Lockheed SR-71 Blackbird

(PD) Photo: United States Air Force
SR-71A loaned from the U.S. Air Force for use in high-speed, high-altitude research at the NASA Dryden Flight Research Center, Edwards, California.

The Lockheed SR-71, unofficially known as the Blackbird and by its crews as the Habu or the sled, was an advanced, long-range, Mach 3 strategic reconnaissance aircraft developed from the Lockheed YF-12A and A-12 aircraft by the Lockheed Skunk Works. The SR-71 line was in service from 1964 to 1998. Clarence "Kelly" Johnson was the man behind many of the design's advanced concepts. The SR-71 was one of the first aircraft to be shaped to reduce radar cross section. However, the aircraft was not stealthy and still had a fairly large radar signature. The aircraft flew so fast and so high that if the crew detected a surface-to-air missile launch, the standard evasive action was simply to accelerate. Thirteen aircraft are known to have been lost, all from non-combat related reasons.

History

Predecessor models

The A-12 Oxcart, designed for the CIA by Kelly Johnson at the Lockheed Skunk Works, 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 plane, "the Article". As the design evolved, the internal Lockheed designation went from A-1 to A-12 as configuration changes occurred, such as substantial design changes to reduce the radar cross-section. The first flight took place at Groom Lake, Nevada, on April 25, 1962. It was 'Article 121,' an A-12, but it was equipped with less powerful Pratt & Whitney J75s due to protracted development of the intended Pratt & Whitney J58. The J58s were retrofitted as they became available. The J58s became the standard power plant for all subsequent aircraft in the series (A-12, YF-12, MD-21) as well as the follow-on SR-71 aircraft. Eighteen A-12 aircraft were built in four variations, of which three were YF-12As, prototypes of the planned F-12B interceptor version, and two were the M-21 variant (see below).

The Air Force reconnaissance version was originally called the R-12 (see the opening fly page in Paul Crickmore's book SR-71, Secret Missions Exposed, which contains a copy of the original R-12 labeled plan view drawing of the vehicle). However, during the 1964 presidential campaign, Senator Barry Goldwater continually criticized President Lyndon B. Johnson and his administration for falling behind the Soviet Union in the research and development of new weapon systems. Johnson decided to counter this criticism with the public release of the highly classified A-12 program and later the existence of the reconnaissance version.

Name and designation

The USAF had planned to redesignate the A-12 aircraft as the B-71 as the successor to the B-70 Valkyrie, which had two test Valkyries flying at Edwards AFB, California. The B-71 would have a nuclear capability of 3 first-generation SRAM's (Short-Range Attack Missiles). The next designation was RS-71 (Reconnaissance-Strike) when the strike capability became an option. However, then USAF Chief of Staff Curtis LeMay preferred the SR designation and wanted the RS-71 to be named SR-71. Before the Blackbird was to be announced by President Johnson on February 29, 1964, 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 plane's designation.

This public disclosure of the program and its designation came as a shock to everyone at Skunk Works and Air Force personnel involved in the program; at this time all of the printed Maintenance Manuals, Flight Crew Handbooks (the source of Paul Crickmoore's page), training slides and materials were still labeled "R-12" (the June 18, 1965 Certificate of Completion issued by the Skunkworks to the first Air Force Flight Crews and their Wing Commander are labeled: "R-12 Flight Crew Systems Indoctrination, Course VIII" and signed by Jim Kaiser, Training Supervisor and Clinton P. Street, Manager, Flight Crew Training Department). Following Johnson's speech, the designation change was taken as an order from the Commander-in-Chief, and immediate republishing began of new materials retitled "SR-71" with 29,000 blueprints altered.

First flight and usage

Although the predecessor A-12 first flew in 1962, the first flight of an SR-71 took place on December 22, 1964, at Air Force Plant 42 in Palmdale, California. The first SR-71 to enter service was delivered to the 4200th (later, 9th) Strategic Reconnaissance Wing at Beale Air Force Base, California, in January 1966. The United States Air Force Strategic Air Command had SR-71 Blackbirds in service from 1966 through 1991.

SR-71s first arrived at the 9th SRW's Operating Location ( OL-8 ) at Kadena Airbase, Okinawa on 08 March 1968. These deployments were code named "Glowing Heat", while the program as a whole was code named "Senior Crown." Reconnaissance missions over North Vietnam were code named "Giant Scale."

