Battlefield Surveillance Aircraft-Experimental (BSAX)

The U.S. Air Force unveiled the Tacit Blue Technology Demonstration Program on April 30, 1996, at the Pentagon. During the Assault Breaker / Pave Mover development, DARPA and the Air Force developed the highly classi?ed Tacit Blue low observable surveillance aircraft whose mission was to be a highly survivable platform for a surveillance radar based on Pave Mover radar technology. The radar was capable of Ground Moving Target Indicator [GMTI] and Synthetic Aperture Radar [SAR] imaging, but had low probability of intercept (LPI) features which made it more difficult to detect than a conventional radar. The aircraft was affectionately (and for obvious reasons) nicknamed "The Whale". Tacit Blue was created to demonstrate that a low observable surveillance aircraft with a low probability of intercept radar and other sensors could operate close to the forward line of battle with a high degree of survivability. Such an aircraft could continuously monitor the ground situation behind the battlefield and provide targeting information in real-time to a ground command center. Tacit Blue validated a number of innovative stealth technology advances.

In January 1975, DARPA issued contracts to McDonnell Douglas and Northrop to design a stealthy manned aircraft, which eventually led to the F-117 Nighthawk. McDonnell Douglas identified what appeared to be appropriate RCS thresholds (although it couldn't design an aircraft to meet those values). Lockheed and Northrop presented concepts that were predicted to meet or exceed the signature goals. Lockheed won the sole Phase 2 award. The conclusion of the XST competition did not end the further development of stealth technology.

In 1976, many government agencies entered various contracts for the study of new stealth technology applications. The work performed by the government's "Blue Team" showed that the USAF should invest in new manned and unmanned vehicles, whose design ensured the lowest radar echo. Thus was created another program, focused on tactical reconnaissance equipment. Northrop was in the unenviable situation where it had lost the competition the new USAF lightweight fighter (LWF) and did not win the XST program. Moreover, even if the tactical demonstrator program invited bids, it was very likely that Lockheed would again win thanks to their experience with stealth technology and the ECHO1 computer program to predict radar reflection. Northrop made significant investments from internal sources in building new wind tunnels and computational technologies. Otherwise Northrop might lose any money on the development of military aircraft until at least the early 1980s.

In December 1976, DARPA and the U.S. Air Force initiated the Battlefield Surveillance Aircraft-Experimental (BSAX) program, which was part of a larger Air Force program called Pave Mover. The BSAX program's goal was to develop an efficient stealth reconnaissance aircraft with a low probability of intercept radar and other sensors that could operate close to the forward line of battle with a high degree of survivability.

DARPA wanted to preserve the expertise that Northrop had developed. It encouraged Northrop to maintain its team, which shortly thereafter engaged in DARPA-sponsored design studies for the BSAX program). Northrop began work in 1976 and was awarded a sole-source contract in 1977 for development of the BSAX aircraft. Based on this work, in 1978 Northrop managed to get a direct contract worth $136 million for the development and construction of BSAX. Thus began the lengthy process of designing the optimal shape of the aircraft, which at one point led to DARPA discussing the possibility of open competition for other companies, but thanks to the lobbying of Northrop managers did not do so. /p>

The BSAX aircraft, later known as TACIT BLUE, was unique in its approach to airborne radar integration, where the airplane was created around the radar instead of integrating a radar with an existing airframe. The company, thanks in particular to the approach of John F. Cashen, managed to deal with a much more complex calculations of radar reflectivity grind of areas and used them to create the aircraft Tacit Blue. Thus opened the way to a new generation of stealth aircraft, which could have much more aerodynamic shapes than the "ultimate flying iron" F-117. Since the aircraft was conceived especially for the smallest of the radar reflectance, Cashen's design was implemented at the expense of the aerodynamics. The classic combat aircraft is designed to bring to the target, the attack and retreat from the battle zone, so that the main focus is on minimizing the radar reflectivity from the front and rear. In contrast, the Tacit Blue was an experimental research machine that had to orbit the combat area in circles, and had to be difficult to detect at short-range practically from every direction. This required an approach of all-aspect stealth. In addition to verifying the stealth deisgn, it had yet another mission, the trial use of SLAR (Side Looking Array Radar) designed by Hughes. It was necessary to cram this oversized radar into a relatively small fuselage and to do so in such a way that the radar had the greatest viewing angle. The resulting shape ws thus largely like a box.

