NORTHROP AP-1 "THE WHALE" (YF-117D 'TACIT BLUE')
This ungainly machine was a prototype for the B-2 bomber's technology... and almost became the J-STARS platform!

Rare ventral view of TACIT BLUE reveals unusual configuration.

Rare shot of TACIT BLUE on approach with landing gear extended.

"Shamu" on its first official presentation at AF Museum in 1996.

Customer:  DARPA/USAF
Main contractor: Northrop (design and manufacture)
Subcontractor: Scaled Composites (composite construction)

Type:  experimental stealth testbed

Program:  Battlefield Surveillance Aircraft, eXperimental (BSAX)

Powerplant: 2 x 5440 lbf. thrust high-bypass Garrett ATF3-6 turbofans

First flight: February 1982

In early April 1976, Lockheed received word that it had officially won Phase I of the XST competition. However, DARPA urged the Northrop team to remain together, and shortly thereafter it successfully submitted studies for a Battlefield Surveillance Aircraft, eXperimental (BSAX) which was to evolve into a highly-successful flight demonstration programme that provided vital data for the subsequent B-2 bomber.

The USAF, DARPA and Northrop teamed up for the TACIT BLUE Technology Demonstration Program from 1978 to 1985. The prototype validated a number of innovative stealth technology advances. Most notably, it was the first aircraft to demonstrate a low radar cross section using curved surfaces, along with a low probability of intercept radar and data link. TACIT BLUE initially 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.

The TACIT BLUE prototype was nicknamed "The Whale" or "Shamu" by the people who worked on it; the real name of the plane, if any, has never been revealed. TACIT BLUE featured a straight, tapered wing with a "Vee" tail mounted on an oversized fuselage with a curved shape. It had 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. Flight control was supplied by a quadruply-redundant, digital fly-by-wire flight system that stabilized the aircraft about the longitudinal and directional axes.

TACIT BLUE owed some of its unique shape and size to the reconnaissance equipment it was designed to carry. A Hughes multi-mode-side-looking radar (SLAR) a predecessor to the ground surveillance radar used in Joint Stars, took up a large part of SHAMU's structure. The SLAR on TACIT BLUE was part of an effort to test if a LPI (low probability of intercept) radar could be flown on a stealth aircraft without compromising its presence. ELINT (Electronic Intelligence) antennas for intercepting enemy communications were also part of TACIT BLUE's reconnaissance systems.

TACIT BLUE was developed as a potential platform for radar sensors developed under the Air Force Pave Mover and Army SOTAS programs. 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 or J-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. 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 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 aircraft made its first flight in February 1982, and subsequently logged 135 hours of flight over a three-year period. The aircraft often achieved three to four flights weekly and several times flew more than once a day.

There were a total of five pilots who flew the aircraft. The first was Dick Thomas, a Northop test pilot. The air Force pilots were Lieutenant Colonel Ken Dyson (who was also a HAVE BLUE pilot) and Lieutenant Colonels, Wes Easter, Don Cornell and Major Dan Vanderhurst. The program cost approximately $165 million and covered development, construction and flight test. As the prime contractor, Northrop received a $136 million contract to provide one complete technology demonstrator and a partially-developed back-up airframe. TACIT BLUE never went into production but yielded valuable engineering data that aided in the B-2A Spirit design. It also served as a basis for several UAVs and especially the later cancelled M-137 TSSAM tri-service weapon.

The TACIT BLUE prototype was finally declassified and unveiled to the public on April 30, 1996, at the Pentagon. It was then sent to the Wright-Patterson Air Force Museum, where it was first exhibited underneath the XB-70 'Valkyrie', and now below Boeing's "Bird of Prey".


Population: 1

Specifications:
 Length: 55.8 ft. (17.0 m)
Wingspan: 48.2 ft. (14.69 m)
Height: 10.6 ft. (3.23 m)
Start mass: 13,500 kg
Loaded weight: 30,000 lbs. (13,606 kg)
Maximum speed: 287 mph (462 km/h)
Service ceiling: 30,000 ft (9,144 m)
Thrust-to-weight: 0.36 lbf/lb (3.6 N/kg)

Crew/passengers: 1

Main sources:

  • United States Air Force fact sheet
  • United States Air Force official public presentation

One of Tacit Blue's thrust high-bypass Garrett ATF3-6 turbofans.

The Whale's air intake was original both in design and location.

Tacit Blue is currently displayed underneath Boeing's Bird of Prey.

The official patch of the Tacit Blue pilots.

Official TACIT BLUE blueprints from Northrop.