we recently explored the fascinating
engineering that made the sr-71 possible
detailing its unique hybrid engines
coolant systems
and much more but we neglected to
explore one of the most fascinating
aspects of its design
the new and exciting material science
that made it possible
the sr-71 speed was not limited by the
power of its engines
it was limited by the heat its structure
could withstand
today we're going to explore titanium a
material that composed
93 of the sr-71 structure
a material that had never been truly
utilized to its full potential
until the sr-71 came along we will
explore its material properties
how it's made and how the engineers of
the sr-71
overcame the challenges they encountered
while using the innovative new material
titanium is one of those words that has
entered common language
it's become synonymous with strength sia
likens titanium to being bulletproof
and yes with the right thickness it is
bulletproof
that's why it's used in the a-10 to
protect the pilot
but in reality the strongest titanium
alloys are only
about as strong as the strongest steel
alloys and their temperature tolerance
is
actually worse while aluminium is
lighter what makes titanium special
is not its tensile strength weight or
high temperature performance
but a combination of all of these
material properties
that made it perfect for the sr-71
when choosing materials for a particular
application engineers will often consult
something called a material selection
diagram
where we plot two material properties on
the x
and y axis this allows us to see
the relative benefits of materials so we
can choose a material
according to our needs here is a
particularly relevant material selection
diagram
for the aerospace industry with density
on the x-axis
and strength the maximum pressure it can
withstand before breaking
on the y-axis our three primary metallic
material choices
for aircraft structures are aluminium
steel
and titanium located here here and here
they spread across the y-axis because
different alloys have different
strengths
steel is by far the heaviest which rules
it out of
most aircraft structures but it still
gets used where its strength
and heat tolerance is needed we can also
see that aluminium
is in fact lighter than titanium but
titanium is stronger than aluminium
a better measure here is the strength to
weight ratio
a ratio found by dividing the metal
strength by its density
after all we can make an aluminium part
stronger
by just adding more material but if we
need to add so much material that the
part is now heavier
than an equivalent strength part made
from titanium
then it's not worth it here titanium
wins
its strength to weight ratio or specific
strength
is better than aluminium yet today very
little titanium is used in aviation
planes primarily use aluminium not
titanium
why is that one reason it's really
expensive
despite titanium being the ninth most
common element
in earth's crust at a percent weight of
0.6 percent
there is more titanium in the earth's
crust than carbon
an element no one considers rare yet in
its purified form
it currently cost about four and a half
thousand dollars per metric ton
aluminium in comparison cost a third of
that at a grand and a half per metric
ton
which itself is a relatively expensive
metal as a result of the high
energy electrolysis refinement process
to boot
that is today's price which has dropped
dramatically
since the sr-71 was created titanium is
expensive because its refinement process
is a nightmare to make titanium we start
with the feedstock
in the form of titanium dioxide with
this chemical formula
this oxide ore called root oil can be
found in high concentrations in these
dark
sandy soils to build the sr-71
the u.s needed to buy vast quantities of
the mineral from the soviets
who had large deposits of rutile to do
this
they purchased the material through
ghost organizations
to hide the final destination of the
material
had the soviets known what they were
helping to build they would not have
sold that material
however the us likely could have just
purchased the material from mines in
australia
this is a relatively common raw material
and is primarily used as a white pigment
for paints
and is even found in sunscreen lotion as
an ultraviolet radiation blocking
pigment
our trouble begins when we need to
separate these two oxygen molecules to
get pure titanium
for iron or refinement we heat it in the
presence of carbon
to force the oxygen to separate from the
iron and bind to the carbon
to form carbon dioxide with aluminium
oxide
its melting point is too high so we
instead dissolve it in a solvent
and then use electrolysis to separate
the oxygen molecule
neither of these methods work with
titanium titanium dioxide is both
thermally stable
and resistant to chemical attack in the
1940s
the first reliable process to produce a
chemically pure form
