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STS-111 Space Shuttle Main Engines
Question and Answer Board

Bernardo from Mexico City
Has Endeavour already been upgraded to a MEDS cockpit?
Well, buenos dias, Bernardo. No, Endeavour has not been upgraded to the MEDS cockpit, or what we call the glass cockpit. At this point, only Atlantis and Columbia have received those upgrades.
Chris from Kansas City
Are there any new plans for a lighter yet faster and more fuel efficient engine? Where are these plans -- in the development stage or are they even farther along?
Chris, rocket scientists are always trying to find new efficiencies in rocket engines and part of that comes from decreasing the weight of the engines. Right now, through Space Launch Initiative, for example, we're trying to find ways to develop engines that reduce the weight and increase efficiency. But you do have to remember that a lot of it is the weight of the vehicle and hopefully through the development of new composite materials, ceramics and other things, we'll be able to drive down the weight of the engines, drive down the weight of the vehicles and achieve those efficiencies.
Kit from Ramona, California
I've heard that rocket engines are more efficient in a thin atmosphere than at sea level and most efficient in a vacuum. So why are launches generally made from sea level rather than from a high point in the mountains? Wouldn't a launch from the mountains save fuel since the rocket engines would be more efficient and because the ship is already at a higher potential energy location in relation to the Earth?
Well, Kit, that's a very good question. Yes, there would be efficiencies launching at higher elevations. But as the song says, there ain't no mountain high enough, or big enough or safe enough for us to have a launch facility like the Kennedy Space Center at a higher elevation, so we'll keep launching from sea level. Another important factor, of course, is latitude as much as altitude. And that's why launches near the equator are much more efficient in achieving certain orbits..
David from Casper, Wyoming
I understand that the Shuttle Main Engines can be gimbaled to aid in steering the shuttle. How far off their standard direction can they be moved (degrees)?
The Space Shuttle Main Engines can actually be gimbaled plus or minus about 12-and-a-half degrees.
Gabe from Inverness, California
How does the power of the STS main engines compare with that of the Saturn V launch vehicle?
Well, each shuttle main engine has about 418,000 pounds of thrust, and there are three on the vehicle, so that's about 1.2 million. The entire vehicle, taking into account the solid rocket boosters, is about seven million pounds of thrust at launch. The Saturn V vehicle in the Apollo program, with five F1 engines, each one generating 1.5 million pounds of thrust, was about seven and a half million pounds at launch. So, close, but not quite.
Callum from Aberdeen
How many engines does STS-111 have?
Well, space shuttle main engines, the SSMEs, there are three of those on each of the orbiters, including Endeavour for STS-111. But then you add the solid rocket boosters and of course the various orbit maneuver engines that round out the propulsion picture on the shuttle.
James from Atlanta, Georgia
During launch, the three SSME's are supplied with fuel (liquid hydrogen) and oxidizer (LOZ), but since these fuels are not hypergolic in nature, what actually ignites the mixture?
Hypergolic fuels of course are fuels that ignite when they come in contact with each other. In a liquid oxygen-liquid hydrogen engine, such as the space shuttle main engine, they are actually ignited by a spark igniter. They are located in the augmented spark igniter chamber, and they actually ignite the fuel.
Julio from Palm Bay
How long is the life of the main engines on the shuttle?
We are making some changes. I believe you're talking about changing the engines out from the orbiters. The engines are supposed to have a theoretical life of about 50 missions. Probably the most that any engine has had to date is 17 or 18 flights, but it's difficult to measure that because you're changing out various components, the different elements of the engine, over time and from mission to mission. But we feel that one of the unique qualities of the space shuttle main engine is the long life by being a reusable engine.
DC from Charleston
The last main engine upgrade was touted as improving safety and efficiency. Are there any radical changes visible on the horizon?
We've been doing upgrades and looking for more safety and efficiency in the engines ever since they were first created a generation ago. And so we're constantly looking for new upgrades, and probably the next major upgrade is the development of an advanced health management system, which would increase safety. So we figure that we will continue to do upgrades to make the engines safer for the flight crews, as well as more reliable and more efficient.
Darrel from Ft. Payne
How much horsepower do the shuttle's main engines produce at the time of lift off?
The three space shuttle main engines generate the maximum equivalent of about 37 million horsepower. The fuel pump alone delivers as much as 71,000 horsepower, the oxygen pump delivers about 23,000. Just as a basis of comparison, the fuel pump alone is probably the equivalent horsepower of 28 locomotives. And with the horsepower of the oxygen pump, that's probably the equivalent of 11 more locomotives.
Shane from Goldsboro
What process does the engine go through in preparation for the next launch once the orbiter has landed from a previous mission?
The engines are removed from the orbiter and taken to the SSME processing facility here at Kennedy Space Center, thoroughly inspected by our team of Rocketdyne engineers and technicians, and given whatever kinds of parts changes need to be made. All of it is under the principle that safety is the most important thing. And we want to make sure that each engine, upon return, is safe for that next flight.
NASA's John F. Kennedy Space Center