In part one of this series we took a look at the early spy flights of the Cold War and the vital intelligence they gathered from behind the Iron Curtain. We mentioned some of the aircraft used on these missions such as the North American B-45 Tornado, English Electric Canberra and the famous Lockheed U-2. There was an ongoing race between the West who were building newer and better spy planes and the East who were developing ever more capable and potent ways to shoot them down. Even as the U-2 entered service it became clear it wouldn’t be long before the Soviets could shoot one of these very high flying spy planes down. The CIA began projects to both improve the U-2’s survivability in Soviet airspace, and also a project to develop an entirely new aircraft to replace the U-2.
On 1st May 1960 what the Americans most feared finally happened. A U-2 flown by CIA pilot Francis Gary Powers was shot down by an S-75 Dvina (NATO reporting name SA-2 Guideline) surface to air missile and crashed near Sverdlovsk (modern day Yekaterinburg). Powers ejected from the aircraft and was captured by the Soviets. This was a massive coup for the Soviets and a huge embarrassment for the Americans as the Soviets paraded Powers in front of the cameras and put him on public trial for spying.
By this time the CIA’s Project Gusto, the codename given to the development of a repacement for the U-2 was already well underway and Convair’s Kingfish design had lost to Lockheed’s Archangel, known within the CIA as Oxcart. The development of Oxcart took on added urgency.
Lockheed were tasked to build an aeroplane which would fly higher and faster than any aeroplane before, and one which was designed to avoid detection by radar. What’s more, they had to do this in total secrecy. A truly colossal task laid ahead where they would be pushing the boundaries of engineering and aeronautics.
The benefits of avoiding radar detection are quite apparent, but we might ask why did the aeroplane also have to fly so high and so fast?
The higher you fly the more difficult it is to shoot you down. Anti-aircraft guns, surface to air missiles and fighter aircraft will all have to leave the ground and get up to the altitude of a very high flying aircraft. There are no anti-aircraft guns which can reach 90,000ft, a surface to air missile would have very little kinetic energy left by the time it reached that altitude, and even if it did the air is so thin the control surfaces of the missile would have very little air to operate effectively. Similarly, even if a fighter could reach such altitudes it won’t have the performance to conduct an intercept and would be left wallowing around in the incredibly thin air trying to accelerate to a speed where it could position itself to bring weapons to bear.
Very high speeds also bring many advantages. It reduces the time in which the enemy can react after detecting you. Ideally by the time they have brought their air defence forces to alert and they’ve got fighters in the air at an altitude where they can possibly shoot you, your high speed spy plane has long since left the area. What’s more, if you’re flying very fast any missiles fired at you from behind or abeam will not have enough kinetic energy to intercept you. Thus, a missile would have to be fired from in front of you. The same for any fighters which have been scrambled to shoot you down; they would have to be in front of you to have any chance of getting a shot on you. This will severely limit the options your enemy has to shoot you down.
There is another advantage of flying incredibly fast and it concerns something called the “blip to scan ratio”. Think of the old radar at Heathrow. You remember, the big red thing that rotated on top of a big concrete column. It took time to rotate and scan the sky. Let’s say one of those old radars takes twelve seconds to complete a full 360 degree scan. If you’re flying at Mach 3.5 you will travel nearly eight miles before that scanner sweeps over your aircraft again. Many long range surface to air missiles, particularly of this era, were command guided. That’s to say the ground radar tracked the target and sent a command signal to steer the missile onto its target. The guidance radars had a much faster scan rate, but even if we say it scans every half a second your Mach 3.5 aeroplane is still going to travel over a third of a mile between scans. The missile simply can’t track you fast enough.
Finally we should talk about radar cross section. This is a measure of how detectable an object is by a radar. It’s expressed in square metres. It’s determined by the material from which the object is made, the size of the object, the size of the object relative to the wavelength of the radar waves, the angles at which the radar waves hit the object and the angles at which they are reflected (i.e. the shape of the object), and the polarisation of the radar waves. A low observable or “stealth” aircraft seeks to reduce its radar cross section as far as possible. It’s important to note that a “stealth” aircraft isn’t invisible to radar. It absorbs or scatters radar waves away from the radar so the radar receives a much smaller “echo”. So small the radar doesn’t register it as a contact. As the stealth aircraft gets closer to the radar the echo gets larger to the point where it does recognise it as a contact. As such I think it may be better to describe “stealth” as radar camouflage.
The technical challenges involved with producing an aircraft with the above qualities is immense. Now remember the CIA gave the go ahead for Lockheed to build the A-12 in January 1960. As you read what follows, I want you to bear in mind that the A-12 first flew in 1962, which was the year the Ford Cortina first went on sale in Britain.
