Space reentry vehicles, part 5

Part 5: More on the detached shock wave

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The term detached shock wave originally referred to an undesirable aerodynamic effect that was found in supersonic projectiles such as bullets.  This type of shock wave was of interest because it had a lot more drag and aerodynamic instability than an attached one.

The first aircraft designed to be supersonic, the Bell X-1, was modelled after the shape of a 0.5 calibre Browning machine gun bullet, which was known to be stable in supersonic flight.

The leading edge of the wings on supersonic aircraft are sharp to avoid this detached shock wave such as the following diagram shows:

In the 1950s the word "detached" came to take on a whole new meaning when NACA engineer Harvey Allen suggested it meant thermal detachment, with some of the heat of friction being channelled around and away from the vehicle.

Harvey Allen, inventor of the bogus thermally detached shock wave concept that was supposed to spare the reentry capsules from burning up like a meteoroid.

This detached shield was necessary to spare space reentry capsules from the huge heat of reentry.  Problem is, the bow shock associated with vehicles with blunt leading edges is not thermally detached at all, and the shock wave, detached or not, increases the temperature of the air, it doesn't decrease it.  This must be so if energy is to be conserved across the shock wave.

Cross section through half a blunted nose cone. Bottom half of cone is replaced with graph at stagnation point.  Kinetic energy must be exchanged for heat across the shock wave. (Diagram from here.)

When the kinetic energy of the air (with respect to the capsule reference frame) is lost, energy is gained in the form of heat.  The heat doesn't sidestep the vehicle.  The heat is created by the craft, or its leading shock wave, due to collision with the air -- the craft is bathed in this heat.  

The blunt shape of the space reentry vehicle increases the heating, it doesn't decrease it.  It would also increase the pressure on the hull to intolerable levels (more on that in part 7). 

Where is the thermally detached shock wave in the case of meteoroids?  They not only burn up almost completely (95 - 99% in larger ones) but are crushed to pieces by the pressure of reentry.

A small distance back from the curved section of the detached bow shock the shape is the same as an ordinary Mach cone -- the bow shock is not fundamentally different to a conical shock.  

Make the shock shape as bowed as you like due to the blunt leading edge, it's still essentially the same shock wave as an "attached" one. Further back all such shocks blend to the same Mach cone.  In fact a sharp shock from a blunt leading edge creates more heat than the thinner attached shock from a vehicle with a sharp leading edge.

Heat is gained across all shock waves such as normal and oblique.  It only cools across expansion waves where the air speeds up instead of slowing down.

The sharper shock of normal and oblique shock waves create more heat, not less, than weaker attached conical Mach waves.  The sharp shock rapidly dissipates as the expansion waves on the sides of the craft meet it, and a short distance downstream the shock is the same Mach cone as that encountered in airplanes with sharp leading edges.

That the bow shock is hotter than conical, so-called "attached" shocks, can be seen in experiments in wind tunnels, for example in the 2002 paper On The Influence Of Spike Shape At Supersonic Flow Past Blunt Bodies. When a small spike is placed over a blunt body the aerodynamic drag goes down and the heating is reduced:

"Blunt front-side bodies are unpropitious for the supersonic flow...an extremely strong shock wave...appears in front of the body, causing an important increase of both pressure and temperature in the vicinity of the stagnation point.

Mounting a spike on the rounded nose of a blunt body moving supersonically can significantly reduce the drag force. In this way, instead of one strong shock in frontal zone, appears a system of conical waves, so that the driving force, and  consequently the fuel consumption, as well as the aerodynamic heating of the body, are reduced."

Hmm, so a sharp leading edge is cooler, not warmer than a blunt leading edge, which is as you'd expect.  Yes, with the increased drag comes increased heating. It's really not that hard to figure out, and it follows from energy conservation laws.  

The pattern of a shock wave is something that can move with the vehicle, but the air itself is only moved a few micrometers -- so the air doesn't actually move with the vehicle. It's not something that can take on the properties of a solid that moves with the craft, that selectively protects it from the heat of friction while still imparting the mechanical drag of friction.

There is no magic bullet that NACA/NASA has discovered here: you can't filter out the mechanical drag part from the heat part of friction. If this was true, it would be the only example in physics of heatless friction ever discovered.

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In the last part of this blog series (part 4) I claimed that even if the entire heat shield of the Apollo reentry capsule was made of solid carbon and boiled and melted in place, removing heat with the phase change, it would still require 2 times as much material as the heat shield actually contained. 

The detached heat shield concept was supposed to be the get-out clause that spared reentry capsules from the huge heat of reentry.  Extraordinary claims require extraordinary evidence, but like so many space race technologies the detached shock wave is something NASA would rather leave in the past and forget.  For example, you won't get updated demonstrations or justifications from NASA for bow shock thermal detachment in a modern wind tunnel.

A detached shock wave was an aerodynamic term that NASA corrupted to its own end to mean thermal detachment.  But it's nothing more than just another bogus space race technology.

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