Cosmox Blogs

Sep 03, 2024

Lockheed SR - 71 Blackbird: The Pinnacle Of Aviation

I can bet all of you as children in the past might always have stopped midway while playing in the park and looking into the sky to see that blazingly fast aircraft penetrating the clouds and approaching the horizon and you might always have thrown tantrums to get that window seat in an aircraft but seeing the military jets in the air might have always left you stunned and desiring to be in one experiencing the skies.

Well, the United States Of America certainly did pass the bar of supersonic aircraft back in 1964 and I am telling you at that time having a supersonic aircraft capable of flying at 85000 feet of altitude almost made all Americans and the world think that alien ships have invaded the earth. So let’s dig deeper into the depths of the atomic age(the Cold War era) when the tensions and military race between the USA and USSR was at its crescendo. And dive into the tech of this engineering marvel the SR-71 that the United States flexed as its reconnaissance asset.

Background

When the Cold War was at its peak both the USA and the soviet union were made fast in developing the most advanced aircraft and the USSR. The United States had designed the relatively slow U-2 aircraft in 1956 which proved to be ineffective compared to the newly designed and highly advanced Soviet Mig 25 developed in 1964, the United States started to lag in this section just like in the space race. But the US has always bounced back and made a giant leap and in this case, also the United States had an ace in the hole with the SR-71 Blackbird. When the CIA approached Lockheed in 1957 to develop an undetectable strategic aircraft. The project was designated the name Archangel which was led by the genius aeronautical engineer Kelly Johnson head of Lockheed's Skunk Works unit in Burbank, California whom you can also call the godfather of this entire series of aircraft.

Design

The SR-71 was designed to be a strategic reconnaissance aircraft capable of flying at a max cruising speed of Mach 3.4(4198.32 km/hr) and an altitude of 85000 feet where it almost touched space. It had the extremely powerful Pratt and Whitney J58 which could produce a thrust of 32,500 pounds which gave SR-71 its flight capabilities. The j58 engine was extensively modified along with the airflow pattern of the engine for a transition from a turbojet to a ramjet for carrying out systematic landing and take-off mechanisms without having to carry the sr - 71 in another aircraft and deploying in the air.

The Engine

The inlet spikes of the SR-71 Blackbird were designed to move forward and backward by 0.66 meters to adjust the inlet and throat area for the desired airflow pattern in the engine and to keep the shockwave in its ideal position when the aircraft is supersonic to minimize the air drag and save energy. The inlet spike stays in the forward position until hitting a speed of Mach 1.6  after which it begins to move backward by 41 millimeters for every 0.1 increase in Mach number so that the shock wave remains in its ideal position which is between the inlet throat and the compressor. Downstream of this normal shock, the air became subsonic. It decelerated further in the divergent duct to give the required speed at entry to the compressor. The capture of the plane's shock wave within the inlet is called "starting the inlet". 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". As you can see in the diagram below how the SR-71’s modified Pratt and Whitney 58 engines allowed for the desired airflow pattern to minimize air drag so that the SR-71 could fly to its potential.

The Sr-71 wet wing fuel tank system

Now as the SR-71 was a strategic reconnaissance aircraft it had no weapons or defense mechanism meaning the aircraft was made completely for surveillance since the weight had to be minimized they used a fuel storage method called a total wet wing fuel tank system meaning it had no additional tanks to carry fuel as the titanium aircraft tanks would be too heavy and the lighter plastic ones would melt due to the intense heat produced because of aerodynamic friction. Now since SR-71 had no weapons the plane had a lot of free internal volume so the engineers stored the fuel within the skin of the plane you might be wondering what if the fuel leaks from the aircraft during flight or catches fire due to the heat, to fix the problem the engineers applied sealant to every gap within its titanium skin so that the fuel would not leak but it still did leak a little bit as the thermal expansion and contraction after every flight deteriorated the sealant and widened the gaps so the sr - 71 had to undergo a maintenance check after every flight. JP-7 fuel was used. It was difficult to ignite. To start the engines, triethylborane (TEB), which 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. 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.

Timeline

-> 24 December 1957: First J58 engine run

-> 1 May 1960: Francis Gary Powers is shot down in a Lockheed U-2 over the Soviet Union

-> 13 June 1962: SR-71 mock-up reviewed by the USAF

-> 30 July 1962: J58 completes pre-flight testing

-> 28 December 1962: Lockheed signs contract to build six SR-71 aircraft

-> 25 July 1964: President Johnson makes a public announcement of SR-71

-> 29 October 1964: SR-71 prototype (AF Ser. No. 61-7950) delivered to Air Force Plant 42 at Palmdale, California

-> 7 December 1964: Beale AFB, CA, announced as a base for SR-71

-> 22 December 1964: First flight of the SR-71, with Lockheed test pilot Robert J "Bob" Gilliland at Palmdale

-> 21 July 1967: Jim Watkins and Dave Dempster fly the first international sortie in SR-71A, AF Ser. No. 61-7972, when the Astro-Inertial Navigation System (ANS) fails on a training mission and they accidentally fly into Mexican airspace

