in this series of two episodes I will
attempt to decipher some of the things
we see when we watch a rocket launch
more precisely I will focus on what we
can learn just by looking at the flames
coming out of the rocket nozzle a word
of warning to avoid a multiple our video
I will have to skip off a mini rocket
propulsion concepts but don't worry I'll
come back to them later in a series of
videos focused on the detailed workings
of rocket engines more information about
this new youtube channel and the
reference documentation at the end of
the video.
The ignition of a rocket engine is
always a critical moment, it goes from
producing no thrust to full thrust in a
few seconds while going through
intermediate regimes it was not really
designed for. It's a period of
instability especially for liquid rocket
engines and it should not last more than
a few seconds to avoid damaging the
engine during ignition one can see the
engines shake, rattle, buckle and spit out
an unstable plume as the combustion is
far from being ideal due to poor
propellants mixing. Thus unburnt fuel is
often ejected in the form of black soot
for kerosene-fueled engines. This
transient phenomenon is not usually
visible for solid boosters as it's
almost instantaneous for them. While the
engines are off you have probably
already noticed all the water spraying
at the base of the rockets. I'm sure
you're familiar with the very loud noise
produced by a rocket launch, a sound
so powerful that sound wave reflecting
off the ground could damage the launch
vehicle or even the payload. For this
reason deluge or water is sprayed under
most launch vehicles to dampen the
sound wave and protect both the engines
and the launch pad. The launch, pad often
thought to be near indestructible, is in
fact very fragile once exposed to the
rocket plume. By the way when a launch
pad has a such water spraying system, the
large smoke-like clouds that can be
seen at liftoff actually consist more of
steam and water droplets than exhaust
products from the engines. One more note,
you can see on this video how Ariane 5
actually produces smoke rings out of its
flame trench due to the startup vortices
around the exhaust plume. During this
initial stage one can also observe the
ignition systems at work. I will start
though by pointing out that the sparks
that you can see here under the space
shuttle rocket nozzles play no role in
the ignition. It's a commonly made
mistake but these parks are here to
consume the unburnt propellants around
the base of the rocket, otherwise they
would accumulate and at some points
possibly generate a small explosion that
could be detrimental to the ignition
sequence. Anyway
let's forget these parts and focus on
the real ignition systems. Let's start
with the merlin engine from SpaceX which
is ignited using a pyrophoric
material. This means that this
material will react instantaneously when
in contact with oxygen; a very useful
feature indeed for an ignition system.
The Merlin engine uses triethyl-borane
mixed with triethyl-aluminum. As its name
indicates triethyl-borane contains boron
which is known to burn with a green
flame when combined with oxygen. During
the startup of the Merlin pressurized
the helium is sent through the turbopumps
to start them up and bring
propellants to the combustion chamber
within a fraction of a second. At the
same time these pyrophoric materials are
released in the combustion chamber to
initiate the flame that will ignite the
main propellant one they are injected a
few moments later. For the merlin engine
these propellants will be oxygen and
kerosene
hence the brief green boron based flame
that precedes the main yellow flame
observe this green glow here and here or
here here at the ignition during the
entry of the stage or here I think you
get the point. Somewhat related to
pyrophoric materials which react in air
instantaneously, hypergolic materials, a
term which is more widely known, react
instantaneously with each other and no
need of any additional oxygen. The titan II rocket who carried the Gemini
spacecraft into space used hypergolic
propellants and thus did not require any
ignition systems since nitrogen
tetroxide and aerozine 50 react on
contact
Unlike the merlin engine the turbopumps of the titans engines would be
started using solid powder charges
that burn for one second
approximately and would bring them up to
their steady state speed. What's funny
with that launch vehicle is that one can
easily hear the startup process when the
powder charge are detonated. Listen in.
And that's not even rockets to make that
type of noise please enjoy the startup
noise of the Merlin's turbopumps.
I love that sound. And now listen to the
rumble of the F-1 engine turbopumps from
Saturn 5's first stage.
By the way you can notice that the
engines are lit-up sequentialy and not
simultaneously for the Saturn 5 as it's
the case for many launch vehicles with
multiple engines such as the Angara
rocket. Listen in. It was a bit fast so
let's play it once again
Finally one last fun ignition system to
investigate is the one from Soyuz. It's
very basic but it's very reliable. They
simply use wooden sticks attached to
each exhaust nozzle. Each of these sticks
containe four pyrotechnic charges
called PZU that will light up the
propellants. At take-off, the sticks burn
up in the exhaust flame while being
blown off by the flow. By the way let's
take advantage of this wonderful view
point and the other sized rockets to
show you the different stages of its
startup sequence. First the turbines in
these engines which power the turbopumps that supply the propellants are
spun-up using gases produced by the
decomposition of hydrogen peroxide.
The valve that regulate the pressure of
these gazes is opened in three steps
which give us three different type of
flames to observe before liftoff.
