now if you happen to have a turbojet
or you're thinking of buying one or if
you watch our video series on this
subject we'll show you how to build one
you really might want to consider adding
an afterburner
[Applause]
uh
[Music]
these little devices can add a
substantial amount of thrust for
relatively low weight low cost and ease
of assembly it's a pretty good thing
now not all jet engines are turbo jet
engines
there are pulse jet engines
pulse detonation engines ram jet engines
and even the very first forays that were
made into the construction of commercial
scale
turbo jet engines were done a little bit
before world war ii where they took
powerful gasoline engines to drive the
compressor stages that compress the air
before the fuel was added and they
produced the exhaust blast
and our very first
videos on the jet series
also used a hybrid type of design where
we took some powerful edfs or electric
ducted fans about five kilowatts each
placed them in series and produced a
modest level of compression
before we added the fuel
heated the exhaust and nearly doubled
the output from the fans alone it really
worked
one of the problems though with any of
these designs is the enormous amount of
energy that's necessary to drive the
compressor stages
just to give you an idea of the
magnitude of that power
if you look at the allison 250 series of
turboshaft engines these are small
turboshaft engines that have been around
for decades
and produce a rotary mechanical output
being driven by the exhaust gases by a
turbine and can range from about 350 to
about 900 shaft horsepower
in those engines
the turbine stage actually sends only
one third of the amount of power that's
extracted to the output shaft
twice as much power as you take out
is actually recirculated to drive the
compressor stages and that's a small
engine
another example is this turbocharger
over here
this single stage compressor just this
little part of this turbocharger at full
power will consume about 250 horsepower
from just this little thing
the power is enormous
and so that's why early on the military
designers decided to take the hot high
pressure high velocity exhaust from the
jet engines
run a turbine and send that power to the
compressor stages
it was lighter and more powerful than
the gasoline engines and hence the birth
of the modern turbo jet engine
we decided to make the same sort of
transition and got away from the hybrid
design and started building jet engines
based on the turbo charger to turbo jet
conversion
now this is actually a pretty popular
project and there's a lot of youtube
videos out there about it because it's
relatively easy to do
because there are millions of these
turbochargers built every year
all of the expensive engineering the
design the precision machining of the
compressor wheels the turbines the
housings the bearings is spread out over
millions of examples
and so all you really have to do to
construct the turbojet is to build the
combustion chamber it's not trivial but
it's a heck of a lot easier than the
other components and if you watch our
videos on the subject we show you how to
do it
now what makes
building an afterburner possible
is that all turbojet engines pass
substantially more air through them than
they actually need to burn the fuel
the reason for that is that if you take
that ideal mixture that golden ratio or
stoichiometric ratio of fuel and air
and you burn the fuel so that you use up
all the oxygen you use up all the fuel
and at the end of the reaction there's
no nothing left behind
you produce a flame temperature that is
so high that you would almost instantly
melt anything downstream of the flame
so what the designers do
in a turbo jet is they take the
compressed air that comes in from the
compressor
they send some of it into the combustion
chamber to burn the fuel
and then as those very hot exhaust gases
are moving down toward the turbine
they allow additional air to dilute that
hot flame more air than was actually
needed to burn the fuel
to bring the temperature down to the
point that it will not melt the turbine
blades this is great because what ends
up happening is at the end product of a
turbojet process you still have a
substantial amount of oxygen in the
exhaust gases
however you've not only cooled the air
by dilution
but the amount of energy that's taken
out by the turbine stages
causes a lot of cooling
shrinking and decrease in pressure of
the gases that are coming out of the
exhaust stage
so what the afterburner does is it takes
advantage of that residual oxygen adds
additional fuel and reheats those
exhaust gases it can double or triple
the volume of those exhaust gases it's
often called a reheater or an
afterburner same principle
by increasing the volume of those gases
if you don't increase the diameter of
the duct through which they're passing
too much
the only way for that much larger volume
to get out is it has to get faster it
has to accelerate
and when you accelerate the same mass of
of air
you increase the momentum mass times
velocity and momentum is another name
for thrust when you're talking about
turbojet engines
this is great i mean when you think
about it it's lightweight it's easy to
build there are no moving parts and
because
you can fabricate these such that there
are no metal contacts to the flame you
can go to extremely high temperatures in
the after burner
so you might think then why aren't