On March 21, 1968 Major (later General) Jerome F. O'Malley and Major Edward D. Payne flew the first operational SR-71 sortie in SR-71 serial number 61-7976 from Kadena AB, Okinawa. During its career, this aircraft (976) accumulated 2,981 flying hours and flew 942 total sorties (more than any other SR-71), including 257 operational missions, from Beale AFB; Palmdale, California; Kadena Air Base, Okinawa, Japan; and RAF Mildenhall, England. The aircraft was flown to the National Museum of the United States Air Force near Dayton, Ohio in March 1990.

From the beginning of the Blackbird's reconnaissance missions over enemy (North Vietnam, Laos, etc.) territory in 1968, the SR-71s averaged approximately one sortie a week for nearly two years. By 1970, the SR-71s were averaging two sorties per week. By 1972, the Blackbird was flying nearly one sortie every day. [24]

While deployed in Okinawa, the SR-71s and their aircrew members gained the nickname Habu (as did the A-12s preceding them) after a southeast Asian pit viper which the Okinawans thought the plane resembled.

In a seventeen-year period of its operational history (from July 21, 1972 to April 21, 1989) the SR-71 flew without a loss of any type. Other operational highlights:

• 3,551 Mission Sorties Flown
• 17,300 Total Sorties Flown
• 11,008 Mission Flight Hours
• 53,490 Total Flight Hours
• 2,752 hours Mach 3 Time (Missions)
• 11,675 hours Mach 3 Time (Total)

32 SR-71 airframes were built, 29 as SR-71As for operational missions and two as SR-71B trainers. The 32nd airframe was fabricated in 1969 as a hybrid trainer designated the SR-71C by mating the back half of an YF-12 wrecked in a 1966 landing accident with a fully functional SR-71 forward section of a static test specimen. Of all SR-71s, 12 (including one trainer) were lost in flight (or ground) accidents. Only one crewmember, Jim Zwayer, a Lockheed flight-test reconnaissance and navigation systems specialist, was killed from a flight accident. The rest of the crewmembers ejected safely or evacuated their aircraft on the ground.

The highly specialized and advanced tooling used in manufacturing the SR-71 was ordered destroyed by then-Secretary of Defense Robert McNamara upon completion of the DoD's existing orders for the aircraft, ensuring no future SR-71 production, and limiting the force to the 32 completed.

The U.S. Air Force retired its fleet of SR-71s on January 26, 1990, allegedly because of a decreasing defense budget and high costs of operation. The reconnaissance aspect of the SR-71 could be performed more cheaply, and often better by imagery intelligence satellites and drones. The SR-71's performance was still unequalled, but eventually there were few things that it could do that could not be done by other devices, and it was very expensive to operate. Also, parts were no longer being manufactured for the aircraft, so other airframes had to be cannibalized in order to keep the fleet airworthy. The USAF returned the SR-71 to the active Air Force inventory in 1995 and began flying operational missions in January 1997. The planes were permanently retired in 1998.

Variants

One notable variant of the basic A-12 design was the M-21. This was an A-12 platform modified by replacing the single seat aircraft's Q bay (which carried its main camera) with a second cockpit for a launch control officer. The M-21 was used to carry and launch the D-21 drone, an unmanned, faster and higher flying reconnaissance device. This variant was known as the M/D-21 when mated to the drone for operations. The D-21 drone was completely autonomous; having been launched it would overfly the target, travel to a rendezvous point and eject its data package. The package would be recovered in midair by a C-130 Hercules and the drone would self-destruct.

The program to develop this system was canceled in 1966 after a drone collided with the mother ship at launch, destroying the M-21 and killing the Launch Control Officer. Three successful test flights had been conducted under a different flight regime; the fourth test was in level flight, considered an operational likelihood. The shock wave of the M-21 retarded the flight of the drone, which crashed into the tailplane. The crew survived the mid-air collision but the LCO drowned when he landed in the ocean and his flight suit filled with water.

The surviving M-21 is on display at the Museum of Flight in Seattle, Washington with a drone. The D-21 was adapted to be carried on wings of the B-52 bomber.

Several D-21s are on display throughout the United States

• Spruce Goose museum in McMinnville, Oregon
• "Celebrity Row" at the Aircraft Maintenance And Regeneration Center (AMARC) located on Davis-Monthan AFB in Tucson, Arizona.
• Tail number 535 is on display at the US Air Force Museum in Fairborn (Dayton), Ohio., An enthusiast site devoted to the Blackbird lists a record time of 64 minutes. The SR-71 also holds the record for flying from New York to London: 1 hour 54 minutes and 56.4 seconds, set on September 1 1974. This is only Mach 2.68, well below the declassified figure of 3.0+. (For comparison, commercial Concorde flights took around 3 hours 20 minutes, and the Boeing 747 averages 6 hours.)

