subfreezing weather that gripped the space center before and during the launch, might have been jarred loose during liftoff to tumble down and gouge out a piece of insulation.
Most experts who talked to The Post discounted the ice theory but agreed that part of the insulation could have come off for other reasons, such as imperfect bonding to the tank's metal wall.
Micci also cited evidence that more than one rupture of the external tank may have occurred. Less than a second after flames appeared at the base of the tank, another fireball appeared between the tank and the orbiter, near the point where the orbiter is attached by metal struts.
"That looked like a second rupture to me," Micci said. "That's one of the points where the forces of the throttle-up are transferred to the tank. Something could have happened there."
The external tank, normally jettisoned and destroyed just before the shuttle reaches orbit, is generally regarded as receiving most of the forces of a launch. It must absorb the thrust of the solid rockets and, more critically, that of the liquid-fuel engines, which is transferred through the connecting struts. Official NASA documents call the tank "the structural backbone of the shuttle system in the launch configuration."
Except for the throttle-down period of a few seconds, the tank must absorb a total thrust of 6.4 million pounds. The tank wall is made of an aluminum alloy that varies in thickness from less than one-tenth of an inch to more than two inches.
Although the solid-fuel rockets may have contributed to the heat that ignited the leaking hydrogen, most experts now do not believe that they failed, causing the explosion.
These rockets consist of heavy cylindrical steel casings, inside which are stacked hollow, cylindrical blocks of solid rocket fuel. At liftoff, this fuel is ignited and burns over the entire inner surface of the hollow core. Because the resulting high pressure gases can escape only downward, they push the rocket upward in a kind of continuous recoil.
There has been speculation that a flaw in a fuel block, or between two blocks, allowed the flame to burn a hole sideways, melting the steel casing and allowing a jet of flame to melt into the adjacent hydrogen tank.
"I don't think this happened," Krier said. "If there had been a jet of flame coming out the side of the solid rocket, it would have thrown the thing into a tumble. We didn't see that. Both the solids flew right on pretty well."
Micci agreed, saying any such jet would have sent the released solid rockets "spinning like a pinwheel. My feeling is they weren't involved, at least not directly."
All of the rocket experts who talked to The Post emphasized that their thoughts were purely speculations but also said they doubted NASA would ever be able to do any better.
"I really doubt they'll ever find out what happened," Krier said.