aviation

No story on the SR-71 would be complete without an understanding and appreciation of just how valuable the KC-135Q model tankers and their crews were to the successful and safe completion of every mission

The Lockheed A-12 and YF-12A aircraft served as the basis for the development of the SR-71, also referred to as the “Blackbird” or long-range, advanced strategic reconnaissance aircraft. The 4200th (later 9th) Strategic Reconnaissance Wing at Beale Air Force Base, California, received the first SR-71 to begin service in January 1966. The first SR-71 flight occurred on December 22, 1964. On January 26, 1990, the US Air Force retired its entire fleet of SR-71s.

For almost two decades, the SR-71 held the record for being the fastest and highest-flying operational aircraft in the world. It could cover 100,000 square miles of the Earth’s surface per hour from 80,000 feet. Two world records for its class were set on July 28, 1976, by an SR-71: an absolute speed record of 2,193.167 mph and an absolute altitude record of 85,068.997 feet.

If it weren’t for air refueling, the SR-71’s range would be relatively limited—roughly 2,000 NM. The aircraft’s range was increased by several air refuelings to the crew’s maximum endurance. Many missions have gone above 12,000 NM. The SR-71 and KC-135Q tankers’ forward basing allowed for increased overall efficiency, shorter range, shorter length missions, and faster reaction times.

There were various ways in which KC-135Q crews and their aircraft were different from other Air Force units. Actually, their boom operators were the only ones qualified to refuel the SR-71, and their aircrews were the only ones certified in Blackbird’s unique radio-silent rendezvous procedures. The Q-model tankers could move JP-4 and JP-7 fuel between different tanks because of special plumbing between the tanks.

They could use JP-4 or JP-7 fuel in their engine. If the SR-71 touched down in an area without JP-7 fuel, the Q-model tankers would fly in with the fuel and refill the aircraft using ground-based transfer hoses. One of the biggest benefits of flying Q-model tankers was that, unlike other SAC tanker crews, their personnel did not have to be on constant alert.

No story on the SR-71 would be complete without an understanding and appreciation of just how valuable the KC-135Q model tankers and their crews were to the successful and safe completion of every mission.

Most SR-71’s operational sorties started off with a refueling after takeoff.

“Many people believe we refueled after takeoff because the aircraft leaked fuel so profusely that we needed to fuel up quickly,” says Col. Richard H. Graham, a former Blackbird pilot, in his book SR-71 The Complete Illustrated History of THE BLACKBIRD The World’s Highest, Fastest Plane. “We had to refuel right after takeoff for only one reason, and it wasn’t because we leaked JP-7 fuel on the ground. Yes, the plane does leak fuel, but not enough to require refueling after takeoff.

“The JP-7 fuel reaches temperatures well over 300 degrees F. during Mach 3 cruise, making the fumes in each of the six fuel tanks very volatile and potentially explosive. The metal skin of the aircraft approaches 400 degrees F., adding to the volatility of the fuel inside the tanks. One of our aircraft limitations was a maximum speed of Mach 2.6 without an inert atmosphere inside the fuel tanks.

“The aircraft had three liquid nitrogen Dewar flasks containing 260 liters of liquid nitrogen, located in the nose wheel well. The only way to ensure 100 percent inert atmosphere in each fuel tank was to refuel the plane in flight completely full of JP-7, allowing ambient air in each fuel tank to vent overboard. Once full of fuel, gaseous nitrogen would now dominate each fuel tank’s empty space above as it burned off JP-7. The nitrogen gas pressurized each fuel tank to 1.5 psi above ambient pressure and inerts the space above the heated fuel to prevent autogenous ignition. This is why we refueled after takeoff. Then we could safely accelerate beyond Mach 2.6.”

Graham concludes:

“There was one other way of achieving tank inerting, called a “Yo-Yo,” but this was a maintenance nightmare. A few of our missions required the SR-71 to accelerate to Mach 3+ right after takeoff with a 65,000-pound fuel load. The Yo-Yo procedure had the crew chief completely refuel the plane to full tanks of 80,000 pounds of fuel. Then, with the nitrogen pressurization system working, they de-fueled 15,000 pounds of JP-7, ending up with a 65,000 pound fuel load and a plane that was capable of going immediately to Mach 3+.”

Photo by U.S. Air Force and Tony Landis Lockheed

‘“Beautiful, beautiful, come back and do another one,” the tower controller said,’ Maury Rosenberg, SR-71 Blackbird pilot

The Lockheed A-12 and YF-12A aircraft served as the foundation for the development of the SR-71, also referred to as the “Blackbird” or long-range, advanced strategic reconnaissance aircraft.

The SR-71 Blackbird is the only reconnaissance aircraft in history to have operated in more hostile airspace and with such complete impunity. It is the fastest aircraft propelled by air-breathing engines. The Blackbird represented the highest point in aviation technology development during the Cold War due to its performance and operational achievements.