It should be noted that any discussion of the SR-71's records and performance is limited to declassified information. Actual performance figures will remain the subject of speculation until additional information is released.

Design

The airframe was made of titanium obtained from the USSR during the height of the Cold War. Lockheed used all possible guises to prevent the Soviet government from knowing what the titanium was to be used for. In order to keep the costs under control, they 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.

The plane was designed to minimize the radar cross-section and as such, the SR-71 was an early attempt at stealth design. However, the radar signature aspects of the SR-71 design did not take into account the extremely hot engine exhaust and the particles in the hot exhaust reflect radar extremely well. Ironically, the SR-71 was one of the largest targets on the FAA (Federal Aviation Administration) long range radars, which were able to track the plane at several hundred miles.

The red stripes found on some SR-71s are there to prevent maintenance workers from damaging the skin of the aircraft. The curved skin near the center of the fuselage is thin and delicate. There is no support underneath with exception of the structural ribs, which are spaced several feet apart.

Air inlets

The air inlets were a critical design feature to allow cruising speeds of over Mach 3.0, yet provide subsonic Mach 0.5, air flow into the turbojet engines. At the front of each inlet was a sharp, pointed moveable 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. 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, 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. 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". So significant was this inlet pressure build-up (pushing against the inlet structure) that at Mach 3.2 cruise, it was estimated that 58% of the available thrust was being provided by the inlet, 17% by the compressor and the remaining 25% by the afterburner. 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). 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 travelled. One 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, only to discover later that they were well ahead of their fuel curve.

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 incorrectly positioned), the shock wave would suddenly blow out the front of the inlet, called an "Inlet Unstart". Immediately, the airflow through the engine compressor would cease, thrust dropped and exhaust gas temperatures would begin to rise. Due to the tremendous thrust of the remaining engine pushing the aircraft asymmetrically along with the sudden deceleration caused by losing 50% of available power, 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. Pilots and RSOs occasionally experienced their pressure suit helmets banging on their cockpit canopies until the initial unstart motions subsided.

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 crew would return to acceleration and climb to the planned cruise altitude.

Eventually, a digital air inlet computer replaced the original analog 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.

Fuselage

Due to the great temperature changes in flight, the fuselage panels did not fit perfectly on the ground and were essentially loose. Proper alignment was only achieved when the airframe warmed up due to the 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 extreme temperatures, the aircraft would leak its 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 air-to-air refueled before departing on its mission. Cooling was carried out by cycling fuel behind the titanium surfaces at the front of the wings (chines). Nonetheless, once the plane landed no one could approach it for some time as its canopy was still hotter than 300 °C. Non-fibrous asbestos was also used, as in non-ceramic automotive brakes, due to its high heat tolerance.

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 seemed to indicate that a shape with flattened, tapering sides would reflect most radar away from the place where the radar beams originated. 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. 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, it was still easy to track by radar (and had a huge infrared signature when cruising at Mach 3+). It was visible on air traffic control radar for hundreds of miles, even when not using its transponder. This fact is further corroborated by the fact that missiles were fired at them quite often after they were detected on radar.

Stealth features were useful mainly for intelligence purposes (hiding the fact that the aircraft was in use). The flight characteristics of the SR-71 made it virtually invulnerable to attempts to shoot it down during its service life, and in fact no SR-71 was ever shot down, despite over 4,000 attempts to do so.

Chines

The chines themselves (sharp edges leading back to the left and right of the nose and along the sides of the fuselage) were an interesting and unique feature.

The Blackbird was originally not going to have chines. At its "A-11" design stage, it looked similar to an enlarged F-104. Lockheed's aerodynamicists were concerned that these large surfaces would negatively affect the aircraft's critical aerodynamic performance. However, the government agencies paying for the project wanted at least an attempt at stealth, despite knowing the airplane would not be "invisible" to radar. Their pressure led Lockheed's aerodynamicists to try adding chines to a few wind-tunnel models near the end of the configuration design process, as the chines would supposedly reduce the aircraft's Radar Cross Section from many angles.

