How does atmospheric re-entry cause compression.

The fiery re entry trope is one we are all well aware of and the fact that it’s something which really happens. It’s also known that the cause is due to compression, not just friction.

Now, I know that compression of air causes an increase in temperature and pressure. You see that in a bicycle pump and is one of the ways a Diesel engine operates. But, in those cases there is a small and closed space, indeed if the timing of a Diesel engine gets off and the exhaust valve opens too soon, air compression won’t happen.

Surely a space craft, Missile Reentry Vehicle and celestial objects are too small compared to the earth for any real compression to happen, the speed should just cause the air to be pushed aside.

How do you think you push air aside, except via compression?

Along with what Gorsnak said, the same principle is involved with an aircraft’s wing although it’s actually decompression on the top of the wing due to how the airfoils shape is designed to make the path for the airflow on top take longer than on the bottom. That difference in distance traveled on the top part of the wing vs the bottom part of the wing is how lift is generated.

If you are in a car, or even in a subsonic plane, that is what happens. The air is compressed, and that overpressure causes the air to move laterally to the side.

If you are going supersonic, then the air doesn’t have the ability to get pushed aside faster than you are pushing through it, hence sonic booms.

In the case of reentry, the vehicle is going so fast that any lateral movement of the air in “trying to get out of the way” is completely swamped by the movement through the air, and it all pretty much just piles up.

Move anything through air, at any speed, and there will be an increase in the air pressure in front and a decrease behind. That increased air pressure in front will cause the air to move off to the side (i.e., from the high pressure region to the lower-pressure region). You’ll end up in an equilibrium where the air accumulates in front at the same rate that it de-accumulates.

For an object moving at subsonic speeds, it’ll take only a small amount of air movement off to the sides to maintain such an equilibrium, and hence only a small pressure difference to drive that movement of air. For a supersonic object, however, the equilibrium pressure is going to be quite high.

It turns out that for atmospheric entry, you can use Newton’s Impact Depth approximation for most of the velocity shedding. In essence, it says that for the hypersonic portion of the reentry, the mass of air that a spacecraft carves out is (roughly) equal to the mass of the spacecraft.

Clearly, the air does eventually get out of the way of the craft. But at hypersonic speeds, that time is long compared to the time it spends in front of the heat shield, and so it slows nearly to the speed of the craft. Only after the kinetic energy of the incoming air has been converted to heat does it get a chance to move off to the side. Hence there’s a nearly complete momentum transfer and Newton’s approximation is valid.

That’s actually false and is a very pervasive myth:

If it were true, it wouldn’t explain how a paper airplane or a balsa wood model plane with flat wings can fly, or how a sail generates lift (pushing you forward against the wind but also heeling the boat over), or how an airplane can fly upside down.

What actually generates lift is some combination of airfoil curvature and angle of attack that results in deflection of air downward. You can see both of those in action when an airliner deploys slats at the front of the wing and flaps at the rear to generate more lift for takeoffs and landings, creating a shape similar to the diagram on the right in the third link above.

And the essential reason for that is that the shock wave is moving in excess of the speed of sound in the medium, so the air up ahead never has any ‘warning’, so to speak, and is immediately trapped at the wave front. This results in a massive degree of compression (in the case of a powerful explosion, the density of air at the shock front can be greater than that of a solid material like steel) and an enormous amount of radiant heating due to compression. For fast moving reentry vehicles, the gas can actually become ionized purely through heating alone, and the ionization can produce high energy ultraviolet radiation which is very ‘hot’. When two shock fronts meet–because you have two different profiles, like two adjacent nacelles, or a shock off of a nose meeting one formed off of a wingtip–you can get what is called shock-shock intensification where the heating is actually multiplied beyond what it would normally be because you have fast moving hot gases running into one another.

I’m not going to get into an argument about Bernoulli’s principle, but the common statement of two flows above and below the wing moving at different speeds is actually true but not really intuitive to the basic cause of lift, which is that there is greater pressure on the bottom side than on the top. (A frequent misstatement is that a two particles diverging at the wing front and following the streams at the rear, which is not guaranteed even for purely laminar flow and certainly not in turbulent flow.) The production of the pressure differential in flight is accomplished by transferring momentum from the moving air flow to the stationary wing (or vice versa, depending on what reference frame you care to use) causing the air on the bottom to gain downward momentum while the wing has a net impulse upward which, in level flight, counteracts the downward force of gravity. This occurs because of a positive angle of attack–that is, the wing is angled upward at the front so that the working fluid is forced to deflect downward to flow around it.

That the air above is moving faster also reduces pressure above (because less is acting to push the wing down than a static ambient pressure) but you aren’t going to get a plane to take off just by blowing wind across the top of the wing; you have to have relative movement between the wing and the flow below it in order to get significant net lift, because the fraction of a psi pressure differential is not enough to even lift the weight of the wing itself, much less a fuselage and engines.