Lets' go back to the previous shot and review
these steps in slow motion.
First, the main oxygen valves are opened and the
PZU you are set off to burn with the
oxygen. Three seconds later the main fuel
valves are opened slightly and the
turbopumps are spun-up. During this
initial thrust phase the four boosters
and the central core are only running at
five percent capacity while several
parameters are monitored before
continuing the launch sequence then five
seconds later the four boosters are
simultaneously transitioned to a high
power regime and they are maintained at
70 percent max power while the central
core remains at five percent max power
hence the long flame at the center due to
poor mixing conditions. This flame
disappears as we enter the last phase of
the launch sequence. During this final
step the four boosters go to 100% power
while the central core goes to 70
percent max power. This is enough to lift
of the Soyuz rockets.
All right, let's rewind and watch it once
again at real speed
 
 
When the engine start up there is often
unwanted combustion near the base of the
vehicle because of unburnt propellants.
Sometimes these burnt gases can move up
the side of the rocket because of
buoyancy. It can be pretty impressive as
seen here permanently with the delta 4
heavy launch vehicle a king of this
phenomenon. Looking at this footage it
would seem something is wrong with the
launch as it's unusual to see the rocket
already half blackened by fire while
still on the launch pad but no while
surprising these flames were predicted
and deemed acceptable by the engineers.
It was even more impressive during the
early flights of the delta 4 heavy when
the rocket was literally on fire as it
cleared the pad and now have a look at
this night launch of a delta 4
once the engines generate enough thrust
after ignition they create a
low-pressure region under the rocket
because of the high velocity of the
exhaust gases this is the venturi effect
predicted by Bernoulli's law. There is also
an entrainment effect on the
surrounding air by the exhaust plume
overall this results in lots of air
getting pulled towards the nozzle
exhaust thus we can see flames that go
up first but then turn around if they
are close enough from the nozzle sucked
in by the exhaust plume and it's low
pressure. As you can see this was
particularly striking with the Saturn 5
rocket. Let's fast forward a few seconds
and let's take a look at these rockets
as they clear the pad we notice right
away that the flames coming out of the
nozzle can take on different colours
Rose yellow orange if burning some
specific hypergolics, ...
white blue violets multicolored and even
transparent for some propellants such as
hydrogen oxygen like here on the space
shuttle. There are several factors that
contribute to this variety of flame
colors for instance solid rocket
boosters spit out large molecules if not
pieces of propellants or soot. All these
warm particles thus emit an intense
light as seen here because of blackbody
radiation like light bulb filaments, your
fireplace, or the Sun. That is why all the
solid rocket boosters have such a bright
yellow white flame there are good
approximations of so-called black bodies
on the other hand engines that use
oxygen and hydrogen as propellants like
on the space shuttle or Ariane 5 produce
barely visible flames as they exhaust
water vapor without any large carbon
containing molecules. Here is the
emission spectrum for hydrogen and
visible light as you can see it's mostly
in blue and violet plus the famous H-alpha
line in the red. But the most
energetic lines the three on the left
here are comparatively brighter at
higher temperatures than the H-alpha
line. Since the nozzle exhaust gases are
still very hot it's no surprise that we
find these blue tones in the flames of
most oxygen hydrogen engines like the
Space Shuttle. But there are a few
exceptions like the RS-68 engine from
the delta 4 launcher despite also using
oxygen and hydrogen as propellants it
produces a yellow flame as can be seen
here in fact the throat of the nozzle is
lined with an ablative carbon layer and
this carbon ends up in the flame giving
it this yellow color so what you see
here in the flames comes from that
ablative coating that is reacting with
the exhaust. Rocket engines usually use
more fuel than oxidizer for reasons that
will be explored in another video. What
matters here is that we have an excess
of hydrogen in the exhaust gases and
these erosions wants nothing more than
react with the oxygen from the
surrounding air this brightens the blue
tones of the flame thanks to the
previously highlighted hydrogen spectrum
this phenomenon is a lot more apparent
with kerosene engines like the ones of
the Falcon 9, Soyouz or Saturn 5
rockets. All these engines use a gas
generator cycle which means a small
quantity of fuel and oxidizer
is diverted from the main
chamber and are instead burnt in a small
separate chamber. This small combustion
chamber next to the main one is what is
called the gas generator as it produces
gas to power the turbine part of the
turbo-pump. That is the turbopump that
feeds propellant from the tank to the
main combustion chamber these gases arethen injected without producing much
thrust through a separate exhaust and
once again
this gas generator usually operates with more
fuel than is optimally required in order
to increase the life of the engine this
fuel rich mixture produces relatively
cool combustion products deprived of
oxygen that are not too damaging to the
metal parts of the turbine to keep it
short here since we will revisit this in
a later video let's remember that the
combustion products from the gas
generator are fuel rich, not particularly
hot, and they have to be dumped overboard
one way or another.