The aerodynamicists discovered that the chines generated powerful vortices around themselves, generating much additional lift near the front of the aircraft, leading to surprising improvements in aerodynamic performance: The angle of incidence of the delta wings could then be reduced, allowing for greater stability and less high-speed drag, and more weight (fuel) could be carried, allowing for greater range. Landing speeds were also reduced, since the chines' vortices created turbulent flow over the wings at high angles of attack, making it harder for the wings to stall. (The Blackbird can, consequently, make high-alpha high-g turns to the point where the Blackbird's unique engine air inlets stop ingesting enough air, which can cause the engines to flame out. Blackbird pilots were thus warned not to pull more than 3Gs, so that angles of attack stay low enough for the engines to always get enough air). The chines act like the leading edge extensions which are used to increase the agility of many modern fighters such as the F-5, F-16, F/A-18, MiG-29 and Su-27. Once these advantages were observed during wind-tunnel tests of Blackbird models, the use of canard foreplanes was no longer needed. (Many early design models of what became the Blackbird featured canards).

When the Blackbird was being designed, no other airplane had featured chines, so Lockheed's engineers had to solve problems related to the differences in stability and balance caused by these then-unusual surfaces. Their solutions have since been extensively used. Chines are still an important part of the design of many of the newest stealth UAVs, such as the Dark Star, Bird of Prey, X-45, and X-47, since they allow for tail-less stability as well as for stealth.

Titanium structures and skin

Before the Blackbird, titanium could only be found in aircraft in high-temperature exhaust fairings and other small parts directly related to supporting, cooling, or shaping high-temperature areas. The decision to build the Blackbird's structure using 85% titanium and 15% composite materials was a first in the airplane industry. The advances made by Lockheed in learning to deal with this material have been used in subsequent high-speed aircraft such as most modern fighters.

Titanium was difficult to work with, expensive, and scarce. In fact, much of the titanium bought by Lockheed to make Blackbirds had to be imported from Russia. Initially, 80% of the titanium delivered to Lockheed had to be rejected due to metallurgical contamination.

One example of the difficulties of working with titanium is the fact that welds made at certain times of the year seemed to be more durable than welds made at other times. It was eventually found that the water supplied to the manufacturing plant came from one reservoir in the summer and another reservoir in the winter: The slight differences in the impurities in the water from these different reservoirs led to differences in the durability of the welds, since water was used to cool the titanium welds.

Studies of the aircraft's titanium skin revealed the metal was actually growing stronger over time due to the intense heating caused by aerodynamic friction, a process similar to annealing.

Major portions of the upper and lower inboard wing skin of the SR-71 were actually corrugated, not smooth. The thermal expansion stresses of a smooth skin would have resulted in the aircraft skin splitting or curling. By making the surface corrugated, the skin was allowed to expand vertically as well as horizontally without overstressing, which also increased longitudinal strength. Despite the fact that it worked, aerodynamicists were initially aghast at the concept and accused the design engineers of trying to make a 1920s era Ford Trimotor — known for its corrugated aluminum skin — go Mach 3.

Engines

The Pratt & Whitney J58-1 engines used in the Blackbird were the only military engines ever designed to operate continuously on afterburner, and actually became more efficient as the aircraft went faster. Each J58 engine could produce 32,500 lb_f (145 kN) of static thrust. Conventional jet engines cannot operate continuously on afterburner and lose efficiency as airspeed increases.

The J58 was unique in that it was a hybrid jet engine. It could operate as a regular turbojet at slow speeds, but at high speeds it became a ramjet. The engine can be thought of as a turbojet engine inside a ramjet engine. At lower speeds the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds the turbojet throttled back and just sat in the middle of the engine, as air bypassed around it, having been compressed by the shock cones and only burning fuel in the afterburner.

Air was initially compressed (and thus also heated) by the shock cones, which generated shockwaves that slowed the air down to subsonic speeds relative to the engine. The air then passed through 4 compressor stages and then was split by moveable vanes: some of the air entered the compressor fans ("core-flow" air), while the rest of the air went straight to the afterburner (via 6 bypass tubes). The air travelling on through the turbojet was further compressed (and thus further heated), and then fuel was added to it in the combustion chamber—it then reached the maximum temperature anywhere in the Blackbird, just under the temperature where the turbine blades would start to soften. After passing through the turbine (and thus being cooled somewhat), the core-flow air went through the afterburner and met with any bypass air.

At around Mach 3, the increased heating from the shock cone compression, plus the heating from the compressor fans, were already enough to get the core air to high temperatures, and little fuel could be added in the combustion chamber without the turbine blades melting. This meant the whole compressor-combustor-turbine set-up in the core of the engine provided less power, and the Blackbird flew predominantly on air bypassed straight to the afterburners, forming a large ramjet effect. No other aircraft does this. (This shows how the temperature tolerance of the turbine blades in a jet engine determine how much fuel can be burned, and thus to a great extent determine how much thrust a jet engine can provide.)