The obvious place to start would be with the appearance of the A-12. If you take a quick look at part one of this series you’ll see a depiction of Archangel-11 which was the design which the CIA selected as the starting point for Oxcart. They were concerned that the radar cross section could be reduced further. A few years previously American physicists had discovered that by flattening or stretching out the circumference of a sphere it was possible to reduce its radar cross section by as much as two thirds. Hence Lockheed added the distinctive chine to the nose and forward fuselage of the aircraft. They also relocated the engines to a new position buried in the wings and removed the single large vertical tail surface and replaced it with two smaller vertical tail surfaces mounted on top of the engine nacelles and canted them inwards. All of these measures reduced the radar cross section. These revisions produced Archangel-12, or A-12.
It’s worth noting that canted tails have featured on many stealth aircraft since; the F-117, F-22, F-35 and Su-57.
Oh, and one interesting aside… flattening a sphere to reduce its radar cross section. This might account for a lot of the so-called “flying saucer” sightings in the late 50s and early 60s.
When you look into the design of the A-12 there’s one thing that keeps cropping up again and again. Heat. The atmospheric friction from flying through the air at incredibly high speeds produces a lot of heat. To give you an idea, at its typical cruise speed of Mach 2.0 Concorde’s wing was hot enough to fry an egg on (although I wouldn’t recommend trying this). In fact, Concorde’s top speed wasn’t determined by its aerodynamics or engine power, it was limited by this frictional heating. If it went much above Mach 2 Concorde’s nose would become dangerously hot.
The first all metal construction aircraft were made by the Germans during the First World War. By the end of the Second World War metal construction was standard for high performance aircraft. The metals used are aluminium based alloys. Aluminium is strong and very importantly for aircraft, light. The problem with aluminium in very high speed aircraft is its low melting point. The frictional heating experienced much beyond Mach 2 will soften structures made from aluminium based alloys and leave them susceptible to damage or failure. This is one of the reasons why fighter aeroplanes hit Mach 2 over sixty years ago and haven’t gotten much faster since.
In 1953 the Bristol aircraft company were given a contract to construct a research aircraft which would determine the viability of using stainless steel construction for aircraft which would fly above Mach 2. The result was the Bristol 188. The results were disappointing and it was shown that stainless steel is simply too heavy to build a useful aeroplane from.
There is another metal we can use though. Titanium. It’s very strong, has a very high melting point, and is very light. Titanium had been used in various parts of aircraft before, mainly for components subjected to very high temperatures (i.e. the engine). Nobody had ever actually built an entire aeroplane out of it before. Why? Because it’s very high melting point made it very difficult to pour and cast. It’s very high toughness made it very difficult to shape and form, and it’s hardness made it difficult to cut and drill. Nevertheless Lockheed decided to build the A-12 from titanium.
The first problem they encountered was where to source the titanium from? At the time the biggest exporter of titanium ore in the world was the Soviet Union. The CIA set up an elaborate network of shell companies and in a great twist of irony, the titanium used to build the A-12 was sourced from the very country it was built to spy on. Lockheed also had to develop new manufacturing techniques as nobody had ever built such large structures and components out of titanium before. The aircraft itself consisted of a titanium frame or skeleton, and the titanium skin was cut into panels which were then bonded onto the frame. The complex curves around the chine were especially challenging, so these were made from small fillets. The distinctive saw tooth pattern which resulted was found to reduce the radar cross section even further as they internally reflected radar waves. This saw tooth pattern would later be used on other stealth aircraft for their access panels, landing gear bay doors, weapons bay doors, etc. The early A-12s had titanium chines, but later on these were superceded by a heat resistant and radar absorbent composite material made from laminated layers of iron ferrite, silicate and ceramic. The exact nature of this material remains classified to this day.
The panels on the top of the wings were also a challenge. These were the largest titanium panels on the aircraft and it was calculated their thermal expansion and contraction would literally tear them off the underlying structure. It was proposed to manufacture and fit these panels with longitudinal corrugations. The corrugations would allow for the thermal expansion and contraction. This idea was opposed by the aerodynamicists who believed the corrugations would add too much drag. As it happened the corrugations worked very well and didn’t impact the aircraft’s performance at all.
Now the engines. From the outset it was intended to use the Pratt & Whitney J58 engine for the A-12, but this engine was still under development and the A-12 first flew with the J79 engine as a substitute. The J79 was famously used on the F-104 Starfighter and F-4 Phantom. Just like nearly everything else on the A-12, the J58 was a very interesting engine. To better understand what’s so interesting about the J58 we need to go over a quick bit of jet engine theory.
The most basic type of jet engine is called a turbojet. In this type of engine air is induced into the engine inlet where it is compressed by a set of engine driven turbines. The compressed air passes into an annular combustion chamber where atomised fuel is introduced and ignited. The fuel-air mixture combusts, as the temperature rises the exhaust gas expands through another turbine (which drives the compressor turbines). The hot exhaust gases are then expanded through an exhaust nozzle and this is where the thrust is generated. You may have heard of an “afterburner” or to give it the correct British name; reheat. In a reheat system fuel is sprayed into the exhaust gas stream after the turbine and reignited which creates even more thrust, but at the cost of greatly increased fuel consumption.
