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Engineeering notes on the SR-71 Blackbird Engineering design is potentially a creative activity. The successful
combination of "outside the box" conceptual thinking and revolutionary engineering adaptations
marked the dawn of a new era beginning in the late 1940s. Realizing the
growing importance of establishing new cold war deterrent factors, the
engineers at Lockheed Skunk Works (Advanced Development Project Office,
Burbank, CA) set forth to cover more ground than was usual for a new reconnaissance aircraft [1]. However, it
became clear to the practicing engineers of the day that the sought after technical
finesse frequently was completely at odds with their workable engineering
reality. With a new open-ended conformal re-mapping of the often rigorous
and fundamental engineering text, a continuation to more advanced engineering
sciences and mathematics was endeavored. In order to push the boundaries the engineer began
thinking in terms of systems. Developing thousands ahead of the times components, the goal was a 'workable
system', and the objective of designing an 'optimum system' was often
abandoned. With time the 'system simulation' gave effort to the optimization
process. One spin-off redefining the boundaries of transformational aerospace
solutions was the digital engineering simulation technique, called the
Finite Element Method (FEM). This method surely rings a bell to nowadays engineers and scientists
with experience within structural analyses and electronic data processing.
In the context of validation, each and every design undergoes a thorough and very costly component testing. Design alternatives are reanalyzed prior to prototype build and testing. Through the application of multistage substructuring finite element technique, the computed information is more complete than by regular testing. This enables decisions to be based on total structural behavior [2], rather than from information obtained from normal testing procedures, which are limited by instrumentation. In other words, the structural design requirements are more in sync with the (aircraft's) performance ceiling. With the advent of large capacity computers and powerful numerical methods the situation of the stress analyst was utterly transformed. Relevant to the matter in hand is the manuscript, "Results of NASTRAN Modal Analyses and Ground Vibration Tests on the YF-12A [3] Airplane", written by the engineers A.R. Curtis (Lockheed-California Co., Burbank, Calif.) and E.E. Kordes (NASA Flight Research Center, Edwards, Calif.). This manuscript was received at The American Society of Mechanical Engineers (ASME) Headquarters, September 19, 1975. Contributed by the Aerospace Division of the ASME for presentation at the Winter Annual Meeting, Houston, Texas, November 30 - December 4, 1975. Here the finite element analysis was carried out in order to evaluate the 10 lowest frequency anti-/symmetric airframe elastic mode shapes of the classified supersonic aircraft. The application program NASTRAN [4] was used to compute mode shapes and obtain a good registration of vibration amplitudes. The variation of amplitudes for basic wing and fuselage modes, calculated with NASTRAN, were compared with the measured resonant frequencies and mode shapes obtained from a ground vibration test of the complete aircraft. Mass properties, elasticity and amount of damping were important variables in this context. With the finite element method the airframe was formulated as an assemblage of structural elements with known or assumed physical properties. The model was built by bar elements, rod elements, shear panel elements and scalar elastic elements. The finite element mesh was chosen to give a good representation of stiffness and mass properties. More than 600 multipoint constraint equations were needed to solve the detailed description of this construction. The structural dynamic characteristics, i.e. flight loads measured in flight and simulated in the laboratory were also compared with a NASTRAN/FEM structural analysis. The YF-12A was engineered at the Lockheed Skunk Works and test flown at the Groom Lake facility [5]. The product of their ambition was a supersonic aircraft that could defeat an attack from Soviet bombers. The interceptor-fighter delights the engineering senses, from the minute you see it. The classical delta-winged, low observable configuration was an imperative for the U.S. Air Force. Stealthier than the U-2 spyplane, but nonetheless unfulfilled when compared to today's fourth generation stealthy fighters. A F-35 (Joint Strike Fighter) and F/A-22 (Raptor) return smaller than a ping-pong ball and a marble respectively. Directional unstability associated with asymmetric loss of thrust, during cruise flight in excess of Mach 3, had a profound influence in choosing the inboard location of the powerplants. The delta-winged vehicle was powered by two Pratt & Whitney JT11D-20A (J58) turbojets, each 32,000 lbs. of thrust with afterburners. This powerplant still stands the test of time. High fuel-air ratios, compressor discharge temperatures and thrust-to-weight ratios were generated. The core engine's combustion system relied on advanced shaping to lower cooling air requirements. The J58 was evaluated for the mass injection precompressor cooling concept (MIPCC), to obtain additional