Performance at low speeds was anemic. Even passing the speed of sound required the aircraft to dive. The reason was that the size of the turbojets were traded to reduce weight but to still allow the SR-71 to reach speeds where the ramjet effect became prominent and efficient; and then the plane became alive and rapidly accelerated to Mach 3.0. The efficiency was then good due to high compression and low drag through the engine and this permitted large distances to be covered at high speed.

Originally, the Blackbird's engines started up with the assistance of an external "start cart", a cart containing two Buick Wildcat V8 engines which were rolled out onto the runway underneath the aircraft. The two Buick engines powered a single, vertical driveshaft connected to a single J58 engine. Once one engine was started, the cart was wheeled over to the other side of the aircraft to start the other engine. The operation was deafening. In later years, the J58s were started with a conventional start cart.

Operational details

Fuel

SR-71 development began using a coal slurry powerplant, but Johnson determined that the coal particles damaged engine components. He then began researching a liquid hydrogen powerplant, but the tanks required to store cryogenic hydrogen did not suit the Blackbird's form factor.

The focus then became somewhat more conventional, though still specialized in many ways. Originally developed for the A-12 Oxcart plane in the late 1950s, the JP-7 jet fuel had a relatively high flash point (60 °C) to cope with the heat. In fact, the fuel was used as a coolant and hydraulic fluid in the aircraft before being burned. The fuel also contained fluorocarbons to increase its lubricity, an oxidizing agent to enable it to burn in the engines, and even a cesium compound, A-50, which disguised the exhaust's radar signature. As a result, JP-7 was claimed to be more expensive than single malt Scotch whiskyTemplate:Fact, which contributed to the $24,000-$27,000/hr cost of operating the SR-71. For comparison, a U-2 costs only one-third as much. On the other hand, a U-2 travels at only one-fourth the speed, cannot carry as much reconnaissance equipment, and is much more vulnerable to interception.

JP-7 is very slippery and extremely difficult to light in any conventional way. The slipperiness was a disadvantage on the ground, since the aircraft leaked fuel when not flying, but at least JP-7 was not a fire hazard. When the engines of the aircraft were started, puffs of triethylborane (TEB), which ignites on contact with air, were injected into the engines to produce temperatures high enough to initially ignite the JP-7. The TEB produced a characteristic puff of greenish flameTemplate:Fact that could often be seen as the engines were ignited. TEB was also used to ignite the afterburners. The aircraft had only 600 cubic centimeters of TEB on board for each engine, enough for at least 16 injections (a counter advised the pilot of the number of TEB injections remaining), but this was more than enough for the requirements of any missions it was likely to carry out.

(CC) Photo: James Gordon
KC-135R refueling NASA SR-71B

The main air refueling aircraft of the U.S. Air Force, especially when the SR-71 was in service, was the KC-135 series. It may be counterintuitive, but refueling tankers are not giant fuel tanks; if their fuselages were filled with fuel, they would be too heavy to take off. With a relevant exception here, KC-135 started as conventional transport aircraft, and had a number of fuel tanks added in aerodynamically efficient places that did not distrupt the weight and balance engineering. With all but one type of KC-135, there was one interconnected set of tanks, because the KC-135 could burn the same fuel it dispensed. One recent variation is the KC-135T (and possibly KC-135R), which is the first tanker — new, non-KC-135 types have the capability — that can be refueled in flight as well as refuel other aircraft, which gives great operational flexibility for very long missions.

For the SR-71 program, however, there was a special variant, the KC-135Q. Q models had two sets of tanks, which were not interconnected: one set carried fuel for the KC-135; I am not certain if it could still refuel aircraft that took the same fuel as the KC-135. The Q model, had a set of tanks that carried JP-7 specifically for the SR-71. While the caption on this picture indicated it was a KC-135R, it may actually have been a KC-135Q.

The KC-135R was the first in a series of KC-135 variants with greater engine efficiency and other improvement that could let it carry more transferrable fuel in a single mission. A Globalsecurity article suggests the T model incorporates both the efficiency improvements of an R and the separate tanking of a Q.

