Editing Lockheed SR-71 Blackbird (section)

==Design==

===Overview===
[[File:Evergreen Aviation & Space Museum Oregon12.jpg|thumb|Tandem cockpits]]

The SR-71 was designed for flight at over [[Mach number|Mach]]&nbsp;3 with tandem cockpits for a crew of two: a pilot; and a reconnaissance systems officer who navigated and operated the surveillance systems. It was extremely important for the pilot and RSO to work well together as a crew.{{sfn|Graham|2008|page=6}}<ref name=crown>{{cite web |url=https://fas.org/irp/program/collect/sr-71.htm |title=Senior Crown SR-71 |work=[[Federation of American Scientists]] |date=7 September 2010 |access-date=17 October 2012 |archive-url=https://web.archive.org/web/20150417003154/https://fas.org/irp/program/collect/sr-71.htm |archive-date=17 April 2015}}</ref><ref name="harvp|Graham|1996">{{harvp|Graham|1996}}</ref>  The SR-71 was designed with the smallest radar cross-section that Lockheed could achieve, an early attempt at stealth design.<ref>{{harvp|Crickmore|2009|pp=30-31}}</ref> Aircraft were painted black. This color radiated heat from the surface more effectively than the bare metal, reducing the temperature of the skin and thermal stresses on the airframe.{{sfn|Merlin|2009|p=16}} The appearance of the painted aircraft gave it the nickname "Blackbird".

===Airframe, canopy, and landing gear===
Titanium was used for 85% of the structure, with much of the rest being [[polymer]] [[composite material]]s.<ref>{{harvp|Merlin|2009}}</ref> To control costs, Lockheed used a more easily worked titanium alloy, which softened at a lower temperature.{{refn|While titanium ores are cheap and abundant, converting those ores into metallic titanium is laborious and expensive. Soviet production of metallic titanium was several times US production, so the CIA{{snd}}without Soviet knowledge{{snd}}used dummy intermediary firms to obtain Soviet-produced metallic titanium for US military use.<ref>{{cite report |url=https://www.cia.gov/readingroom/docs/CIA-RDP86T00591R000200170005-0.pdf |title=The Soviet Titanium and Its Role in the Military Buildup |publisher=Directorate of Intelligence |access-date=15 January 2025}}</ref><ref>{{harvp|Rich|Janos|1994}}</ref><ref>{{cite web |url=https://nationalinterest.org/blog/buzz/titanium-russia-was-secret-ingredient-sr-71-blackbird-207691/ |title=Titanium from Russia Was the Secret Ingredient in the SR-71 Blackbird |date=3 December 2023 |first1=Maya |last1=Carlin |work=National Interest |accessdate=27 January 2025}}</ref>|group=N}} The challenges posed led Lockheed to develop new fabrication methods, which have since been used in the manufacture of other aircraft. Lockheed found that washing welded titanium requires [[distilled water]], as the chlorine present in tap water is [[corrosion|corrosive]]; [[cadmium]]-plated tools could not be used, as they also caused corrosion.<ref>{{harvp|Rich|Janos|1994|p=213-214}}</ref> Metallurgical contamination was another problem; at one point, 80% of the delivered titanium for manufacture was rejected on these grounds.<ref>{{harvp|Rich|Janos|1994|p=203}}</ref><ref>{{harvp|McIninch|1971|p=5}}</ref>

The high temperatures generated in flight required special design and operating techniques. Major sections of the skin of the inboard wings were corrugated, not smooth. Aerodynamicists initially opposed the concept, disparagingly referring to the aircraft as a Mach 3 variant of the 1920s-era [[Ford Trimotor]], which was known for its corrugated aluminum skin. But high heat would have caused a smooth skin to split or curl, whereas the corrugated skin could expand vertically and horizontally and had increased longitudinal strength.

Fuselage panels were manufactured to fit only loosely with the aircraft on the ground. Proper alignment was achieved as the airframe heated up, with [[thermal expansion]] of several inches.<ref>{{harvp|Graham|1996|p=47}}</ref> Because of this, and the lack of a fuel-sealing system that could remain leak-free with the extreme temperature cycles during flight, the aircraft leaked [[JP-7]] fuel on the ground prior to takeoff,<ref>{{harvp|Graham|1996|p=160}}</ref> annoying ground crews.{{r|iwm20211103}}

The outer windscreen of the cockpit was made of three layers of glass with cooling sections between them.{{citation needed|date=December 2024}} The ANS navigation window was made of solid [[quartz]] and was [[ultrasonic welding|fused ultrasonically]] to the titanium frame.<ref>{{cite journal |last1=Burrows |first1=William E. |title=The Real X-Jet |journal=Air & Space Magazine |date=1 March 1999 |url=https://www.airspacemag.com/military-aviation/the-real-x-jet-12377380/ |access-date=16 January 2018}}</ref> The temperature of the exterior of the windscreen could reach {{convert|600|°F|°C|abbr=on|sigfig=2}} during a mission.<ref>{{harvp|Graham|1996|p=41}}</ref>

[[File:16 19 037 SR71.jpg|thumb|Detail of SR-71A at the [[Museum of Aviation (Warner Robins)|Museum of Aviation]], [[Robins AFB]] showing red lined no-step areas. Not shown on this museum exhibit is the additional NO STEP wording on operational aircraft which showed to which side of the line the warning was applicable.]]
The Blackbird's tires, manufactured by [[Goodrich Corporation|B.F. Goodrich]], contained aluminum and were inflated with nitrogen. They cost $2,300 each and generally required replacing within 20 missions. The Blackbird landed at more than {{convert|170|kn|mph km/h|sigfig=2}} and deployed a drag parachute to reduce landing roll and brake and tire wear.<ref>Blackbird diaries, Air & Space, December 2014/January 2015, p. 46.</ref>

===Shape and threat avoidance===
[[File:Sr71 1.jpg|thumb|[[Contrail|Water vapor]] is condensed by the low-pressure vortices generated by the chines outboard of each engine inlet.]]

