Max q

Uh, as we look on board here down the rocket back, down towards the Earth, which we are screaming away from.

You can see in the bottom right the graphic representing that all of the engines have lit here on this Falcon 9 rocket, which is less than 45 seconds into flight and already very quickly approaching, uh, max Q, which is the maximum dynamic pressure that the rocket will be under here as we're looking at almost two Gs and 1,000 kilometers per hour.

Altitude now about 6,000 feet and climbing.

We'll bring Greg and Frank back in, and I know, I think we've each had a chance to see a rocket here or there.

And the point at which these two competing effects meet has a name.

It's called max Q. Max Q typically happens about one minute after launch, 10 kilometers or so above the ground.

Max Q is also the moment at which the physical stresses on the rocket are the greatest.

This means it's also stressful for all the humans involved as well.

If you're listening to a launch, you can often hear the announcer's voice state that the rocket has passed through Max Q and then relaxed noticeably.

A rocket headed into orbit will begin deviating from pure vertical once it clears the launch tower, but after max Q, most of that effort will go into achieving the horizontal orbital velocity.

Whether launching into orbit or even faster to reach the escape velocity, the speeds needed to reach space and stay there are incredibly large.

A nice way to summarize this idea is that getting into space is easy.

We are 53 seconds into this flight.

Rocket will be heading into max Q. Falcon 9 is supersonic.

Picking up speed and altitude very quickly.

Engines will throttle down slightly to reduce the mechanical pressure and the mechanical stress on the rocket.

Vehicles are designed to pass through this band quickly and to fly through it at near-zero angle of attack, and many throttle down as they approach it.

Shortly afterward comes maximum dynamic pressure, universally known as max Q.

Dynamic pressure is the aerodynamic pressure the vehicle experiences due to its motion, and it depends on air density multiplied by the square of velocity.

At liftoff, density is high, but velocity is nearly zero, so dynamic pressure is small.

as the vehicle accelerates velocity climbs faster than density falls and dynamic pressure rises steeply then as altitude increases further density begins falling faster than velocity rises and dynamic pressure falls away again Somewhere between those two trends, there is a peak, and for most launch vehicles it arrives around 60 to 90 seconds into flight, at an altitude of roughly 10 to 15 kilometres, which is 6 to 9 miles.

Max Q is the moment of greatest aerodynamic loading, and the vehicle's structure is sized for it with margin.

Many vehicles reduce thrust as they approach it, accepting slower acceleration in exchange for lower loads.

The space shuttle's main engines throttled down to as low as 67% through this region, and the call of throttle up afterward became one of the most familiar phrases in launch commentary.