of titanium was developed called the
crawl process
this process made the sr-71 possible it
begins
by first converting the titanium dioxide
to titanium chloride
to do this titanium dioxide is mixed
with chlorine and pure carbon
and heat it any oxygen or nitrogen
leaking in will ruin the process
so this has to be done in relatively
small batches
in a sealed vessel once this process is
complete
we have titanium chloride we then need
to purify the titanium chloride
from any impurities in the titanium ore
through
distillation where we heat the product
and separate titanium chloride
using its lower boiling point this
titanium chloride vapor
is fed into a stainless steel vessel
containing molten magnesium
at 1 300 kelvin titanium is highly
reactive with oxygen
at higher temperatures so the vessel
also needs to be sealed
and filled with aragon here the titanium
chloride reacts with the magnesium
which itself is an expensive metal to
form titanium
and magnesium chloride this reduction
reaction
is extremely slow taking between two and
four days
then once the reaction is complete we
need to let the product cool
before removing the magnesium chloride
products through high temperature
distillation once again
the magnesium and chlorine are recycled
with electrolysis
another energy intensive process at this
stage we have titanium sponge
which needs further processing still
typically a porous metal like this
would simply be heated and compressed
into rolls of sheet metal or some other
form of useful end product
but titanium as we said will react with
oxygen
and nitrogen if heated this high we
can't do that
so the titanium sponge is compressed
into an electrode
along with any alloying metals needed
heat is then generated
through an electric air current inside
another sealed vessel
this form of heat needs no oxygen this
melts the electrode to form a large
titanium ingot
this process results in an incredibly
expensive material that becomes even
more expensive
as a result of the difficulty the
engineers found when attempting to form
it
into its final shape the engineers of
the sr-71 were among the first people in
history to make real use of the material
in that process they ended up throwing
away a lot of material
some through necessity some through
error
at times the engineers were perplexed as
what was causing problems but thankfully
they documented and catalogued
everything which helped find trends in
their failures
they discovered that spot welded parts
made in summer
were failing very early in their life
but those welded in winter were fine
they eventually tracked the problem to
the fact that the burbank water
treatment facility
was adding chlorine to the water they
used to clean the parts
to prevent algae blooms in the summer
but
took it out in winter chlorine as we saw
earlier
reacts with titanium so they began using
distilled water from this point on
they discovered that their cadmium
plated tools were leaving trace amounts
of cadmium on the bolts
which would cause galvanic corrosion and
cause the bolts to fail
this discovery led to all the cadmium
tools being removed from the workshop
however the largest wastes were caused
by the lack of appropriate forging
presses
in the united states titanium alloys
require much higher pressure to deform
during forging than aluminium alloys or
steel alloys
the best forage in the united states at
the time could only produce 20
of the pressures needed to form these
titanium parts
clarence l johnson the manager of skunk
works at the time
pleaded for the development of an
adequate forging press
which he stated would need to be a 250
000 ton metal forming press because of
these inadequacies and forming
capabilities
the final forging dimensions were
nowhere near the design dimensions
and much of the forming process had to
be completed
through machining meaning most of the
material was cut away to form the part
resulting in 90 percent of the material
going to waste
when your raw material cost this much
this kind of waste
really hurts to add insult to injury
drill bits and other machining tools
were being thrown away
at a rapid pace titanium is a difficult
material to machine
precisely because of its qualities that
made it suitable
for use in the sr-71 this is a material
selection diagram with thermal
conductivity on the x-axis
and thermal expansion on the y-axis here
we can see that titanium has low values
for both
among the lowest four metals its low
thermal expansion
made accommodating thermal expansion as
the plane heated up
easier but measures still had to be made
to prevent it causing stress
the skin panels were fastened to the
underlying structure with oblong holes
which would allow the skin to expand and
contract without the fasteners causing
buckling
and the skin over the wing was also
corrugated to prevent warping during
expansion
this is actually quite noticeable you
can see the sections that are corrugated
