All About the SR-71 Blackbird (Some of it is technical)

This post is going to get very technical. Each slide (if technical) will have a layman’s explanation of the image, and then a technical description. Additionally the last pictures contain stories from former pilots.

The Basics

The Lockheed Martin SR-71 Blackbird was developed in the 1960s by the Skunkworks (division of LM) to replace the LM U-2 which was difficult to fly at its normal altitude, and flew slowly enough to be vulnerable to anti-aircraft missiles.

Mission requirements ended up creating this majestic bird. Designed to fly above Mach 3 at over 24 km (80000 ft), it became the fastest manned plane ever put into production. The original plane was a single seat spy plane for the CIA called the A-12, but the USAF converted it to a two seat configuration and ordered it as the SR-71.

Stealth 1

The Blackbird was designed primarily as a reconnaissance aircraft (attempts were made to weaponize, but little room was found for munitions). As such the designers found it prudent to make the vehicle harder to detect after a U-2 was shot down. The shape of the plane (wide flat body, chines running down sides of fuselage) is designed to deflect or scatter radio waves, and the general shape of the body (falcon cross section) helps disperse radio waves further.

Stealth 2

The Northrop Grumman B-2 Spirit was developed in the mid 1980s as a stealth bomber. Notice that the shape of the cabin is a similar shape to the SR-71, but was calculated using computers and technology that was completely unavailable in the 1960s.

Propulsion 1 (Technical)

Here’s an awesome video that explains how the engines work.

The SR-71 has to be able to start on the ground, takeoff, ascend to 24 km, and fly at Mach 3 for extended periods of time. The propulsion engineers made an extremely intricate engine that could work well across all the altitude and velocities the plane could encounter.

Note that Mach number is not a constant, it is dependent on the air temperature (only), but the plane will react the same at the same Mach number regardless of temperature.

At Mach 1 a shockwave develops, and as the velocity increases the the shockwave half angle drops (the blue line on the >M1 inlet cones). The inlet cone is designed to be movable to position the shockwave properly, and the shockwave angle and position control the velocity of the flow. A Ramjet requires that the combustion takes place below the speed of sound, so the variable inlet cone is necessary to guarantee the shockwaves will drop the flow velocity below the speed of sound.

Looking at the picture:

Mach 0:

The inlet spike is pushed fully forward to allow maximum airflow through the inlet, the shockwave bypass doors are open, and the “suck-in” doors are open to allow maximum airflow through the turbojet, maximum cooling, and allow for the afterburners to engage.

Mach 0.5:

As the velocity increases to Mach 0.5 the suck-in doors can close since there is sufficient air through the inlet cone. The rear doors are still left open to feed the afterburner, but the suck-in doors are closed to prevent too much air leaving the system (notice the centerbody bleed is exiting the system due to higher than necessary entrance pressure).

Mach 1.5:

Now the fun can begin. The shockwave develops at M1, but until the velocity increases to some point between M1.5-2.5 the cone does not need to move back. The ejector flaps (nozzle area) still remains at minimum since there is not very much air flowing through the ramjet. At this stage the turbojet is still active.

Mach 2.5:

The ram inlet begins moving backward to reduce the inlet area, add oblique shock, and increase compression of the shockwaves. The turbojet is either idling, or completely off, and the nozzle begins expanding to allow for a larger expansion ratio to keep the thrust maximized.

Mach 3.2:

The inlet cone has become full backed, and the inlet cross section is at a minimum. The air has to be slowed by 2/3 before it can enter the combustion chamber, so the pressure increases by a lot compared to the Mach 2.5 scenario. The nozzle has fully expanded to the largest exit area to help regulate exit pressure, and maximize thrust.

Note that thrust increases as the vehicle moves at a higher velocity. The air compresses more the faster the vehicle goes, and the pressure thrust due to the underexpanded flow will increase the more pressure the body builds.

Propulsion 2 (Technical)

These shock diamonds are due to the effect of the pressure of the flow exiting the engine. If the flow is at a lower pressure than the ambient conditions then the flow will compress, and create half a diamond. Since the fluid has momentum it will compress higher than the ambient, and will then expand. That would create the second half of the diamond. This continues until the flow either drops below the speed of sound, or until the flow settles to ambient.

It also looks awesome!

Aerodynamic Heating (Somewhat Technical)

As a vehicle increases in velocity it will cause the skin of the vehicle to heat up. At Mach 3 the leading edges will heat to 470 degrees Celsius (876 degrees Fahrenheit), and the rest of the skin will heat to around 260 degrees Celsius (500 degrees Fahrenheit). A lot of people have heard stories of how the SR-71 leaks fuel on the ground so that when it heats up it seals, and that is not the only interesting fact about the skin.