A jet engine needs subsonic air at the compressor inlet. Ideally at around Mach 0.4. If you try to introduce supersonic air to a jet engine… well let’s just say the engine will stop working, quite spectacularly. How do supersonic aircraft manage to keep their engines working then? They use ramps and diverters in the intake to slow down the air so it reaches the compressor inlet at the correct speed. Back in the 1950s there was also another popular method called a shock cone. A shock cone is, as the name suggests a conical body placed in the air intake. At supersonic speeds the tip of the cone produces a shock wave. The air behind the shock wave is slowed to subsonic speed and compressed. If we can position the cone so the shock wave intersects with the lip of the air intake we’re in business. What’s more, inside the air intake the cone reverses and creates an expanding volume. This slows the air down even more, and the ram effect of the higher speed air coming into this expanding volume compresses it even further. This is even before we’ve got to the compressor turbine in the engine! More compression means more air mass in the combustion chamber. This means we can burn more fuel. This means we can make a bigger bang, release more energy and generate more thrust.
The astute puffin will be remembering that the angle of the supersonic shockwave becomes more acute the faster the aircraft travels through the air. This means that an aircraft like the A-12 with an especially large supersonic speed range will not be able to keep the shockwave coincident with the lip of the air intake. What can we do about that? Make the shock cone articulated so it can move fore and aft to keep the shock wave at the air intake lip. Now we’re cooking on gas! This is proper engineering!
That’s not all. The J58 is what’s called a bypass turbojet. It takes high pressure air bled from the compressor stages and pipes it back in to the other end of the engine just before the exhaust nozzle. Lots of people mistakenly think that at high speed the J58 bypasses all of the air past the core and effectively becomes a ramjet. This is not the case. There is always air going through the engine core on the J58, even at Mach 3+. The bypass starts at higher speeds and the high pressure air is instead used to augment the effect of the afterburner. Effectively it’s a turbocharged afterburner and allows the engine to produce a lot of extra thrust at very high speeds, just as a more traditional jet engine starts to lose thrust. This is the extra thrust that allows the A-12 to punch through Mach 3. Oh, and the air passing down those bypass ducts is also quite handy for cooling the engine core.
Just how hot did the A-12 get? At Mach 3+ parts of the airframe reached over 370 degrees Celcius. They needed to develop a special high flashpoint fuel as regular kerosene wasn’t suitable. Thus the A-12 used JP-7. I’m told you can toss a lit match into a bucket of JP-7 and it won’t ignite. The fuel itself acted as a heatsink for the airframe. Speaking of which, they couldn’t find a suitable material to construct the linings of the fuel tanks from. So instead the aircraft’s skin was the lining of the fuel tanks. Remember that thermal expansion? The skin panels were cut to allow for that thermal expansion, so on the ground on a very cold day the aircraft would leak fuel. Quite a good thing then that JP-7 was very hard to set light to.
New hydraulic fluid and engine oil had to be developed which could function properly across the massive temperature ranges this aircraft was operated at. At operational speeds the cockpit would reach temperatures of 60 degrees Celcius, the pilot needed not only a pressure suit but a special climate controlled pressure suit to keep him cool whilst flying the aircraft. The landing gear bays would get so hot the rubber on the tires would melt. The solution was to develop new tires where the rubber was impregnated with aluminium powder which acted as a heat sink and stopped the rubber melting.
How can you fix the perspex of the canopy to the titanium frame which will reach over 370 degrees Celcius in flight? Lockheed developed ultrasonic welding just for this purpose. Ultrasonic welding is a widely used method for bonding dissimilar materials in manufacturing today. Have you got one of those cheap Bic cigarette lighters? That’s put together using ultrasonic welding. Pretty much all of the plastic food packaging you’ve got was sealed with ultrasonic welding as it’s very hygienic and thus well suited to the food industry.
Navigating the aircraft also presented significant new challenges. At the time the latest thing was an Inertial Navigation System or INS. This system uses gyroscopes and accelerometers to measure the aircraft’s motion. You enter the longitude and latitude of the aircraft before taking off and the system monitors the aircraft’s motion and extrapolates its position. An INS system will have an inherent measurement error, and over time that error will grow. If you’re in an A-12 travelling at nearly 40 miles per minute this error will rapidly become unacceptable. A new navigation method was required. Thus a subsidiary of Northrop developed an astro-navigation system for the A-12 (based on a navigation system which had been developed to guide ballistic missiles). This system used a special camera tuned to see starlight wavelengths and peered through a quartz window on the upper spine of the A-12. The system contained a database of stars and measured their position, allowing it to extrapolate the aircraft’s three dimensional position to within an accuracy of 300m, even when travelling at over Mach 3.
From it’s first flight from Groom Lake in 1962, it took five years of intense development under a veil of total secrecy to develop the A-12 into an operational aircraft ready to undertake its first missions in 1967.
Now remember puffins, all this was going on in the time you might have been driving around in your brand spanking new factory fresh Cortina. Just for a second imagine what must be going on behind closed doors in a desert somewhere right now as you read this.
In the next part we’ll look at how the A-12 was used operationally and the several derivative aircraft it spawned.
© Æthelberht 2019
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