The Roar of Innovation
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.
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."
"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."
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.
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.
Editor's note: Our webcast can support approximately 3000 simultaneous viewers. The audience will probably exceed that number, however. If you can't establish a connection on Nov. 8th, please come back later and view the replay, which we will maintain on our server indefinitely.more information
Live! Space Shuttle Main Engine Test -- Tune in on Nov. 8th to see the test, or come back later for replays.
Space Shuttle Main Engine Specifications -- (MSFC) shuttle main engine performance figures
A-1 Test Stand -- (Stennis) An aerial view of the engine testing complex at the Stennis Space Center.
Space Shuttle Main Engine Enhancements -- (MSFC) Engineers are constantly improving the shuttle's main engines.
Space Shuttle Basics -- (spaceflight.nasa.gov) this reference manual includes pages about the space shuttle's main engines, its solid rocket boosters and external fuel tank.
Rocket propellants and their specific impulses -- Specific impulse is a number engineers use to rate an engine's fuel efficiency. Higher values are better. The shuttle's specific impulse, 450 seconds, is near the maximum for a chemical rocket. The specific impulse of a nuclear rocket would be 900 to 1,000 seconds.
Why use liquid hydrogen as fuel? The amount of energy produced by hydrogen, per unit weight of fuel, is about three times the energy contained in an equal weight of gasoline and nearly seven times that of coal.
NASA Human Spaceflight -- (Spaceflight.nasa.gov) news and information about NASA's space shuttles.
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