Do larger less dense objects heat less while entering the atmophere?

Question

When a space craft enters the atmosphere, it gets hot and heat shielding is needed. Do objects with more surface area and less density heat up less then heavier, smaller objects? Could an object with a large enough weight to area ratio be air cooled and not burn? I understand that when entering the atmosphere it is not like hitting water since the air gradually becomes more dense. In theory, would a feather get too hot and burn on reentry or would it make it to the ground intact? I'm thinking a rotating aerographene sphere.

Answer 1

The heat to be dissipated by an object re-entering from orbit is $64$ megajoules per kilogram. There is no dependence here on density: that is the amount of kinetic energy it starts with and has to lose.

In a hyper-gradual re-entry the object can radiate the heat as it goes. But slow deceleration requires lower drag than we know how to attain.

In a fast re-entry the surface of the object (which is what is heated) will get extremely hot and may burn or vaporise. But now the re-entry time begins to matter. To conduct heat from the surface to the inside takes time, and if the re-entry is fast enough, the heat pulse won’t have time to penetrate before the re-entry is over and no more heat is coming in.

Between slow and fast there is a worst case where radiation cools the outer surface but conduction has time to cook the contents. That is why (for example) high-heat-capacity buffers - of beryllium, for instance - end up doing more harm than good.

Answer 2

Do lighter objects heat less while entering the atmosphere?

They heat more, not less. The square-cube law comes into play. Heating is proportional to an object's surface area and inversely proportional to mass. All other things being equal, that means heating is inversely proportional to the square of an object's size. Small meteors tend to vaporize quite high in the atmosphere where the air is still very thin. Larger ones heat to the point where they explode lower in the atmosphere where the air is much thicker. Even larger ones can survive entry and hit the Earth, and sometimes they're still cold.

Answer 3

They heat less, not more.

Braking begins when retarding force (relative air flux times frontal area) becomes significant. For a less-dense object (effectively, high area per mass), braking begins higher in the atmosphere, where air pressure (and thus mean free path) is a lower pressure. Thermal dissipation is thus more radiative, and less mechanical. Objects with greater area per mass then see lower peak temperatures on reentry.

This issue was first confronted with warheads. Given 1950s technology, ICBM warheads needed brutal braking, via massive heat shields, to survive. This slowed them, higher and sooner, which increased the odds of detection and some sort of countermeasure. As heat shield technology improved (better ablators), designers went to ‘pointier’ warheads, maintaining their high speed deeper into the air above the enemy. Because they could.

The Shuttle orbiter, then, got a ridiculous frontal area… remember, it first entered ‘broadside,’ like a barn door, not like an airplane in flight. The orbiter, when first entering, is not an airplane, and does not need to conform to expectations of how a plane ‘should’ fly. Only later in reentry does it pitch down to ‘airplane’ flight. This barn-door attitude makes heating gentler.

Analogous to skydiving, “space-diving” has been proposed. A space-diver needs no ablative heat shield if the frontal area reaches some high value. That area has been calculated (for a ‘normal’-weight diver, whatever that means) to be a mattress-like area. One could then space-dive by hanging onto a reasonable-scale shield, with no ablatives. Presumably a fatter space-diver needs a bigger shield.

And of course, the Virgin tourism craft have then adopted the “shutttlecock” principle, increasing air-exposed area while keeping the control mechanisms as far to the rear as possible. No ablative shielding is needed, and not just because the speed was lower in the first place.