SR-71 Blackbird: the Untouchable

(To Lorenzo Pasturenzi)
15/10/19

In the second half of the 50 years, with the worsening of the Cold War between the Western bloc and the new Soviet bloc (Warsaw Pact, 1955), it became extremely necessary for American intelligence to be able to get as much information as possible about the state of the Soviet armies and on the possible deployment of the great protagonists of this conflict, the intercontinental missile systems.

In an age where the space race was in its infancy and the spy-satellites a reality present only in some futuristic projects top secret, the importance of aerial reconnaissance grew enormously.

The United States lined up the U-2 "Dragon Lady", entered online in the 1955 but already considered obsolete in the 1958 due to the rapid advancement of missile technology and anti-aircraft systems. The CIA then contacted Lockheed, a leading company in the defense sector, laying the foundations for the birth of a new "non-detectable" and "non-collapsible" reconnaissance aircraft.

Code name: "Archangel Project"

The need for a new reconnaissance was made even more stringent the 1 ° May 1960, when the U-2 piloted by Francis Gary Powers was shot down over the skies of the Soviet Union, triggering a very serious diplomatic crisis between the two blocks. The CIA then added new requirements to the Archangel project, requiring a sharp reduction in the radar signature of the different projects presented by Lockheed. A new project ensued, OXCART, in which NASA also participated, interested in the technological advances that the development of the new aircraft would have entailed.

"Kelly" Johnson, visionary engineer at the head of the development team, gave birth in the 1961 to the A-12 (photo), direct ancestor of the SR-71. There were three main versions designed: an interceptor, a reconnaissance aircraft and a bomber. The interceptor and bomber versions were discarded (especially so as not to take away funds from the development of the XB-70 Valkirie), while the reconnaissance was renamed SR-71.

In the 1967, after a long gestation, the fastest piloted plane driven by an exoreactor of human history entered service.

The 31 units built established a series of records that last today: maximum tangent altitude in sustained flight (26000 m) and maximum flight speed reached (3529 km / h), with the addition of different mileage records classic (for example, West coast-East coast in 1h and 8 minutes). Above all, however, the aircraft succeeded very well in the task for which it was designed: none of the 31 specimens entered the service was never shot down on mission, thus earning the affectionate nickname of "Untouchable".

THE TECHNIQUE

The engineering problems faced in the realization of a machine of such complexity were many. Above all, the need to find a suitable engine that would provide sufficient thrust and that could operate at the most disparate conditions of altitude and speed (from the subsonic regime at low altitudes to the supersonic regime at 26000 m). The choice fell on the Pratt & Whitney J58, an engine still under development designed to operate at high Mach numbers.

The apparatus consisted of a classic turbojet with afterburner, however with an interesting peculiarity. To overcome the problem of the stalling of compression stages at high speeds, unable to manage the incoming air flow for Mach of flight above 2.2, a bulkhead was conveyed which conveyed the incoming flow, adequately compressed by the action of the intake air, directly to the afterburner chamber, thus excluding the compressors themselves, the main combustion chamber and the turbines from the cycle. The engine thus transformed became a stator (ramjet).

The ramjet is a type of aeronautical propeller that does not contain mobile mechanical organs inside it (turbines and compressors), but uses the shock waves developed by the inlet during the supersonic flight to compress the flow and then carry out its combustion in the chamber and ejection by nozzle.

A fundamental component of this process was therefore the particular air intake, the geometry of which allowed the compression of the fluid entering the engine through the generation of a train of oblique shock waves, making it suitable for combustion directly in the afterburner. This solution even made it possible to improve consumption thanks to the elimination of energy losses linked to the operation of turbomachines excluded from the new thermodynamic cycle!

The design of the air intake thus became the fulcrum around which the engineers' efforts were concentrated. As we have said, this had to be able to supply the engine with the right amount of air at the right speed and pressure both for the wide range of altitudes and speeds faced by the aircraft and for the type of propulsion adopted. It was therefore decided to use a complex retractable spine geometry followed by a convergent-divergent duct with variable geometry.

For subsonic speeds the plug was completely extracted, maximizing the engine capture area. In the supersonic field, the apparatus was pushed back about 4 cm for each increment of 0.1 Mach.

The plug form allowed to generate oblique shock waves, less intense compared to normal shock waves that would have been obtained with other conformations since they act only on the component perpendicular to the velocity vector, which slowed down and compressed the fluid. After having undergone the action of this first train of shock waves, the air was conveyed into the convergent-divergent duct composed of the external walls of the engine and the initial section of the spine itself, in which other oblique waves were formed and a normal shock wave. The goal of the engineers was to position this normal shock wave in the throat of the duct, since in this zone the velocity of the fluid was close to Mach 1 and therefore the wave, whose intensity depends on the Mach of the flow on which it acts, would be was very weak thus causing little energy loss.

To maintain the shock wave in the throat at various speeds, bypass ducts were designed which, by subtracting a certain amount of air, managed the pressure and temperature inside the intake. Moreover, the movement of the pin allowed to modify the geometry of the convergent-divergent duct, so as to obtain a speed close to the Mach 1 in the throat for the various flight speeds. By achieving these operating conditions, the air intake was started, ie it caused little energy loss and provided the ramjet with a high pressure flow. This engineering masterpiece led to the birth of an engine capable of delivering 145 KN of thrust and to operate at the various flight regimes required of the aircraft.

Even the structure project was not without its problems. In fact, the high Mach flight involved a high overheating of the aircraft cell (so as to change its color from black to blue!) Due to the friction developed by the contact between the air flow and the walls. In several areas the 300 ° C were reached, therefore the classic aluminum alloys used at the time could not be used. In their place, an almost entirely titanium coating made its debut (90% of the cell), which in spite of the high costs did not suffer a degradation of the mechanical properties at high temperatures.

The tank project was also particularly complex. The high temperatures in fact caused an expansion of the materials used, so to avoid breakages it was necessary to leave free cavities that the material could occupy. The tanks designed in this way became watertight only at high speeds, while during takeoff and landing fuel leaks occurred!

The SR-71 was also the first aircraft for which structural solutions were used to reduce its radar footprint, greatly diminishing its radar cross-section (a measure of a body's ability to reflect the radar signal in the direction of the receiving radar) . Special radar-absorbing panels were studied and the aircraft was painted black to reduce the emissivity linked to structural heating (hence the nickname "Blackbird"). It can therefore be said that it was the first project to focus its development around the requirements of stealthiness.

Photo: US Air Force / NASA / CIA / web