afterburners put on all jet engines
the problem is very low fuel economy
you can double the fuel flow in an
afterburner double the fuel flow that
you're normally using in a uh in the
basic turbojet engine and increase the
thrust maybe 60 or 70 percent
and that might be acceptable if you're
trying to take off from a carrier deck
or you're in a combat maneuver but if
you're flying a commercial jet across
the pacific you're going to probably end
up in the water
now the reason for that is not because
the afterburner
fails to burn all of the fuel it does
it's an efficient way to burn the fuel
the reason for this is a fundamental
fundamental physical property that is
inescapable and it's
present
in the operation any kind of internal
combustion engine and a jet engine is an
internal combustion engine
this principle is so important that even
though i reviewed this
some time ago i'm gonna go through it
again because i think it'll help to
understand
what i'm talking about
if i take this air piston here
and i plug the output hole from this
and i compress the gas inside of the
cylinder my muscles are doing work on
the gas inside the cylinder force times
distance
if i allow the piston to move back the
gas to expand the gas does work on my
muscles
one unit of work in one unit of workout
no big deal if however
i take this piston and i compress the
gas twice
then i place it in a heat source and
begin heating the gas to twice its
absolute temperature so if it's 300
kelvin goes to 600 kelvin if it's 0
degrees celsius goes to 300 degrees
celsius
at that point the pressure of the gas
molecules inside roughly doubles
now when i allow it to expand the force
at every point in that expansion is
twice as great as it was when i
compressed it so i get two units of work
out
one unit in
heated up
two units out one net unit of work okay
do it again but this time i push the
cylinder in much harder and i bring the
pressure to four times its original
level
now again i heat the gas
same amount of heat same number of air
molecules and i bring the pressure up to
roughly twice its original point
now when i allow the piston to move back
it's moving farther and at greater force
and so i get two large units of work in
the point being
that the more work you invest in the
beginning in the compression stage the
more usable work you can get out at the
end now for your physicists out there
yes i am rounding off some of the ideal
gas law numbers but the underlying
principle remains
more compressive work in
more usable work out
and this is one of the reasons why a
diesel engine is more efficient than a
gasoline engine a typical gasoline
engine will have a compression ratio of
maybe eight nine ten to one
a diesel engine maybe eighteen to twenty
to one and a high performance turbo jet
engine can exceed fifty to one five zero
to one it's a very efficient heat engine
and that's what the problem is with the
efficiency of an afterburner because the
turbine stage has removed a lot of work
energy from the fuel from the gas
the pressure drops inside of the
afterburner and so when you're adding
the fuel you're adding it at a lower
compression ratio than when it was burnt
inside of the turbojet there's no way to
avoid that
but if you want to take off from a
carrier jet deck
it's the way to go
and if we really don't care about fuel
prices you know they're as low as
they're ever going to get right now then
we don't care either we just want more
power
so with that basis in physics behind us
let's go outside and let's fire up the
turbojet engine
all right this is the turbojet and as
you can see it looks pretty complicated
you can put down your paper and your
pencil if you're interested in
designing something like this or using
some of the ideas that we have we're
going to follow this video with a series
of short few minute videos that are
going to break this down into all the
subsystems including the source for the
parts that we're using and how we build
this and how we operate this so for
today you can just watch me go through
the abbreviated startup sequence and
enjoy so let's get going
all right let's get some headphones
now we're going to start by turning on
the oil pump and the fuel pump
we're going to adjust the oil pressure
to about 40 psi
and the fuel pressure to about 50 psi
now when i turn on the starter fan
everything is going to get so loud
everything will have to be in text boxes
you're not going to hear me
all right here we go
so
so
[Music]
all right one of the nice things about
having the starter fan on here is that
when we're done with a run
rather than waiting an hour for this
thing to cool off as it radiates its
heat we can run this at a low level for
about 10 minutes and we can
exchange parts so i'm going to do that
right now
[Applause]
okay
okay so now let's get into the nitty and
the gritty of the actual afterburner
design
now this is a nozzle i took this off the
jet engine that we just demonstrated
and almost invariably you're going to
want to have a nozzle on the output of
any kind of a turbo jet engine
the reason for that is that the exhaust
gases that come out behind the turbine
or in the case of a turbocharger from
the x deucer
are still under some residual pressure
and you want to convert that pressure
into greater velocity what the nozzle
does is just like putting your thumb
over the end of a garden hose it causes
the gas
molecules to speed up more momentum more
thrust