-> 5 February 1968: Lockheed ordered to destroy A-12, YF-12, and SR-71 tooling

-> 8 March 1968: First SR-71A (AF Ser. No. 61-7978) arrives at Kadena AB, Okinawa to replace A-12s

-> 21 March 1968: First SR-71 (AF Ser. No. 61-7976) operational mission flown from Kadena AB over Vietnam

-> 29 May 1968: CMSgt Bill Gornik begins the tie-cutting tradition of Habu crews' neckties

-> 13 December 1969: Two SR-71s deployed to Taiwan

1970s - 1980s

-> 3 December 1975: First flight of SR-71A (AF Ser. No. 61-7959) in "big tail" configuration

-> 20 April 1976: TDY operations started at RAF Mildenhall, United Kingdom with SR-71A, AF Ser. No. 61-7972

-> 27–28 July 1976: SR-71A sets speed and altitude records (altitude in horizontal flight: 85,068.997 ft (25,929.030 m) and speed over a straight course: 2,193.167 miles per hour (3,529.560 km/h))

-> August 1980: Honeywell starts the conversion of AFICS to DAFICS

-> 15 January 1982: SR-71B, AF Ser. No. 61-7956, flies its 1,000th sortie

-> 21 April 1989: SR-71, AF Ser. No. 61-7974, is lost due to an engine explosion after taking off from Kadena AB, the last Blackbird to be lost

-> 22 November 1989: USAF SR-71 program officially terminated

1990s

-> 6 March 1990: Last SR-71 flight under the Senior Crown program, setting four speed records en route to the Smithsonian Institution

-> 25 July 1991: SR-71B, AF Ser. No. 61-7956/NASA No. 831 officially delivered to NASA Dryden Flight Research Center at Edwards AFB, California

-> October 1991: NASA engineer Marta Bohn-Meyer becomes the first female SR-71 crew member

-> 28 September 1994: Congress votes to allocate $100 million for the reactivation of three SR-71s

-> 28 June 1995: First reactivated SR-71 returns to USAF as Detachment 2

-> 9 October 1999: The last flight of the SR-71

Final retirement

Now after seeing the timeline many of you might be wondering what the reasons for it getting retired and not returning after having such great performance and being one of the most valuable assets to the USA military, well there many many reasons for it being one of the main reasons were:

-> The USAF had not budgeted for the aircraft, and UAV developers worried that their programs would suffer if money was shifted to support the SR-71s

-> The allocation requiring yearly reaffirmation by Congress, and long-term planning for the SR-71 was difficult.

-> In 1996, the USAF claimed that specific funding had not been authorized, and moved to ground the program. Congress reauthorized the funds, but, in October 1997, President Bill Clinton attempted to use the line-item veto to cancel the $39 million (~$68.8 million in 2023) allocated for the SR-71.

-> In June 1998, the U.S. Supreme Court ruled that the line-item veto was unconstitutional. All this left the SR-71's status uncertain until September 1998, when the USAF called for the funds to be redistributed; the USAF permanently retired it in 1998.

Well, all the above reasons coupled with the end of the Cold War ended the sole motive of developing such expensive and non cost efficient aircraft.

Groundbreaking achievements of SR-71

The SR-71 was the world's fastest and highest-flying air-breathing operational manned aircraft throughout its career and it still holds that record. On 28 July 1976, SR-71 serial number 61-7962, piloted by then Captain Robert Helt, broke the world record: an "absolute altitude record" of 85,069 feet (25,929 m). Several aircraft have exceeded this altitude in zoom climbs, but not in sustained flight. That same day SR-71 serial number 61-7958 set an absolute speed record of 1,905.81 knots (2,193.2 mph; 3,529.6 km/h), approximately Mach 3.3. SR-71 pilot Brian Shul states in his book The Untouchables that he flew more than Mach 3.5 on 15 April 1986 over Libya to evade a missile.

The SR-71 also holds the "speed over a recognized course" record for flying from New York to London—distance 3,461.53 miles (5,570.79 km), 1,806.964 miles per hour (2,908.027 km/h), and an elapsed time of 1 hour 54 minutes and 56.4 seconds—set on 1 September 1974, while flown by USAF pilot James V. Sullivan and Noel F. Widdifield, reconnaissance systems officer (RSO). This equates to an average speed of about Mach 2.72, including deceleration for in-flight refueling. Peak speeds during this flight were likely closer to the declassified top speed of over Mach 3.2. For comparison, the best commercial Concorde flight time was 2 hours 52 minutes and the Boeing 747 averaged 6 hours 15 minutes.

Conclusion

Well with the development of its successor Lockheed Martin SR - 72 (colloquially referred to as "Son of Blackbird") in development with flight capabilities of Mach 6 and altitude up to 80,000 feet we can safely say that aviation technology never loses its motive and fuel to touch the space and merry go round the entire globe would continue on and with sr - 72 the skies would never be empty and would always have after images of aircraft penetrating through the atmosphere.