Rocket exhaust dynamics question

[Maxhiker14]

I was looking at pictures from a rocket I flew a couple years ago and it reminded me of a question I've been wondering about for a while.


Just aft of the nozzle the exhaust appears straight and clean, then several inches further down the flame changes character completely, becoming brighter and much wider. What causes this and why do we see it on some motors but not others?
My best guess is that is has something to do with the exhaust being under-expanded at the nozzle exit, but it's not clear to me why the expansion happens further downstream.

 

[Maxhiker14]

On a possibly related note- I've noticed some motors have appear to have a gap between the nozzle and flame. I've also been curious about why this is. Here is one example:

 

[OverTheTop]

What you are seeing are "Mach diamonds". They are created in the exhaust by shock wave interactions in the exhaust. I can't remember the details, but I remember reading an excellent description a few years back. Will try to dig it out...

The gap varies due to different nozzle geometries and air pressure during flight. They can be under-expanded, over-expanded, or if conditions are just right you get optimal expansion, at least until the pressure changes a little and you get off optimum.

You see rocket stages have their booster motors generally designed for operation at around sea-level pressures (eg SpaceX Merlin engine), or a bit higher altitude if they get keen. Upper stages have a different exhaust bell and are generally matched to the vacuum of space (eg SpaceX Merlin Vacuum engine). Both SpaceX Merlin engines are largely identical, except for the size and curve of the exhaust bells.

https://en.wikipedia.org/wiki/SpaceX_Merlin

 

[Maxhiker14]

Yeah, mach diamonds are awesome! I'm familiar with them and why they form. My question is more about why there is a transition from the mach diamond part of the flame to the lower brighter/wide part of the flame.
Here's another good example:

In this picture, it kinda looks like the flame is being compressed down to some critical point where it suddenly expands?

 

[OverTheTop]

Ok. I think turbulence in the boundary layer between the exhaust and the atmosphere eventually gets to the point where the shockwaves are no longer reflected back into the exhaust, so the Mach diamonds cease to exist.

 

[Alan15578]

I was looking at pictures from a rocket I flew a couple years ago and it reminded me of a question I've been wondering about for a while.
View attachment 635322

Just aft of the nozzle the exhaust appears straight and clean, then several inches further down the flame changes character completely, becoming brighter and much wider. What causes this and why do we see it on some motors but not others?
My best guess is that is has something to do with the exhaust being under-expanded at the nozzle exit, but it's not clear to me why the expansion happens further downstream.

My guess would be that the motor produces a fuel rich exhaust. As the exhaust stream slows down it allows mixing with the atmospheric air and the additional oxygen permits some external combustion.

 

[Alan Whitmore]

My guess would involve the temperature of the exhaust gas, which, in some cases, is so hot directly out of the nozzle that none of the atoms in the exhaust flow has cooled enough to have its electrons drop down one or more orbital levels and release that energy as light. I've always used that hypothesis to explain the peculiar "red/white/blue" or "french flag" appearance of propellants using copper chromite as burn rate catalyst; the copper atoms cool and reach the point where they begin to emit blue light, and somewhat later the 'chromite' portion cools enough to emit red light. [Or maybe it's the other way around, I don't have such a photo in front of me right now. I'll look for one.] In the middle the emission spectra overlap and the flame looks white against a clear sky, and purple against a dark background like trees.
I'm not a physicist, merely a retired geneticist, so this explanation may be undiluted codswallop, but it works for me.

 

[AlexBruccoleri]

My guess would involve the temperature of the exhaust gas, which, in some cases, is so hot directly out of the nozzle that none of the atoms in the exhaust flow has cooled enough to have its electrons drop down one or more orbital levels and release that energy as light. I've always used that hypothesis to explain the peculiar "red/white/blue" or "french flag" appearance of propellants using copper chromite as burn rate catalyst; the copper atoms cool and reach the point where they begin to emit blue light, and somewhat later the 'chromite' portion cools enough to emit red light. [Or maybe it's the other way around, I don't have such a photo in front of me right now. I'll look for one.] In the middle the emission spectra overlap and the flame looks white against a clear sky, and purple against a dark background like trees.
I'm not a physicist, merely a retired geneticist, so this explanation may be undiluted codswallop, but it works for me.

The static temperature right out of the nozzle is low....and it gets warmer as the gas slows down when interacting with the ambient air. Note the stagnation temperature is high right out of the nozzle.

 

[drewnickel]

My opinion: What you are seeing is almost certainly un-burned material being shot out of the nozzle. Even in professional motors some of the fuel does not react, and most of the time solid propellants are burning a bit fuel rich just due to the fact that you need extremely high concentrations of AP to fully react with all of the available fuel. High percentages of AP means high solids loading, which means the propellant is more viscous and difficult to work with (sometimes to the point of needing to be packed rather than poured). This un-burned material is extremely hot but in an environment depleted of oxidizers because all of the oxidizer has been consumed within the motor, but when it gets out into the open it is exposed to oxygen in the air which allows it to burn. The reason for the delay is that the flow is initially fairly steady/laminar, which means that the un-burned fuel cannot cross stream lines into the air to be oxidized. When the exhaust becomes turbulent, mixing is allowed and so the rest of the fuel is consumed outside of the motor and produces a lot of heat and consequently a brighter exhaust. Can see this on the Saturn V pad cams from the apollo era where there is dark sooty exhaust from the F1 engine's film cooling which is initially laminar and unable to mix and burn, but later becomes turbulent and produces an almost blinding exhaust plume.