Why is LH2/LOX exhaust invisible? According to black-body radiation for 3,000 kelvin, shouldn't it be red or orange? Yes, i know the exhaust is water (super-heated steam) and what we really see above our boiling water on the stove is mist, but after reading about black body radiation i see no reason why the actual steam from a rocket engine wouldn't be visible as a bright blur, red or orange according to black-body radiation. Also, i seem to remember reading somewhere that invisible rocket exhaust is dangerous, so some craft add a chemical additive to the propellant to make it visible, but i can't for the life of me remember what it was or where to find this info again. Anyone know? thanks in advance
What do you mean the exhaust is invisible. The steam produced is quite visible in the atmosphere. As far as black body radiation the comubustion temeprature is high which is why we see the bright light right at the nozzle, but the water produced will almost immediately lose energy and cool due to the steam being at atmospheric pressure. In space there is no visible exhause because the water vapor disperses immediatley due to the vacuum of space. I think the dangerous exhaust is from the combustion of hydrazine and Hydrogen Peroxide but I could be remebering that wrong.
I think the OP means the flame, not the exhaust. Clean flames aren't dense enough with solid objects to see. A candle flame, on the other hand, is made up of glowing soot particles. http://en.m.wikipedia.org/wiki/Flame
The flame is blue, sort of blends in with the sky. Kerosene/LOX produces carbon particles which glow yellow/white making for a much more brilliant flame.
Hmm, Russ_Watters is distinguishing between the flame and the exhaust, which I thought were synonymous. Can anyone define the difference? If one is the act of combustion and the other is the result of combustion, that still seems a pretty fine line. However, upon closer examination of youtube videos that show the shuttle engines (which produce superheated steam) firing from the ground, I do see a slight reddish glow in the plume and sometimes even shock diamonds. Also, when looking directly into the engine nozzles, they appear white. This is all pretty confusing.
Trailing from the shuttle engines is a small blue glow of a few dozen feet and a cloud of white vapor a thousand miles long. One is flame, the other exhaust.
Because it's steam. Steam is clear. The exhaust temperature isn't 3000K, although parts of the interior of the engine reach that temperature. ?? You mean, so people don't accidentally walk under the space shuttle during a launch? Can't imagine a scenario where seeing the exhaust would save someone who otherwise would have died.
Hi DrZygote214. Please Register or Log in to view the hidden image! The Space Shuttle Main Engines work as a 'pre-burner' system where the combustion takes place before the hot gases enter the final rocket chamber and exit via the exhaust nozzle of that chamber. Naturally, to avoid the problem of hot Oxygen attack on rocket materials, including exhaust Expansion Bell surfaces, the fuel-oxidizer mixture is Rich in Hydrogen so the exhaust is 'reducing' and not 'oxidizing' in chemical effect. This hydrogen rich exhaust leaves the nozzle and expansion bell stages, such that the steam and hydrogen radiation characteristics are as you see them, with hydrogen radiating very pale blue light and the steam gases hardly in visible light. The pale blue is NOT the result of burning hydrogen, but of the hydrogen temperature and radiation characteristics at that temperature at those compression/shock waves 'focused hot spots'. That unburned Hydrogen exhaust component from the expansion bell then finally 'burns' as it eventually mixes with outside ambient airflow Oxygen, creating more steam and some flame regions far behind the actual rocket itself. And IIRC certain HydroCarbon compounds are added to hydrogen gas/Hydrogen-rich Natural Gas/Methane in the lab (bunsen burners and the like) so that the practically invisible 'pale blue' flame of burning Hydrogen is rendered more visible/orange color and so easily seen before one puts one's hand over it unawares that it is 'active combustion' going on! I think the same is done for burning gas systems in domestic stove-top burners, for the same safety and awareness reason, that the flame is visible deeper blue-orange? For similar safety reasons (to do with unburned gas situations where there is a leak or unattended burner not lit properly etc), certain 'foul smelling' compounds are added to the hydrogen/natural gas to warn people (via sense of smell) that there is a leak/danger of explosion of the gas which would otherwise give no warning of impending danger. Good luck in your future questions/discussions, DrZygote214. Please Register or Log in to view the hidden image!
Well, when you put it that way... Always thought that white trail was from the solid rocket boosters and no trail from the main engines. Well no, not that scenario. I know the Shuttle doesn't use any additives the purposes of visible flame. I was trying to remember some other craft that burn H2/O2 that does this, and still can't and maybe getting confused with something else. I was thinking more along the lines of jet planes at airports. There have been wake incidents before and jet exhaust is not that visible either. Okay, but that doesnt really explain the color either. According to black body radiation, to get pale blue you'd need...8 to 10 thousand kelvins... Also, i don't think ive ever seen the engine or exhaust colored blue. Every video, i see it red/orange/white inside the engine, and a reddish blur, if at all, in the exhaust. If you can find a youtube link ill check it out. BTW here's another good idea (I did this last time but couldn't find one) Is there a video showing the main engines burning in the vacuum of space, like the de-orbit burn? Could not for the life of me find a video of this Please Register or Log in to view the hidden image! Hey, that might be what i was remembering! Thanks everyone, things are inclining towards sense again instead of the other way.
The trail from the main engines is the same sort of trail you get from a jet engine - usually invisible, but when conditions are right and the water condenses/freezes, you can see it.