Among the engines that use this famous
gas generator, must employ an exhaust
pipe, or simply "exhaust", to discard the
burnt gases it's easily spotted here on
both sides of the Vulcain 1 engine that
powered the first stage of Ariane 5 until
2004. By the way they are to exhaust
here because Vulcain 1 has two gas
generators for two independent turbo
pumps for fuel and oxidizer.
Sometimes the exhausts are clearly
visible during lunches as shown here
with the Atlas rocket. You see the yellow
plume on the side ? In this zoomed-in view
we can clearly see the exhaust pipe the
flame coming out of it looks dark
because of the relatively cold temperature
of the burnt gas as mentioned
earlier. The same observation can be made
for several launch vehicles like the
Saturn 1 where one can spot four
different exhaust nozzles.
In this footage we can see three of
these nozzles spitting out this cold gas
that is mostly useless for propulsion
similarly this nozzle is perfectly
visible next to the Merlin engine from
SpaceX. And you can see them here
alongside the YF-20 engines on the
Chinese Long March Rockets in other
versions of gas generator cycle engines
for example the F1 engine of Saturn 5's
first stage, the J-2 engine of Saturn 5's second stage, Ariane 5's Vulcain 2
engine or even the groundbreaking V2; these cold gases are
not discarded via a separate nozzle.
Instead they are re-injected in the main
flow path through a series of small
orifices sometimes upstream of the
throat as seen here on the V2 engine, but
usually downstream of the nozzle throat
for the more modern engines. These few
hundred degree Celsius cold fuel-rich
mixture cannot burn there as the
exhaust gases are also devoid of oxygen
so it's still not contributing much
thrust to the rockets but it's forming a
protective film along the nozzle walls
that shield them from the thermal stress
of the hottest gases. Take a look at the
another exit these kind of dark curtains
that make the exhaust look dirty is in
fact just the film of cooler gases that
has been shaped to a cylinder around the
flame
since these cooler gases are products of
a fuel rich mixture these curtains
contain unburned fuel that can react
with the oxygen from the surrounding air
once out of the nozzle this is not
dissimilar to the burning of the excess
hydrogen around the space shuttle that
we saw earlier it takes some time for
the fuel and oxygen to mix with each
other and since the exhaust speed is
very high the fuel and oxygen only start
reacting with each other a few tens of
centimeters downstream of the nozzle
exit there a new flame front is formed
from the mixing of the cold few rich
gases and the surrounding air the speed
of this flame front
more or less matches the speed of the
outer layer of the exhaust which gives
the impression that the flame is trying
to catch up with the rocket engine
without ever succeeding this post-combustion
of course provides no useful
thrust to the engine as it occurs
outside of the nozzle it is just a
secondary effect from the release of
unburned fuel but it can be damaging for
the engine as it increases its thermal
load. This phenomenon is very apparent on
the Saturn 5 here as it clears the launch
pad. More complicated now what do you
think is happening here for the Saturn
1b this rocket had eight h1 engines with
open cycles that is with gas generators
only the four outer engines discarded
the gas generator exhaust into the main
combustion chamber upstream of the
throat just like the v2 that heavily
inspired the h1 engine and so the dark
curtains followed by the bright flame
fronts are only visible for these four
exhausts the inner engines exhausted
these gases through separate external
nozzles and thus their flames remained
clear can you see the difference ? This
other view also highlights the
differences between the four inner and
the four outer engines.
Regarding the small white pieces that
rain down the launchpad after ignition
there are simply bits of water ice that
come off the launch vehicle as it starts
vibrating when the engines are on this
ice forms before the launch as cryogenic
propellants like oxygen are loaded in
the rocket the water vapor from the
surrounding air does not just condensate but forms an ice sheet along the
very cold walls of the tanks this is why
it's very common to see this ice rain
for most of launches as most of the
space launchers use at least liquid
oxygen as propellant as we witness the
liftoff of an Atlas 5 rocket from an
onboard camera highlighting some of the
phenomena we described earlier this
episode is already 18 minutes long and
we still are only a few meters of the
ground however in my opinion some of the
most interesting features seen in
rocket flames only occur later in the
flight thus let's plan on meeting again
soon for the next episode in this series
focused on rocket flames this
accelerated footage of the attack 5
launch gives you a taste of what will be
discussed next time sources and
references are from widely available
public sources such as books or
scientific papers or these references
are listed in the description of the
video this episode was just a test for
me as I have no clue about how my very
poor English accent will be received by
an international audience please let me
know what's your opinion about this
video in the comment section and if the
feedback is good I will make new videos
on this channel I also take a moment to
thank all of you that may support me
through patreon thanks to you this
channel will improve its quality and
also hopefully expand its horizons by
shooting episodes from exciting varied
places
thanks to all of you who watched this
video and hopefully see you all very
soon for the sequel of this episode on
rocket flames
