ANCILLARY AIRCRAFT EQUIPMENT | travisheritagecenter

ANCILLARY
AIRCRAFT
EQUIPMENT

Ancillary equipment refers to individual units or devices that form part of a larger system, each essential to enabling the aircrew and aircraft to successfully complete its mission. From World War II through the Cold War, advances in airborne electronics transformed aviation from visual flying into a discipline of precision navigation, reliable communication, and accurate weapons delivery. Systems such as the AN/ARG-41 radar and the ART-13 and BC-375-E radios expanded aircraft awareness and command capability across oceans and continents. Precision instruments like the K-3 computing gunsight and the legendary Norden bombsight revolutionized air combat and bombing accuracy. In the jet and nuclear age, the B-52 MD-1 Astro Navigator, Polar Converter, and Tracking Computer enabled truly global operations—including transpolar missions—independent of ground-based navigation aids. Together, these technologies illustrate the evolution of airborne systems that allowed aircrews to fly farther, communicate more effectively, and strike with unprecedented accuracy.

AN/APG-41 RADAR

The AN/APG-41 radar was mounted above the B-47 Stratojet’s tail gun turret and was used to detect, track, and target incoming enemy aircraft, providing firing data directly to the gunner. Early performance estimates for the prototype B-47 suggested the bomber would be fast enough to match or outrun contemporary fighter designs still on the drawing board. As a result, its defensive armament was limited to a single tail turret, initially armed with two .50-caliber AN/M2 Browning machine guns, later replaced by two 20 mm cannons, all intended to be directed by an automatic radar-controlled fire-control system.

The B-47 was designed with a three-man crew housed in a pressurized forward compartment. The pilot and copilot sat in tandem beneath a long, fighter-style bubble canopy, while the navigator/bombardier occupied a compartment in the nose. The copilot also served as the tail gunner, operating the remotely controlled, radar-directed turret, while the navigator doubled as bombardier.

The tail armament was contained in the M24A1 remotely operated turret, mounted at the extreme rear of the fuselage. During the early years of the Cold War, the B-47 and its tail gun system were key elements of the United States’ strategic nuclear deterrent, providing long-range strike capability against potential adversaries. However, the system was constrained by the relatively primitive radar technology of the era, rapid improvements in enemy interceptor performance,

and evolving air combat tactics. As the Cold War progressed, newer strategic bombers—most notably the B-52 Stratofortress—replaced the B-47 in U.S. Air Force service, relying increasingly on speed, altitude, and electronic countermeasures rather than traditional tail gun defenses.

The closest the B-47 came to direct combat occurred during RB-47 reconnaissance operations. These aircraft flew thousands of high-risk “ferret missions,” designed to provoke and locate Soviet radar and air defense systems. In the event of war with the Soviet Union—a possibility that appeared increasingly real during this tense period—the intelligence gathered on these flights would have been critical. At least five RB-47s were fired upon, and three were shot down. RB-47 crews returned fire using their tail turrets, although it remains uncertain whether any enemy aircraft were destroyed. These engagements represent the only confirmed instances of B-47s firing their weapons in combat.

On 1 July 1960, an RB-47H reconnaissance aircraft was shot down by a Soviet MiG-19 while operating in international airspace over the Barents Sea. The copilot reported that the MiG-19 successfully jammed the aircraft’s MD-4 fire-control system radar, causing the gunsight display to “white out” and rendering the tail turret ineffective. This incident starkly demonstrated a key vulnerability of radar-dependent defensive systems and underscored the growing importance of electronic warfare in modern aerial combat.

BC-375-E

ART-13 RADIO

The AN/ART-13 radio transmitter was developed by Collins Radio Company in the late 1930s for military aviation use. Introduced just prior to World War II, it quickly became the standard high-frequency (HF) transmitter used by the U.S. military. Designed for reliability and flexibility, the ART-13 supported both voice (AM) and telegraphy (Morse code) communications, making it suitable for a wide range of operational requirements.

The ART-13 was commonly installed aboard U.S. military aircraft, including bombers and transport aircraft, where it provided essential long-range communications for missions such as strategic bombing, transport operations, and command and control. Its rugged construction and dependable performance made it especially valuable in the demanding conditions of combat. The transmitter is easily recognized by its large tuning dial and analog controls, characteristic of mid-20th-century radio technology.

