Author Topic: Rocket plume colors  (Read 15571 times)

Offline AnimatorRob

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Rocket plume colors
« on: 11/11/2007 02:29 PM »
Is there any rhyme or reason to the color of the exhaust plume from various rockets? In particular, the SSME ( H2 - O2 ) has a virtually invisible exhaust while the RS-68 ( also H2 - O2 ) has a very bright orange exhaust.

Online DaveS

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Re: Rocket plume colors
« Reply #1 on: 11/11/2007 02:36 PM »
Ablative vs regenerative cooling of the nozzle. SSME is the latter while the RS-68 is the former.
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Offline AnimatorRob

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Re: Rocket plume colors
« Reply #2 on: 11/11/2007 04:27 PM »
I had a suspicion that was the reason. Thanks!

Offline Proponent

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RE: Rocket plume colors
« Reply #3 on: 11/12/2007 06:07 AM »
Other H2-O2 engines have nearly invisible exhausts.  Kerosene-O2 has a yellowish exhaust caused by the carbon in the exhaust plume.  Solids have very bright (and smoky) exhausts as a result of all of the glowing solid particles.

Offline simonbp

RE: Rocket plume colors
« Reply #4 on: 11/12/2007 07:45 AM »
That's the engineer's response. Here's the astrophysicist's:

Basically, in a LH2/LOX engine, you're creating a H+/O2- plasma in the combustion chamber, meaning the electrons are stripped from their atoms leaving an electron-ion soup. The hotter the plasma, the higher the transition energy as the electrons fall back back down to the ions, and that energy is released as light. The higher the energy, the bluer the light emitted. But the energy levels are quantised (thanks Max Plank), so only certain colours of light can be emitted. These are the spectra lines of Hydrogen (O2- and OH- have lines too, but they're in the infrared). Most LH2/LOX engines are very efficient, so they run very hot, and most the light emitted is in the ultraviolet, with a little bit in the blue and violet. For contrast, watch the H2 bleed-off flame on next STS night launch; it burns much cooler, and so has the bright red colour of the H-alpha line. Those bright orange flames around the pad on a Delta IV launch are also mainly due to H-alpha.

Liquid hydrocarbon also have those lines, but their colour is mainly the product of the microscopic particles of carbon soot glowing like a perfect blackbody (same deal as the filament in a light bulb). Because blackbody colour is not quantised, the colour of the soot (and thus the flame) becomes bluer as the temperature of the flame increases (as anyone who has used a gas stove will tell you).

The colour of the STS solids (bright white) is also due to microscopic soot, but this time it's made of Aluminum Nitrate crystals that have just been oxydised, and are glowing very hot, meaning most of the blackbody emission is in the UV, meaning they look bright white.

It's just rocket science. :)

Simon ;)

Offline MATTBLAK

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RE: Rocket plume colors
« Reply #5 on: 11/12/2007 09:07 AM »
Excellent discussion!!
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Offline zeke01

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RE: Rocket plume colors
« Reply #6 on: 11/12/2007 11:20 AM »
Great!  Now can someone explain those ghostly blue 'diamonds' in the J2 and SSME exhausts?   The first 'diamond' is really noticeable, just below the exit plane of the engine bells when at full power.

This phenomenon bugs me to no end, prolly some recombination reaction involving ionized OH, H, free electrons, due to different temperatures/pressure differences within the exhaust?

Am I close?  Are there details?  A paper/writeup anywhere?

Thanks!

Offline Jim

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RE: Rocket plume colors
« Reply #7 on: 11/12/2007 11:38 AM »
Quote
zeke01 - 12/11/2007  7:20 AM

Great!  Now can someone explain those ghostly blue 'diamonds' in the J2 and SSME exhausts?  

Thanks!

Shock diamonds.  They occur in jet engines while in afterburner

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

Offline zeke01

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RE: Rocket plume colors
« Reply #8 on: 11/12/2007 02:09 PM »
So the blue color is from the excess hydrogen being burnt in the exhaust flow, which fits with the hydrogen emission spectrum above.

Exceeelent.  My mind is at ease once more.

Thanks.

Offline usn_skwerl

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RE: Rocket plume colors
« Reply #9 on: 11/12/2007 03:15 PM »
heres a great shot of the shock diamond off the SSME

its still fascinating to see the exhaust cross section decrease in size after leaving the nozzle. I'll never get tired of seeing that phenomena
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Offline DMeader

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Re: Rocket plume colors
« Reply #10 on: 11/12/2007 04:13 PM »
So I've learned a couple of things today.

Simonbp, thanks very much for that explanation. A recent "Mythbusters" episode showed hydrogen burning in air with a distinctive reddish/yellowish flame, and that answers my question why.

Also, Jims link has diagrams explaining the exhaust cross section decrease in the SSME plume. Something else I've always wondered about.

Thanks guys!