quite clearly here this didn't affect
machining difficulties but the extremely
low thermal conductivity did
machining metals produces a lot of heat
that can damage the tool
and cause unfavorable material
properties in the titanium
like hardening which means the metal
under the fresh cut
is no harder and therefore even more
damaging to the tool
this is usually minimized with coolant
but titanium's low
thermal conductivity means very little
heat is transferred
into the coolant to deal with this lower
machining speeds need to be used
along with high volumes of coolant which
is also expensive
this slows the rate heat is generated
and increases the rate it is removed
this slower machine speed makes the
process incredibly slow
but this is offset by taking deeper cuts
in each pass
which has the added benefit of cutting
under the work hardened layers
titanium is also more sensitive to dull
tools
as its stiffness is quite low machine is
referred to metals like this
as being gummy they tend to form long
chips that can clog the work area
and cause all sorts of problems if not
properly managed
they can ruin the work surface and
damaged the tools
the engineers at lockheed gradually
learned these lessons and developed
better tools for the job
when the first version of the sr-71 was
being constructed
the drill bits used to cut the holes for
the rivets could only drill
17 holes before they were unusable and
needed to be discarded
by the end of the sr-71 program they had
developed a new drill bit
that could drill 100 holes and then be
sharpened for further use
by the end of the program the engineers
found enough improvements to save
19 million dollars on the manufacturing
program
it's pretty clear that titanium is
expensive and extremely difficult to
work with
had aluminium been an option for the
sr-71
with a little bit of added weight the
engineers would have jumped at the
opportunity
but aluminium simply cannot deal with
the temperatures that steel
and titanium can this is the material
selection diagram displaying
several metals specific strength as a
function of temperature
this is ultimately what made titanium so
attractive for the sr-71
titanium alloys maintain a great deal of
their strength
up to temperatures as high as 450
degrees celsius
the same cannot be said for aluminium
what i find fascinating
is the titanium's max operating
temperature is less a function of loss
in strength
but a function of oxidation pure
titanium is highly reactive to oxygen
which forms an
oxide layer on the outside of the metal
which is brittle
this oxide layer has some benefits as it
provides
excellent corrosion resistance which is
why many submarines use titanium
to resist attack from salt water but at
higher temperatures
this oxide layer and titanium are
soluble to oxygen
which means the oxygen can permeate
through the outer oxide layer
and diffuse into the metal causing the
oxide layer to grow
and eventually helps dangerous cracks to
form
the primary titanium alloy used in the
sr-71
was 13 vanadium 11 chromium and 3
percent aluminium
both chromium and aluminium form
thermally stable oxide layers on the
outer skin of the metal
which prevents oxygen from diffusing
further into the metal and causing it to
become even more brittle
which raises the max operating
temperature of the metal
while the vanadium acts as a stabilizer
for a crystal structure
referred to as the beta phase which
leads to a material with higher tensile
strength
and better formability with an ability
to heat treat to a higher strength
in my humble opinion advancements in
material science like
this have the largest knock-on effect in
the advancement of human technologies
so much so that we name entire eras of
human history
after the materials we developed during
that time
during world war ii the development of
aluminium alloys
suitable for aviation allowed for the
emergence of some incredible planes
and with that some incredible tactics
like aerial invasions
a method of invasion that first emerged
in world war
ii i just released the fifth episode of
the logistics of d-day
on nebula the streaming platform i
created with my youtube friends
in this episode i explore the tactics of
the allied aerial landings
in normandy i explored the technologies
that helped the planes navigate to their
drop zones in an era before gps
where the airborne troops landed and why
and even explore
some of the wooden gliders that were
used to carry heavy equipment
into the battlefield i am currently
working on the next episode which
explores the immense logistics required
to build frontline airfields to
facilitate close air support
the first episode of this series is
available for free
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clearing away to the beaches which
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the aerial landings
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