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Drag

Rockets and landing · level 3, lesson 3 of 6

A question to start

About a minute after launch, rocket commentators often say "max Q". Engineers watch it nervously, and some rockets turn their engines down just before it. What is happening?

The idea

As a rocket pushes through the air, the air pushes back on it. This force is drag. It acts against the motion, so on the way up it pulls the rocket back down.

Drag depends on two things that change as a rocket climbs:

  • Speed. The faster the rocket, the harder the air pushes. Twice the speed gives about four times the drag.
  • How thick the air is. Near the ground the air is thick. Higher up it gets thinner and thinner. At 10 km, about the height planes fly at, it is a third as thick; at 30 km, only about 1.5% as thick.

At first the rocket is slow, so drag is small. Then it speeds up fast, and drag grows. But it is also climbing into thinner air, which makes drag smaller. Somewhere in between, usually about a minute after launch and roughly 10 to 15 km up, the push of the air is at its greatest. That moment is max Q (Q is the letter engineers use for the air's push). The rocket's structure feels the most strain then.

max Qtimepush of the airabout 1 minute

Above about 50 km there is so little air that drag hardly matters. In space there is none.

On Mars, the air near the ground is about as thin as Earth's air 30 km up. A rocket leaving Mars feels very little drag, which is one less problem for the trip home. But the thin air is also a problem when landing: there is not enough of it to slow a ship down easily.

Worked examples

1. Your hand out of the car window. At 30 km/h you feel a gentle push; at 60 km/h, about four times as much.

2. Why rockets are pointed and thin. A thin, pointed rocket has less air to push aside than a wide, flat one, so its drag is smaller.

Try it

  1. Which way does drag push on a rocket going up?
  2. A rocket doubles its speed in the same air. About how much does its drag change?
  3. Why does drag get smaller again after max Q, even though the rocket is still speeding up?
  4. Would a rocket leaving Mars feel more or less drag than one leaving Earth? Why?
Answers
  1. Down, against its motion.
  2. About four times as much.
  3. The air is getting thinner very quickly as it climbs.
  4. Much less, because the air of Mars is much thinner.

Common mistake

"Drag is biggest at the end, when the rocket is fastest." By then the rocket is above almost all the air. Drag is biggest at max Q, where speed and thick air together give the biggest push.

Where it leads