Answers Your Questions
| From: Jimmy Xu, of Toronto,
To: Kelly Beck, lead flight director
Question: What is the maximum dynamic pressure, and why does the shuttle throttle down at max-q?
Answer: Dynamic pressure -- also known as q-bar -- is the force exerted on the shuttle by the atmosphere as the shuttle is passing through it, and it is measured as a force per unit area (pounds per square foot). The thicker the atmosphere and the faster the shuttle is traveling through the atmosphere, the greater is this force. If you've ever stuck your hand out of a car window while the car was moving at higher speeds, then you can picture the speed/force relationship.
Maximum dynamic pressure, or max-q, is the point at which this force peaks, and occurs during ascent about a minute after liftoff. The combination of the shuttle's rapid acceleration and relatively low altitude -- that is, denser atmosphere -- produce this peak pressure. In order to keep the force exerted on the shuttle below what it is designed to handle, the main engines are programmed to throttle back to about two-thirds of their rated power -- about 67 percent. In addition, the solid rocket boosters' propellant is loaded in such a way that the SRB thrust also is reduced during this period. Lowering the thrust lowers the acceleration, so the shuttle speed increases at a slower rate. The slower speed, compared to what it would be if throttling didn't occur, results in reducing the maximum dynamic pressure the shuttle endures. As the shuttle climbs higher, and the atmosphere thins, the thrust is returned to its normal value.