Have you eve heard that hoary old trope about race cars? The one that says ‘if it looks right, it is right?’ It’s far from a hard-and-fast rule, and motorsport history isn’t short of stunning cars with winless CVs, but there’s certainly a grain of truth to it…and it turns out it also applies to planes, none more so than the Lockheed SR-71 Blackbird.
Perhaps unsurprisingly given its role, the amount of money it cost and the intelligence of the people charged with building it, the SR-71 story is awash with fascinating facts and half-truths. Perhaps the most telling of these (and certainly the most impressive) is that this plane, one now well over half a century old, remains the fastest airfraft in the world. Well, the fastest that we members of the global populace lacking top-level US military clearance know.
The ‘Blackbird’ grew from the dramatic shooting down of Gary Powers’ U2 spy plane in 1960. It effectively signalled that the days of the sub-sonic CIA-funded U2 being able to traverse soviet territory with impunity were over, and that something far faster would be required in this, an age where satellite technology was still in its infancy. The also CIA had more reason than ever to keep a beady eye on the communist world, what with the soviets having tested both an atomic and a hydrogen bomb by the time Powers’ was show down, with the promise of more (and larger) tests to come.
Luckily for the west, the CIA could call upon the immense design and engineering talents of one Kelly Johnson. One of the founders of Lockheed’s infamous ‘Skunkworks’ facility, Johnson had the penned the U2 and so knew precisely what he was working with – and more importantly still, what had to be done for it to be improved.
Johnson had already been given the go-ahead to commence work on a project Archangel in 1958, with 11 different but similar designs built in short order. All were designed to fly high and fly fast, and all had the same elongated, pencil-thin fuselage The CIA decided that the A-10 variant was the most promising but recognised that it would need extensive work in order that its radar signature be reduced, which in turn resulted in the creation of 12 new planes called A-12. This would in turn birth a number of related prototypes, including the YF-12 (for intercepting and downing nuclear bombers) and the M-21 (designed to carry drones).
It was from the A-12, then the Y-12 that the SF-71 was developed, the need for such a craft only becoming more pronounced as the sixties clicked by and the Vietnam war heated up. Lyndon Johnson announced the existence of the YF-12 interceptor in 1964 as a means of putting the wind up the soviets. This move also acted as the death knell for the project itself while also safeguarding the secrecy (and future survival) of the SR-71 programme.
The A-12 and its SR-71 successor had much in common, not least their Pratt & Whitney J58 afterburning turbojet engines. These were needed to propel the plane to heights of 85,000ft (and higher) and speeds of Mach 3.2 (the plane’s projected cruising speed) and above, high and fast enough to evade contemporary Surface to Air (SAM) systems. In afterburner mode the twin engines provided a massive 68,000lb of static thrust at sea level.
It soon became clear that the engines would need to be comfortable working in a wide array of different speeds, up to and above 2000mph, and at immense height. As such a compressor bleed system was developed, a means of propelling the SR-71 above Mach 2. A conventional turbojet capacity was used for take-off and climbing to twice the speed of sound, after which the variable ‘spike’ at the front of each of its engine nacelles would retract, allowing a permanent compressor ‘bleed’ to the afterburner. This meant that the engines would effectively function as hybrids, able to perform as both turbojets and ramjets depending on the section of the mission brief the SR-71 happened to be fulfilling.
It won’t come as a surprise that flying at well over twice the speed of sound places unique demands on airframe construction, which is why 85% of the SR-71 was made from titanium, covertly sourced via a series of shell companies from the very people the plane would eventually spy on – the soviets! Flying at high speed would cause the panels to expand and shrink, and as such many of its outermost panels were corrugated in order that they could expand as and when required.
The SR-71’s fuel tanks were designed to expand and fit precisely only when the plane was at high speed and as such leaked when parked. While the degree to which its six centrally mounted reservoirs leaked their contents has been overstated over the years, there’s no doubt that they did drip and drop liberally, so. SR-71 ground crew even developed the DPM scale, or Drops Per Minute, to measure how much and how quickly fuel seeped out while the jet was parked.
The extreme temperatures generated by the SR-71’s massively varied flight window also had the effect of annealing the plane, the constant increases and decreases in temperature serving to strengthen its bodywork over the course of its life. The whole craft would expand and shrink by a good few inches over the course of a flight, all of which had to be taken into account by both its initial designers (the CIA) and those tasked with keeping it running.
Height and speed were undoubtedly the key weapons in the SR-71’s arsenal, but they were paired with an early attempt at low observance technology, or as it’s better known today, stealth. While primitive by the standards of the F117 and B2 Spirit which would come later, the Blackbird’s unique appearance and radar absorbent paint were designed to reduce its radar signature as much as possible.
The most obvious evidence of this commitment to stealth were the SR-71’s ‘chines,’ the sharpened edges running the length of its fuselage and one of the plane’s most iconic features. They were also found to have an unexpected aerodynamic benefit and provided greater lift thanks to the vortices they generated.
The importance of stealth to the SR-71’s mission window decreased as its career progressed, with even Kelly Johnson, its designer, admitting in later years that SAM and radar technology had progressed at a faster rate than low observance technology.
The very nature of the SR-71 means that there is simply too much advanced engineering to cover here, though a few points worth noting are its nitrogen filled tyres, the fact that it was started with a specially designed rig consisting of a pair of Buick V8s powering a single vertical drive-shaft, and that it guzzled JP-7 fuel at an immense rate – and this conventional propellant was only selected after the mooted coal slurry (yes, really!) and liquid hydrogen powerplants were rejected.
Just 32 airframes were built of which 12 were lost, though none through enemy action. The plane flew from 1964 until retirement in 1990, after which NASA continued to use the plane as a test bed for a further 9 years. The plane spent the majority of its career based in either Okinawa or RAF Mildenhall, from which they conducted overflights of soviet territory, as well as North Vietnam, North Korea and the odd incursion into communist China.
The SR-71’s career was finally brought to end thanks to a mix of internal Pentagon politicking, advances in satellite technology and the exorbitant costs associated with keeping the fleet airworthy. Despite this a number of the Blackbird’s records remain unbroken a full two decades on from retirement, perhaps the most impressive being Brian Shul’s speed record of Mach 3.5, set while evading a Libyan SAM in April 1986.
As already stated no SR-71 was ever lost to enemy action over the course of the airframe’s extended career, and much of this is down to the turn of speed it possessed. The Russian MIG 25 ‘Foxbat’ could get close (though only if its pilot was willing to subject the engines to near terminal damage) and Saab Viggens occasionally achieved a radar lock while the the Blackbird crossed the Swedish coast, but that really was it.
No other plane has looked as brilliant or flown so fast, so it goes without saying that the Lockheed SR-71 Blackbird is a shoe-in for Retropower Hero status.