Airborne Awesomosity

Boeing B-47 Stratojet, The Untouchable

The dawn of jet age came with the scream of six J35 turbojets on December 17th, 1947. It was 44 years to the day after the Wright brothers’ first flight, and after four years of rapid development, Boeing’s new bomber was ready for its first flight. It was only eight years after the first jet powered aircraft had struggled into the sky, and only five years after the introduction of the B-29, the most advanced bomber of World War II. Though heavier than the B-29, the XB-47 looked much more delicate. Its thin, swept back wings had their root in captured German research, and had no room for fuel. It was so radical that even Bob Withington, the aerodynamicist who had designed and built Boeing’s wind tunnel, and proved the benefits of the 35-degree sweep angle, looked at the taxiing prototype and thought “That’s a mighty strange-looking airplane. I wonder if it will actually fly.”

On the ground, it looked like a crocodile out of water, but the XB-47 flew, and once the second prototype was fitted with new GE J47 engines in 1948, it changed the meaning of the word. It was the first plane to fulfill the promise of the jet engine. It didn’t eke out some small advantage over piston powered aircraft, it cruised more than twice as fast and half-again as high as the B-29. The introductions of the true supersonic F-100 Super Sabre and Mig-17 were six and eight years away, respectively. Until then, no fighter could come close to the B-47.

610 mph, 44,500 feet, MC Hammer’s old refrain, writ large.

When it entered Strategic Air Command service in 1951, the B-47 was the ultimate weapon. In order to reach any target on earth, B-47 crews made air-to-air refueling so routine that the entire maneuver was often done in radio silence. Their speed and altitude meant there was little warning, and even if they were spotted, there was no defense that could stop them. They carried fissile nuclear bombs up to 25 Megatons in size [1], and some tested air-launched guided nuclear missiles. Reconnaissance variants with eleven cameras and ten ‘photoflash’ bombs made some of the first overflights of the Soviet Union. Electronic warfare variants carried over a dozen broadband and selective jammers, extra chaff to overwhelm radars, and recorded and monitored every kind of Soviet signal, including some of the first space flights [2]. Others monitored weather patterns and the spread of nuclear fallout.

B-47s had hydraulically boosted controls, downward ejection seats for navigators, anti-skid wheel tachometer relays that opened solenoid valves to release brake pressure on a locked wheel, and inerting systems that sublimated CO2 inside the fuel tanks. The B-47 wasn’t just a Cadillac with power-everything and afterburning taillights. A Stratojet was a Duesenberg.

[youtube]https://www.youtube.com/watch?v=6cIgTAtj4E4[/youtube]

B-47s weren’t just all engine with a big fuel tank taped on. The Air Force trained its pilots to do full-lock barrel rolls, and 2.5 to 3G Immelmann turns at 530 or so miles per hour. (Check out the jaw-dropping roll response at 6.33.) The engines were actually the weak link in the design. Even with water-methanol injection, they strained to develop a mere 7,200lbs of thrust each [3], and loosing an engine on takeoff required a fast reaction that was the opposite of propeller planes, lifting the slowed wing instead of correcting the yaw with the rudder. Takeoff distances were over 10,000 feet unless 33 1000lb-thrust JATO rockets were used.

Most frustratingly, J47s took over 20 seconds to spool from idle to full power. This lack of response was the most dangerous for the point where recovering from a high speed stall could immediately cause a low speed stall, and vice versa. For landing, where full power may be needed to pull back up and circle around again, a parachute was deployed to slow the jet with the throttles pushed forward. Landing was also tricky because of the bicycle main landing gear. Touching the nose first made the B-47 bounce back and forth with escalating violence, like an out-of-control playground spring rider.

Necessity is the mother of awesome.

Still, B-47s could reach over 550 mph at low levels, and over 610 mph at altitude. It was fast enough to experience control reversal, where the aileron twists the wing, and the whole wing deflects air in the opposite of the intended direction. The Air Force put a speed limit on the airframe until lift-reducing spoilers were added on top of the wings for lateral control.

The surface to air missiles deployed between 1954 and 1957 were what ended the high-altitude bomber’s reign, eventually downing even CIA U2s. Invading bombers now had to fly as low as possible to avoid detection and get back out of range before a battery could respond [4]. The battleground was tilted back to level, and the technology race had shifted its focus. The long shadows of the Intercontinental Ballistic Missiles and mutually assured destruction that followed the B-47 are probably the biggest reasons it is overlooked, while World War II’s B-17 and B-29 and the still flying B-52s are celebrated.

