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Kerosene + LOX rockets have big, bright exhaust plumes, at least in the atmosphere; second stages are are a different matter: Why is the flame of the Falcon 9's 2nd stage (nearly) invisible?

But the kerosene burning Black Arrow looks spooky to me in that it seems to be hovering above the ground. Only after careful inspection can I detect a faint exhaust glow below it in the photo below.

Question: How did the kerosene-burning Black Arrow have transparent exhaust? What is it about the chemistry that makes the kerosene exhaust plume almost invisible?

From @PearsonArtPhoto's answer to Has any rocket been launched into orbit from a plain flat surface?

enter image description here

From Norbert Brügge's West_Europe/BlackArrow/Gallery (from Space Launch Vehicles):

From Norbert Brügge's West_Europe/BlackArrow/Gallery http://www.b14643.de/Spacerockets_1/West_Europe/BlackArrow/Gallery/Arrow.htm

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    $\begingroup$ It didn't use kerosene, it used a fear of spiders and snakes! $\endgroup$
    – BrendanLuke15
    May 27, 2021 at 18:42
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    $\begingroup$ I think this is probably not able to be fully answered without discussing mixture ratios. A (long-ish chain) hydrocarbon-fuel-rich flame is going to be pretty bright, but a lean flame is going to be anywhere between invisible and blue. $\endgroup$
    – Tristan
    May 27, 2021 at 19:38
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    $\begingroup$ As usual, exposure is everything; this video has a brief bit of color footage of the R0 test flight (at about 5:40), showing a distinctly visible, if pale, exhaust plume. $\endgroup$
    – Russell Borogove
    May 28, 2021 at 16:33
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    $\begingroup$ @RussellBorogove thanks for the link. Ironic how there's color video for the failure but only b&w for the success. That roll oscillation was very interesting. $\endgroup$
    – Organic Marble
    May 28, 2021 at 17:49
  • $\begingroup$ @RussellBorogove excellent video, one of their best! $\endgroup$
    – uhoh
    May 29, 2021 at 0:42

2 Answers 2

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Typically, kerosene-LOX engines run fuel-rich, with an oxidizer:fuel mass ratio of about 2.5:1 (as compared to 3.5:1 for complete combustion), leaving significant amounts of unburned carbon soot in the exhaust, which glows yellow as it combusts with oxygen in the atmosphere.

The oxidizer:fuel mass ratio for complete combustion of kerosene with hydrogen peroxide is a little over 7:1 (varying with the exact formulation of the fuel).

The Gamma-8 engines of the Black Arrow, however, ran leaner than most kerosene engines, with a oxidizer mass ratio of 8:1. The oxidizer was 85% concentration hydrogen peroxide (i.e. 15% water), so the actual ratio would still be slightly fuel-rich, but there would be much less unburned carbon in the exhaust compared to most kerosene-LOX engines, hence little external combustion, and no bright yellow plume.

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    $\begingroup$ How did it survive that? I really like the description I ran into once--running a rocket engine oxygen-rich tends to make it run engine-rich. Or does this engine run cool enough that it's not destroyed by the oxygen? $\endgroup$
    – Loren Pechtel
    May 28, 2021 at 3:09
  • $\begingroup$ My initial answer was apples-to-oranges; I was considering the kerosene-LOX stoichiometric ratio rather than the kerosene-H2O2 ratio. With the oxidizer being 85% peroxide, I think the engine is still running just a little bit fuel rich. $\endgroup$
    – Russell Borogove
    May 28, 2021 at 16:24
  • $\begingroup$ @LorenPechtel it's definitely running cooler than a stochiometric LOX-oxidised engine. Chemical reaction rates tend to depend exponentially on temperature. So even if the rocket did run engine-rich, the cool temperature might have made it acceptable. (These engines aren't reusable after all, it's enough if they survive a few test burns and the actual launch.) $\endgroup$
    – leftaroundabout
    May 28, 2021 at 22:09
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I think Wikipedia's Bristol Siddeley Gamma; Advantages of kerosene / peroxide engines can shed some light on the missing light.

The combustion formula of kerosene and hydrogen peroxide is

$$\mathrm{CH_2+3H_2O_2}\to\mathrm{CO_2+4H_2O},$$

and you can see that the exhaust is mostly water, which results in a clean, transparent flame and increases thrust performance because of its low molecular mass.

On the other hand, LOX and Kerosene is different because the combustion formula is

$$\mathrm{CH_2 + 1.5O_2}\to\mathrm{CO_2 + H_2O}$$

with much less water as exhaust.

Also, the space startup Skyrora is using the same combination as the Black Arrow, yet their rocket does not burn as cleanly. I think this is due to the black arrow having an 8:1 oxidizer-fuel mass ratio, while Skyrora is using a 6:1 ratio. This is explained in more detail in Russell's answer.

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    $\begingroup$ @duck Pure water exhaust from h2/o2 engines has a bright blue glow. So there must be more to it than the amount of water. $\endgroup$
    – Organic Marble
    May 27, 2021 at 19:58
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    $\begingroup$ Could it have something to do with the catalyst decomposing the high test peroxide? It produces a lot lower temperature than kerosene and LOX. $\endgroup$
    – Duck
    May 27, 2021 at 20:05
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    $\begingroup$ Thanks for the edit; we're getting closer but "less water" or "more CO2" are not yet explaining why one makes so much bright light and the other almost none. $\endgroup$
    – uhoh
    May 27, 2021 at 21:12
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    $\begingroup$ I realized that a lot of the peroxide decomposes without reacting with the fuel, so the 8:1 ratio wasn't as extreme as it looked, and didn't have the time/energy to do the math. Stoichiometric balance is at about 7.2:1, and the oxidizer here is only 85% peroxide, so it's a slightly rich mix (~6%? excess fuel, contra the usual ~40% rich kerolox mix), not extremely lean as it seemed at first glance. Answer updated and restored. $\endgroup$
    – Russell Borogove
    May 28, 2021 at 16:26
  • $\begingroup$ @Arsenal see above comment, and the answer you're looking for. $\endgroup$
    – uhoh
    May 29, 2021 at 0:17

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