The SR-71 was the second operational aircraft,<ref name="cia.gov">{{cite web|url=https://www.cia.gov/news-information/featured-story-archive/2015-featured-story-archive/oxcart-vs-blackbird.html|archive-url=https://web.archive.org/web/20151208074002/https://www.cia.gov/news-information/featured-story-archive/2015-featured-story-archive/oxcart-vs-blackbird.html|url-status=dead|archive-date=8 December 2015|title=OXCART vs Blackbird: Do You Know the Difference? |website=Cia.gov}}</ref> after the [[Lockheed A-12]],<ref name="cia.gov" /> designed to be hard to spot on [[radar]]. Early studies in [[stealth technology]] indicated that a shape with flattened, tapering sides would reflect most radar energy away from a beam's place of origin, so Lockheed's engineers added [[Chine (aeronautics)|chines]] and canted the vertical control surfaces inward. Special [[radar-absorbing material]]s were incorporated into [[serration|sawtooth-shaped]] sections of the aircraft's skin. [[Cesium]]-based fuel additives were used to somewhat reduce the visibility of exhaust plumes to radar, although exhaust streams remained quite apparent. Ultimately, engineers produced an aircraft with a wing area of about {{cvt|170|sqm|sqft|order=flip}} but a radar cross-section (RCS) of around {{cvt|10|sqm|sqft|order=flip}}.<ref>{{harvp|Graham|1996|p=75}}</ref> Johnson later conceded that Soviet radar technology advanced faster than the stealth technology employed against it.<ref>Hott, Bartholomew and George E. Pollock [https://web.archive.org/web/20030216054101/http://web.ics.purdue.edu/~gpollock/The%20Advent%2C%20Evolution%2C%20and%20New%20Horizons%20of%20United%20States%20Stealth%20Aircraft.htm "The Advent, Evolution, and New Horizons of United States Stealth Aircraft."] ''archive.is''. Retrieved: 7 February 2014.</ref>

While the SR-71 carried [[Radar jamming and deception|radar countermeasures]] to evade interception efforts, its greatest protection was its combination of high altitude and very high speed, which made it invulnerable at the time. Along with its low radar cross-section, these qualities gave a very short time for an enemy [[surface-to-air missile]] (SAM) site to acquire and track the aircraft on radar. By the time the SAM site could track the SR-71, it was often too late to launch a SAM, and the SR-71 would be out of range before the SAM could catch up to it. If the SAM site could track the SR-71 and fire a SAM in time, the SAM would expend nearly all of the [[delta-v]] of its boost and sustainer phases just reaching the SR-71's altitude; at this point, out of thrust, it could do little more than follow its ballistic arc. Merely accelerating would typically be enough for an SR-71 to evade a SAM;<ref name="SR71 Blackbird 2006"/> changes by the pilots in the SR-71's speed, altitude, and heading were also often enough to spoil any radar lock on the plane by SAM sites or enemy fighters.<ref name="harvp|Graham|1996">{{harvp|Graham|1996}}</ref> At sustained speeds of more than Mach 3.2, the plane was faster than the Soviet Union's fastest interceptor, the [[Mikoyan-Gurevich MiG-25]],{{refn|The [[MiG-25|Foxbats]] could sustain speeds of Mach 2.83, but they also had an emergency option to reach Mach 3.2{{snd}}after which the engines would have to be repaired or replaced.<ref name=GlobalAircraftMiG-25 />|group=N}} which also could not reach the SR-71's altitude.<ref name=GlobalAircraftMiG-25>{{cite web |url=http://www.globalaircraft.org/planes/mig-25_foxbat.pl |title=MiG-25 Foxbat |website=globalaircraft.org |access-date=31 May 2011 |archive-url=https://web.archive.org/web/20141223100252/http://www.globalaircraft.org/planes/mig-25_foxbat.pl |archive-date=23 December 2014}}</ref> No SR-71 was ever shot down.<ref name="Landis_p98-101"/>

The SR-71 featured chines, a pair of sharp edges leading aft from either side of the nose along the fuselage. These were not a feature on the early A-3 design; Frank Rodgers, a doctor at the Scientific Engineering Institute, a [[Central Intelligence Agency|CIA]] [[front organization]], discovered that a cross-section of a sphere had a greatly reduced radar reflection, and adapted a cylindrical-shaped fuselage by stretching out the sides of the fuselage.<ref>Suhler 2009, p. 100.</ref> After the advisory panel provisionally selected Convair's FISH design over the A-3 on the basis of RCS, Lockheed adopted chines for its A-4 through A-6 designs.<ref>Suhler 2009, ch. 10.</ref>

Aerodynamicists discovered that the chines generated powerful [[vortex|vortices]] and created additional [[Lift (force)|lift]], leading to unexpected aerodynamic performance improvements.<ref>''AirPower'' May 2002, p. 36.</ref> For example, they allowed a reduction in the wings' [[angle of incidence (aerodynamics)|angle of incidence]], which added stability and reduced drag at high speeds, allowing more weight to be carried, such as fuel. Landing speeds were also reduced, as the chines' vortices created turbulent flow over the wings at high [[Angle of attack|angles of attack]], making it harder to [[Stall (flight)|stall]]. The chines also acted like [[leading-edge extension]]s, which increase the agility of fighters such as the [[Northrop F-5 Freedom Fighter|F-5]], [[General Dynamics F-16 Fighting Falcon|F-16]], [[McDonnell Douglas F/A-18 Hornet|F/A-18]], [[Mikoyan MiG-29|MiG-29]], and [[Sukhoi Su-27|Su-27]]. The addition of chines also allowed the removal of the planned [[Canard (aeronautics)|canard]] foreplanes.{{refn|See [[:File:Blackbird-Canards.JPG|Blackbird with Canards]] image for visual.|group=N}}<ref>Goodall 2003, p. 19.</ref><ref>''AirPower'', May 2002, p. 33.</ref>

