Space shuttle Endeavour and its six-member STS-134 crew head toward Earth orbit and rendezvous with the International Space Station in May 2011.

Originally published June 30, 2011

Next time you’re in a car driving down the highway, put your hand out the window. Feel that? That’s dynamic pressure.

When you’re driving at highway speed, the air rushing past puts about 10 pounds per square foot of pressure on your hand. As you’ve probably noticed, the faster you’re moving, the more the air pushes on you.

Now imagine how that would feel at 1,000 miles per hour.

As the space shuttle flies through the Earth’s lower atmosphere during the first two minutes after liftoff, the high speeds combined with the relatively dense air create immense loads on the shuttle.

Just one minute after liftoff, the shuttle has already reached the typical cruising altitude of an airliner, but is traveling about 1,000 mph -- nearly twice as fast as an airliner. The orbiter’s guidance system steers the shuttle during the ride uphill into orbit, much like an airliner’s autopilot.

The Flight Design and Dynamics team at Johnson Space Center in Houston designs steering commands that keep the shuttle’s structural loads within certified limits, particularly on the orbiter’s wings and tail and at the connections between the main components (orbiter, external tank, and solid rocket boosters).

The structural loads on the shuttle at any particular moment are dependent on the dynamic pressure, which in turn depends on the density of the air at that altitude and how fast the shuttle is moving through that air.

The shuttle accelerates as it climbs through the atmosphere, so you’d expect the speed and dynamic pressure to increase. However, the density of air decreases with altitude, so you’d also expect the dynamic pressure to decrease.

The “sweet spot” where the shuttle’s speed and air density combine for the highest dynamic pressure is known as “max Q.” (Max Q is also the name of the band composed of NASA astronauts, but that’s a story for another day.) Max Q for the shuttle usually peaks around 700 pounds per square foot about one minute after liftoff.

The goal of trajectory planning during the first stage - the first two minutes after liftoff before the SRBs separate - is to optimize the performance of the solid rocket boosters and shuttle main engines while limiting the dynamic pressure at max Q. A big part of that has to do with winds from the ground up to 60,000 feet.

We’ll talk next time about how they do that.