unique hybrid engines, coolant systems and much more, but we neglected to explore one

of the most fascinating aspects of it's design. The new and exciting material science

that made it possible.

The SR-71's speed was not limited by the power of it's engines. It was limited by

the heat it's structure could withstand. Today we are going to explore Titanium, a

material that composed 93% of the SR-71s structure. A material that had never been truly utilized

to its full potential until the SR-71 came along. We will explore it's 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 bullet proof. That's why it was 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 it's 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 axes.

This allows us to see 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 structure are Aluminium, Steel

and Titanium. Located here, here and here. They spread across the y-axis because different

alloys have different strengths. [1]

Steel is by far the heaviest, which rules it out of most aircraft structures, but it

still gets used where it's 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 metals

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. It's strength to weight ratio, or specific strength, is better than

Aluminium, yet today very little titanium is used in everyday objects. Planes primarily

use aluminium, not titanium.

Why is that?

One reason: It's really expensive, despite titanium being the 9th most common element

in earth's crust at a percent weight of 0.6%. There is more titanium in the earth's

crust than carbon, an element no-one considers rare.

Yet, in it's purified form it currently costs about four and half thousand dollars

per metric tonne. Aluminium in comparison costs a third of that at a grand and a half

per metric tonne, which itself is a relatively expensive metal as result of it's high energy

electrolysis refinement process. [2] To boot, that is today's price which has dropped

dramatically since the SR-71 was created.

Titanium is expensive, because it's refinement process is a nightmare.

To make Titanium, we start with a feedstock in the form of Titanium Dioxide, with this

chemical formula. [3]

This oxide ore called rutile can be found in high concentrations in these dark sandy

soils. To build the SR-71 the US 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 organisations to hide the final destination of material. Had the Soviets known what they

were helping build, they would not have sold 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 those two oxygen molecules to get pure titanium.

For Iron ore refinement, we heat it in the presence of carbon to force the oxygen to

separate from the iron and bind with carbon to form carbon dioxide. With aluminum oxide,

it's 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 thermal 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 Kroll Process. This process made the SR-71 possible. [4]

It begins by first converting the titanium dioxide to titanium chloride. To do this titanium

dioxide is mixed with chlorine and pure carbon and heated. 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 it's lower

boiling point.

This Titanium Chloride vapour is fed into a stainless steel vessel containing molten

magnesium at 1300 kelvin. Titanium is highly reactive with oxygen at high temperatures,

so the vessel also needs to be sealed and filled with argon. 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 between 2 and 4 days. Then once the reaction

is complete we need 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 be simply heated and compressed into rolls of sheet metal or

some other form of useful end product. But Titanium will react with oxygen and nitrogen

if heated this high, we can't do that. [5]

So the titanium sponge is compressed into an electrode along with any alloying alloys

needed. Heat is then generated through an electric arc 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. [6]

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 to 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 the 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 summer, but took it out in winter. [7]

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 bolts, which would cause galvanic corrosion and cause the bolts to fail. This discovery

led to all cadmium tools to be 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. [8] The best forge in the United States at that

time could only produce 20% of the pressure 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.

[9] Because of these inadequacies in 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% of the material going to waste. When your raw material costs 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 it's qualities

that made it suitable for use in the SR-71.

This is a material selection diagram with thermal conductivity on the x axes and thermal

expansion on the y. Here we can see that titanium has low values for both. Among the lowest

for metals. [10]

It's 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. [11]

This didn't affect machining difficulties, but the extremely low thermal conductivity

did. Machining materials produce a lot of heat can damage the tool and cause unfavorable

material properties in the titanium, like hardening. Which means the metal under the

fresh cut is now 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. [12] This slower machine speed makes the process incredibly slow, but this

is offset by taking larger 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 it's stiffness is quite low. Machinists

refer 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 damage 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 which could drill 100 holes and then be sharpened for further use. [13] By the end

of the program the engineers had 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 a 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. [14]

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 that

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 oxide layer to grow and eventually helps dangerous cracks to form.The primary

titanium alloy used in the SR-71 was (B-120VCA) [15] was thirteen percent vanadium, eleven

percent chromium and three 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 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 the ability to heat treat to higher strength.

[16]

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 WW2, 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 2. I just released the 5th 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 explore the technologies that

helped the planes navigate to their dropzones in an era before GPS, where the airborne troops

landed and why, and even explore some of the wooden gliders they used to carry heavy equipment

into the battlefield.