Skip to content

Heat shields

Rockets and landing · level 3, lesson 6 of 6

A question to start

Pump up a bicycle tyre quickly and feel the end of the pump: it gets warm. What has this to do with a spacecraft glowing as it comes back through the air?

The idea

When you squash air quickly, it gets hot. That is why the pump warms up.

A ship coming back from space hits the top of the air at many kilometres a second. It slams into the air ahead of it faster than the air can get out of the way. The air piles up in front of the ship and is squashed very hard and very fast, into a layer called a shock wave. Squashed that hard, the air gets hotter than the surface of the Sun in places, and glows. Some of that heat reaches the ship.

It is mostly squashing, not rubbing, that makes the heat. The ship's speed gets turned into heat in the air around it, and the ship slows down a lot.

The heat shield is a thick layer on the front of the ship that takes the heat. Two kinds are used:

  • Ablative shields slowly char and flake away, carrying the heat off with them. Apollo, the Mars rovers Curiosity and Perseverance, and SpaceX's Dragon capsules use them.
  • Tiles that do not burn away but are very bad at passing heat through them. The Space Shuttle used them; you could hold a tile by its corners moments after the middle had been glowing hot.
shock waveheat shieldair rushes in

At Mars. The Perseverance rover's capsule hit the top of the Mars air at about 20,000 km/h. Its heat shield, 4.5 m across, reached about 1,300 °C. Mars's air is thin, but the ship goes so fast that it still gets very hot.

Ships coming home to Earth from the Moon, or from Mars, are faster still, about 11 to 12 km a second. Their heat shields must take even more.

Worked examples

1. The blunt end first. Capsules come in blunt end first. The wide, round shape keeps the hot shock wave a little way in front of the ship, so less heat reaches it than if it were pointed.

2. A shooting star. A shooting star is a grain of dust, often smaller than a grain of rice, hitting the air at tens of kilometres a second. It has no heat shield, so it glows and burns away.

Try it

  1. Why does a bicycle pump get warm?
  2. What makes a returning spacecraft so hot: rubbing, or squashing the air?
  3. How does an ablative heat shield protect a ship?
  4. Why do capsules come in blunt end first?
Answers
  1. Squashing air quickly heats it.
  2. Mostly squashing the air in front of it.
  3. It chars and flakes away, carrying the heat off with it.
  4. The blunt shape keeps the hot shock wave a little way off, so less heat reaches the ship.

Common mistake

"There is no air on Mars, so landing there is cool." Mars has thin air, enough to heat a ship that comes in at thousands of metres a second to well over 1,000 °C. Every Mars lander needs a heat shield.

Where it leads