It is made nearly completely of Titanium, and at the time of production the US had very few Titanium mines, and the Soviet Union had a near monopoly. CIA set up shell corporations to purchase enough Titanium for the aircraft.

Additionally the skin on the wings was ridged so that as the plane heated up the skin would not crack or buckle due to differential heat expansion across different areas of the plane.

Since the SR-71 heated so much during flight the fuel had to have an exceedingly high flash point (ignition with a flame), and so JP-7 was created to meet the requirements. JP-7 is so hard to ignite that a match dropped into a bucket of it will be extinguished. As such a specialized chemical called Triethylborane (TEB) that flashes green upon contact with air was used to ignite the afterburners. It is the same chemical that SpaceX uses to ignite its Merlin engines. The plane held enough TEB to ignite the afterburners about 6-7 times, so mission duration was limited by the number of refueling it could make.

Refueling

The SR-71 had to refuel within 10-20 minutes of takeoff as it had to takeoff nearly at empty, and much of the fuel leaked out as well. The SR-71 was nearly at its stall speed during refueling, and the KC-135 tanker was nearly at its maximum. As the SR-71 took on fuel it could not fully keep up with the tanker since its turbojets were in fact very weak on their own.

Some pilots would ignite a single afterburner, trim the rudders hard the opposite direction, and fly diagonally to provide enough thrust to keep up with the KC-135.

Story Time 1: Putting Libyan Air Defense To Shame With Speed

In April 1986, following an attack on American soldiers in a Berlin disco, President Reagan ordered the bombing of Muammar Qaddafi’s terrorist camps in Libya. My duty was to fly over Libya and take photos recording the damage our F-111’s had inflicted. Qaddafi had established a ‘line of death,’ a territorial marking across the Gulf of Sidra, swearing to shoot down any intruder that crossed the boundary. On the morning of April 15, I rocketed past the line at 2,125 mph.

I was piloting the SR-71 spy plane, the world’s fastest jet, accompanied by Maj Walter Watson, the aircraft’s reconnaissance systems officer (RSO). We had crossed into Libya and were approaching our final turn over the bleak desert landscape when Walter informed me that he was receiving missile launch signals. I quickly increased our speed, calculating the time it would take for the weapons-most likely SA-2 and SA-4 surface-to-air missiles capable of Mach 5 – to reach our altitude. I estimated that we could beat the rocket-powered missiles to the turn and stayed our course, betting our lives on the plane’s performance.

After several agonizingly long seconds, we made the turn and blasted toward the Mediterranean ‘You might want to pull it back,’ Walter suggested. It was then that I noticed I still had the throttles full forward. The plane was flying a mile every 1.6 seconds, well above our Mach 3.2 limit. It was the fastest we would ever fly. I pulled the throttles to idle just south of Sicily , but we still overran the refueling tanker awaiting us over Gibraltar.

Source: Sled Driver – Brian Shul

Story Time 2: The Fastest Gun In The West

We trained for a year, flying out of Beale AFB in California , Kadena Airbase in Okinawa, and RAF Mildenhall in England. On a typical training mission, we would take off near Sacramento, refuel over Nevada, accelerate into Montana, obtain high Mach over Colorado, turn right over New Mexico, speed across the Los Angeles Basin, run up the West Coast, turn right at Seattle, then return to Beale. Total flight time: two hours and 40 minutes.

One day, high above Arizona , we were monitoring the radio traffic of all the mortal airplanes below us. First, a Cessna pilot asked the air traffic controllers to check his ground speed. ‘Ninety knots,’ ATC replied. A twin Bonanza soon made the same request. ‘One-twenty on the ground,’ was the reply. To our surprise, a navy F-18 came over the radio with a ground speed check. I knew exactly what he was doing. Of course, he had a ground speed indicator in his cockpit, but he wanted to let all the bug-smashers in the valley know what real speed was ‘Dusty 52, we show you at 620 on the ground,’ ATC responded. The situation was too ripe. I heard the click of Walter’s mike button in the rear seat. In his most innocent voice, Walter startled the controller by asking for a ground speed check from 81,000 feet, clearly above controlled airspace. In a cool, professional voice, the controller replied, ‘ Aspen 20, I show you at 1,982 knots on the ground.’ We did not hear another transmission on that frequency all the way to the coast.