now the design of a nozzle
is in theory
pretty difficult to model because there
are so many variables
it depends on the ar ratio of the
turbocharger this is a value that
determines the mass flow versus
compression ratio of a particular model
and it varies it also depends on what
kind of pressures you're running inside
of your combustion chamber even the
bypass ratio and the fuel that you're
using it's pretty difficult to model
but in practice it's actually very easy
to do
what you do is you obtain a flange that
will mount onto your turbocharger
and then place a rather aggressive taper
on the output
aggressive being that if you start with
a ratio of two to one in area so not
diameter but the area of the output is
half the area of the input
this is almost certainly going to be a
little too much
once you've fabricated that bring it out
to the engine connect it up and run it
up
measure your fuel flow your pressure
your thrust your temperatures
then take it off
bring it inside and slice a couple of
millimeters off opening up the aperture
just a little bit and test it again
do this several times until the numbers
that you care about stop getting better
you're done it's that easy and it might
sound like a little bit of work but it's
fun because that's why you built the
turbocharger in the first place into a
jet engine you want to mess around with
it
now the afterburners themselves are
relatively easy to build because of the
fact that there's so much aftermarket
availability
of compatible components to
turbochargers you can get flanges of
different sizes and styles t3
t4
four bolt
flares tapers tubing that all sort of
work together so that all you really
have to do is cut the tubing
drill some holes and do some welding it
makes it a lot easier for you to do
simply because you don't have to
fabricate the tubes
now the designers of a military
afterburner
face much bigger challenges because if
you're going to put an afterburner on a
fighter jet you want it as light as
compact and as efficient as possible you
don't want a huge stove pipe sticking
out of the back of a fighter jet
the problem is that turbojet exhaust
velocities even in our little turbojets
approach mach 1.
and so literally you only have a few
milliseconds for all of the fuel to be
added
mix and blend evaporate burn and expand
before it gets out of the exhaust duct
that's tough
and it has to happen inside of the
afterburner
if you have the burn continue outside of
the afterburner it may look cool may
look like a nice blow torch but it does
nothing because all of the expansion of
the gas is happening in the free
atmosphere it has to happen in the
confines of the duct in order for that
velocity to accelerate and produce more
thrust
so what the professionals do
is they install what are called
turbulators or flame holders
these are little obstructions that are
placed in the output of the exhaust of
the jet engine often they're shaped like
v
grooves in rings or in
bars across the output
and they do two things one is they
increase the turbulence and the mixing
of the fuel and the air that's important
but more important what they do is they
produce a shadowed area of relatively
low velocity gas right behind them
that's important because it's very
difficult if not impossible to maintain
a stable flame
in an exhaust velocity that exceeds the
flame front propagation velocity it's
one of the reasons why you could put out
an oil well fire with a stick of
dynamite
so by placing these things in in the
exhaust you improve the stability of the
flame
but anything that you put in the exhaust
path obstructs some of the exhaust
coming out of the engine so can decrease
the efficiency of the underlying jet so
it's a very delicate balance that they
have to perform to get
good performance overall
we're not putting these afterburners on
a fighter jet and so we can take
advantage of a few tricks that makes
this a lot easier to do
first of all
we can make the ducting that the app
much larger in
moving through it more slowly
then we can make it much longer so that
more slowly moving gas has to travel a
longer distance this gives us more time
for the reaction to occur
in addition the flare or the increase in
diameter of the tubing also introduces
turbulent mixing
gas that's traveling right up the axis
the center portion of the output from
the jet engine is moving fastest the gas
that's
sliding along the walls is moving more
slowly and so what you get is sort of a
rotating toroid or donut or snow smoke
ring as the gases are moving up the end
of the tube
this can be enough to give you a stable
afterburner function
we found it was a little iffy so we
decided to add some flame holders and i
found it's kind of a neat and convenient
way to do this and it's very flexible
right at the point where the tube has
increased in diameter and the gas
velocity is decreased drilled eight
holes four pairs 180 degrees apart and
then welded on some threaded studs over
those holes
then
you take a solid rod like this piece of
tungsten welding rod and you cut it to
slightly longer than the outside
diameter of the tube
insert it into opposing holes and then
just take some screws or some set screws
and lock it into position
what's nice about that is you can add
one two three up to four
flame holders of varying diameters
an even better way that we did in sort
of version 2.0 is we took some thin wall
stainless steel tubing