===Propulsion system or powerplant===
====Complete powerplant====
The SR-71 used the same powerplant as the A-12 and [[YF-12]]. It consists of three main parts: inlet, J58 engine and its nacelle, and ejector nozzle. "Typical for any supersonic powerplant the engine cannot be considered separately from the rest of the powerplant. Rather, it may be regarded as the heat pump in the over-all system of inlet, engine, and nozzle. The net thrust available to propel the aircraft may be to a large extent controlled by the performance of the inlet and nozzle rather than by the physical potentialities of the engine alone."<ref>https://apps.dtic.mil/sti/citations/ADB183548,'Inlet-engine matching considerations', Obery and Stitt, NACA Conference on Turbojet Engines for Supersonic Propulsion, AD B183548, Section VII, Paper 2.</ref> This is illustrated for the Blackbird by the thrust contributions from each component at Mach 3+ with maximum afterburner: inlet 54%, engine 17.6%, ejector nozzle 28.4%.<ref>Campbell, David. [https://arc.aiaa.org/doi/10.2514/3.60402 "F-12 Series Aircraft Propulsion System Performance and Development"], "Table 1 Propulsive thrust distribution"</ref>

When stationary and at low speeds the inlet caused a loss in engine thrust. This was due to the flow restriction through the inlet when stationary. Thrust was recovered with ram pressure as flight speed increased (uninstalled thrust {{cvt|34000|lb(f)|kN}}, installed at zero airspeed {{cvt|25500|lb(f)|kN}} rising through {{cvt|30000|lb(f)|kN}} at 210&nbsp;knots, unstick speed).<ref>{{cite book |url=https://www.sr-71.org/blackbird/manual/1/1-7.php |title=SR-71 Flight Manual |via=SR-71 Online |section=Section 1 |pages=1–7}}</ref>

At supersonic speeds not all the airflow approaching the inlet capture area entered the inlet. At supersonic speeds an intake always adapts to the engine requirements, rather than forcing air into the engine, and the unwanted air flows around the outside of the cowl, causing spillage drag. More than half the air approaching the capture area had to be spilled at low supersonic speeds and the amount reduced as the design speed was approached because the inlet airflow had been designed to match the engine demand at that speed and the chosen design point ambient temperature. At this speed the spike shock touched the cowl lip and there was minimal spillage (with its attendant drag) as shown by Campbell.<ref name="Inlet development">{{cite journal |url=https://www.jstor.org/stable/44657533 |title=F-12 Inlet Development – Fig.4 – Inlet airflow |first=David H. |last=Campbell |journal=SAE Transactions |volume=83 |year=1974 |pages=2832–2840 |publisher=SAE International|jstor=44657533 }}</ref> The inlet and engine matching was also shown by Brown,<ref>Brown, William. "J58/SR-71 Propulsion Integration, attachment to CIA-RDP90B001170R000100050008-1, Fig. 3 'Inlet and engine airflow match'".</ref> who emphasized the benefit of increased engine airflow at higher Mach numbers that came with the introduction of the bleed bypass cycle. These two authors show the disparity between inlet and engine for the Blackbird in terms of airflow and it is further explained in more general terms by Oates.<ref>"Aerothermodynamics of Aircraft Gas Turbine Engines", Oates, Air Force Aero Propulsion Laboratory, Figure 13.1.17 'Elements of Inlet Airflow Supply Determination', (a) and (b).</ref>

Engine operation was adversely affected when operating behind an unstarted inlet. In this condition the inlet behaved like a subsonic inlet design (known as a pitot type) at high supersonic speeds, with very low airflow to the engine. Fuel was automatically diverted, by the fuel derich system, from the combustor to prevent turbine over-temperature.<ref>A-12 Utility Flight Manual, 15 September 1965, changed 15 June 1968, Fuel Derich System.</ref>

All three parts were linked by the secondary airflow. The inlet needed the boundary layers removed from its spike and cowl surfaces. The one with the higher pressure recovery, the cowl shock-trap bleed, was chosen as secondary air<ref name="Inlet development"/> to ventilate and cool the outside of the engine. It was assisted from the inlet by the pumping action of the engine exhaust in the ejector nozzle, cushioning the engine exhaust as it expanded over a wide range of pressure ratios which increased with flight speed.<ref>https://www.sae.org/publications/technical-papers/content/740832/, "J58/YF-12 Ejector Nozzle Performance" pp. 1, 2.</ref>

Mach&nbsp;3.2 in a [[standard day]] atmosphere was the design point for the aircraft. However, in practice the SR-71 was more efficient at even faster speeds and colder temperatures. The specific range charts showed for a standard day temperature, and a particular weight, that Mach&nbsp;3.0 cruise used {{cvt|38000|lb/h|kg/h}} of fuel. At Mach&nbsp;3.15 the fuel flow was {{cvt|36000|lb/h|kg/h}}. Flying in colder temperatures (known as temperature deviations from the standard day) would also reduce the fuel used, e.g. with a {{cvt|-10|C|F|order=flip}} temperature the fuel flow was {{cvt|35000|lb/h|kg/h}}.<ref>{{cite book|url = https://books.google.com/books?id=6svmtOFa1JIC| pages=165, 166| title=SR-71 Revealed: The Untold Story| isbn=978-1-61060-751-3| last1=Graham| first1=Richard H.| publisher=Zenith Imprint}}</ref> During one mission, SR-71 pilot [[Brian Shul]] flew faster than usual to avoid multiple interception attempts. It was discovered after the flight that this had reduced the fuel consumption.<ref name="Shul">Shul and O'Grady 1994</ref> It is possible to match the powerplant for optimum performance at only one ambient temperature because the airflows for a supersonic inlet and engine vary differently with ambient temperature. For an inlet, the airflow varies inversely with the square root of the temperature, and for the engine, it varies with the direct inverse.<ref>https://archive.org/details/sim_journal-of-aircraft_november-december-1968_5_6/mode/2up Design and Development of an Air Intake for a Supersonic Transport Aircraft, "Effect of Ambient Temperatures", p.518</ref>

<gallery widths="270px" heights="203px" mode="packed" class="center" caption="Powerplant">
File:1 - Seattle.jpg|The inlet extends from the spike tip to the four sets of three louvers that vent the spike boundary layer bleed overboard through four spike support struts. The more-forward louvers vent the forward bypass bleed. The engine extends from there to the ejector nozzle blow-in doors (shown open) and the nozzle extends from there to the ejector flaps (shown closed).
File:SR71 J58 Engine Airflow Patterns.svg|Diagrams show operation of the air inlet, flow through the engine (primary air), nacelle flow past the engine (secondary air), and flow into the ejector nozzle (primary, secondary and tertiary air).
File:Pratt & Whitney J58 ground test.jpg|This picture of an uninstalled engine being tested illustrates the need for cooling air around the exhaust duct. The engine, when installed as part of the powerplant, has secondary cooling air at {{Convert|1200|F|C|round=50}} passing over the afterburner duct which is heated internally by combustion up to {{Convert|3200|F|C|round=50}}. The heating, followed by the primary nozzle restriction, accelerates the exhaust to sonic speed as it leaves the primary nozzle (shown). The ejector nozzle (not shown) surrounds the primary exhaust with secondary and tertiary air to cushion its expansion in the ejector nozzle.
</gallery>

