Answers Your Questions
| From: Sarah, of Antioch, Ill.
To: Kelly Beck, lead flight director
Question: What does it mean when you say that the "three liquid fuel main engines are throttling forward in a three step fashion?"
Answer: At main engine start -- beginning at 6.6 seconds prior to liftoff -- the three SSMEs (Space Shuttle Main Engines) are commanded to 100 percent of the rated power level. At a velocity of 60 feet per second, approximately 3 seconds after liftoff, the SSMEs are commanded to 104 percent. When the shuttle approaches the period of maximum dynamic pressure -- that is, where the force exerted by the air as the shuttle pushes its way through the atmosphere reaches the maximum amount -- at 617 fps or about 30 seconds after liftoff, the SSMEs are commanded to the 67 percent thrust level. This maintains the stresses on the vehicle below the maximum acceptable levels. Then, at 1,397 fps or about 60 seconds after liftoff, since the atmosphere is now less dense, the engines are commanded back to the 104 percent level, where they remain until shortly before main engine cutoff. The SSMEs are again throttled back at that point to limit the acceleration to no more than three times the force of gravity. This throttling back is done to lower the stresses on the crew and payloads.