Index Home About Blog
From: (Henry Spencer)
Subject: Re: Footprints under lunar module
Date: Wed, 23 Jun 1999 20:59:11 GMT

In article <7kqjjt$h58$>,
Kim Keller <> wrote:
>I'm pretty sure the effect is due only to the drop in atmospheric pressure.
>I can't imagine there being any unburned fuel escaping the combustion
>chamber, but I could be wrong on that.

There's generally lots of unburned fuel, because almost all rocket fuel
combinations perform better if run fuel-rich.  (The extra fuel, or its
decomposition products, improves the gas properties of the exhaust enough
to make up for carrying the extra mass.)  However, the same is true of
combinations like LOX/LH2 and N2O4/UDMH, whose plumes are nearly invisible
even at launch.

The bright yellow flame of a LOX/kerosene rocket is definitely from carbon
particles in the exhaust.  Current opinion is that they form when fuel is
cooked while still in the form of droplets; the glow is largely absent
when the fuel is introduced into the chamber as vapor.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Let's try a diagram
Date: Wed, 7 Jul 1999 18:57:07 GMT

In article <>,
Simon Bradshaw <> wrote:
>> -all artists impressions of landing LMs show a bright engine exhaust...
>This is a common error in space art...
>In fact, of the commonly-used propellants, only solid propellants and
>LOX/Kerosene seem to have very luminous plumes (owing to exhausts rich in
>particulates or carbon).  N2O4/UDMH and LOX/Hydrogen both give almost
>transparent exhaust plumes.  So does H2O2/Kerosene, which somewhat
>surprises me: I would have expected an appearance similar to that with LOX
>as oxidiser.

That *might* be because the major peroxide/kerosene systems have injected
kerosene into the hot-gas stream from decomposed peroxide, which gives
much more efficient atomization and vaporization.  That might matter,
because apparently the carbon particles in the conspicuous LOX/kerosene
exhaust are the result of kerosene *droplets* being heated strongly;
systems which inject a hydrocarbon as a gas reportedly don't show them.
So maybe the rapid vaporization in the peroxide/kerosene system gives
the kerosene no chance to react with itself.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: Doug Jones <>
Subject: Re: Saturn V exhaust
Date: Sat, 17 Jul 1999 13:05:40 -0700

Matt Jackson wrote:
> I have noticed something curious in footage film of Saturn V launches,
> where the exhaust on the first stage kerosene/LOX engines seems to be
> dark and sooty up to a few feet after leaving the nozzle, and then
> burns brightly onwards.  What is actually happening here?

Delayed mixing and secondary combustion of fuel-rich gases.  IIRC,
the extremely rich turbine exhaust was introduced into the nozzle a
few feet up from the exit, giving slightly better performance than a
simple vent. These gases were relatively cool and very fuel-rich
(mixture ratio less than 0.4:1 O:F), really were sooty and black,
and caught fire after a few feet of mixing.

But what's really pretty are lean flames at around 2.8:1-  purple
violet like an afterburning jet engine.  A mother to cool, though.

Doug Jones, Freelance Rocket Plumber

From: Doug Jones <>
Subject: Re: Saturn V Engines
Date: Sun, 07 Nov 1999 11:55:40 -0800 wrote:
> I know this has been asked before and I always
> intend to pay attention to the answer, but I
> keep forgetting to do so.
> So: what is the black area in the rocket exhaust
> of the 1st stage Saturn V engines directly below
> the nozzle? Ablating material? Unburnt fuel?

IIRC, the F-1 turbine exhaust was injected into the main exhaust in a
scroll a few feet above the nozzle exit.  This gas was the result of very
rich burn, around 0.3:1 O:F ratio (3+ is stochiometric) so it was really
black smoke.  It exited the nozzle at relatively low temperature, then
ignited after mixing with the air.  The overall mixture ratio of the engine
was about 2.2:1, still quite rich, explaining the very bright yellow-orange
afterburning plume of the engine.

When run leaner, around 2.8:1, the plume of a LOX-kerosene engine gets
smaller, dimmer, and shifts to an eerie purple color, which I think may be
emission lines from carbon monoxide.

Doug Jones
Rocket Plumber, XCOR Aerospace

From: (Henry Spencer)
Subject: Re: Saturn V Engines
Date: Sun, 7 Nov 1999 20:52:37 GMT

In article <8046d4$9sq$>,  <> wrote:
>...what is the black area in the rocket exhaust
>of the 1st stage Saturn V engines directly below
>the nozzle? Ablating material? Unburnt fuel?

In general, light emission from rocket exhaust plumes is complicated; they
aren't simple uniform flames.  They have shock waves bouncing back and
forth inside, and various other complexities.  Also, since rockets
generally perform best if run a bit fuel-rich, there is actual combustion
happening when the plume hits atmosphere, to complicate things further.

In the case of the Saturn V's F-1 engine, however, what's happening is
fairly simple.  The F-1's pumps are run by a turbine, driven by gas from a
very fuel-rich gas generator.  The spent turbine gas -- which is very
smoky and sooty, as you'd expect from very-fuel-rich combustion of
kerosene -- is dumped into the exhaust nozzle partway down, partly just to
get rid of it and partly because it's much cooler than the real exhaust
and helps keep the hot stuff away from the nozzle wall.  (The dump
manifold is the tapered ring that runs around the nozzle about halfway
down.)  So the emerging exhaust has an outer layer of sooty turbine gas,
and that's the black area.  After a short distance, it's mixed enough
with air to ignite, which is why the black disappears fairly abruptly.
The space program reminds me        |  Henry Spencer
of a government agency.  -Jim Baen  |      (aka

Index Home About Blog