====Inlet====
The engine inlets needed internal supersonic diffusion since external compression used on slower aircraft caused too much drag at Blackbird speeds. Their features included a centerbody or spike, spike boundary-layer bleed slots where normal shock was located, a cowl boundary layer bleed "shock trap" entrance, streamlined bodies known as "mice", forward bypass bleed ports between the "mice", rear bypass ring, louvers on external surface for spike boundary layer overboard, and louvers on external surface for front bypass overboard. Venting this bypass overboard produced high drag: {{cvt|6000|lb(f)|kN}} at cruise with 50% door opening, compared to the total aircraft drag of {{cvt|14000|lb(f)|kN}}.<ref>https://authors.library.caltech.edu/records/6s4e6-b2j60, AE107_SR-71_Case_Study_51-120, "Drag Penalty Of Overboard Bypass At Cruise".</ref> Designer David Campbell holds a patent on the inlet's aerodynamic features and functioning,<ref>{{cite web |title=Supersonic inlet for jet engines |url=https://patents.google.com/patent/US3477455A/en |work=patents.google.com}}</ref> which are explained in the "A-12 Utility Flight Manual"<ref>A-12 Utility Flight Manual, 15 September 1965, changed 15 June 1968, Air Inlet System.</ref> and in a 2014 presentation by Lockheed Technical Fellow Emeritus Tom Anderson.<ref>Tom Anderson. "SR-71 Inlet Design Issues And Solutions Dealing With Behaviourally Challenged Supersonic Flow Systems", Lockheed Martin Corporation, presented at AEHS Convention 2014.</ref>

When an inlet was operating as an efficient supersonic compressor—a status called "started"—supersonic diffusion took place in front of the cowl and internally in a converging passage as far as a terminal shock where the passage area began to increase and subsonic diffusion takes place. An analog control system was designed to hold the terminal shock in position.

But in the early years of operation, the analog computers could not always keep up with rapidly changing inputs from the nose boom. If the duct back pressure became too great and the spike was incorrectly positioned, the shock wave become unstable and would shoot quickly forward to a stable position outside the cowl. This "inlet [[unstart]]" would often extinguish the engine's afterburner. The asymmetrical thrust from the other engine would cause the aircraft to yaw violently. [[Stability Augmentation System|SAS]], autopilot, and manual control inputs would attempt to regain controlled flight, but extreme yaw would often reduce airflow in the opposite engine and stimulate "sympathetic stalls". This generated a rapid counter-yawing, often coupled with loud "banging" noises, and a rough ride during which crews' helmets would sometimes strike their cockpit canopies.<ref>{{harvp|Crickmore|1997|pp=42-43}}</ref> One response to a single unstart was unstarting both inlets to prevent yawing, then restarting them both.<ref name="landis_jenkins_p97">{{harvp|Landis|Jenkins|2004|p=97}}</ref> After wind-tunnel testing and computer modeling by NASA Dryden test center,<ref>{{cite web |year=2004 |title=NASA Dryden Technology Facts – YF-12 Flight Research Program |url=https://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-17-DFRC.html |url-status=dead |archive-url=https://web.archive.org/web/20190912045121/https://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-17-DFRC.html |archive-date=12 September 2019 |access-date=9 March 2019 |publisher=NASA |location=US}}</ref> Lockheed installed an electronic control to detect unstart conditions and perform this reset action without pilot intervention.<ref>{{harvp|Rich|Janos|1994|p=221}}</ref>

During troubleshooting of the unstart issue, NASA discovered that the vortices from the nose chines were entering the engine and reducing engine efficiency. To fix this, the agency developed a computer to control the engine bypass doors. Beginning in 1980, the analog inlet control system was replaced by a digital system, Digital Automatic Flight and Inlet Control System (DAFICS),<ref>{{Cite web |title=SR-71 Online - SR-71 Flight Manual: Section appendix, Page A-2 |url=https://www.sr-71.org/blackbird/manual/appendix/a-2.php |access-date=21 August 2023 |website=www.sr-71.org}}</ref> which reduced unstarts.<ref name="landis_jenkins_p83">{{harvp|Landis|Jenkins|2004|p=83}}</ref>