But while it flew, the Stratojet had it all: high and low-altitude speed and maneuverability, tapered, unmatched looks that set the tone for the jet age, cutting edge technology, and a design that has been the template for almost every large jet in the seven decades since its introduction. While it was not retired unsurpassed like the SR-71, and it didn’t serve for decades like the most successful military and civilian aircraft, neither was it obsolete before it was introduced like so many Toasters favorites. It was a quantum leap in technology that ruled the skies before being replaced by slightly improved descendents that would never match its air dominance.

Does it get any better than a fighter canopy AND a bubble nose?

[1] Nuclear cores were kept outside the bomb assemblies until just before a drop.

[2] Two Electronic Countermeasures variants remained in service with the Navy until 1977.

[3] Vaporizing liquid water-methanol created cooler, denser intake air. This increased power, as well as keeping temperatures down and allowing more fuel to be burned. The engines were also run very rich for as much power and cooling as possible. For perspective, the total thrust of all six engines on a B-47 was 43,200 lbs. A single GE90 turbofan on a 777 made 127,900 lbs of thrust during testing. And the 777 has two. The B-47 carried 14,610 to 18,405 gallons of fuel including two 1,780 gallon drop tanks, which was only fuel that could be dumped. This was three times the fuel of a B-29. A 777 carries up to 53,515 gallons, weighs up to 766,000 lbs to the B-47E-IV’s max of 230,000 lbs, and flies up to 13,422 miles to the B-47’s 4990. Six times the power, more than three times the efficiency per pound, that’s progress. Aside: Looking for sources on that led me to a very strange follow on to the spelling article.

[4] The toss bombing practiced by B-47 crews was a credible response in terms of reducing SAM effectiveness, but not in the sense of getting a bomber clear of its own nuclear blast. The high loading also contributed to fatigue failures in the bolts that attached the wings. For an in-credible response to ballistic missiles to ensure mutual destruction, look no further than Minimum-Interval Take Off (below) and Cartridge Starting B-52s (link).

[youtube]http://www.youtube.com/watch?v=LU6DoQOd6z0[/youtube]

Sources and Further Reading:

Obituary, Holden ‘Bob’ Withington, 1917-2011

B-47 and Cactus, Public Domain via Wikipedia

Front View, Public Domain via Wikipedia

Rocket Assisted Take Off, Public Domain via Wikipedia

Bubble Nose, Public Domain via Wikipedia

The Dawn of Discipline, by Walter J. Boyne, Air & Space magazine, July 2009

I Made The First Jump, Col. ‘Chic’ Henderson, Popular Mechanics, March 1955

B-47 Ejection, via ejectorseats.co.uk

Aircraft Landing Gear Design Principles And Practices, by Norman S. Currey

Whys and Hows of Water Injection on Jet Engines, via kls2.com

Water Injection, via engineering.purdue.edu

Guinness World Record For Most Powerful Jet Engine

777-200LR/300ER Technical Specifications, via Boeing

777 Distance Record, via Boeing

  • They always looked like the baby B-52 to me.

    • fodder650

      That isn't to far from the truth actually. The B-52 took all the lessons learned from the B-47 and upsized it.

      • skitter

        The first two B-52 prototypes even had a bubble canopy, but the Air Force requested side-by-side seating.

      • Number_Six

        The B-52 must have been dull as dishwater to fly after having enjoyed the aerobatic qualities of the B-47.

        • alanbdahl

          In his book Tex Johnston mentioned that he barrel rolled the B-52 during testing too, not just the Dash-80 707 prototype. I also recall that someone did a loop in one too but I can't remember if that was Johnston or another pilot.

          • Number_Six

            There's likely nothing Tex Johnston couldn't barrel roll or loop but it still seems like a much riskier proposition in the BUFF.

          • skitter

            A barrel roll, when done at 1G, is a pretty safe, easy maneuver. It just takes a slight nose up and increase in power. Still, it's unusual for pilots to be required to do it in 60+ ton planes. A loop, Immelmann or otherwise, takes a lot of energy to fly straight up and over. Most planes short of an F-15 are at their slowest at the top of the loop, upside down, and risk stalling in the wrong direction and orientation. And the lower the power to weight, as with the woefully underpowered B-47s and -52s, the more Gs and stress will be needed to convert entry speed into up-and-over.

  • FЯeeMan

    Vladimir, are you sure ve're safe here?
    Yes Yuri, I'm sure ve're safe here, vhy do you ask?
    Vell, Vlad, I see several long black smudges coming toward us, wery high up in the sky…

    Jet engines burn much cleaner these days.

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