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Wednesday, May 13, 2009
Design and operational details
The flight instrumentation of SR-71 BlackbirdA particularly difficult issue with flight at over Mach 3 is the high temperatures generated. As an aircraft moves through the air, the air in front of the aircraft compresses and this heats the air, and the heat conducts into the aircraft's airframe. To help with this, high temperature materials were needed and the airframe was substantially made of titanium, obtained from the USSR, at the height of the Cold War. Lockheed used many guises to prevent the Soviet government knowing what the titanium was to be used for. In order to control costs, Lockheed used a more easily-worked alloy of titanium which softened at a lower temperature. Finished aircraft were painted a dark blue (almost black) to increase the emission of internal heat (since fuel was used as a heat sink for avionics cooling) and to act as camouflage against the sky.[citation needed] The aircraft was designed to minimize its radar cross-section, an early attempt at stealth design.[11]
[edit] Air inlets
Operation of the air inlets and air flow patterns through the J58.The air inlets were a critical design feature to allow cruising speeds of over Mach 3.2, yet provide subsonic Mach 0.5 airflow into the turbojet engines. At the front of each inlet was a sharp, pointed movable cone called a "spike" that was locked in the full forward position on the ground or when in subsonic flight. During acceleration to high-speed cruise, the spike would unlock at Mach 1.6 and then begin a mechanical (internal jackscrew powered) travel to the rear.[12] It moved up to a maximum of 26 inches (66 cm).
The original air inlet computer was an analog design which, based on pitot-static, pitch, roll, yaw, and angle-of-attack inputs, would determine how much movement was required. By moving, the spike tip would withdraw the shock wave, riding on it closer to the inlet cowling until it just touched slightly inside the cowling lip. In this position shock-wave spillage, causing turbulence over the outer nacelle and wing, was minimized while the spike shock-wave then repeatedly reflected between the spike centerbody and the inlet inner cowl sides. In doing so, shock pressures were maintained while slowing the air until a Mach 1 shock wave formed in front of the engine compressor.[13]
The backside of this "normal" shock wave was subsonic air for ingestion into the engine compressor. This capture of the Mach 1 shock wave within the inlet was called "Starting the Inlet". Tremendous pressures would be built up inside the inlet and in front of the compressor face. Bleed tubes and bypass doors were designed into the inlet and engine nacelles to handle some of this pressure and to position the final shock to allow the inlet to remain "started." It is commonly cited that a large amount of the thrust at higher mach numbers comes from the inlet. However, this is not entirely accurate. Air that is compressed by the inlet/shockwave interaction is diverted around the turbo machinery of the engine and directly into the afterburner where it is mixed and burned. This configuration is essentially a ramjet and provides up to 70% of the aircraft's thrust at higher mach numbers.
Ben Rich, the Lockheed Skunkworks designer of the inlets, often referred to the engine compressors as "pumps to keep the inlets alive" and sized the inlets for Mach 3.2 cruise (where the aircraft was at its most efficient design point).[14] The additional "thrust" refers to the reduction of engine energy required to compress the airflow. One unique characteristic of the SR-71 is that the faster it went, the more fuel-efficient it was in terms of pounds burned per nautical mile traveled. An incident related by Brian Shul, author of Sled Driver: Flying the World's Fastest Jet, was that on one reconnaissance run he was fired upon several times. In accordance with procedure they accelerated and maintained the higher than normal velocity for some time; afterwards they discovered that this had reduced their fuel consumption.[15]
In the early years of the Blackbird programs the analog air inlet computers would not always keep up with rapidly-changing flight environmental inputs. If internal pressures became too great and the spike was incorrectly positioned the shock wave would suddenly blow out the front of the inlet, called an "Inlet Unstart." The flow of air through the engine compressor would immediately stop, thrust would drop, and exhaust gas temperatures would begin to rise. Due to the tremendous thrust of the remaining engine pushing the aircraft asymmetrically an unstart would cause the aircraft to yaw violently to one side. SAS, autopilot, and manual control inputs would fight the yawing, but often the extreme off-angle would reduce airflow in the opposite engine and cause it to begin "sympathetic stalls." The result would be rapid counter-yawing, often loud "banging" noises and a rough ride. The crews' pressure-suit helmets would sometimes bang on the cockpit canopies until the initial unstart motions subsided.[16]
One of the standard counters to an inlet unstart was for the pilot to reach out and unstart both inlets; this drove both spikes out, stopped the yawing conditions and allowed the pilot to restart each inlet. Once restarted, with normal engine combustion, the plane could accelerate and climb to the planned cruise altitude.[citation needed]
The analog air inlet computer was later replaced by a digital one. Lockheed engineers developed control software for the engine inlets that would recapture the lost shock wave and re-light the engine before the pilot was even aware an unstart had occurred. The SR-71 machinists were responsible for the hundreds of precision adjustments of the forward air by-pass doors within the inlets. This helped control the shock wave, prevent unstarts, and increase performance.[citation needed]
[edit] Fuselage
To allow for thermal expansion at the high operational temperatures the fuselage panels were manufactured to fit only loosely on the ground. Proper alignment was only achieved when the airframe warmed up due to air resistance at high speeds, causing the airframe to expand several inches. Because of this, and the lack of a fuel sealing system that could handle the thermal expansion of the airframe at extreme temperatures, the aircraft would leak JP-7 jet fuel onto the runway before it took off. The aircraft would quickly make a short sprint, meant to warm up the airframe, and was then refueled in the air before departing on its mission. Cooling was carried out by cycling fuel behind the titanium surfaces at the front of the wings (chines). On landing after a mission the canopy temperature was over 300 °C (572 °F), too hot to approach. Non-fibrous asbestos with high heat tolerance was used in high-temperature areas.[14]
[edit] Stealth
There were a number of features in the SR-71 that were designed to reduce its radar signature. The first studies in radar stealth technology seemed to indicate that a shape with flattened, tapering sides would avoid reflecting most radar energy toward the radar beams' place of origin. To this end, the radar engineers suggested adding chines (see below) to the design and canting the vertical control surfaces inward. The plane also used special radar-absorbing materials which were incorporated into sawtooth shaped sections of the skin of the aircraft, as well as cesium-based fuel additives to reduce the exhaust plumes' visibility on radar.