<gallery widths="252px" heights="189px" mode="packed" class="center" caption="Inlet">
File:Lockheed SR-71A Blackbird at Evergreen Aviation & Space Museum (6586637283).jpg|Entry to the inlet. Behind is the outer wing and hinged portion of the nacelle that encloses the engine. The spike is shown in the forward position (for speeds below Mach 1.6). Just discernible behind the cowl lip are spike boundary layer bleed slots where the normal shock is located at higher speeds when the spike has moved rearwards, the cowl bleed "shock trap" ram intake, streamlined bodies ("mice") and, between the mice, the forward bypass door openings<ref>Tom Anderson. SR-71 Inlet Design Issues And Solutions Dealing With Behaviourally Challenged Supersonic Flow Systems, Lockheed Martin Corporation, presented at AEHS Convention 2014, 'Inlet Diffuser Photos'.</ref> that dump unwanted air externally through the front louvers and cause nacelle drag.<ref>https://authors.library.caltech.edu/records/6s4e6-b2j60, AE107_SR-71_Case_Study_51-120 "Drag Penalty Of Overboard Bypass At Cruise"</ref> When the landing gear is down, ambient air flows in reverse through the bypass to supplement the front inlet flow into the engine.
File:Lockheed SR-71A Blackbird at Evergreen Aviation & Space Museum (6586640345) (2).jpg|A rear view of the inlet where air enters the engine. Two features were added after flight testing highlighted the need: 1) "mice," visible as streamlined shapes, added to reduce the diffusion rate after pilots noted rumbling; and 2) rear bypass doors added to prevent unstarting the inlet during descents with low engine flow.<ref>{{cite conference |last=Anderson |first=J. Thomas |year=2013 |title=How Supersonic Inlets Work – Details of the Geometry and Operation of the SR-71 Mixed Compression Inlet |url=https://www.enginehistory.org/Convention/2014/SR-71Inlts/SR-71Inlts.shtml |conference=AEHS convention |access-date=October 26, 2024 |website=Engine History}}</ref> The ring of doors is at the extreme rear of the inlet as shown by their accompanying rear-turning scoop, extending from 7 o'clock to 5 o'clock, which directs the air through the nacelle to the ejector nozzle.<ref>{{cite web |url=https://ntrs.nasa.gov/citations/19750003899 |title=Continuous-output terminal-shock-position sensor for mixed-compression inlets evaluated in wind tunnel tests of YF-12 aircraft inlet |work=NASA Technical Reports Server |date=December 1974 |publisher=NASA |pages=Figures 7, 8 |last1=Dustin |first1=M. O. |last2=Cole |first2=G. L. |last3=Neiner |first3=G. H. }}</ref> The door actuator<ref>{{cite web |url=https://ntrs.nasa.gov/citations/19740030201 |title=Preliminary results of flight tests of the propulsion system of the YF-12 airplane at Mach numbers to 3.0 |work=NASA Technical Reports Server |date=November 1973 |publisher=NASA}}</ref>
File:Inlet shock waves at Mach 2.jpg|[[Schlieren]] flow visualization of shock waves for started and unstarted inlet at Mach 2
</gallery>

====Engine and nacelle====
{{main|Pratt & Whitney J58}}
The engine was an extensively re-designed version of the J58-P2, an existing supersonic engine which had run 700 development hours in support of proposals to power various aircraft for the US Navy. Only the compressor and turbine aerodynamics were retained. New design requirements for cruise at Mach 3.2 included:
* operating with very high ram temperature air entering the compressor, at {{Convert|800|F|C|sigfig=2}}
* a continuous turbine temperature capability {{Convert|450|F-change}} hotter than previous experience ([[Pratt & Whitney J75]])
* continuous use of maximum afterburning
* the use of new, more expensive, materials and fluids required to withstand unprecedented high temperatures

The engine was an afterburning turbojet for take-off and transonic flight (bleed bypass closed) and a low bypass augmented turbofan for supersonic acceleration (bleed bypass open). It approximated a ramjet during high speed supersonic cruise (with a pressure loss, compressor to exhaust, of 80% which was typical of a ramjet). It was a low bypass turbofan for subsonic loiter (bleed bypass open).<ref>https://authors.library.caltech.edu/records/6s4e6-b2j60, AE107_SR-71_Case_Study_321-450, p. 27.</ref><ref>A-12 Utility Flight Manual, 15 September 1965, changed 15 June 1968, 'Start Bleed And Bypass Valve Actuation', Figure 1-7</ref>

Analysis of the J58-P2 supersonic performance<ref name="roadrunnersinternationale.com">Bob Abernethy. https://www.roadrunnersinternationale.com/pw_tales.htm, 'More Never Told Tales of Pratt & Whitney'.</ref> showed the high compressor inlet temperature would have caused stalling, choking and blade breakages in the compressor as a result of operating at low corrected speeds on the compressor map. These problems were resolved by Pratt & Whitney engineer Robert Abernethy and are explained in his patent, "Recover Bleed Air Turbojet".<ref>{{cite web | url=https://patents.google.com/patent/US3344606A/en | title=Recover bleed air turbojet }}</ref> His solution was to 1) incorporate six air-bleed tubes, prominent on the outside of the engine, to transfer 20% of the compressor air to the afterburner, and 2) to modify the inlet guide vanes with a 2-position, trailing edge flap. The compressor bleed enabled the compressor to operate more efficiently and with the resulting increase in engine airflow matched the inlet design flow with an installed thrust increase of 47%.<ref name="roadrunnersinternationale.com"/><ref>William Brown. J58/SR-71 Propulsion Integration, attachment to CIA-RDP90B001170R000100050008-1, Fig. 3, 'Inlet and engine airflow match'.</ref> A continuous turbine temperature of {{Convert|2000|F|C}} was enabled with air-cooled first stage turbine vane and blades. Continuous operation of maximum afterburning was enabled by passing relatively cool air from the compressor along the inner surface of the duct and nozzle. Ceramic thermal barrier coatings were also used.

The secondary airflow through the nacelle comes from the cowl boundary layer bleed system which is oversized (flows more than boundary layer) to give a high enough pressure recovery to support the ejector pumping action. Additional air comes from the rear bypass doors and, for low speed operation with negligible inlet ram, from suck-in doors by the compressor case.

<gallery widths="220px" heights="165px" mode="packed" class="center" caption="Engine/nacelle">
File:Pratt & Whitney J58-JT11D-20K turbojet engine, 1962 - Lockheed SR-71A Blackbird, 1966 - Evergreen Aviation & Space Museum - McMinnville, Oregon - DSC01037.jpg|View of J58 engine shows some features required for flight at Mach 3.2: titanium inlet guide vanes and first stage compressor blades for lighter weight at high ram temperatures, transonic first stage compressor blades and low hub/tip ratio compressor entry, both scaled from the bigger Mach-3 J91 engine compressor, 2-position flaps on the inlet guide vanes and three of the six bypass tubes.
File:Pratt & Whitney J58 18.jpg|The afterburner was rated for continuous operation at {{cvt|3200|F|C|-2}} made possible with ceramic coatings (colored white) on duct liner and flame holders<ref>https://ntrs.nasa.gov/citations/20090018047, "History Of Thermal Barrier Coatings For Gas Turbine Engines: Emphasising NASA's Role from 1942 to 1990".</ref> and compressor bleed air cooling the duct and nozzle (above Mach 2.1 when the bleed was flowing). The nozzle is fully open, the maximum afterburning position. The main purpose of the variable nozzle area was to control engine operation which it did in conjunction with varying heat release in the afterburner.
File:Lockheed SR-71A Blackbird at Evergreen Aviation & Space Museum (6586637067).jpg|The inlet at left was depressed when the engine ran at high power settings with inadequate inlet ram (stationary and low flight speeds). The lower-than-ambient pressure in the inlet brought in extra air through the spike bleed and forward bypass louvers shown on the inlet external surface. Adequate secondary cooling air came in through the suck-in doors (shown open on the hinged nacelle).
</gallery>

