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 Nov. 6, 2002: They can lift 375,000 pounds and outrun a speeding bullet. They consume fuel that's colder than interstellar space, yet their exhaust is as hot as a small star. Three of them working together unleash as much power as 23 Hoover dams.
They're the space shuttle's main engines (SSMEs). Onlookers who have seen the shuttle take off and land many times might be excused for thinking the engines must be old technology. What else could be so reliable? In fact, says NASA engineer Everett Runkle, the shuttle's engines are cutting-edge. "The SSMEs are the best-performing engines on Earth," he says. They have a specific impulse (a measure of fuel efficiency) of 450 seconds--higher than any other chemical rocket. "If we wanted a better combination of power and 'gas mileage,' we would have to go to nuclear propulsion." Above: A space shuttle main engine undergoes a full-power test firing at the Stennis Space Center in Mississippi. [more] Even more remarkable, perhaps, is their resiliency. Consider this: Almost every rocket engine ever launched lies in pieces at the bottom of the sea or scattered across remote terrains. "Most rocket engines are built to fly just once," explains Runkle. "They deliver their payload to space and then fall back to Earth." An Apollo moon rocket is a good example. "When we launched Saturn V rockets during the Apollo program," he recalls, "the first two stages of the rocket would be discarded during ascent. They ended up in the Pacific Ocean. A third stage, the S-IVB, was either crashed into the Moon or sent into orbit around the Sun." The shuttle's main engines, on the other hand, are not disposable. "We bring 'em intact back after every trip," he says. Most have flown 20 or more times and they are designed for 100 missions. How do these engines work? "It's a little like a car engine," he answers. "In an automobile, gas is injected into a combustion chamber and a spark plug lights it off. The explosion pushes a piston, which provides force to move the car."  Above: A schematic diagram of a space shuttle main engine. [more] "The shuttle uses a different kind of fuel: liquid hydrogen (LH2) and liquid oxygen (LO2). The two are sprayed as fine mists into a combustion chamber where a high-tech spark plug starts the fire going. It's a non-stop fire fed by a continuous stream of LH2 and LO2 droplets." Temperatures in the combustion chamber can reach 6,000o F (3300o C), or about the same as the surface temperature of an M-class star. That's ironic considering the temperature of the hydrogen fuel, -423o F (-253o C), is colder than most of the gas which fills interstellar space. The liquid oxygen at -298o F (-183o C) is cold, too, but not so cold as the hydrogen. Hot gases stream out of the engine's nozzles and thus provide thrust. The exhaust is mostly water vapor--hence the white clouds that billow around the engines during a shuttle launch. "Our engines are very clean," notes Runkle. "They don't pollute the environment."  Left: The shuttle's three main engines are fed liquid oxygen and liquid hydrogen from the giant orange-colored external fuel tank.
Although the shuttle's engines are the creme de la creme of chemical rockets, NASA engineers are constantly working to improve them. "We started out 20 years ago with a model-T, now it's a V8," says Runkle. Recent innovations include a "large throat" combustion chamber. The throat is where LH2 and LO2 enter the chamber for burning. The new throat is about 10 percent larger than the old one--a change that reduces pressure and temperature and, thus, wear and tear on the engine. The fuel pumps have been improved, too. Each engine has two turbopumps that supply up to 970 pounds (440 kilograms) of liquid oxygen per second and up to 162 pounds (73 kilograms) of liquid hydrogen fuel per second to the combustion chamber. "These pumps are so powerful," says Runkle, "they could shoot a stream of liquid hydrogen 40 miles high." The shuttle's original pumps were assembled using welds. After each flight they had to be removed and the welds carefully inspected. Engineers have designed new pumps which are cast in whole from improved heat- and pressure-resistant materials. "Fewer welds and tougher materials means we can go more missions without replacing components," says Runkle.  These and many other improvements require constant testing. Indeed, at the Stennis Space Center in Mississippi, shuttle engines are test-fired about once a week on average. The firings are attention getting, to say the least. White smoke billows miles-high into the atmosphere. The ground vibrates, the air shakes, and nearby onlookers feel the rumble in their bones. It can be a thrilling experience.
Right: A large crowd watches a shuttle main engine test firing at NASA's Marshall Space Flight Center. Most shuttle engine tests are not open to the public, but one slated for Nov. 8th will be. Thousands of people will visit the Stennis Space Center this Friday and watch the test from a distance of a few hundred yards. If you don't live in Mississippi ... no problem. Science@NASA will broadcast the test live on the web. You can hear the roar of the engines, watch their exhaust, and listen to the voice of a narrator describing it all. It's the next best thing to being there. Click here to access the webcast. Live commentary begins at 5:30 p.m. CST on Nov. 8th (23:30 UT); the test itself is slated for 6:00 p.m. CST. And don't forget to turn up the speakers on your computer. You won't want to miss the roar of innovation. | 