Life support

Crews flying the SR-71 at 80,000 feet faced three main survival problems: 1) With a standard pressure demand oxygen mask, human lungs can not absorb enough of 100% oxygen above 43,000 feet to sustain consciousness and life, 2) above 63,000 feet, body fluids boil and death is very rapid Template:Fact, and 3) the instant heat rise pulse on the body when exposed to a mach 3 air flow during ejection would be about 450 degrees (F). To solve these problems, the David Clark Company was hired to produce protective full pressure suits for all of the crew members of the A-12, YF-12, MD-21 and SR-71 aircraft. These suits were later adopted for use on the Space Shuttle during ascent.

In addition, at mach 3+ cruise the external heat rise due to the compression of air on the vehicle would even heat up the inside of the windshield to 250 degrees (F) and cooling of the crew members was vital. This was achieved by cooling the air with an air conditioner. The air conditioner dumped the heat from the cockpit into the fuel prior to combustion via a heat exchanger.

After a high altitude bailout, an oxygen supply would keep the suit pressurized. The crew member would then free fall to 15,000 feet before the main parachute was opened allowing the high heat rise to bleed off as the crew member slowed down and descended. To demonstrate this full pressure suit capability, crew members would wear one of these suits and undergo an altitude chamber explosive decompression to 78,000 feet or higher while chamber heaters would rapidly turn on to 450 degrees (F) and then be turned down at the rate experienced during a real life free fall.

Since the cabin altitude of the SR-71 stayed at 27,000 - 29,000 feet during flight, crews flying a low-subsonic flight (such as a ferry mission) would wear either their full pressure suit or standard USAF hard hat helmets, pressure demand oxygen masks and nomex flying suits.

Astro-Inertial Navigation System (ANS)

Blackbird precision navigation requirements for route accuracy, sensor pointing and target tracking preceded the development and fielding of GPS (the Global Positioning System and its family of position determining satellites). U-2 and A-12 Inertial Navigation Systems existed, but US Air Force planners wanted a system that would bound inertial position growth for longer missions envisioned for the R-12 / SR-71.

Nortronics, the electronics development organization of Northrop, had extensive astro-inertial experience, having provided an earlier generation system for the USAF Snark missile. With this background, Nortronics developed the Astro-Inertial Navigation System for the AGM-87 Skybolt missile, which was to be carried and launched from B-52H bombers. When the Skybolt Program was cancelled in December 1962, the assets Nortronics developed for the Skybolt Program were ordered to be adapted for the Blackbird program. A Nortronics "Skunkworks" type organization in Hawthorne, California completed the development and fielding of this system, sometimes referred to as the NAS-14 and/or the NAS-21.

The ANS primary alignment was done on the ground and was time consuming, but brought the inertial components to a high degree of level and accuracy for the start of a mission. A "blue light" source star tracker, which could detect and find stars during day or night, would then continuously track stars selected from the system's digital computer ephemeris as the changing aircraft position would bring them into view. Originally equipped with data on 56 selected stars, the system would correct inertial orientation errors with celestial observations. The resulting leveling accuracies obtained limited accelerometer errors and/or position growth.

Rapid ground alignments and air start abilities were later developed and added to the ANS. Attitude and position inputs to on-board systems and flight controls included the Mission Data Recorder, Auto-Nav steering between loaded destination points, automatic pointing and/or control of cameras at control points and optical or SLR sighting of fix points (this mission data being tape loaded into the ANS prior to take-off).

The ANS was located behind the RSO station and tracked stars through a round, quartz window seen in photos of the upper fuselage. Cooling in the Blackbird mach 3.0+ cruising environment was a serious development challenge resolved by Lockheed and Nortronics engineers during the early test phases. The ANS became a highly reliable and accurate self-contained navigation system.

Note: The original B-1A Offensive Avionics Request For Proposal (RFP) required the installation and integration of a NAS-14 system, but cost cutting changes later deleted it from the B-1. Some U2-Rs did receive the NAS-21 system, but newer Inertial and GPS systems replaced them.

Sensors and Payloads

Original capabilities for the SR-71 included optical/infrared imagery intelligence systems, side-looking airborne radar (SLAR), electronic intelligence (ELINT) gathering systems, defensive systems (for countering missile and airborne fighter threats) and recorders for SLAR, ELINT and maintenance data.

