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Okay folks, it’s been long enough. Time to finally address the biggest point of confusion on #AvGeek twitter: what exactly is going on with vapor cones?

Let’s just say much of what you’ve read is (probably) wrong. Wonder no more: a thread... 1/n
2/n We’ve all seen the photos & videos—whether it’s a jet flying over the beach or a rocket during its ascent—with the beautiful vapor cone (you also hear shock collar or even shock egg). Inevitably the exclamations follow: “sonic boom!” “Mach 1” & “breaking the sound barrier”
3/n And I totally understand. It’s a nuanced concept & a quick Google search brings up all kinds of misinformation. I could not find a single article that seemed to truly “get it”. So no shaming here—it’s debunking time
4 You know you’re headed down the wrong path when people start talking abt the Prandtl-Glauert singularity, the result of a simplified eq used to explain the difficulty of “breaking the sound barrier” before supersonic flight was understood, it’s known to be invalid for Mach>0.7!
5/n So needless to say you cannot totally trust Wikipedia on this one—just look at all the disclaimers above this article!!

...should I suggest some edits after this?…
6 w/ all that said there is ONE online resource I found that does a satisfactory job of explaining vapor cones. It’s a wonderful write-up from who else—NASA—explaining many forms of condensation on many aircraft that I highly recommend for further reading…
7/n However it’s 163 pgs long and a bit technical depending on your background, so while it was a fun review for me, let me try to put this in 280 character we go
8/n Back to our original question: what do you see in this photo? Quite simply it’s condensation that is the result of a rapid EXPANSION of air. It is this expansion of air that creates a drop in temperature and pressure that can lead to condensation in humid conditions.
9/n Generally you aren’t looking at a shock wave but another related compressible flow feature: an EXPANSION FAN. Just like you get shock waves when high speed flow is compressed (encounters an obstacle), you get expansion fans when high speed air flow is given room to expand.
10/n When you have a shock wave on a super/transonic aircraft, there is often an expansion fan somewhere downstream (which in turn will be followed by another shock to return the flow to the original state). You could even conceptually think of it as the opposite of a shock wave.
11/n Notice the two primary locations where we see expansion fans for these fighter jets:
-Expansion and acceleration over (and under) the wing
-Expansion across the canopy

Then the fans end (and condensation is vaporized) when processed by the shock downstream.
12/n In the NASA report they go into some detail abt condensation (a topic that gets complex quickly) & actually did some modeling to show where we would expect condensation for a transonic airfoil. The colors are relative humidity—green to white are high enough for condensation
13/n I should also say that often you see vapor cones @ high subsonic Mach #s (M~0.9) because flow LOCALLY ACCELERATES over a wing (& expands) to supersonic velocities, creating a shock on the wing followed by an expansion fan, which causes condensation
14/n So you can actually get vapor cones across a range of Mach numbers—often vapor cones happen at subsonic speeds and the presence of a vapor cone by no means says a vehicle is moving at M=1 (on the contrary it’s quite unlikely the speed is EXACTLY M=1)
15/n It’s worth noting that the condensation is caused by the temp drop (which is a consequence of the pressure drop) because the lower temp air molecules drop below the local dew point & lose their ability to hold water (warm air holds more water)
16/n Condensation is a fickle thing. Just because you drop below the dew point does not guarantee condensation. You can have supersaturated air but it needs particles (dust, pollution, smoke, etc) to kick-start the nucleation process & droplet formation
17/n Combine this with the fact that water vapor (humidity) is not uniformly distributed throughout the atmosphere, and you often get these flickering vapor cones that are NOT caused by changing vehicle velocities or intermittent “breaking the sound barrier” (which isn’t a thing)
18/n While we’re here: What’s a shock wave? It’s a sharp discontinuity created by an object moving faster than the speed of sound, caused by the fact that at this speed air molecules can’t communicate fast enough to other molecules to move out of the way. So nature makes a shock.
19 This creates a big jump in pressure (you may have heard of an N-wave) & the wave travels to your ears, which your brain depicts as a loud BOOM. It doesn’t happen when you “break the sound barrier” but this wave travels around any trans/supersonic aircraft generating shockwaves
20 You normally can’t see a shock wave: they’re only O[100 nm] thick (similar to visible light wavelengths, ~1000x more thin than the avg human hair or a handful of molecular collision mean free paths). They create a temperature jump that VAPORIZES water, eliminating condensation
21/n However these sharp gradients can bend light like a lens, so with the right perspective or appropriate equipment (like a shadowgraph or schlieren setup) you can make the thin shock waves really pop (see next tweet for another example).
22 Compare these cases @ M=0.98. On the left is an F1 Lightning (wind tunnel) model & the shocks show up as white lines behind the canopy & just upstream of the fin. Now look @ the vapor cone around an F6 @ the same speed. The shocks are actually where the vapor cones terminate!
23/n Of course, you see condensation in other low pressure, low temperature features like wakes during high angle of attack maneuvers and tip vortices of wings or rotors—same concept just different flow features
24 So to recap (or TLDR) when you see a vapor cone it’s the result of condensation caused by an EXPANSION FAN & it’s not a
-Sonic boom
-Shock wave
-Sound barrier
-Portal to another dimension

& not related to the Prandtl-Glauert singularity & might not even be @ supersonic speed!
Honestly, there are so many articles on this topic that are wrong you start to wonder if you’re losing your mind. Thank goodness for the NASA Technical Reports Server 😂

Getting ready for the replies like...
(but in all seriousness I love your questions so keep them coming)
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