Technically, the AN/ART-13 operated in CW, MCW, and AM modes and covered low-frequency (LF), medium-frequency (MF), and high-frequency (HF) bands up to 18.1 MHz. It featured ten preset, autotuned VFO channels, allowing crews to rapidly select assigned frequencies in flight. Power output was approximately 100 watts, using an 813 vacuum tube as the final amplifier. Under favorable atmospheric conditions, the ART-13 enabled communications between aircraft and ground stations separated by thousands of miles.

After World War II, the AN/ART-13 remained in service with the U.S. military and was also adopted by other nations. The Soviet Union produced near-identical copies, designated RSB-70 and R-807, for use in their military aircraft. It is widely believed that these copies were based on ART-13 units recovered from U.S. B-29 bombers that made emergency landings in Soviet territory during the war—an event mirrored by the Soviet reverse-engineering of the B-29 itself, which resulted in the Tupolev Tu-4.

As radio technology advanced, the AN/ART-13 was eventually phased out and replaced by more modern solid-state and digital communication systems, marking the transition from vacuum-tube aviation radios to the next generation of airborne communications.

BC-375-E RADIO TRANSMITTER

The BC-375-E U.S. Army radio transmitter, shown here mounted above a TU-7-B transmitter tuning unit, was manufactured by General Electric and installed in many World War II–era bomber aircraft, including the B-17 Flying Fortress and B-24 Liberator. Its design was derived from the BC-191F ground-based radio, modified specifically for airborne use.

Commonly installed in B-17s, the SCR-287 radio set—a combination of the BC-375 transmitter and the BC-348 receiver—was fitted to thousands of bombers. In a typical B-17 installation, the transmitter was mounted beneath the radio operator’s table, while the receiver sat on top. Other aircraft installations placed the transmitter on the opposite side of the fuselage from the radioman’s position. While layouts varied from aircraft to aircraft, they consistently reflected the greater importance of receiving over transmitting during combat operations.

Most BC-375 transmissions were extremely brief—often just a word or two—and nearly all were sent using continuous wave (CW) Morse code. For the first decade or more of military aviation radio operations, Morse code was the primary means of communication; voice transmission was rarely used. The BC-375-E served primarily as a liaison radio, allowing bombers to communicate with command bases on the ground. To reduce weight and complexity, only the tuning units necessary for the specific mission frequencies were carried aboard bombing missions. Other aircraft types, such as transports or search-and-rescue planes, often carried additional radio equipment to support more varied communication requirements.

By the later years of World War II, the BC-375 had become increasingly obsolete for bomber use. The decision to manufacture approximately 100,000 units reflected early-war planning assumptions. Midway through the war, engineers at Collins Radio Company introduced the far more advanced ATC/ART-13 transmitter. This newer system delivered greater RF power in a smaller package, required fewer tuning units for frequency flexibility, featured automatic tuning, and provided excellent audio quality.

As a result, ART-13 transmitters were installed in newer bomber aircraft even while BC-375 units were still being delivered. By the end of the war, many BC-375 transmitters remained unopened in storage, considered obsolete for bomber operations. However, this assessment did not apply universally. The BC-375 continued to see service in transport aircraft and search-and-rescue planes well after World War II. Though no longer state-of-the-art, it remained reliable, durable, and well-suited to those mission profiles.

B-52 MD-1 ASTRO NAVIGATOR

Before the advent of GPS, one of the primary navigation methods for ships, aircraft, spacecraft, and missiles was celestial navigation, also known as astronavigation. This technique relied on measuring angles to the Sun or selected reference stars and calculating position using highly accurate astronomical data that defined the precise locations of these bodies at known times.

In early U.S. Air Force aircraft—such as the first B-52 bombers—celestial navigation was performed manually by a navigator using a sextant, sighting through a dedicated astrodome in the top of the aircraft. In the late 1960s, this manual process was replaced on the B-52E and B-52F models by an automatic astro-tracking system coupled to an analog computer. This system automatically identified stars, measured their positions, and continuously updated the aircraft’s inertial navigation system (INS).

