While never going into production, the odd-looking Tacit Blue was arguably one of the most important programs for the legacy Northrop Corporation. Over a span of three years starting in 1982, Tacit Blue proved that the company’s technological breakthroughs in low observables were real – paving the way for the successful B-2A Spirit bomber and a slew of other stealthy designs over the next 35 years.
Tacit Blue began with Northrop’s participation in the 1975/76 Experimental Survivable Testbed (XST) competition, which was won by Lockheed Martin and its eventual F-117A Nighthawk “Stealth Fighter.” Despite the loss, the Department of Defense and the Defense Advanced Research Projects Agency (DARPA) viewed Northrop’s low observable team as a national asset and decided it needed to be retained.
In 1978, Northrop was asked to work on a new type of vehicle that would work as a battle management and control platform that could fly unseen near a conflict’s front lines. This Battlefield Surveillance Aircraft – Experimental (BSAX) would scan adversary ground forces to ascertain their composition and positions.
Military commanders would then be able direct air forces, artillery and tactical missiles against high-priority targets. This meant the aircraft had to carry an active radar system and fly undetected with 360 degree all-aspect, low observability near the forward edge of battle area.
Critical to this mission was a new type of radar built by Hughes Aerospace Corporation, which could direct its radar beam in different directions without physically needing to move. Because of its size, the aircraft was essentially designed around the radar by constructing a box and wrapping the low observable features around it. Thus Tacit Blue’s basic shape looked like a butter dish with wings.
Many engineering problems had to be overcome, but three in particular necessitated critical breakthroughs in low observable technology. They included being able to control the amount and intensity of radar detections; incorporating flowing curvature surfaces for improved aerodynamics and stealth, and the ability to operate against low-frequency radars.
Contrary to popular perception, stealth aircraft are not invisible to radar. But while detection cannot be eliminated, it can be redirected and minimized. Careful attention to an aircraft’s design can reduce the ability of radar detection along its aligned edges. By keeping the number of those edge detections to the fewest amount necessary, the detections are only briefly noticeable and usually don’t last long enough to track.
The second breakthrough was in shape. Due to computer program limitations, early low observable designs featured flat-plate, faceted surfaces. Although faceted surfaces were effective enough in reducing radar return, they proved very troublesome aerodynamically. Smoothly flowing, complex curvature surfaces provided the answer. Not only were sweeping curves stealthy and more elegant looking than facets, they were much more aerodynamic.
The third breakthrough occurred when the U.S. Air Force decided the vehicle also needed to be able to operate against low-frequency radars. Learning to defeat those additional systems proved critical in assuring battlefield survivability not only for Tacit Blue, but for all follow-on designs.
Two curved, butterfly tails graced the aft end of the Tacit Blue prototype, resembling whale flukes. That likeness gave rise to the aircraft’s unofficial nickname of “Whale” with the people who worked on the program starting to call themselves “Whalers.”
First flight for the “Whale” took place, Feb. 5, 1982, and it was not the easiest vehicle to fly. However, the program conducted 135 test flights during the test period and successfully validated the low observable design features and radar sensor technology.
Tacit Blue did not go into service as its battlefield surveillance role was given to a standoff platform based on a commercial airline design. Using the sensor systems developed for Tacit Blue, the E-8 Joint Surveillance Target Attack Radar System, or Joint STARS, came to be.
For Northrop, the success of Tacit Blue provided assurance to the Air Force that the then under-development B-2 stealth bomber would work as advertised. Thirty-five years later, Tacit Blue’s influence continues to shape Northrop Grumman’s designs for stealthy aircraft well into the foreseeable future.