Tacit Blue featured a straight tapered wing with a Vee tail mounted on an oversized fuselage with a curved shape. The aircraft has a wingspan of 48.2 feet and a length of 55.8 feet and weighed 30,000 pounds. A single flush inlet on the top of the fuselage provided air to two high-bypass turbofan engines. Tacit Blue employed a quadruply redundant, digital fly by wire flight control system to help stabilize the aircraft about the longitudinal and directional axes. Like Have Blue, Tacit Blue had to have quadrupled back-up electro-mechanical flight management system, since it was manifestly the most unstable aircraft in the world.

Although the machine had surfaces with a few composites absorbing radar waves, it was in large part built of aluminium. For the sake of saving costs a number of components were taken from the existing machines. Within the structure was part of the chassis of the F-5E, McDonnel Douglas ejection seat, and Garrett ATF3-6 engines used for aircraft to Falcon 20. The location of the engines deep in the fuselage, as was done for the Have Blue type, in turn significantly reduced the noise of the aircraft. However, the best work was carried out by engineers on the reduction of infrared emission. The products of combustion are ejected quite far behind common engine nozzles, out of view from the ground. Than combustion products are cooled, so much so that the infrared emissions evaluation experts note that it is the coldest aircraft ever seen. The same can be said about the engine condensation trails.

The first prototype was built at the beginning of 1980. Only one complete airframe was ever flown, although a second airframe shell was constructed to serve as a backup. Introductory flight was scheduled for 04 February 1982. Test pilot Dick Thomas was not at all enthusiastic about the anticipated flight characteristics, but the pilot successfully performed the first flight of the Tacit Blue demonstrator. The aircraft often flew three to four flights weekly and several times flew more than once a day. The last was 14 February 1985. Five pilots subsequently logged 135 flights (300-plus flight hours) over a three year period.

TACIT BLUE was developed as a potential platform for radar sensors developed under the Air Force Pave Mover and Army SOTAS programs. At the beginning of the program the possibility of further development of the Tacit Blue and evaluation of the platform was provisionally envisaged. Through its stealth properties it could operate much closer to the front than a conventional aircraft. The main changes would have been a larger tail surfaces and wing moved more into the center of the fuselage. The aircraft, however, had substantial shortcomings, which would have reduced its value in the operational role, so the machine remained only a technology demonstrator.

In 1982, the Undersecretary of Defense for Research and Engineering (USDRE) combined the SOTAS and Pave Mover efforts into a joint program, later designated Joint STARS. From 1982-1984, the services, OSD, and Congress wrestled over the development of requirements for the joint program, as well as the appropriate platform for the sensor. At the time, one option under active consideration was a two-phased program in which the radar would initially be deployed on ten conventional aircraft, with subsequent production focused on a stealth platform derived from the TACIT BLUE test aircraft. The technology surrounding Tacit Blue entered heavily into the debates surrounding the JointSTARS and the platform choices. But the inherent impossibility of making the radar truly undetectable was realized and the "unobservable/undetectable" approach to JointSTARS was abandoned. In May 1984, the Chiefs of Staff of the Air Force and Army made the final decision to put the Joint STARS radar on a 707 platform.

TACIT BLUE validated a number of innovative low observable developments and represented the first successful use of curved surfaces to achieve signature reduction instead of the two-dimensional approach employed on HAVE BLUE. The resulting aerodynamic benefit was instrumental in achieving lower drag and resultant higher speeds and longer endurance. The curved surface stealth technology developed for Tacit Blue was a key element in the design of the B-2. According to the Air Force, "TACIT BLUE was one of the most successful technology demonstrator programs in Air Force history, meeting all program objectives and most low observable and sensor performance goals." The program's radar development also led to the E-8 Joint Surveillance Target Attack Radar System (Joint STARS).

The program cost approximately $165 million and was executed under a contract to Northrop Corp. as the prime contractor. TACIT BLUE was developed and tested at several different locations and flown by both Air Force and contractor pilots. The aircraft had been in storage since 1985, and is now on display at the US Air Force Museum at WPAFB, OH.

"TACIT BLUE was a leading edge program that took innovative stealth technologies out of laboratory and onto the flightline. The team of professionals who worked on this successful program serve as an example of the what can be achieved when industry and government work together," said Arthur L. Money, Assistant Secretary of the Air Force (Acquisition).


Span 48 ft. 2 in.
Height 10 ft. 7 in.
Length 55 ft. 10 in.
Weight 30,000 lbs.
Engines Two Garrett ATF3-6 high-bypass turbofan engines
Design Operational Speed 287 mph/250 knots
Operating Altitude 25-30,000 ft.
Armament None
Crew One
Cost Approx. $165 million

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