The Real X-Jet
From Transformers to the X-Men, the Blackbird is still Hollywood's favorite futuristic jet. Here's the real story of its birth.
- By William E. Burrows
- AirSpaceMag.com, March 01, 1999
(Page 3 of 7)
Pratt & Whitney executive Arnold J. Gunderson crystallizes the engineering challenge by pointing out that while the J-58 had parts that were machined to thousandths of an inch, the whole engine got so hot at cruise that it expanded two and a half inches in width and grew six inches in length. (The airframe expanded and contracted as well, which was why fuel leaked from its wings.) That problem was solved, says Gunderson, by putting the gear box that drives the generators, hydraulic pumps, and other subsystems on the bottom of the engine, where it effectively floats, and hanging the engine itself from the top of the nacelle so it could expand and contract without stressing its parts.
The heat also made the A-12 and the SR-71 trickier to land than to launch. Descending too quickly and shock-cooling the airframe and engines would cause a sudden shrinking of parts that could be more stressful than the expansion that occurred during heat-up. Pilots had to avoid cooling the engine at a rate that could cause shrinkage and make the ends of the turbine blades rub against their seals, ruining the engine.
Mach 3.2 was the A-12’s (and then the SR-71’s) design point. The airplane could hit Mach 3.6 and, in theory, Mach 4 if the air was cold enough. But, as Gunderson explains, “Kelly designed the airplane to fly at Mach 3.2 at 80,000 feet. The airplane cruises there. It is very, very comfortable there because that’s the point at which it was designed to fly, where every system is snug and tight-fitting.... All of the enhancements and refinements we did over the many, many years of the program were done to reinforce that. Moving away created more problems than it was worth. Every time we flew faster and higher, we ran into areas where we had not so much experience. Things would go wrong. You’d swallow the [aero]spikes, you’d flame out.”
Unlike their engines, 93 percent of the airplanes’ structure and skin were made of titanium. The metal’s quality varies widely, and the first batch of titanium sponge (which looks like a giant soap pad) had to be bought by the CIA from the Soviet Union and smuggled past U.S. Customs. Ironically, the Soviets had most of the world’s supply of the metal and exported a product of exceptionally high quality. The plane’s landing gear was the largest titanium forging produced in the United States and the only titanium landing gear in use in any U.S. aircraft. Forging and cutting special metals to tolerances as tight as .005 inch, then welding them together, were exotic specialties that were virtually developed from scratch.
Welding brought with it new mysteries. Skunk Works engineers were at a loss to explain why A-12 wing panels that were spot-welded in the summer failed early, while those that were welded in the winter held together indefinitely. They traced the trouble to a characteristic of titanium: It is absolutely incompatible with chlorine. “We finally traced the problem to the Burbank water system, which had heavily chlorinated water in the summer to prevent algae growth, but not in winter,” Johnson related in a classified CIA journal article in 1982. “Changing to distilled water to wash the parts solved the problem.”
The A-12’s inlets, which include forward air bypass doors that automatically manipulate the air—and the famous spikes—were designed under Lockheed engineer (and later Skunk Works president) Ben Rich’s supervision and are the key to the airplane’s performance. The B-58 Hustler bomber used fixed spikes in front of its General Electric J-79s. Like his counterparts at Convair, Rich found that air hitting the unprotected front of a turbojet at high Mach numbers creates insurmountable pressure problems.
The spike’s purpose was to control the supersonic shock wave and, by working in combination with doors and openings to bleed away excess air, prevent supersonic airflow from entering the compressor intake. It accomplished this trick by moving back 26 inches into the throat of the inlet on a programmed schedule as the speed built up in a manner somewhat like the way the nozzle on a garden hose changes to adjust the water’s flow. At Mach 3.2, each inlet swallowed about 100,000 cubic feet of air a second.
When a spike failed to work perfectly, which happened repeatedly for many years, supersonic air pressure instantly built up and choked the compressors. They reacted without warning by violently spitting the shock wave back out. This event was the dreaded “unstart,” which, at Mach 2 or higher, would cause a thunderous bang that the crew heard and felt, while the airplane jerked so violently in the direction of the unstarted inlet that, according to Lockheed Martin’s Garfield Thomas, “it was like running into a brick wall.” He remembers one unstart yaw violent enough that the pilot “slammed his head so hard against the sill on the window [that it] cracked the helmet and knocked him semi-conscious.” A computer restarted the inlet, Thomas adds, and the pilot survived.