The Blackbird was built to reach altitudes of up to 85,000 feet and cruise at “Mach 3+,” three times faster than the speed of sound, or more than 2,200 miles per hour.

The following tape transcription, which was recorded in 2018 at the Western Museum of Flight, reports SR-71 Pilot Maury Rosenberg discussing the one occasion when he chose to ask for a “fly-by” over the Sacramento airport while returning to Beale Air Force Base (AFB), where he was supposed to land.

The request was eagerly approved by the air tower crew, who wanted him to fly the SR-71 “down the ramp” (much closer to the tower and other buildings).

Rosenberg recalls:

‘I think it was the 1982 or 83 Toronto Airshow. I was a participant; we took an SR-71 and three crews went. One crew flew it in. One crew flew on the second-day airshow. And the third crew, which was myself, flew just a couple of flybys on the third day of the airshow and then actually flew the aircraft back to Beale Air Force Base. So when I came back into the Sacramento area and I was descending down to land at Beale, we had a lot of fuel. We had to work on the holiday, and I asked the backseater You want to make an approach at Sac Metro? And he said Can we do that? And I said Why not?

‘So, when we got over to approach control I asked if Sac Metro was available for an approach and they said they’d switch this over to Sac Metro and they cleared us for an approach. So, as we were coming down final and we had configured the airplane with the gear down and we’re about two miles out on final, they switched us over to tower.

‘I asked the tower Would you like a flyby down the runway or down the ramp? The guy said Down the ramp! When I said okay, I sucked the gear up, pushed the power up. We started heading towards the ramp, towards the tower. As we were approaching it, I rolled the plane up away from the tower and lit the afterburners. We went around and made a pass. The tower controller said, “Beautiful, beautiful, come back and do another one”. I said I better not, I’m gonna go to Beale. So, we went back to Beale and we landed.

‘As I mentioned it was a holiday weekend and Colonel Lonnie met us when we pulled in the hangar after we landed which was the normal procedure. I thought well that’s nice that he came out. As I came down the ladder from the aircraft, he looked at me and he said Maury, do we have any regulations that say we can’t make approaches at Sacramento Airport? I said No sir.
I want one on my desk at seven o’clock tomorrow morning.

‘Now there’s a continuation to this story. I retired in 87 and flew with United Airlines. I was a Boeing 767 co-pilot and in 87, I think it was either the fall of 87 or the winter of 88 right after the New Year, I was flying a 767 from Washington DC to San Francisco and an air traffic controller wanted to know if they could ride in the cockpit. The captain of the plane signed it off. So, this young lady was the air traffic controller. I was flying, it was my leg from DC to San Francisco.

‘We’re flying and he’s just talking to her halfway across the country and at some point, he asked her how long she’d worked at San Francisco Airport. She hadn’t been there that long, she said she used to work at Sac Metro. I looked over my shoulder at this young lady and I said Were you working Sac Metro in 1982? She looked at me and she said yeah, I was, why do you ask? I said Were you working the tower when the SR-71 buzzed it? Oh yeah, I was how did you know that and I told her it was me and she started laughing. She said you scared the hell out of people, there were people in the terminal that dived to the ground. They thought the airplane was going to hit the turf.’

Check out the Habubrats SR-71 and Born into the Wilde Blue Yonder Facebook pages for awesome Blackbird photos and stories.

Photo by Unknown

Members of the SR-71 Blackbird crew have reported that on occasion, they came down in search of a tanker, not so much because they were running out of gas but because their gas was getting too hot

Colonel Richard “Butch” Sheffield, the Reconnaissance Systems Officer (RSO) for the SR-71 Blackbird, noted in his unpublished book “The Very First” that the pressure suits of the Blackbird crew members began to warm up a little when the aircraft’s fuel ran low. On the other hand, adding more fuel caused them to cool down. Therefore, the fuel was used as a heat sink for the SR-71.

Since the SR-71 was an incredibly engineered, complex aircraft, they had to figure out a means for the Blackbird to handle the massive amount of heat produced by its fast flight.

‘Flying at over Mach 3 is a thermal problem. Everything is too hot, including any air you slow down to interact with the vehicle. You are trying to make the vehicle (and the pilots inside) survive for hours in a pizza oven while they are getting cozy with two 500 million BTU/hour flamethrowers,’ Iain McClatchie, an aviation and turbine engine expert, says on Quora.

‘When you look at a graph like this, your first impression might be that the vehicle is this glowing hot thing slicing through the icy -52 C air at 80,000 feet. So naturally, you think of the air as cooling the airplane down.

‘Not so much. The air has to change to the vehicle’s speed to touch the vehicle, and that requires work. That work heats the air. At Mach 3.2, the stagnation temperature of the air is 740 F, which is hotter than every (labeled) point on the above graph! (The nacelles around the engine afterburners, unlabeled, are in fact hotter than the air around them.)