Imagery systems used on the Blackbird were diverse. At the simple end of the spectrum, SR-71s were equipped with a Fairchild tracking camera of modest resolution and a HRB Singer infrared-tracking IR camera, both of which ran during the entire mission to document where the aircraft flew and answer any post-flight "political" charges of overflight. Further advances included equipping Blackbirds with two of ITEK's Operational Objective Cameras (OOC) that provided stereo imagery left and right of the flight track or an ITEK Optical Bar Camera (OBC) that replaced the OOCs and was carried in the nose in place of the SLR. The ultimate advance in imagery was the HYCON Technical Objective Camera (TEOC) that could look straight down or up to 45 degrees left or right of centerline. SR-71s were equipped with two of them, each with a six-inch resolution and the ability to show such details as the painted lines in parking lots from an altitude of 83,000 feet. In the later years of the SR-71's operational usage, the infrared camera use was discontinued.

Side-looking radar, built by Goodyear Aerospace in Arizona, was carried in the removable nose section (which could be loaded with the SLR antenna in the maintenance shop before installation on the Blackbird). It was eventually replaced by Loral's Advanced Synthetic Aperture Radar System (ASARS-1) and built and supported by Goodyear. Both the first SLR and ASARS-1 were ground mapping imaging systems and could collect data in fixed swaths left or right of centerline or from a spot location where higher resolution was desired. As an example, in passing abeam of an open door aircraft hangar, ASARS-1 take could provide meaningful data on what was the hangar's contents or whether the hangar was empty.

ELINT gathering systems, called the Electro Magnetic Reconnaissance System (EMR) built by AIL could be carried in both the left and right chine bays to provide a wide view of the electronic signal fields the Blackbird was flying through. Computer loaded instructions looked for items of special Intelligence Interest.

Defensive systems, built by several leading electronic countermeasures (ECM) companies included (and evolved over the years of the Blackbird's operational life) Systems A, A2, A2C, B, C, C2, E, G, H and M. Several of these different frequency/purpose payloads would be loaded for a particular mission to match the threat environment expected for that mission. They, their warning and active electronic capabilities, and the Blackbird's ability to accelerate and climb when under attack resulted in the SR-71's long and proven survival track record.

Recording systems captured SLR phase shift history data (for ground correlation after landing), ELINT-gathered data, and Maintenance Data Recorder (MDR) information for post flight ground analysis of the aircraft and its systems' overall health (humorous stories accompanied some of the flight crews' discovery that the voice track in the MDR recorded interphone conversations between pilot and RSO and tanker aircraft crew members during refueling hook-ups).

In later years of its operational life, a data-link system was added that would allow ASARS-1 and ELINT data from about 2,000 nm of track coverage to be downlinked if the SR-71 was within "contact" with a mutually equipped ground station.

Flight Simulator

The Link Simulator Company's SR-71 Flight Simulator was developed during 1963 – 1965 under a deep "black" security blanket because it (and the team Link assigned to it) were given access to CIA Oxcart and USAF R-12 / SR-71 clearances, the complete list of names of classified vendors supplying parts and software that had to be simulated, the total aircraft performance envelope data and a government-produced satellite photo montage of almost the entire continental United States to provide optical imagery for the RSO's portion of the Flight Simulator. This later capability was mounted on a separate, large, rectangular glass plate (approximately 6 feet by 12 feet in size) over which moved an optical sighting head that traveled at the scaled speed and direction of the Blackbird during its simulated flight. Realistic and accurate images were then displayed in the Optical View Sight and SLR RCD (Radar Correlator Display) in the RSO cockpit. Imagery was not provided to the pilot's simulator, which like the RSO simulator, had translucent window panels with varying degrees of lighting to change a simulated flight from daylight to night flying conditions.

Instructor positions were behind both the pilot's and the RSO's cockpits with monitoring, malfunction and emergency problem controls provided. The simulator halves could be flown as separate cockpits with different training agendas or in a team mode where intercom, instrument readings and air vehicle/sub-systems performance were integrated. Although most simulator flights were in a flight suit "shirt sleeve" environment, selected flights during a crew's checkout training were made with the crew wearing the complete David Clark Company's Full Pressure Suit.

In 1965, when the first Beale AFB Instructor Pilot/RSO crew (wearing civilian clothes only) visited the Flight Simulator during USAF checkout and acceptance trials at Link's upper New York state facilities, they were surprised to park in front of a busy, active grocery store and then be escorted quietly to a side door that led them into a hidden, rear portion of the building that was Link's highly classified "Skunkworks" type facility for the Blackbird program. Total secrecy was so complete that no one in the New York township site was aware of what was going on behind the busy checkout stands selling food-stuffs and beverages.