thrust with reduced thermal stresses. I.e. injecting water and liquid oxygen into the inlet airflow, in order to increase the mass flow density. This concept was shelved. However, many manufacturing techniques were transferred to other powerplant programs. The multiple oblique shock, spike-type diffusers are readily observable on the turbojets air-induction system. Due to engine design limitations, it was necessary to provide diffusers that would decelerate the inlet rushing air to subsonic velocity, with a minimum loss of stagnation pressure. Multiple oblique shocks were required ahead of the normal shock wave. The latter acts perpendicular to the airflow at the engine cowl inlet. A compromise had to be reached, since the boundary-layer bleed (flow separation) along the surface of the spike increases with the adverse pressure gradient originating from the oblique shocks. The supercritical mode of operation was chosen to achieve maximum engine mass flow, and a margin of security at different angles of attack and speed. Care was taken to avoid shock repulsion, i.e. starving the engine of air with consequent engine flame-out. However, one inlet problem recurred at a certain supersonic speed. The moveable spike would fail to precisely diffuse the normal shock wave to the cowl inlet, for maximum rampressure. This condition specific to the YF-12A & SR-71, called an 'unstart', drew everyone's attention on board. The two-man crew would then face up to the challenge of an asymmetric loss of thrust. With persistent situational awareness the pilot unloaded the aircraft, by relaxing the stick-pressure. Reverting to fail-safe procedures, in the proper sequence and with no time cushion, gave moments of unrelieved suspense. At that speed and altitude the sudden sideways acceleration would bring the aircraft off the chosen flight path by several miles. The two Pratt & Whitney powerplants operated over a wide range of flight conditions, like those encountered during a climb to above 80,000 ft (24.4 km) altitudes and Mach 3+ speeds. The ramjet cycle [6] was the optimum engine cycle to really speed up this aircraft. A shift from the turbojet cycle. Supersonic speeds were needed to obtain a good thermal efficiency and maximum afterburner thrust from this cycle. Thus, specific thrust is proportional to flight Mach number and burner total temperature ratio. High temperature performance composites [7]. Aerospace structural composites have a higher mechanical strength than for any metals. Specific strength and stiffness (low density) are improved by a factor of 4. The thermal expansion coefficient is negligible for practical purposes. As regards to the YF-12A, in the late 1950's the stakes could hardly be higher. The engineers needed a radar absorbent, high temperature structural composite that could withstand aerodynamic heating, up to 425 deg C (leading surfaces). Forged titanium profiles with high-temperature fatigue performance, wherever needed, were used for the primary structure. The cover panels were fixed to the spanwise spars by stand-off clips, reducing the effects of differential thermal expansion. The high specific stiffness of composites curtailed the high load level associated with panel buckling. Needless to say, this aircraft was far ahead of its time, and could not have been built without using carbon composites and thermoplastics. A cause of much expense, the production of the YF-12A for the U.S. Air Force was cancelled in the mid-sixties. The SR-71 flew successfully, CIA reconnaissance missions, for many years. The YF-12A & SR-71 were both engineering benchmarks. The research and skills set by developing these vehicles, provided a window to a whole new threshold of engineering capabilities and territory of complexity. Nowadays the SR-71 aircrafts are engaged with new roles. Linking up the cross-boundary-discoveries in science and engineering of NASA near earth-space effort for the 21th century. E.g. the NASA-788/SR-71 is duty-bound for the development of the rocket engine for the next generation reuseable shuttle. [1] Deterring an adversary (specifically the Soviet Union) from a strategic attack. [2] A structure implies anything fabricated, manufactured, or erected that must withstand an imposed load. The continuum response is described by differential equations. These equations are solved by using discretionary methods. [3] State-of-the-art experimental fighter interceptor (1962). Predecessor to the SR-71 Blackbird, reconnaissance aircraft. [4] Originally developed by NASA, is the world's leading software for both static and dynamic structural problems. [5] Designated as Area 51 by the old Atomic Energy Commission (AEC). [6] Supersonic inlets are an integral part of ramjet and scramjet engines. A supersonic combustion ramjet is also called a scramjet. In other words, for the scramjet the diffused air never becomes subsonic. The scramjet operates at hypersonic speeds. Isentropic cone diffusers operate best in the Mach range 3 to 3.5. [7] Usually a chosen stacking sequence of carbonfibers, mixed with an epoxy resin. This combination is cured under raised pressure and temperature. The leading suppliers are Cytec Engineered Materials, Hexcel and Hitco Carbon Composites. Years ago, Raytheon Aircraft was the first company to be supported through the Pentagon's engineering initiative. Ron Certitude |