What causes the formation of beads in rocket exhaust plumes?

[Artlav]

Not sure if it's chemistry or physics, but the latter seems more like it.

Let's take a look at pretty much any large rocket with smoke-less exhaust during it's launch, for example the recent one:

As you can see, there are a kind of beads in the exhaust plume at seemingly constant intervals. The higher the thrust, the longer the intervals.

What causes these beads? I think they are related to the nozzle's shape, but if so, why are there several of them? What prevents it from expanding at where the first bead is, and keep going? Or, does it look this way for some other reason, like temperature/luminosity/etc?

 

[rcgldr]

Those are 'shock diamonds'. Afterburners in jet engines also produce these:

http://en.wikipedia.org/wiki/Shock_diamonds

http://www.aerospaceweb.org/question/propulsion/q0224.shtml

http://science.nasa.gov/headlines/y2007/04may_methaneblast.htm?list208336

 

[sir-pinski]

It is likely that the beads in rocket exhaust are a result of the physics of the rocket engine rather than chemistry. These beads are most likely droplets of unburned fuel or oxidizer that are expelled from the engine due to the high pressure and velocity of the exhaust gases.

The shape and design of the nozzle can play a role in the formation of these beads. The nozzle is designed to accelerate the exhaust gases and create a specific shape to the exhaust plume. The presence of these beads could be due to the shape of the nozzle causing a disturbance or turbulence in the exhaust flow, causing some of the fuel or oxidizer to form into droplets.

Additionally, the temperature and composition of the exhaust gases can also play a role in the formation of these beads. The exhaust gases are extremely hot and can cause some of the fuel or oxidizer to vaporize and then condense into droplets as it cools down in the surrounding air.

In conclusion, the presence of beads in rocket exhaust is most likely a result of the physics of the rocket engine, including the shape of the nozzle and the high temperature and velocity of the exhaust gases. While chemistry does play a role in the combustion process, it is the physics of the rocket engine that ultimately determines the appearance of the exhaust plume.