‘I know; it seems unbelievable. Use this handy graph and see for yourself.

‘Basically, the shocks from the airplane heat the air around it, but the vehicle itself cools the air in contact with it down. Once the airplane passes by, all that disturbed air tumbles to a stop, leaving a path of hot air through the upper atmosphere.

‘So back to life in the pizza oven. The basic solution is (a) leave most of the airframe hot and make it out of stuff like titanium and stainless steel that are strong when hot, and (b) start with a large amount of cold fuel and then dump heat from critical areas into the fuel before burning it. When decoupling from an aerial tanker, half the SR-71’s weight was fuel.

‘A special type of kerosene fuel, JP-7, was developed for the SR-71 to be good as a heat sink. It boils away at 285 °C at 1-atmosphere pressure, which is the upper end of the kerosene range. When the plane tanked up at 30,000 feet, the kerosene might start below 0 C. At speed, it would be used to cool the avionics and cockpit, and by the time it arrived at the engine, it would get up to 177 °C. It was then used as hydraulic fluid for the various engine actuators, primarily the variable geometry nozzle. By the time it got to the fuel injectors, it had gotten up to 316 °C (but wasn’t boiling because it was at several atmospheres of pressure). At cruise, the burner cans were at 330 kPa (about 3.3x the pressure at sea level), so the fuel still didn’t boil as it left the nozzles but the droplets would have evaporated very quickly.’

McClatchie continues;

‘JP-7 is mostly a mix of hydrocarbons centered around C12H26 (dodecane). The graph above shows the vapor pressure of dodecane as a function of reciprocal absolute temperature. That makes it a bit hard to read. 0.0024, for instance, is 417 Kelvin, which is 143 Celsius. Liquids start to boil when their vapor pressure is greater than the ambient pressure. I’ve labeled the boiling point of dodecane at 2900 Pa, which is the absolute pressure at 80,000 feet, and 13000 Pa, which is the minimum absolute pressure in the SR-71 fuel tanks. Note that the dodecane component of JP-7 starts to boil at 162 °C at sea level… quite a bit less than the advertised 285 °C, which is actually when the stuff boils away completely.

‘The flash point of JP-7 is 60 °C. The fuel was held in tanks whose walls were formed of the skin of the vehicle. Since fuel vapor against the top skin of the vehicle would be well over 60 °C during cruise, if air was allowed in any ignition source in the tank, it would cause deflagration and destruction of the vehicle. Instead, nitrogen gas from a 260-liter liquid nitrogen dewar was used to pressurize the tanks. This would have mostly been an issue during descent when the ambient pressure rose and extra gas was needed to fill the tank ullage space.

‘But nitrogen gas fill was not enough. The fuel was heated in the tank by the bottom surface of the vehicle, just as water in a pot is heated by the flame on a stove. In this case, the fluid was over a meter deep in the tank and consequently took longer than a pot to boil. At ambient cruise pressure, the fuel would have begun to simmer in the tanks at 116 °C. By pressurizing the tank to 10 kPa over ambient, the tolerable tank temperature rose by 33 °C. This temperature limit put a time limit on how long the SR-71 could stay at cruise before it began to lose fuel to boiling.

‘The fuel pumps in the tanks raised the fuel pressure so that boiling was no longer a problem once in the fuel system. The limit of how much heat could be absorbed by the fuel was rather set by its coking temperature — the temperature at which the fuel begins to deposit varnish on the interior of whatever plumbing it is in. I don’t have a specific number on JP-7, but it must be higher than 316 C. There was another experimental hydrocarbon blend developed called JP-900, which resists coking up to 482 °C. This was intended for a higher-speed vehicle that was never built.

‘They were not able to get the wind tunnel behavior to match the actual behavior of the airplane. Kelly Johnson speculated that this was because, during cruise, the fuel sitting against the lower skin of the fuselage and inner wing kept that portion of the airframe cooler than the upper skin. This caused the vehicle to bow from the differential temperature expansion, which would have made the wings slightly anhedral and would have made the vehicle unstable in roll.’

McClatchie concludes;

‘Heating of the fuel while in the tanks caused yet another problem. As I said earlier, the engines can take the fuel at a maximum temperature of 177 °C. So as the fuel in the tanks heats up, its ability to absorb heat on the way to the engine decreases. Flight crews have said that they sometimes came down looking for a tanker, not so much because they were running out of gas but because their gas was getting too hot.

‘Using fuel as a heat sink is common in fast jets. The Concorde did it, the F-15, F-16, F-18, F-22, and F-35 do it, and probably all other supersonic aircraft.’

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Photo by Unknown, NASA, U.S. Air Force and Lockheed Martin