Source: Sled Driver – Brian Shul

Story Time 3: How Low Can You Go

As a former SR-71 pilot, and a professional keynote speaker, the question I’m most often asked is “How fast would that SR-71 fly?” I can be assured of hearing that question several times at any event I attend. It’s an interesting question, given the aircraft’s proclivity for speed, but there really isn’t one number to give, as the jet would always give you a little more speed if you wanted it to. It was common to see 35 miles a minute. Because we flew a programmed Mach number on most missions, and never wanted to harm the plane in any way, we never let it run out to any limits of temperature or speed. Thus, each SR-71 pilot had his own individual “high” speed that he saw at some point on some mission. I saw mine over Libya when Khadafy fired two missiles my way, and max power was in order. Let’s just say that the plane truly loved speed and effortlessly took us to Mach numbers we hadn’t previously seen.

So it was with great surprise, when at the end of one of my presentations, someone asked, “What was the slowest you ever flew in the Blackbird?” This was a first. After giving it some thought, I was reminded of a story that I had never shared before, and relayed the following.

I was flying the SR-71 out of RAF Mildenhall, England, with my back-seater, Walt Watson; we were returning from a mission over Europe and the Iron Curtain when we received a radio transmission from home base. As we scooted across Denmark in three minutes, we learned that a small RAF base in the English countryside had requested an SR-71 fly past. The air cadet commander there was a former Blackbird pilot, and thought it would be a motivating moment for the young lads to see the mighty SR-71 perform a low approach. No problem, we were happy to do it. After a quick aerial refueling over the North Sea, we proceeded to find the small airfield.

Walter had a myriad of sophisticated navigation equipment in the back seat, and began to vector me toward the field. Descending to subsonic speeds, we found ourselves over a densely wooded area in a slight haze. Like most former WWII British airfields, the one we were looking for had a small tower and little surrounding infrastructure. Walter told me we were close and that I should be able to see the field, but I saw nothing. Nothing but trees as far as I could see in the haze. We got a little lower, and I pulled the throttles back from the 325 knots we were at. With the gear up, anything under 275 was just uncomfortable. Walt said we were practically over the field—yet, there was nothing in my windscreen. I banked the jet and started a gentle circling maneuver in hopes of picking up anything that looked like a field.

Meanwhile, below, the cadet commander had taken the cadets up on the catwalk of the tower in order to get a prime view of the flypast. It was a quiet, still day with no wind and partial gray overcast. Walter continued to give me indications that the field should be below us, but in the overcast and haze, I couldn’t see it. The longer we continued to peer out the window and circle, the slower we got. With our power back, the awaiting cadets heard nothing. I must have had good instructors in my flying career, as something told me I better cross-check the gauges. As I noticed the airspeed indicator slide below 160 knots, my heart stopped and my adrenalin-filled left hand pushed two throttles full forward. At this point, we weren’t really flying, but were falling in a slight bank. Just at the moment that both afterburners lit with a thunderous roar of flame (and what a joyous feeling that was), the aircraft fell into full view of the shocked observers on the tower. Shattering the still quiet of that morning, they now had 107 feet of fire-breathing titanium in their face as the plane leveled and accelerated, in full burner, on the tower side of the infield, closer than expected, maintaining what could only be described as some sort of ultimate knife-edge pass.

Quickly reaching the field boundary, we proceeded back to Mildenhall without incident. We didn’t say a word for those next 14 minutes. After landing, our commander greeted us, and we were both certain he was reaching for our wings. Instead, he heartily shook our hands and said the commander had told him it was the greatest SR-71 flypast he had ever seen, especially how we had surprised them with such a precise maneuver that could only be described as breathtaking. He said that some of the cadet’s hats were blown off and the sight of the planform of the plane in full afterburner dropping right in front of them was unbelievable. Walt and I both understood the concept of “breathtaking” very well that morning, and sheepishly replied that they were just excited to see our low approach.

As we retired to the equipment room to change from space suits to flight suits, we just sat there—we hadn’t spoken a word since “the pass.” Finally, Walter looked at me and said, “One hundred fifty-six knots. What did you see?” Trying to find my voice, I stammered, “One hundred fifty-two.” We sat in silence for a moment. Then Walt said, “Don’t ever do that to me again!” And I never did.

A year later, Walter and I were having lunch in the Mildenhall Officer’s Club, and overheard an officer talking to some cadets about an SR-71 flypast that he had seen one day. Of course, by now the story included kids falling off the tower and screaming as the heat of the jet singed their eyebrows. Noticing our HABU patches, as we stood there with lunch trays in our hands, he asked us to verify to the cadets that such a thing had occurred. Walt just shook his head and said, “It was probably just a routine low approach; they’re pretty impressive in that plane.” Impressive indeed.

Source: Brian Shul