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While a booster on the shuttle had a bit more than twice the thrust of the shuttles engines at take-off, I'd venture a guess that it produced on the order of a thousand time more visible light. In fact, this may be an example of both the brightest and dimmest large rocket engine exhausts for heavy, sea-level launch.

Are there any tables, or approximate data for the relative visible-light brightness of major rocket engines? I'm not looking for IR or UV or anything of particular military or defense interest, just the ordinary commercial and civilian rockets that people can watch launch.

What originally got me thinking about this is that this question was marked as a duplicate of this question within 2 hours, apparently with the idea that any old rocket launch can be seen for about the same distance, perhaps about 100 km away.

I was originally wondering if there is really a "standard rocket brightness" or if some rockets used today are still far brighter or dimmer than others, but as @RussellBorogove points out in comments a large number of launches include solid fuel boosters which tend to be very bright.

So I'd like to ask this question about brightness on the basis of individual propellant combinations, not total launch brightness.

My guess is that for a large engine at sea level, the brightnesses rank like this:

SRB > RP-1/LOX > UDMH/N2O4 > LCH4/LOX > LH2/LOX

Am I wrong?

below: Space Shuttle Discovery from here. Scroll aaaaaaaall the way down to get a better estimate of the total brightness. Note that the hydrogen/oxygen flames are so dim that you can still see right through the flames.

enter image description here

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    $\begingroup$ In my guessing, i've chosen the order of propellant combinations partly based on what I've seen in pictures here in SXSE and on the idea that solids and long-chain carbons have more potential to make soot which incandesces, and H2/O2 is nearly invisible. $\endgroup$
    – uhoh
    Commented Mar 11, 2017 at 17:06
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    $\begingroup$ Don't forget the contribution of smoke to visibility centripetalnotion.com/images/shuttleshadow.jpg $\endgroup$
    – Organic Marble
    Commented Mar 11, 2017 at 17:22
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    $\begingroup$ @OrganicMarble "dim" is a subjective word. It's a comparison, but in no way a premise to my question. In this case I've said "so dim that you can see through them" in obvious direct comparison to the SRB exhaust. Even a 100 mA green LED at ~20km is magnitude +0! $\endgroup$
    – uhoh
    Commented Mar 11, 2017 at 18:12
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    $\begingroup$ Worth noting that almost all current medium & heavy lift rockets have a kerosene or solid (i.e. bright yellow plume) first stage or booster component. Even the major exception, Delta IV Heavy, uses an ablative nozzle which contributes a little bit of carbon incandescence to its hydrogen-oxygen exhaust plume. $\endgroup$
    – Russell Borogove
    Commented Mar 11, 2017 at 20:34
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    $\begingroup$ There have been 3 boosterless Delta IV medium launches in 2003 and 2006, prior to the RS-68A engine upgrade, military comm and weather sat missions. I think that configuration is little-used because it's less cost-effective for small payload launches than Atlas V. Ariane 5 can't get off the pad without boosters -- core engine is 1015kN, gross weight at liftoff about 1.7x that. $\endgroup$
    – Russell Borogove
    Commented Mar 12, 2017 at 19:15

1 Answer 1

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The different brightness is caused by the fuel. The liquid fuel rockets burn hydrogen with oxygen, both are gases and the reaction product is hot water vapour, also a gas. But pure gas flames emit very little light, that is why gas mantles are used for camping gas lanterns. The hot solid mesh emits far more light than the hot gas heating the mesh.

The solid fuel boosters contain aluminium powder and an oxidizer. Aluminium and magnesium are known to burn very bright.

The reason is Kirchhoff's law of thermal radiation; the emissivity for visible light is much bigger for solids than for gases.

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    $\begingroup$ @uhoh: Carbon from incomplete combustion, I'd guess, basically the same as in a candle flame. $\endgroup$
    – Ilmari Karonen
    Commented Dec 1, 2017 at 15:13
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    $\begingroup$ Carbon, yes -- most engines run fuel-rich. $\endgroup$
    – Russell Borogove
    Commented Dec 1, 2017 at 17:40
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    $\begingroup$ Since kerosene is a large molecule with long carbon chains, and as RussellBorogove points out the engines may be fuel-rich, there is probably plenty of soot, and it is probably the solid particles of soot glowing with thermal radiation. Same exact situation for the wax vapor burning in a candle flame (wax and kerosene are similar) where it's the hot, glowing soot particles that make the light, not individual molecules. $\endgroup$
    – uhoh
    Commented Dec 2, 2017 at 1:30
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    $\begingroup$ Acetylen is a small molecule with a short carbon chain of only two atoms, but it burns bright in air when used in carbide lamps. When burnt with oxygen for welding, brightness is much smaller. $\endgroup$
    – Uwe
    Commented Dec 3, 2017 at 18:54
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    $\begingroup$ Despite being a small molecule, acetylene burns with quite a sooty flame, so there are tiny particles of very hot carbon present, which efficiently radiate light. $\endgroup$
    – Steve Linton
    Commented Jun 29, 2019 at 22:22

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