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Rocket exhaust is bright in visible light, we can still see through it but when there's hot, glowing soot it's so bright that we think it's opaque, but it's not. See answers to How does the camera make the exhaust of the Electron's RP-1/LOX exhaust transparent?

But radio is much lower frequency than light and if there is ionization present in exhaust then the plasma frequency associated with the electrons might be high enough to attenuate or otherwise interact with radio transmissions between a spacecraft and something else.

You can convince yourself that flames have ionization by watching the videos below and reading the One Tube Radio blogpost Flame Speaker Science Fair Project but this doesn't necessarily mean that the electron density is high enough to become opaque to RF.

Question: Is there any information at all that describes and quantifies the radio opacity of rocket exhaust? Has this (or related interactions) ever been discussed in detail or even measured in the context of spaceflight?


The two embedded videos below illustrate the plasma component of a flame by applying a high voltage to a flame and accelerating the ions in the flame, which are heavy enough to make sound by transferring their motion to other molecules as well. People also make diodes and triodes from flames and use them to rectify and/or amplify radio signals and these frequencies require the participation of the lighter electrons; see for example these videos:

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    $\begingroup$ Radio properties of rocket exhaust plumes are relevant to military rocket applications; there's probably some good info in the Agard lectures on rocket plume technology but I'm not up to wading through the academics right now. Looks like most of the considerations are regarding emission and radio reflection from the plume, rather than transmission through it, though. $\endgroup$ – Russell Borogove Jun 18 '20 at 1:50
  • $\begingroup$ @RussellBorogove thanks! I've slightly modified the language to allow for all of those types of interactions. $\endgroup$ – uhoh Jun 18 '20 at 1:53
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Question: Is there any information at all that describes and quantifies the radio opacity of rocket exhaust? Has this (or related interactions) ever been discussed in detail or even measured in the context of spaceflight?

I approach this with dread due to my well known aversion to anything dealing with electromagnetic waves; but yeah, there is such information. Fortunately you don't ask for details, just whether it exists.

The paper Radio frequency attenuation by a rocket plume using diffraction theory and finite element modeling doesn't appear super informative by itself, but it has a good biblio showing some of the work that has been done.

Here are some examples that show prior work has been done:

K. Kinefuchi, I. Funaki, and T. Abe. Prediction of in-flight frequency attenuation by rocket plume applying diffraction theories.Journal of spacecraft and rockets,50, (2013).

L. Glen McMillion. A simple method for predicting RF attenuation through a rocket exhaust plume.U.S. ArmyResearch Office, (1997).

L. D. Smoot and D. L. Underwood. Prediction of microwave attenuation characteristics of rocket exhausts.AIAA6th solid propellant rocket conference,65-181(1965).

B. van der Veek, S. Chintalapati, D. R. Kirk, H. Gutierrez, and R. F. Bun. Modeling and validation of ku-band signal attenuation through rocket plumes.Journal of Spacecraft and rockets,50, (2013)

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    $\begingroup$ Thank you for your speedy answer; dread is good, in moderation at least. Under advisement by RB's comment I'd opened up the wording of the question to also allow for affects other than attenuation, but I'd missed the blurb that you've quoted. I've inserted "(or related interactions)" to it. Since you talk about diffraction this actually improves agreement. $\endgroup$ – uhoh Jun 18 '20 at 5:05

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