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otherusesIn aerospace engineering , the maximum dynamic pressure , often referred to as maximum Q or max Q , is the point at which aerodynamic stress on a vehicle in atmospheric flight is maximized. It is an important factor in the structural and mission design of rockets , missiles , and other aerospace vehicles which travel through an atmosphere ; the flight envelope may be limited to reduce the total structural load on a vehicle near max Q.
Dynamic pressure, q , is defined mathematically as : where ? is the air density , and v is the vehicle speed . For a typical launch of a rocket from the ground into space, dynamic pressure is
zero at lift-off, when the air density ? is high but the vehicle's speed v = 0
zero outside the atmosphere, where the speed v is high, but the air density ? = 0
always non-negative, given the quantities involved
Therefore, there will Rolle's Theorem|always be a point where the dynamic pressure is maximum. The point where that occurs is max Q.
In other words, below max Q, the effect of the vehicle acceleration overcomes the decrease in air density so as to create more dynamic pressure (opposing kinetic energy) acting on the craft. Above max Q, the opposite is true. The dynamic pressure acting against the craft decreases as the air density decreases, ultimately reaching 0 when the air density becomes zero.
During a normal Space Shuttle launch, for example, max Q occurred at an altitude of approximately 11 km (35,000 ft). cite web| last = Jackson | first = Douglas T. | title = Space Shuttle Max-Q | work = Aerodynamics Questions | publisher = AerospaceWeb.org | date = 2001-05-06 | url = http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml | accessdate = 2007-02-12 The three Space Shuttle Main Engine s are throttled back to about 70% of their rated thrust as the dynamic pressure approaches max Q; cite web| last = Heiney | first = Anna | title = Launch Blog | publisher = NASA | date = 2007-08-08 | url = http://www.nasa.gov/mission_pages/shuttle/shuttlemissions/sts118/launch/launch-blog.html | accessdate = 2011-05-22 combined with the nozzle design of the Space Shuttle Solid Rocket Booster|solid rocket boosters , which reduces the thrust at max Q by 1/3 after 50 sec of burn, the total stresses on the vehicle are kept to a safe level.
During a typical Project Apollo|Apollo mission, max Q occurred between 13 and 14 km of altitude (43,000–46,000 ft). cite web| last = Woods | first = David | coauthors = O'Brien, Frank | title = Apollo 8, Day 1: Launch and Ascent to Earth Orbit | work = Apollo Flight Journal | publisher = NASA | date = 2005-08-21 | url = http://history.nasa.gov/ap08fj/01launch_ascent.htm | accessdate = 2007-02-14 cite web| last = Brandt | first = Tim | coauthors = Woods, David | title = Apollo 16, Day One Part One: Launch and Reaching Earth Orbit | work = Apollo Flight Journal | publisher = NASA | date = 2004-10-29 | url = http://history.nasa.gov/ap16fj/01_Day1_Pt1.htm | accessdate = 2007-02-14
The point of max Q is a key milestone during a rocket launch, as it is the point at which the airframe undergoes maximum mechanical stress.
