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...from common observation, I can easily conclude there is a lot of infrared (from burning, superheated fuel), ending somewhere near yellow of visible spectrum; there should be next to none of UV and higher. But how far up does it reach? Do the engines emit a considerable amount of microwaves?

This question is motivated by "How does a Falcon 9 booster know how far away the ground is?", and relevant available options.

The obvious solution is "reflected waves", but they must be in a spectrum not obscured by the engines. Ultrasound, or anything audio is right off, with the enormous level of audible noise. A laser (visible or IR) distance measurement will have the dot lost in the bright flame. I wonder how viable microwaves are, though - essentially, a radar.

For example, during a spacecraft reentry, the communicational blackout is caused by the plasma of superheated air and ablator, radiating on a spectrum broad enough to flood both long waves and x-rays with enough noisy energy to make them not viable for communication. Rocket exhaust is much less energetic, but it's still one of most energetic large-scale emissions achieved by technology and I wouldn't be surprised if it exceeds the standard "visible+infrared" range of "burning stuff".

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    $\begingroup$ I think the spectrum of a rocket flame is going to vary a lot depending on the type. (Kerosene, Hydrogen, various solids, hybrid etc.) A quick example here. $\endgroup$
    – Andy
    Commented Jun 20, 2016 at 13:55
  • $\begingroup$ @Andy: The best answer would contain a full overview. A good one will give the brackets. An acceptable one will give a typical example. $\endgroup$
    – SF.
    Commented Jun 20, 2016 at 14:07
  • $\begingroup$ @Andy: The problem with the paper you linked is that it cuts the graphs off way early, the highest going to 800nm and it seems the cut-off point is the limit of their equipment rather than end of the emission band. Only the calcium graph is "extinguished" reasonably on both ends, but does anyone seriously use calcium in rocket fuel? I mean, I know you can load up any crap into the hybrid engine and it will still work, but does any calcium compound contribute anything? $\endgroup$
    – SF.
    Commented Jun 20, 2016 at 14:20
  • $\begingroup$ By the way I wouldn't expect the article I linked to be acceptable as an answer - just an example of the sort of research that has been done previously (yes that one's only around the visible range). It has one item in the references mentioning IR and UV for example; if someone can find a copy of that that it might make an answer. $\endgroup$
    – Andy
    Commented Jun 20, 2016 at 14:36
  • $\begingroup$ @Andy: Definitely so. I'm most interested in the upper bond of the spectrum: how much interference does the engine cause to microwaves=radar. $\endgroup$
    – SF.
    Commented Jun 20, 2016 at 19:21

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The engine plume will basically be black body radiation. There will be significant variations in intensity depending on the emissivity of the fuel (ie: SRB vs Hydro/LOX) but the spectrum should correspond to exhaust temperature.

The graph below shows the emission spectrum from a PolyMethylMethacrolate (PMMA) solid fuel grain. Other than the UV peak at .3 microns, it is consistent with the black body –like spectrum at 5270K.

enter image description here https://www.stellarnet.us/oh-emission-spectra-of-hybrid-rocket-motors-using-pmma-and-htpb

enter image description here http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html

Further into the infrared, there are peaks due to CO2 emission

enter image description here https://www.researchgate.net/publication/252874469_Characterizing_and_overcoming_spectral_artifacts_in_imaging_Fouriertransform_spectroscopy_of_turbulent_exhaust_plumes/figures?lo=1

Solid fuel often has additives, such as aluminum, which provide additional bands in the IR spectrum.

Your question indicates you are looking for a “quiet” portion of the electromagnetic spectrum so you can get a high signal/noise ratio for your altimeter. Microwave (radar) looks like a safe bet.

Since the Falcon9 only lands on designated pads, a radio beacon system may be more practical than an altimeter, since the beacons could give a 3D fix.

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