cut it to slightly less than the inside
diameter of the tube
located it over the holes and then drove
screws into each end that allows us to
lock this into position
this also gives us the ability to add a
wide variety of different diameter tubes
and another little trick is in the
middle position put this in a vise with
a couple of metal blocks and squish the
middle together so that when these stack
they can stack more closely together and
you make the segment where the flame
holder is a little bit thinner just
makes it more convenient
and it works
i know a fair amount of engineering as
i'm sure a lot of you do too
and there's always a risk that you can
get a little cocky you can think you
know a little bit more than you actually
do
and that's particularly egregious when
you ignore real world evidence because
it kind of gets between you and your
beautiful design or your theory
and i'm guilty of both
the autoignition temperature of kerosene
or jet fuel is 210 degrees celsius which
is about a third to a quarter of the
exhaust gas temperature coming out of
the turbojet
so i was certain that as long as i got
real good mixing of the fuel in the air
it would just burst into flame
despite the fact that i was aware
that most if not all
military afterburners include a
secondary ignition source the
afterburner
and i had seen a video from a really
good channel on turbojet engines called
agentjz you might want to check them out
about 10 years ago they ran a j79 turbo
jet engine that had been outfitted with
an afterburner and during the test the
afterburner secondary ignition system
failed to light and they ended up
blowing jet fuel into the field behind
the test shed didn't work but i just
plowed right ahead put the afterburner
on the engine ran everything up and
it didn't work
so
i ended up drilling a couple more holes
welding on some larger studs and
threading them to put a spark plug
inside of the afterburner ran it up
again with a second ignition system
worked like a champ
you live and you learn
now i'm still not certain
why it doesn't work the best theory i
have at this point is that autoignition
temperatures are published for
atmospheric oxygen levels
and although there still is substantial
amount of oxygen in the gas coming
through the afterburner the turbojet has
consumed a substantial amount as well
so it may be that the autoignition
temperatures are much more sensitive
to oxygen concentration than i had
thought
now if you know the answer
or you have a different theory
put it in the comments section below i
read them all and maybe you come up with
a better answer and i'd be really
interested in hearing about it
now
this is generally where the amateurs
stop
it's not where the pros stop and not
where we decided to stop
if you're going to bother to invest in
putting an afterburner on a jet engine
you want the most punch out of that
afterburner you can get in other words
you want the flame temperatures in there
as high as possible
however nearly 3000 degrees will melt
just about anything you could make the
duct out of so what they do cleverly is
they build a second inner
shell or cylinder of thin steel
and perforate it with thousands of tiny
little holes
that cylinder
sits inside of the duct with a little
annular space between the duct wall and
the cylinder
and some of the exhaust gases that come
from the turbojet
actually travel around in between that
gap and bleed in through those holes
providing a little inner buffer layer
between the hot flame and the steel
it works if you take a look at a typical
example of an afterburner firing up on a
fighter jet you'll see that the gases
inside are incandescently hot but the
walls if they're glowing at all are
usually glowing a relatively dull color
or they're not even glowing it does work
so that's what we did in version 2.0
fabricated a container tube or an outer
tube out of thin wall stainless steel
tubing
welded a flange on the back so that we
could mount this on the output from the
jet engine
and then placed a feed through for the
spark plug and for the fuel
this tube here is about three and a half
inches od or about eight and a half
centimeters and it's about a half a
meter long
at the far end we put a small reduction
in diameter a little nozzle but you'll
notice that this nozzle is substantially
smaller than the one that was used on
the same engine because we need to get
out a much larger volume of gas that's
been created by the additional heat
inside of this tube
is the inner liner tube
what i did with this is took a shorter
piece of stainless steel tubing this is
about 40 centimeters or 16 inches long
placed it on a lathe and just use the
lathe to scribe lines about one
centimeter apart
and then drew lines with the tool 16
lines like this to produce a grid to
locate the position of the holes
then
welded a cone on this end
and threaded the end of the cone to
accept a quarter npt fitting
sort of a plumbing type fitting
and i have a right angle your lock
fitting right in here that screws in and
in the end of this fitting i placed
a misting fuel nozzle like this
now because i didn't want to reach in in
order to be able to get this inside
i simply took the inside of the tubing
of the right angle fitting and threaded
it for 1 8 npt a much finer thread like
this
put this thing in the lathe and turn
down the outer mushroom shape of the end
of the nozzle this is the exact same
nozzle turned down and before
