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max Q is the point of maximum dynamic pressure, the point at which aerodynamic stress on a spacecraft in atmospheric flight is maximized.
Considering the definition of dynamic pressure: q = ? v² / 2, where q is the amount of aerodynamic pressure, ? (rho) is the air density and v is the vehicle speed, we have that such quantity:
- is zero at lift-off, when the air density ? is high but the vehicle's speed v=0
- is zero outside the atmosphere, where the speed v is high but the air density ?=0
- is always positive (or zero, as above) given the quantities involved
Therefore, there will necessarily be a point where the dynamic pressure is maximum: that point is precisely max Q.
In other words, below the max Q point, the effect of the spacecraft acceleration overcomes the decrease in density as to create more dynamic pressure (opposing kinetic energy) acting on the craft. Above the max Q point, the opposite is true, and the dynamic pressure acting against the craft decreases as the air density decreases, ultimately reaching a Q of 0 where the air density is zero.
As explained in the above link, "Q" or "q" is simply a quantity called dynamic pressure, which is a combination of air density and speed. During the launch of any rocket, the speed increases since the rocket accelerates, but air density decreases the higher the rocket goes. There is a point where the combination of increasing speed and decreasing density is a maximum, and this is the point called "max q." Rockets, aircraft, missiles, and other vehicles are all designed to withstand only a certain maximum q before they will suffer structural damage, so the term is used throughout aerospace engineering and not just by NASA.
During a normal Space Shuttle launch, for example, max Q is at an altitude of around 11 km (35,000 ft).
During a typical Apollo mission, max Q occurred approximately between 13 km and 14 km of altitude (43,000–46,000 ft).
The point of max Q is a key milestone during a rocket launch, because that is the point at which the airframe undergoes the most mechanical stress
. The Space Shuttle Challenger disintegrated shortly after max Q.
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