Shock wave causes an adverse pressure gradient which causes an airfoil to stall. Also because gas density is much lower boundary layer becomes laminar and unable to prevent separation.

Correct me if information above is incorrect. Also are there any sources of stall? Like how much AoA affect separation? Can an airfoil stall (just because of hypersonic rarefied conditions) at 0° AoA?


2 Answers 2


Hypersonic rarified conditions means you're almost in space.

You are asking can you stall? Maybe the first question should be can you fly?

Correct me if I'm wrong but it seems that you are trying to apply a real life exemple to a totally different area of flight. The stall you are referring to happens in transonic flight when the shock wave appearing on the extrados forces the flow to detach.

In subsonic/transonic flight lift is generated through an acceleration of the flow around the extrados resulting in a pressure loss on the succion side of your wing. If the flow detach you're losing this acceleration effect of the flow following the surface of the wing and thus you lose lift.

In supersonic flight the shock waves starts attached to the leading edge of the wing. The shock deviates instantly the flow and the more the flow needs to be deviated the stronger the shock and the more pressure increases. Lift is generated through the difference of deviation in the flow betew intrados and extrados resulting in pressure difference on both side. The boundary layers here doesn't play a huge role and I've never heard of supersonic flight stall.

In hypersonic rarefied condition the shock detach from the body and you have very high temps, and complexe chemistry hapenning inside the layer between the shock and your surface thus making the flow hard to simulate. First of all you don't really use wings in this mixture of high energetic particles so speaking of stall doesn't make sens. Then lift is mostly done the same way as in supersonic, through the differential increase of pressure being the shock but the chemistry between the few molecules of air can create weird stuff resulting for exemple in asymmetrical lift on a surface for very complexe not always well known reason.

  • $\begingroup$ Maybe the OP has thought up a design where part of the airflow over the aircraft is still subsonic even when the aircraft is moving at over mach 5? :-P $\endgroup$
    – Vikki
    Commented Apr 17, 2021 at 0:52

I guess you got a few things confused.

Rarefied flows are characterized by long distances between molecular collisions compared to the gradients of speed or pressure, so the molecules will not settle at the equilibrium limit. More precisely, the Knudsen number – defined as the ratio of the mean free path to some characteristic length – is greater than that required to approximate the flow as a continuous medium.

There are two possible conditions for rarefied flows:

  1. Either flight in near-space (or conditions in near vacuum such as in chemical vapor deposition) where gas density is extremely low, or
  2. flow inside conditions with extremely high gradients, such as in thin shock and boundary layer regions (like in the small gap between a hard disk and its magnetic head).

Next, you mention shock-induced stall. So I assume you mean the second condition. But that doesn't happen in hypersonic flow, but in transsonic flow when only some region of the flow field is supersonic (but most is still subsonic). Hypersonic rarefied flows (which can be found in reentry vehicles in the upper atmosphere) have no such thing as attached or separated flow, because these terms can only meaningfully be used for continuum flows.

Like how much AoA affect separation? Can an airfoil stall (just because of hypersonic rarefied conditions) at 0° AoA?

We have many questions here covering separation and stall angle of attack, all of which will tell you that separation depends less on AoA but pressure gradients.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .