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From the late years of World War II to today's computer enabled design changes, stealth has been a major factor in the improvement of reconaissance and attack aircraft. The term "stealth", is thought to have been coined in 1966 by Charles E. "Chuck" Myers, combat pilot and later an exec at Lockheed,. When we think of stealth today, immediately images of the B-2 bomber or the F-117A Nighthawk fighter come to mind. However, in the closing years of World War II, stealth was embodied in overhead recon aircraft...in this particular case, stealth in the visual light spectrum. No, not invisibility, but damn close. The idea was actually quite simple. It was discovered that at high altitudes (above 20,000 feet in WWII) the shadow under an aircraft could not be disguised. Bright white paint didn't work, since the aircraft naturally blocked light from the sun on the underside, and thus always looked like a dark dot in the sky. Enter a bright pack of engineers. Adding bright incandescent lamps across the underside and especially under the wing, there were able to make the recon aircraft invisible for a few moments during daylight recon missions...the underside now at a color intensity much closer to the sky. Armed with a rehostat, one can imagine the flight engineering peering up at the daylight brightness or overcast and making manual adjustments to their new camoflauge.
Moving forward on the historical timeline, we ignore the electromagnetic jamming phase of stealth technology, since this in of itself quickly became its own headlight on a dark highway, essentially making an ECM equipped aircraft a bigger target than it was before!
The next major breakthrough in the technology was learned as part of the efforts to build a U-2 and SR-71 at the Lockheed Skunk Works. The term RCS (Radar Cross Section) became the watchword of the day, with engineers and aerodynamicists fighting over design features that used to only be the purvue of the aircraft flight characteristics designer.
Begun as CIA projects, the U-2 and SR-71 were intended to be invisible to the enemy sensors and therefore able to spy down on the land of the opposition virtually undetected and with impunity. However, the U-2 program managers realized that stealth alone was not enough, every attempt at stealth was found to have an achilles heel. In the case of the U-2, it was simply speed. It was, to put it mildly, a sitting duck. Almost immediately after its initial testing, it was seen that despite its rather low RCS (about like a small automobile), the U-2 would eventually be shot down...in fact they gave it only a two to three year immunity to defense.
Work was quickly begun on upgrading an existing "blue sky" design Lockheed had been thinking about, and the YF-11 project was born. Using speed as a defense against intercept, the YF-11 was also designed to work in the near-space region called "The Black", altitudes at or above 80,000 feet. This rarified region of the atmosphere, so near the Stratosphere, is hell to reach with a ground launched rocket, and certainly difficult for the most modern of aircraft in the late 1960s. So with a combination of lower RCS (the SR-71 is thought to have an RCS more like the RCS that of a human standing), fiery speed (Mach 3+), and high altitude (equal to or greater than 80,000 feet), the YF-11 and its follow-on the A-12 and SR-71 made for difficult work for Soviet interception.
And when Gary Powers was shot down in his U-2, the U.S. had been operating the SR-71 for at least a year, and the Soviets were seriously considering they were tracking UFOs. It was years later that they managed to field an aircraft and missile combination that could threaten the SR-71, however, there was never a shoot down (at least in our limited public knowlege).
So we have the first progression in stealth technologies...an additional 2 to 3 miles of safe cruise capability during WWII, to an Radar Cross Section of an automobile in the early 60s, and then to the RCS of a man in the mid 70s. All in 20 years time. What would the next 20 years hold? Hold on, its coming!
Returning to the 1970s, the B-70 had shown the way to produce a high speed bomber. The design centered on using variable geometry wings and some pretty incredible engines, a monstrous bomber prototype with a nice delta wing configuration (wings swept back) easily broke bomber size aircraft flight records five years before. Enter the design of the B-1B bomber.
In 1974, DARPA (Defense Advanced Research Projects Agency) was commissioned to come up with a leap forward in RCS research...reducing the signature down to something more akin to a small RPV (Remotely Piloted Vehicle), i.e. a large remote control hobby aircraft with about a 6 foot wingspan.
At that time, test ranges for testing RCS were already at their limits (this was the second time this had occurred, the first is rumored to have been when the YF-11 designers created a minor drama as that design could barely be seen by the current technology on most of the test ranges in the 1960s).
Also, computer technology (software and hardware) were not yet sophisticated enough to predict the RCS of a particular design, ensuring terrific delays as physical models were built, placed on a pole down range, and then illuminated with a radar set. In fact, shapes were at first trial and error until they had developed a set of rough guidelines to at least start a design.
According to Air & Space 3,
Northrop lost the stealth fighter contract, but was able to continue their design work through funding of the Tacit Blue project, a predecessor to the E-8 Joint Stars battlefield radar system. Tacit Blue was an ungainly design, however, it pioneered a number of important breakthroughs in shaping and skin design that are used in all stealth aircraft today.
Meanwhile, Lockheed was able to create an incredible design for the stealth fighter, once again driving the range setups down to their RCS measuring limits. Using a different shaping approach than Northrop, the F-117 design uses facets, many panels at strange angles to each other, much like the facets of a crystal found in a geode.
In fact, the breakthroughs were based upon a little known piece of work from Russia, a monograph by Pyotr Ufimstsev, an optical theoretical physicist. Through the steps described in computing RCS for different parts of an aircraft, then summing them, the Lockheed team was finally able to computer model their aircraft designs. According to the popular A&E "Wings" program on cable TV, a program called "Echo 1" was used to define RCS components as long as they were combinations of flat surfaces. Subsequently, this led to faster and more accurate shaping, and thus allowing Lockheed to quickly come up with and prove a workable stealth aircraft design. But as the shapes allowed had to be flat panels, the result is the faceted design one sees in the stealth fighter today.
As it was, the stealth competition was quite close, so close in fact, that Northrop has remained in the competition for stealth aircraft of all types, finally winning the B-2 stealth contract a few years later, and making a very close effort in the ATF (Northrop prototyped the YF-23, a sleek and very effective runner up for the Advanced Tactical Fighter program).
Over time, the computer simulations became the king of stealth design, but in another article in Air and Space ("The (Tacit) Blue Whale"), clearly shows how some designs were literally found by molding in clay prior to a test model being created for use on the RCS test range (this particular reference...molding clay...was made in reference to Tacit Blue in the Air and Space article 3).
One would have to believe that the F-22, with is numerous curved surfaces, has benefited from a much more sophisticated RCS modeling simulator, thus allows for simulation of curved surfaces as well as the flat. Of course, the RCS information, even in general terms, for the F-22 is classified.
In summary, the early stealth designs of the U-2 and the SR-71 were crucial in the later designs of the future. For example, the F-117A, several new RPVs, the B-2, and the F-22 ATF are all children of the early stealth designs stemming from the U-2 and SR-71. The B-2 gained alot from the Northrop stealth fighter entry, as well as from Tacit Blue, a stealthy prototype to the Joint Stars Battelfield Surveillance system. And obviously, Lockheed took great advantage of their early RCS software and experience with the F-117 in order to design and produce the F-22.
Sources are listed in the MILNET Bibliography:
1 Popular Science, May 1997, pgs. 54-59. [MILNET #70]
2 The World's Great Stealth and Reconaissance Aircraft, [MILNET #56]
3 Air & Space Magazine, May 1997, "The Invisible Men", pgs. 18-27, August 1996, "The (Tacit) Blue Whale, pgs. 50-55. [MILNET #78]