THE US AIR FORCE has taken the wraps off another of its classified stealth projects with the unveiling of the Tacit Blue technology demonstrator. The Tacit Blue was used to test low-observable technologies eventually used in the Northrop Grumman B-2 bomber and other stealthy aircraft and missile programmes.

The USAF/Defense Advanced Research Projects Agency Tacit Blue stealth-technology demonstrator was successfully flown during the early 1980s, a few years after the Have Blue prototype of the Lockheed Martin F-117 stealth fighter was used to test faceted surfaces, also to achieve low observability.

Despite the relative age of the programme, the USAF remains sensitive about various aspects of the Tacit Blue. The released images have been heavily doctored to remove some features and alter others, including the rear-fuselage section and wing leading edge.

The Tacit Blue represented the first attempt at using "...curved linear or Gaussian surfaces to achieve signature reduction", says Lt Gen George Muellner, the principal deputy for the US Air Force's acquisition chief. He says that the aircraft's classified radar cross-section was smaller than that of a bat.

Initially, the project was created to demonstrate that a stealth surveillance aircraft with a low probability of intercept (LPI) radar - in this case a Hughes Aircraft ground-surveillance multi-mode radar - and other sensors could operate close to the forward line of battle with a high degree of survivability.

The Tacit Blue was an element of the Pave Mover programme which led to the Northrop Grumman E-8 Joint Surveillance Target Attack Radar System. The E-8 aircraft - a militarised Boeing 707 - carries a 7.3m-long radar antenna, which displays either moving or fixed ground targets.

Early in the programme, the USAF verified the LPI radar's capabilities, and decided that the ground-surveillance mission could be achieved by using larger aircraft patrolling 90km (50nm) back from the front line.

The USAF then decided to use the Tacit Blue as a generic testbed for low-observable technology.

The emergence of the Tacit Blue will fuel speculation as to whether it was succeeded by another manned low-observable reconnaissance platform.

The single-seat, 13,620kg, Tacit Blue was powered by two AlliedSignal (formerly Garrett) ATF3-6 high-bypass turbofan engines. A quadruple-redundant digital flight-control system (FCS), built by General Electric, was installed.

Northrop won a sole-source contract in 1978 and completed fabrication of the 17m-long Tacit Blue demonstrator at Hawthorne, California, four years later.

A second airframe shell - later scrapped - was constructed to serve as a back-up. The total programme cost $165 million, of which prime contractor Northrop received $136 million.

The aircraft had its maiden flight on 5 February, 1982, and subsequently logged 250h in 135 daylight sorties over a three-year period. Average mission duration was 2.5h. The Tacit Blue was flown at several undisclosed locations, completed its final flight on 14 February, 1985 and was then stored.

In the released images, the Tacit Blue features a straight, tapered wing with a "V'" tail mounted on an oversized fuselage with a curved shape. A single flush inlet on the top of the fuselage provided air to its two turbofan powerplants.

In the flight tests, the aircraft was found to be unstable in pitch and yaw, and relied on the GE FCS to stabilise the aircraft about the longitudinal and directional axes.

Five company and USAF test pilots flew Tacit Blue, saying that "...it responded well, but flew like a big aircraft".

The USAF says that the Tacit Blue can now be revealed because technology resulting from the secret project is in operational use in the Northrop B-2 stealth bomber. The terminated Northrop Grumman Tri-Service Standoff Attack Missile, the Lockheed Martin F-22 fighter and the Lockheed Martin Darkstar UAV also gained from the project.

Source: Flight International