Shuttle Boosters Like A Controlled Bomb
Thursday, January 30, 1986
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.