fabricating so that this entire
apparatus can slip in from the end and i
can change different nozzle sizes this
produces about an 80 degree spread of
fuel into the cone and into the
beginning part of the
tube here
then welded four studs on each end 90
degrees apart that keep this centered
inside of here
and then began drilling holes
the design of the holes is similar and
based on the same philosophy of the
flame holder inside of a turbojet engine
we start out with a large number of very
tiny holes that provide mixing of the
fuel and the air and a relatively
low air velocity
then we complement that by threading
some of those holes and placing some
fine thread
button
cap head screws
this provides the flame holders and then
right behind that mixture of fuel and
air we locate the port for the spark
plug
that fuel that begins burning here is
still burning pretty rich and as this
moves down it begins to gain more and
more air to assure that we get a full
burn of the fuel air mixture
finally near the end i increased the
number of holes to make sure that we
didn't get any flow resistance from the
large quantity of air that's coming into
our gas exhaust coming into the
afterburner engine
an interesting thing you can see
something that i didn't optimize
is the color differential here
i probably should have started these
holes a little bit lower maybe a little
smaller maybe
not so many of them because as the gas
was flowing into the tube and the
pressure was dropping on the outer
annular space between these two tubes we
were getting less and less protective
cooling at this end and this end got a
little bit hotter it didn't melt it it
certainly didn't make it not function
but you can see it's somewhat of an
iterative
trial and error type of process
240 holes here 120 holes here 60 holes
here three millimeters five millimeters
that's it
now the way this assembles
is
let's see if i can get this lined up
the inner tube
feeds into this outer tube with minimal
clearances so i have to sometimes wiggle
this around a little bit to get it to
fit
[Music]
there we go piece of cake
then as i slide this in
when this hole lines up with the spark
plug
this input for the your lock fitting
lines up with the input of the fuel
we get this in here like this
get the pipe to seat in here tighten
these nuts like this
install the spark plug like this
and we're ready to go
so let's go outside hook this guy up and
i'll show you how it works
all right so we've installed this on the
end of the turbojet we're using a little
flare coupling to allow us to use a v
band clamp that takes the two flanges
and holds them together and by tightly
screwing this together it squeezes this
and puts a lot of pressure on the two
approximate faces
this is the fuel supply for the kerosene
or the jet fuel that goes into the
afterburner this is a check valve that
prevents retrograde flow if we had any
kind of a problem we're not going to
feed hot gases into the fuel system this
is the spark plug and this is a
secondary support system that just
provides additional support so that
we're not just levering everything off
of the end of the
turbojet so now we're ready to go let's
get this thing fired up
all right so now what we're going to do
is run it up the same as we did the last
time except now i have the separate fuel
pump
and the ignition system for the
afterburner
so here we go
turn on the oil pump
and adjust it to about 40 psi
the fuel pump
adjusted to about 50 psi
all right
then we hit the ignition
and turn on the fan it's going to get
pretty loud
so
so
so
[Applause]
[Applause]
[Applause]
it works
now the interesting thing
about an afterburner is that
you can put almost anything you want
through there to burn
you can use jet fuel or kerosene you can
use diesel fuel
you can use alcohol isopropyl alcohol
and even rum
disappointing thing is it's so hot uh
once you fire it up you can't smell the
rum so but it does work
in addition you can use the same design
to put other things through the
afterburner that you don't burn
so what we're going to do is we're going
to refit this with model 1.0 and we're
going to put some fog juice through it
we'll see what happens
okay so we took out the mark ii
afterburner and put in mark one
this one does not have any ignition
system hooked up to it and we took off
the accelerating cone on the end but
left the flame holders and this is what
we're going to be pumping the fog juice
into i think you'll like it
all right so we're going to run it up
one more time we're going to use the
afterburner fuel system not the ignition
system to pump in the fog juice
[Applause]
all right
here we go gonna be loud
[Music]
do
[Music]
man i love my job
this is a heck of a lot of fun
so in any case you can use these
afterburners for a variety of different
purposes and if you want to produce fog
and you don't want to just do say a high
school performance of river dance or a
little disco performance but you want to
say cover the intersection of a road or
maybe a small town
this is your baby
we're also going to be following this up
as i said with several very short videos
that break down the subsystems of this
unit so that you can reproduce this and
we'll give you all the parts and pieces
and
plans and ideas about what you need to
know in order to be able to make this
work
and i want to thank you very much for
watching
stay safe
you have a lot of fun
and we'll see you soon
[Music]
you