Imagine a mixture so dangerous that a tiny spark could trigger a massive explosion.

Now imagine making millions of pounds of the stuff, pouring it into canisters, and trucking those canisters from northern Utah to Florida.

This is the job of Thiokol Propulsion, makers of rocket motors for NASA, the U.S. military and commercial launching companies like Lockheed Martin.

Through an automated process requiring relatively little human assistance, Thiokol mixes chemicals in giant bowls. What comes out is a gummy substance called solid rocket fuel, a highly inflammable substance that has the consistency of a pencil eraser.

The process is a lot like making a cake. But no one gets to lick the mixing beaters.

First, powders of aluminum -- the actual fuel -- and ammonium perchlorate -- an oxidizer -- are mixed.

Then, a liquid is stirred in and the mixture is hardened.

Very carefully, the contents are poured into casings, which become rocket motors and boosters.

Then, with a delicate touch, the fuel is either shipped or sent by train from Utah to the launch site, usually Florida's Cape Canaveral.

The process, though considered routine, must be treated with extreme caution.

"The beauty, or in some cases the danger, of solid propellant is that you carry the fuel and the oxidizer in one mix, so that once its started it doesn't need an external source of oxygen to burn," said Thiokol's business development manager Lee Rulis. "It's contained in the fuel itself."

"You're talking a lot of energy in a confined place," Rulis said. "It's something you don't want to mess with. Like anything else, it can get out of hand."

Once, it did.

In the mid-1980's, flames erupted at Thiokol's plant, scorching a mixing bowl, after a bit of fuel that dripped out of the bowl ignited.

But that's Thiokol's only major accident at the plant. And the plant operation was thoroughly reviewed afterwards.

Solid rocket propellant is extremely useful in the aerospace world. Unlike liquid fuel, it doesn't leak.

And solid propellant can be stored for decades, making it appealing for military uses like missiles. (Thiokol is currently refurbishing the motors of the Air Force's Minuteman intercontinental ballistic missiles, which were originally developed during the late 1950s.)

However, liquid propellant is more powerful, making solid propellant a good choice for secondary boosters.

Thiokol makes the world's largest solid rocket motor ever flown: the space shuttle's reusable solid rocket motor.

The motor is filled with 1,107,000 pounds of propellant, and generates an average thrust of 2,600,000 pounds for around 120 seconds.

After launch, the motor is recovered at sea and studied for abnormalities.

Annually, NASA pays Thiokol about $400 to $500 million, the military around $75 million, for their motors.

Thiokol also makes strap-on boosters for Lockheed Martin's Athena rocket, Japan's H 2-A rocket, among others.

It's been a slow year for Thiokol -- only two shuttles have launched so far, with one more planned for December.

But on a busy year, the Thiokol staff works virtually non-stop, year-round, pouring and mixing ingredients.

The work gets routine -- or as routine as handling an explosive material can get.

"We're very careful," Rulis said. "Each move is considered a critical move."