In 1965, the Flight Simulator was transferred to Beale AFB, California and the 9th Strategic Reconnaissance Wing's SAGE building, which provided vault level security for it plus the Wing Headquarters, Flight Mission Planning, and Intelligence Analysis / Exploitation of Blackbird mission products.

Besides SR-71 flight crew training and currency usage, the Flight Simulator was used several times by Lockheed and CIA operatives to analyze Groom Lake A-12 problems and accidents with similar assistance provided for SR-71 flights at Edwards AFB. Another unique feature was that an actual flight mission tape for the SR-71 ANS could be loaded into the Flight Simulator's digital computers, which had been designed and programmed by Link engineers to emulate the Nortronics ANS. During Category II testing at Edwards AFB, some types of ANS navigation errors could be duplicated in the Flight Simulator at Beale AFB with Link engineers often then assisting in software fixes to the main ANS flight software programs.

At the conclusion of SR-71 flying at Beale AFB, the Flight Simulator (minus the RSO optical imagery system) was transferred to the NASA Dryden facility at Edwards AFB in support of NASA SR-71 flight operations. Upon completion of all USAF and NASA SR-71 operations at Edwards, the Flight Simulator was moved in July, 2006 to the Frontiers of Flight Museum on Love Field Airport in Dallas,Texas (www.flightmuseum.com) and with support from the Museum and Link (now, L-3 Communications Simulation and Training Division) it is intended to be available for viewing by Museum visitors.

Myth and lore

The plane developed a small cult following, given its design, specifications, and the aura of secrecy that surrounded it. Some conspiracy theorists speculated that the true operational capabilities of the SR-71 and the associated A-12 were never revealed. Most aviation buffs speculate that given a confluence of structural and aerodynamic tolerances, the plane could fly at a maximum of Mach 3.3 for extended periods, and could not exceed Mach 3.44 in any currently known configuration.Template:Fact Specifically, these groups cite the specific maximum temperature for the compressor inlet of 427 °C (800 °F). This temperature is quickly surpassed at speeds greater than Mach 3.3. Mach 3.44 is given as the speed at which the engine enters a state of "unstart". Some speculate that the former condition can be alleviated by superior compressor design and composition, while the latter might be solved with improved shock cones. It is known that the J58 engines were most efficient at around Mach 3, and this was the Blackbird's typical cruising speed.

The SR-71's Pratt & Whitney J58 engines never exceeded test bench values above Mach 3.6 in unclassified tests. Given the history of the plane, the advanced and classified nature of much of its original design, and most importantly, the fact that no SR-71 exists in a form that is immediately airworthy, it may never be known what the true design tolerances of the aircraft were, or if these tolerances were ever approached in flight. This undoubtedly contributes to the mystique of the SR-71.

The SR-71 was the first operational aircraft designed around a stealthy shape and materials. The most visible marks of its low radar cross section (RCS) are its inwardly-canted vertical stabilizers and the fuselage chines. Comparably, a plane of the SR-71's size should generate a radar image the size of a flying barn, but its actual return is more like that of a single door. Though with a much smaller RCS than expected for a plane of its size, it was still easily detected, because the exhaust stream would return its own radar signature (even though a special cesium compound was added to the fuel to reduce this signature). Furthermore, this is no comparison to the later F-117, whose RCS is on the order of a small ball bearing.

Succession

Much speculation exists regarding a replacement aircraft for the SR-71, most notably an aircraft identified as the Lockheed Aurora. The fact that the SR-71 was still able to perform its duties with an excellent service record at the time of its retirement, that the need for its reconnaissance duties had not subsided at the time of its retirement, and that it was retired then pressed back into active service for a short time before being quickly retired again, give credibility to the rumors of a successor aircraft. Whether that aircraft is the Lockheed SR-91 Aurora is still unknown to the general public.

Such a successor may be linked to a classified project rumored to exist at the Lockheed Skunk Works in the early 1980s to build a hybrid scramjet-powered reconnaissance aircraft capable of speeds near Mach 5. Production of the aircraft may have been incorporated into the 1988 Department of Defense budget, with the aircraft becoming operational around 1989. The fact that none of the systems suggested as replacements for the SR-71 are capable of effectively fulfilling the SR-71 duties, with regard to time sensitive reconnaissance and penetration of highly defended areas, gives additional weight to the existence of an undisclosed replacement. Its functions appear to have shifted to reliance on satellites for imagery intelligence and some signals intelligence, as well as unmanned aerial vehicles (UAV). Some UAVs are long-endurance and stealthy(e.g., MQ-4 Global Hawk).