====Ejector Nozzle====
The nozzle had to operate efficiently over a wide range of pressure ratios from low, with no inlet ram with a stationary aircraft, to 31 times the external pressure at {{Convert|80000|ft|m|abbr=on}}. A blow-in door ejector nozzle had been invented by Pratt & Whitney engineer Stuart Hamilton in the late 1950s<ref>Jack Connors. https://arc.aiaa.org/doi/book/10.2514/4.867293, "The Engines of Pratt & Whitney: A Technical History", ISBN 978 1 60086 711 8, p. 328.</ref> and described in his patent "Variable Area Exhaust Nozzle".<ref>"Variable Area Exhaust Nozzle", U.S. Patent 3,062,003</ref> In this description the nozzle is an integral part of the engine (as it was in the contemporary Mach 3 General Electric YJ93.<ref>"Variable Geometry Exhaust Nozzles and Their Effects on Airplane Performance", SAE 680295, p. 5.</ref> For the Blackbird powerplant the nozzle was more efficient structurally (lighter) by incorporating it as part of the airframe because it carried fin and wing loads through the ejector shroud. The nozzle used secondary air from two sources, the inlet cowl boundary layer and rear bypass from immediately in front of the compressor. It used external flow on the nacelle through the tertiary blow-in doors until ram closed them at Mach 1.5. Only secondary air was used at higher speeds with the blow-in doors closed.

At low flight speeds the engine exhaust pressure at the primary nozzle exit was greater than ambient so tended to over-expand to lower than ambient in the shroud causing impingement shocks. Secondary and blow-in door air surrounding the exhaust cushioned it preventing over-expansion. Inlet ram pressure increased with flight speed and the higher pressure in the exhaust system closed, first the blow-in doors and then started to open the nozzle flaps until they were fully open at Mach 2.4. The final nozzle area did not increase with further increase in flight speed (for complete expansion to ambient and greater internal thrust) because its external diameter, greater than nacelle diameter would cause too much drag.<ref>Design For Air Combat, Ray Whitford, {{ISBN|0 7106 0426 2}}, p. 203</ref>

<gallery widths="220px" heights="165px" mode="packed" class="center" caption="Ejector-nozzle">
File:Lockheed SR-71A Blackbird at Evergreen Aviation & Space Museum (6586638859) (2).jpg|Ejector nozzle at the rear of the powerplant. The engine nozzle (left) is the first component in the exhaust system, followed by the secondary and tertiary air flows and ejector nozzle. The tertiary doors are open. There is a fixed convergent/divergent shroud and the ejector nozzle trailing flaps are at their minimum area (closed). These nozzle and door positions correspond with full afterburner up to transonic speed, after which the doors close and flaps start to open. Secondary air from the inlet passes between the engine and nacelle and joins the blow-in door air to control the expansion of the engine exhaust through the shroud and trailing flaps.
File:SR-71A taking off with afterburner RAF Mildenhall 1983.JPEG|A similar viewing angle, unstick speed 210 knots, to the "exploded" view, and with the same operating configuration: afterburner nozzle open, blow-in doors open and trailing flaps closed due to low internal pressure with low speed low inlet ram. Note the dark con-di shroud. Air entering the blow-in doors joins secondary air from the inlet and flows over the fixed shroud surface and trailing flaps while surrounding the exhaust from the engine.
</gallery>

=== Fuel ===
[[File:Boeing KC-135Q refueling SR-71.JPEG|thumb|alt= KC-135 and SR-71 during an "in-flight" refueling|An SR-71 refueling from a [[KC-135Q Stratotanker]] during a flight in 1983.]]
[[File:A right side view of a KC-10 Extender aircraft refueling an SR-71 Blackbird aircraft in-flight during testing DF-ST-83-03355.jpg|thumb|An SR-71 refueling from a [[KC-10 Extender]]. KC-10s were added in the mid-1980s as additional tankers.{{sfn|Graham|2008|page=113}}]]

[[JP-7]] fuel was used. It was difficult to ignite. To start the engines, [[triethylborane]] (TEB), which [[pyrophoricity|ignites on contact with air]], was injected to produce temperatures high enough to ignite the JP-7. The TEB produced a characteristic green flame, which could often be seen during engine ignition.<ref name="Shul" /> The fuel was used as a heat sink for the rest of the aircraft to cool the pilot and the electronics. An electric starting system was not possible due to the limited capacity of the cooling system, so the chemical ignition system was used.<ref>{{cite tech report |url=https://www.cia.gov/readingroom/document/cia-rdp90b00170r000200280025-6 |title=J58/SR-71 Propulsion Integration Or The Great Adventure Into The Technical Unknown |first=William |last=Brown |publisher=Pratt & Whitney Aircraft Group |id=attachment RDP90B00170R000200280025-6 to CIA-RDP90B00170R000100050008-1 |via=CIA}}</ref>

On a typical mission, the SR-71 took off with a partial fuel load to reduce stress on the brakes and tires during takeoff and also ensure it could successfully take off should one engine fail.<ref name="harvp|Graham|1996|p=46">{{harvp|Graham|1996|p=46}}</ref> Within 20 seconds, the aircraft traveled {{convert|4500|ft|abbr=on}}, reached {{convert|240|mph|abbr=on}}, and lifted off. It reached {{convert|20000|ft|abbr=on}} of altitude in less than two minutes, and the typical {{convert|80000|ft|abbr=on}} [[cruising altitude]] in another 17 minutes, having used one third of its fuel.{{r|iwm20211103}} It is a common misconception that the planes refueled shortly after takeoff because the fuel tanks, which formed the outer skin of the aircraft, leaked on the ground. It was not possible to prevent leaks when the aircraft skin was cold and the tanks only sealed when the skin warmed as the aircraft speed increased. The ability of the sealant to prevent leaks was compromised by the expansion and contraction of the skin with each flight.<ref>{{cite web |last1=Boudreaux |first1=Stormy |title=The SR-71 Experience |url=https://www.youtube.com/watch?v=J5qrMTtSUV8&t=2129s |website=YouTube |date=22 October 2022 |publisher=Air Zoo |access-date=17 September 2023}}</ref> However, the amount of fuel that leaked, measured as drops per minute on the ground from specific locations, was not enough to make refueling necessary.<ref>{{harvp|Graham|2013|p=110}}</ref>