Online Damon Hill

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Re: Rocket plume colors
« Reply #11 on: 11/13/2007 09:21 PM »
This is the basis for an in-flight engine diagnostic system to detect the presence of stuff that isn't supposed to be in the exhaust plume, like tiny bits of metal or burning metal that would be the precursor of a potentially catastrophic failure; this could allow for a graceful engine shutdown.  Might work as a check for proper mixture ratios and that combustion is taking place efficiently according to the desired model.  The system has to be fast and not prone to false alarms since said failure modes can run to destruction in a fraction of a second, but often enough may run on for several seconds before getting rude and disgraceful.  The idea is that engines should be failproof, but realistically we'd settle for fail>soft<...

Existing systems monitor temperature (especially turbopump bearings) and vibration levels, but monitoring flame color for a real-time spectrographic  analysis adds a new safety check.

Offline pippin

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RE: Rocket plume colors
« Reply #12 on: 11/13/2007 09:36 PM »
Quote
zeke01 - 12/11/2007  4:09 PM

So the blue color is from the excess hydrogen being burnt in the exhaust flow, which fits with the hydrogen emission spectrum above.

Exceeelent.  My mind is at ease once more.

Thanks.

Err...No, it's from ion/electron recombinations in the exhaust plasma, otherwise it would not glow within the engine (unburnt hydrogen is not burnt there due to lack of oxygen) and excessive excess hydrogen in the exhaust could actually burn red/yellow when being burnt with oxygen from air.

Offline meiza

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Re: Rocket plume colors
« Reply #13 on: 11/13/2007 10:58 PM »
Did anybody notice in the latest SpaceX Merlin 1C test run it looks as if the flame is green in the beginning!
http://e.photos.cx/vlcsnap-254136-a26.jpg">

Offline pippin

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Re: Rocket plume colors
« Reply #14 on: 11/13/2007 11:58 PM »
Copper?

Offline meiza

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Re: Rocket plume colors
« Reply #15 on: 11/14/2007 12:23 AM »
That's the same thing I thought... The Merlin has a copper combustion chamber.
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Online Damon Hill

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Re: Rocket plume colors
« Reply #16 on: 11/14/2007 02:13 AM »
Could be the copper oxidizing a bit, could be the TEB ignitor; boron also burns green.

Offline luke strawwalker

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RE: Rocket plume colors
« Reply #17 on: 11/15/2007 03:34 PM »
Ok I've got a question that's been bugging me for years and never saw an explanation for it and never had anybody knowledgeable to ask, so here goes...

When you watch the video of the Saturn V liftoffs (and I've noticed this on some ICBM launches as well, with storable propellant engines IIRC) you'll notice that the flame is VERY dark as it exits the engine nozzle with just a faint orange/black striped glow and then it becomes a brilliant yellow-orange white a few feet behind the nozzle.  This 'white flame' seems to be "chasing" the exhaust up towards the nozzle but always stays a few feet behind the nozzle rim.  Why is that??  Is it something similar to the 'Mach Diamond' effect or is it caused by something else??  Why would the exhaust appear 'dark' inside the nozzle and directly behind it (it SHOULD be hottest in the combustion chamber and nozzle and cool as it expands down and out of the nozzle shouldn't it?) but then get suddenly 'brilliant' a few feet behind the nozzle exit??  I noticed that the flow coming out of the nozzles looks very laminar in the dark part and then turns more turbulent where it turns brilliant white.  

I've noticed this on some ICBM test videos where the engine exhaust will look pretty dark or even glow "black" or have black stripes or mach diamonds in the exhaust plume at liftoff or shortly thereafter.  It's not a camera artifact or something is it??  I've noticed it in videos under a lot of different conditions though... I seem to remember noticing it on a Titan I or II silo launch video (from inside the silo) and on a Matador Cruise Missile launch test from the ramp outside the shelter... but it's MOST noticeable in any Saturn V liftoff video, as the F-1's pass the tower cameras on the way up....  

Just wondering and hope someone knows... it's driving me nuts... :)  OL JR :)
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Online dmc6960

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Re: Rocket plume colors
« Reply #18 on: 11/15/2007 04:06 PM »
I believe that is the gas generator exhaust which is routed into the nozzle instead of out its own port.
-Jim

Offline Citabria

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Re: Rocket plume colors
« Reply #19 on: 11/15/2007 04:31 PM »
That's right. The gas generator exhaust is much cooler (and fuel-rich) than mainstage exhaust and so it is routed through those large tapered ducts you see surrounding the F-1 nozzle where it exits and cools the inside of the nozzle extension. The effect is even more apparent in films of Saturn 1 launches, because the outer 4 H-1 engines have those ducts and the inner 4 do not. You can see the darker exhaust from the outer engines but the inner engines show less or none of the darker band.

BTW, the Titan II hypergolic exhaust did not glow as much as RP-1, so the Mach diamonds were more visible.

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