Thread: Atmospheric deceleration of an object falling from space

A conversation going on in another thread about dropping things from orbit as kinetic energy weapons got me curious about how much atmospheric drag would slow down such a weapon. Suppose you have a heavy (a few tons?) mass that's falling directly down toward Earth (normal to the surface) at orbital velocities, and the object has been designed to minimize drag and hit at the highest possible speed. Does anyone have any idea how much such an object might slow down? It would only be in the thicker parts of the atmosphere for something like 10 seconds, so it intuitively seems like there wouldn't be time for it to slow down much, but I don't really have any idea how to calculate it. I suspect that the simple drag equations that I learned in physics class would be insufficient for something traveling at 10+ km/sec.

Any calculations would only be an approximation.........

There are SO many variables. - Temperature, Air pressure & densities. Humidity.

There there is the object itself......Shape; skin drag, base drag.....etc, etc, etc

Originally Posted by Scifor Refugee

A conversation going on in another thread about dropping things from orbit as kinetic energy weapons got me curious about how much atmospheric drag would slow down such a weapon. Suppose you have a heavy (a few tons?) mass that's falling directly down toward Earth (normal to the surface) at orbital velocities, and the object has been designed to minimize drag and hit at the highest possible speed. Does anyone have any idea how much such an object might slow down? It would only be in the thicker parts of the atmosphere for something like 10 seconds, so it intuitively seems like there wouldn't be time for it to slow down much, but I don't really have any idea how to calculate it. I suspect that the simple drag equations that I learned in physics class would be insufficient for something traveling at 10+ km/sec.

They come in really fast.

ICBM re-entry bodies (the bomb part) are ballistic and come from near-orbital trajectories at high speedl, sufficiently high that they glow white hot. That is why the outer bodies are constructed of material that is capable of withstanding extremely high temperature. I'm not certain of the drag coefficient at those velocities but it generally is not too large at supersonic speeds.

Originally Posted by DrRocket

They come in really fast.

ICBM re-entry bodies (the bomb part) are ballistic and come from near-orbital trajectories at high speedl, sufficiently high that they glow white hot. That is why the outer bodies are constructed of material that is capable of withstanding extremely high temperature. I'm not certain of the drag coefficient at those velocities but it generally is not too large at supersonic speeds.

Well yes, I know they come in really fast, but I'm wondering about approximately how much they slow down by the time they reach the ground. Would an object that comes in more-or-less normal to the ground at 10 km/sec still be traveling about that fast when it hit? Or would the drag slow it down to "only" 5, 1, 0.5, etc. km/sec? Smallish meteors seem to slow down quite a lot, but then again they don't have a shape that minimized hypersonic drag...

Edit: I've found some sources saying that ICBM warheads can impact at up to 4 km/sec, but I don't know how much that's limited by their flight path (they probably won't be coming in normal to the surface), the need to slow down to have time for terminal guidance, etc.