The overall effectiveness of these designs is still debated; Ben Rich's team could show that the radar return was, in fact, reduced, but Kelly Johnson later conceded that Russian radar technology was advancing faster than the "anti-radar" technology Lockheed was using to counter it.[17] The SR-71 made its debut years before Pyotr Ya. Ufimtsev's ground-breaking research made possible today's stealth technologies, and, despite Lockheed's best efforts, the SR-71 was still easy to track by radar and had a huge infrared signature when cruising at Mach 3.2 or more. It was visible on air traffic control radar for hundreds of miles, even when not using its transponder.[18] SR-71s were evidently detected by radar, as missiles were often fired at them.
In the end, the SR-71's greatest protection was its flight characteristics, which made it almost invulnerable to the attack technologies of the time; over the course of its service life, not one was shot down, despite over 4,000 attempts to do so.[19]
Development
[edit] Predecessors
The A-12 OXCART, designed for the CIA by Clarence Johnson at the Lockheed Skunk Works,[3] was the precursor of the SR-71. Lockheed used the name "Archangel" for this design, but many documents use Johnson's preferred name for the aircraft, "the Article". As the design evolved, the internal Lockheed designation progressed from A-1 to A-12 as configuration changes occurred, such as substantial design changes to reduce the radar cross-section.
The first flight, by an A-12 known as "Article 121", took place at Groom Lake, Nevada, on 25 April 1962 equipped with the less powerful Pratt & Whitney J75 engines due to protracted development of the intended Pratt & Whitney J58. The J58s were retrofitted as they became available, and became the standard power plant for all subsequent aircraft in the series (A-12, YF-12, M-21) as well as the follow-on SR-71 aircraft.
Eighteen A-12 family aircraft were built. One was a pilot trainer with a raised second cockpit for an Instructor-Pilot and 12 were reconnaissance A-12s to be flown operationally by CIA pilots. Three were YF-12A prototypes of the planned F-12B interceptor version, and two were the M-21 variant.
[edit] SR-71
The SR-71 designator is a continuation of the pre-1962 bomber series, which ended with the XB-70 Valkyrie. During the later period of its testing, the B-70 was proposed for a reconnaissance/strike role, with an RS-70 designation. When it was clear that the A-12 performance potential was much greater, the Air Force ordered a variant of the A-12 in December 1962.[4] Originally named R-12,[5] the Air Force version was longer and heavier than the A-12. Its fuselage was lengthened for additional fuel capacity to increase range. Its cockpit included a second seat and the chines were reshaped. Reconnaissance equipment included signals intelligence sensors, a side-looking radar and a photo camera.[4] The CIA's A-12 remained a better reconnaissance tool than the Air Force's R-12, however; the A-12 flew higher and faster,[6] and with only one pilot it had room to carry a superior camera[6] and more instruments.[7]
During the 1964 campaign, Republican presidential nominee Barry Goldwater continually criticized President Lyndon B. Johnson and his administration for falling behind the Soviet Union in the research and development of new weapons systems. Johnson decided to counter this criticism by announcing the YF-12A Air Force interceptor (which also served as cover for the still-secret A-12)[8] and, on 25 July 1964, the Air Force reconnaissance model. Air Force Chief of Staff General Curtis LeMay preferred the SR (Strategic Reconnaissance) designation and wanted the RS-71 to be named SR-71. Before the July speech, LeMay lobbied to modify Johnson's speech to read SR-71 instead of RS-71. The media transcript given to the press at the time still had the earlier RS-71 designation in places, creating the myth that the president had misread the aircraft's designation.[9][10]
This public disclosure of the program and its renaming came as a shock to everyone at the Skunk Works and to Air Force personnel involved in the program. All of the printed maintenance manuals, flight crew handbooks,[5] training slides and materials were labeled "R-12" and the 18 June 1965 Certificates of Completion issued by the Skunk Works to the first Air Force Flight Crews and their Wing Commander were labeled "R-12 Flight Crew Systems Indoctrination, Course VIII". Following Johnson's speech the name change was taken as an order from the Commander-in-Chief, and immediate reprinting began of new materials, including 29,000 blueprints, to be retitled "SR-71".
Lockheed SR-71
The Lockheed SR-71 was an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed A-12 and YF-12A aircraft by the Lockheed Skunk Works as a Black Project. The SR-71 was unofficially named the Blackbird, and called the Habu by its crews, referring to an Okinawan species of pit viper.[1] Clarence "Kelly" Johnson was responsible for many of the design's innovative concepts. A defensive feature of the aircraft was its high speed and operating altitude, whereby, if a surface-to-air missile launch were detected, standard evasive action was simply to accelerate. The SR-71 line was in service from 1964 to 1998, with 12 of the 32 aircraft being destroyed in accidents, though none were lost to enemy action.[2]
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