What is an entry burn?

Coming down, a booster runs into thickening air that cannot get out of the way fast enough. Compressed air gets extremely hot — the heat comes from compression, not friction, which is the explanation usually given and the wrong one.

Firing three engines into the airflow pushes a cushion of the rocket’s own exhaust ahead of it, so the worst of the heating happens out in the exhaust rather than against the vehicle. The booster is, in effect, holding the hot part at arm’s length.

On a Falcon 9 the entry burn runs from about T+6:09 to T+6:30 — a little over twenty seconds. In that time the booster falls from roughly 55 km to 40 km and slows from about 5,000 km/h to 3,200 km/h.

It is worth noticing what this replaces. A capsule solves the same problem with an ablative heat shield that chars away and has to be inspected or replaced. A booster solves it by spending propellant instead — propellant it had to carry all the way up. Neither is free; they are simply different bills.