The SR-71 also required [[Aerial refueling|in-flight refueling]] to replenish fuel during long-duration missions. Supersonic flights generally lasted no more than 90 minutes before the pilot had to find a tanker.<ref>Marshall, Elliot, The Blackbird's Wake, Air and Space, October/November 1990, p. 35.</ref>

Specialized [[Boeing KC-135 Stratotanker|KC-135Q]] tankers were required to refuel the SR-71. The KC-135Q had a modified high-speed boom, which would allow refueling of the Blackbird at near the tanker's maximum airspeed. The tanker also had special fuel systems for moving [[JP-4 (fuel)|JP-4]] (for the KC-135Q itself) and JP-7 (for the SR-71) between different tanks.<ref>{{harvp|Graham|1996|pp=38–39}}</ref> As an aid to the pilot when refueling, the cockpit was fitted with a [[peripheral vision horizon display]]. This unusual instrument projected a barely visible [[artificial horizon]] line across the top of the entire instrument panel, which gave the pilot [[subliminal stimuli|subliminal]] cues on aircraft attitude.<ref>{{harvp|Crickmore|2004|p=233}}</ref> If a KC-135Q was not available any tanker with JP-4 or JP-5 could be used in an emergency to avoid losing the aircraft, but with a Mach 1.5 speed limit.<ref name="harvp|Graham|1996|p=46"/>

On hot days, when approaching the maximum fuel load of {{cvt|80285|lb|kg|round=5}},<ref name="harvp|Graham|1996|p=46"/> the left engine had to be run with minimum afterburner to maintain probe contact.{{sfn|Graham|2008|page=120}}

===Astro-inertial navigation system{{Anchor|Astro-Inertial Navigation System|Astro-inertial navigation system}}===
Nortronics, [[Northrop Corporation]]'s electronics development division, had developed an [[astro-inertial guidance]] system (ANS), which could correct [[inertial navigation system]] errors with [[Celestial navigation|celestial observations]], for the [[SM-62 Snark]] missile, and a separate system for the ill-fated [[AGM-48 Skybolt]] missile, the latter of which was adapted for the SR-71.<ref>Morrison, Bill, SR-71 contributors, Feedback column, [[Aviation Week and Space Technology]], 9 December 2013, p. 10</ref>{{verify source|date=February 2012}}

Before takeoff, a primary alignment brought the ANS's inertial components to a high degree of accuracy. In flight, the ANS, which sat behind the reconnaissance systems officer's (RSO's), position, tracked stars through a circular quartz glass window on the upper fuselage.<ref name="Shul"/> Its "blue light" source [[star tracker]], which could see stars during both day and night, would continuously track a variety of stars as the aircraft's changing position brought them into view. The system's digital computer [[ephemeris]] contained data on [[List of selected stars for navigation|a list of stars used for celestial navigation]]: the list first included 56 stars and was later expanded to 61.<ref name="manual4-3">[http://www.sr-71.org/blackbird/manual/4/4-3.php "SR-71A-1 Flight Manual, Section IV, p. 3."] ''sr-71.org''. Retrieved: 13 December 2011.</ref> The ANS could supply altitude and position to flight controls and other systems, including the mission data recorder, automatic navigation to preset destination points, automatic pointing and control of cameras and sensors, and optical or SLR sighting of fixed points loaded into the ANS before takeoff. According to Richard Graham, a former SR-71 pilot, the navigation system was good enough to limit drift to {{cvt|1000|ft|m}} off the direction of travel at Mach&nbsp;3.<ref>{{cite web|url=https://www.youtube.com/watch?v=CeBu6mRDaro|title=SR-71 Pilot Interview Richard Graham Veteran Tales|work=YouTube|date=6 August 2013 }}</ref>

===Sensors and payloads===
[[File:SR-71 Defensive System B.jpg|thumb|The SR-71 Defensive System B]]

The SR-71 originally included optical/[[Thermographic camera|infrared imagery]] systems; [[side-looking airborne radar]] (SLAR);<ref>{{cite web |title=SR-71 Online – SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-86.php |website=Sr-71.org |pages=4–86}}</ref> [[electronic intelligence]] (ELINT) gathering systems;<ref>{{cite web |title=SR-71 Online – SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-99.php |website=Sr-71.org |pages=4–99}}</ref> defensive systems for countering missile and airborne fighters;<ref>{{cite web |title=SR-71 Online – SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-123.php |website=Sr-71.org |pages=4–123}}</ref><ref>{{cite web |title=SR-71 Online – SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-129.php |website=Sr-71.org |pages=4–129}}</ref><ref>{{cite web |title=SR-71 Online - SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-132.php |website=Sr-71.org |pages=4–132}}</ref><ref>{{cite web |title=SR-71 Online - SR-71 Flight Manual: Section 4 |url=https://www.sr-71.org/blackbird/manual/4/4-146.php |website=Sr-71.org |pages=4–146}}</ref> and recorders for SLAR, ELINT, and maintenance data. The SR-71 carried a [[Fairchild Camera and Instrument|Fairchild]] tracking camera and an [[Thermographic camera|infrared camera]],<ref>{{cite book |url=https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/books-and-monographs/a-12/Archangel-2ndEdition-2Feb12.pdf |archive-url=https://web.archive.org/web/20120926211605/https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/books-and-monographs/a-12/Archangel-2ndEdition-2Feb12.pdf |url-status=dead |archive-date=26 September 2012 |title=Archangel: CIA's Supersonic A-12 Reconnaissance Aircraft |edition=2nd |first=David |last=Robarge |publisher=CSI Publications |date=January 2012 |access-date=19 March 2019}}</ref> both of which ran during the entire mission.