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Comments (9)
Bouroughs did a great job of describing the machine development. However he did overlook the J-58 engine's need for a ground based Starter Cart (2 Buick Wildcat V-8s) and that the JP-7 required an "exotic" igniter called TEB ( Tri-Ethyl-Borate).
To see some of the personal experiences on the A-12, go to the following web sites:
http://www.habu.org/a-12/06932.html
http://www.wvi.com/~lelandh/m21_Crash.html
http://www.roadrunnersinternationale.com/oxcart_pilot_training.html
There are a number of pilot's stories under the Roadrunners...
Posted by Edward Power on June 11,2011 | 12:26 PM
This is a very interesting piece. I am a retired Navy Civil Servant. I was an electronics engineer for 25 yrs with the Navy Civil Service. My Dad worked for Lockheed during the Kelly Johnson years. He was superintendant over the skunk works tooling dept, 3rd under Kelly Johnson. His name was Edwin C. Mort. He passed away early January of 1989.
This piece answers many questions I have had about the "Black Bird".
Posted by Richard E. Mort on June 11,2011 | 08:58 PM
My father worked for General Electric from 1945-84
and did work with a lot of other companies during
that time.
One such company was the Skunk Works during Kelley Johnson's time and I remember the Blackbird being
refered to as the YF-12A.
My father worked with a lot of space exploration programs(Including the X-15 during Harrison "Stormy" Storms time)
defense projects and commercial aviation jobs(The SST program with Boeing until President Nixon axed it in 1971)
I've always been interested in what my father did BUT just
didn't have the math or science knowledge to persue it. I
still read LOTS of science fiction. Sir Arthur C Clarke
was both our favorite author.
My father lived from 1919-1995 and I was born in 1949.
Thanx! for letting me ramble on a bit.
Great piece on the Blackbird...
06 16 11
Posted by Don Ducker Jnr on June 16,2011 | 09:11 AM
Now 12 years later Annie Jacobsen has written Area 51, and in there is a lot of information about Oxcart, the Skunkworks and other formerly classified projects.
Posted by PeterK on June 23,2011 | 05:54 PM
The SR-71 also served as the artistic inspiration for a cover illustration (http://www.isfdb.org/wiki/images/a/a9/ANLGJUL68.jpg) for a novella by Dean McLaughlin in the July 1968 issue of Analog Magazine. The artist was the late Frank Kelly Freas who depicts an SR-71-like aircraft looming over WWI fighters as a result of the 20th Century airplane arriving unexpectedly over France in 1918. The story somewhat humorously depicts the frustration of the modern airplane's sole pilot who discovers that his modern missiles can't shoot down wood and fabric biplanes with tiny radial engines.
Posted by Diana Powe on June 24,2011 | 05:33 AM
Excellent work Mr.Burrows. Nothing captures my imagination like the Blackbird. Not even my beloved Corsair.
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Posted by Hank Armstrong on June 24,2011 | 10:19 AM
It is just breathtaking to think of the 'Murderers' Row' of engineering, design, and fabrication talent that Kelly Johnson surrounded himself with in the 50's and 60's at Lockheed's SkunkWorks. Of course, the Cold War-driven unlimited budgets and different gov't/defense contractor relationships of that time enabled a lot of innovation, but to imagine the history-altering aircraft these guys dreamed up and built is just amazing. With slide rules and calipers and feeler gauges . . . .
Like Ben Rich spoke about in his book, SKUNK WORKS, these groups of guys in white shirts and ties and pocket protectors, justifying their blueprints over cigarettes and coffee and slide rules to crusty, seen-everything fabricators whose experience tempered their ideas must have been an intoxicating atmosphere. I often think THAT's why it was buried behind layers of Black World secrecy: They were just having too much fun making history, one rivet at a time.
Posted by Joey Wilson on July 8,2011 | 01:20 AM