"Yeah, sure, unless the pressure drop is in these here strands of spaghetti," Jeremy said. He gestured toward one of the clumps of mangled tubing behind the engine bell. "They channel the LH2 into the walls of the engine nozzle and combustion chamber before outletting them to the preburners--"

Nimec raised his palm to stop him.

"Whoa," he said. "Back up a second. I'm still not clear on how less equals more in this instance."

"That's 'cause a very important word I used to describe the state of the liquid hydrogen must've slipped past you," Jeremy said. "Namely cryogenic."

Annie saw Nimec holding back his irritation at being patronized.

"As Jeremy said," she broke in, "the SSMEs operate with a high level of efficiency. That's in part because the propellants are used for multiple purposes. To remain in a liquid state hydrogen has to be kept super-cold ... to give you an idea of how cold, bear in mind that it vaporizes at any temperature above minus 423 degrees Fahrenheit. As a solution to the problem of critical engine overheating, the designers of the SSMEs found a means to divert some of the liquid hydrogen fuel throughout the engine with a system of ducts before it finally makes its way to the preburners. There's a pair in each of the main engines, and their function is to ignite the very hot hydrogen vapors that result from the combustion process before they can accumulate and ignite in the engine bell. If you ever watch a video of a liftoff in slow motion, you'd can see the preburners shooting the gas out below the bells as thousands of tiny fireballs."

Nimec looked at her. "So you're telling me that a significant dropoff in LH2 pressure would have caused the engine to overheat and the preburners to fail... leading to an explosion of the free hydrogen vapors in the engine bell."

"That's what Jim was telling us. Or trying to. He would have known where in the engine the liquid hydrogen pressure had critically decreased just by looking at a gauge on his instrument panel. But with everything happening so fast ... the cabin filling with smoke ..."

"He never finished saying what he wanted to."

"Which was that the LH2 pressure was dropping in the preburner ducts."

Their gazes met. Nimec saw the moist, overbright look in her eyes, realized she was fighting back tears, and found himself on the verge of reaching out with a comforting hand. Instead he stiffened, caught wholly off guard by the impulse.

Turning to Jeremy, he said, "When we were on the tram you mentioned the difference between knowing what happens given a certain set of conditions, and understanding why it happens."

Jeremy visibly wavered.

"I was talking about snowflakes," he said.