As the SR-71 had a second cockpit behind the pilot for the RSO, it could not carry the A-12's principal sensor, a single large-focal-length optical camera that sat in the "Q-Bay" behind the A-12's single cockpit. Instead, the SR-71's camera systems could be located either in the fuselage chines or the removable nose/chine section. Wide-area imaging was provided by two of [[Itek]]'s [[Operational Objective Camera]]s, which provided stereo imagery across the width of the flight track, or an [[KA-80 Optical Bar Camera|Itek Optical Bar Camera]], which gave continuous horizon-to-horizon coverage. A closer view of the target area was given by the [[HYCON Technical Objective Camera]] (TEOC), which could be directed up to 45° left or right of the centerline.<ref name="auto">{{harvp|Crickmore|1997|p=74}}</ref> Initially, the TEOCs could not match the resolution of the A-12's larger camera, but rapid improvements in both the camera and film improved this performance.<ref name="auto"/><ref>{{harvp|Crickmore|1997|p=563}}</ref>

SLAR, built by [[Goodyear Aerospace]], could be carried in the removable nose. In later life, the radar was replaced by Loral's Advanced [[Synthetic Aperture Radar]] System (ASARS-1). Both the first SLAR and ASARS-1 were ground-mapping imaging systems, collecting data either in fixed swaths left or right of centerline or from a spot location for higher resolution.<ref name="auto"/> ELINT-gathering systems, called the Electro Magnetic Reconnaissance System, built by AIL could be carried in the chine bays to analyze electronic signal fields being passed through, and were programmed to identify items of interest.<ref name="auto"/><ref>{{harvp|Crickmore|1997|p=77}}</ref>

Over its operational life, the Blackbird carried various [[electronic countermeasures]] (ECMs), including warning and active electronic systems built by several ECM companies and called Systems A, A2, A2C, B, C, C2, E, G, H, and M. On a given mission, an aircraft carried several of these frequency/purpose payloads to meet the expected threats. Major Jerry Crew, an RSO, told ''[[Air & Space/Smithsonian]]'' that he used a [[Radar jamming and deception|jammer]] to try to confuse [[surface-to-air missile]] sites as their crews tracked his airplane, but once his threat-warning receiver told him a missile had been launched, he switched off the jammer to prevent the missile from homing in on its signal.<ref>{{cite journal|url=http://www.airspacemag.com/flight-today/blackbird-diaries-180953373/ |title=Blackbird Diaries {{!}} Flight Today |journal=Air & Space Magazine |page=45 |date=December 2014 |access-date=24 July 2015}}</ref> After landing, information from the SLAR, ELINT gathering systems, and the maintenance data recorder were subjected to postflight ground analysis. In the later years of its operational life, a datalink system could send ASARS-1 and ELINT data from about {{convert|2000|nmi|km|abbr=on}} of track coverage to a suitably equipped ground station.{{Citation needed|date=February 2012}}

===Life support===
[[File:Brian Shul in the cockpit of the SR-71 Blackbird.jpg|thumb|right|SR-71 pilot [[Brian Shul]] in full flight suit]]
[[File:SR71 crew.jpg|thumb|The crew of a NASA Lockheed SR-71 Blackbird standing by the aircraft in their pressurized flight suits, 1991]]

Flying at {{convert|80000|ft|m|abbr=on|sigfig=2}} meant that crews could not use standard masks, which could not provide enough oxygen above {{convert|43000|ft|m|abbr=on|sigfig=2}}. Specialized protective [[pressurized suit]]s were produced for crew members by the [[David Clark Company]] for the A-12, YF-12, M-21 and SR-71. Furthermore, an emergency [[Ejection seat|ejection]] at Mach&nbsp;3.2 would subject crews to temperatures of about {{convert|450|°F|°C|-1}}; thus, during a high-altitude ejection scenario, an onboard oxygen supply would keep the suit pressurized during the descent.<ref name=weaver/>

The cockpit could be pressurized to an altitude of {{convert|10000|or|26000|ft|m|abbr=on|sigfig=1}} during flight.<ref>Donald 2003, p. 172.</ref> The cabin needed a heavy-duty cooling system, as cruising at Mach&nbsp;3.2 would heat the aircraft's external surface well beyond {{convert|500|°F|°C|-1}}<ref name="PM06/91">''[[Popular Mechanics]]'', June 1991, p.&nbsp;28.</ref> and the inside of the windshield to {{convert|250|°F|°C|-1}}. An air conditioner used a [[heat exchanger]] to dump heat from the cockpit into the fuel prior to combustion.<ref>{{Cite book|title=Stratonauts: Pioneers Venturing into the Stratosphere|last=Ehrenfried|first=Manfred|year=2013|publisher=Springer |isbn=978-3-319-02901-6}}</ref> The same air-conditioning system was also used to keep the front (nose) landing gear bay cool, thereby eliminating the need for the special aluminum-impregnated tires similar to those used on the main landing gear.<ref name="SR-71 Maintenance (July 1984 - December 1989)">{{cite web|url=http://www.blackbirds.net/u2/c_bennett/bbird-03.html |title=SR-71 Maintenance |website=Blackbirds.net |access-date=29 October 2015}}</ref>

Blackbird pilots and RSOs were provided with food and drink for the long reconnaissance flights. Water bottles had long straws which crewmembers guided into an opening in the helmet by looking in a mirror. Food was contained in sealed containers similar to toothpaste tubes which delivered food to the crewmember's mouth through the helmet opening.<ref>{{Cite book|title=Sled Driver|last=Shul|first=Brian|publisher=Midland Publishing Limited|year=1992|isbn=978-1857800029|location=Earl Shilton, Leicester, England|pages=38–40}}</ref><ref name="Dowling">{{cite web |last=Dowling |first=Stephen |title=SR-71 Blackbird: The Cold War's ultimate spy plane |url=https://www.bbc.com/future/article/20130701-tales-from-the-blackbird-cockpit |website=[[BBC Future]] |date=2 July 2013 |access-date=30 November 2024 |archive-date=15 October 2024 |archive-url=https://web.archive.org/web/20241015211701/https://www.bbc.com/future/article/20130701-tales-from-the-blackbird-cockpit |url-status=live}}</ref>