Although GPS would eventually supersede astro-navigation for most applications, these systems remained in service for decades. A maintenance tag dated 1983 on this unit indicates that automated astro-tracking was still operational well into that period.

Automatic star trackers were widely used on other aircraft, including the SR-71 Blackbird, RC-135 reconnaissance aircraft, B-58 Hustler, and P-3 Orion maritime patrol aircraft. Notably, even the most modern U.S. Air Force bomber, the B-2 Spirit, retains an automatic star-tracking navigation system as a GPS-independent backup.

Automated celestial navigation has also played a critical role in strategic and tactical missile systems. Missiles such as Polaris, Poseidon, Trident, MX (Peacekeeper), SM-62 Snark, and AGM-28 Hound Dog employed star-tracking systems to correct inertial drift and ensure long-range accuracy—an approach that remains relevant for systems designed to operate in GPS-denied environments.

B-52 POLAR CONVERTER

The Polar Converter is a specialized analog computer designed to rapidly and accurately convert rectangular (Cartesian) coordinates into polar coordinates. It is a critical component of the B-52 Stratofortress bombing navigation system, designated AN/ASB-15, and interfaces directly with the aircraft’s navigation, targeting, and autopilot systems to ensure precise coordination during bombing operations.

In aviation—particularly during strategic bombing missions—precise target location is essential for mission success and for minimizing unintended damage. Aircraft sensors and tracking computers typically determine a target’s position in terms of horizontal and vertical displacement relative to the aircraft, expressed as rectangular coordinates. While suitable for raw measurement, these coordinates are inefficient for real-time navigation and weapons delivery.

The Polar Converter transforms this rectangular data into polar coordinates, which are more practical for bombing computations and better account for Earth’s curvature. Continuously updated inputs—including true airspeed, aircraft heading, distance to target, and wind velocity—are supplied to the Polar Converter by the tracking computer. Using these inputs, the system solves the bombing problem in real time.

As a result, the AN/ASB-15 system provides:

Time-to-go information prior to bomb release
Heading error indications to both the pilot and the bombardier
Automatic steering corrections to the autopilot to guide the aircraft to the release point
Automatic bomb-door opening at the proper moment
Precise weapon release at the calculated release point

Together, these functions enabled the B-52 to deliver weapons with a high degree of accuracy, even under adverse conditions, long before the advent of digital computers and satellite navigation.

Polar Converter cut-a-way

B‑52 TRACKING COMPUTER

The tracking computer is a core component of the AN/ASB‑15 bombing and navigation system. It incorporates mechanical computing mechanisms used to determine the data required to solve both navigational and bombing problems.

The tracking computer continuously maintains and updates critical parameters, including airspeed, aircraft heading, distance to target, and wind velocity. Using these inputs, it computes solutions to navigation and bombing problems and sends them to the ballistics control unit. The ballistics control then applies any required offsets before forwarding the corrected signals to the polar converter.

As part of the bombing solution, the tracking computer performs several critical functions. It displays time‑to‑go until bomb release and heading error to both the pilot and the operator, supplies heading‑error signals to the autopilot for automatic steering to the bomb release point, commands the bomb doors to open at the proper time, and initiates weapon release at the precise moment required.

K-3 COMPUTING GUN SIGHT

The Sperry K‑3 is a computing gun sight used in the Sperry upper gun turrets installed on the B‑17 bomber. The K‑4 is mechanically identical but fitted with a different mounting arrangement for use in the Sperry lower gun turrets on the B‑17.

Figure 1 shows two K‑3 sights along with a computing element from a third unit. The remaining photographs provide various views of these devices. Figure 9 reproduces a page from the Gunner’s Information File, illustrating the lower turret with the K‑4 sight prominently located at the center.

To operate the sight, the gunner first inputs range information by estimating the size of the attacking aircraft and adjusting its image in the optical sight until it fits within the reticles. The gunner then tracks the target by moving the K‑3—mounted on a movable head—keeping the aircraft image centered in the reticles.

These sight movements drive the internal computing unit, which uses the entered range data and built‑in ballistic information to calculate the required deflection (lead). The system then automatically positions the turret so that the guns are aimed ahead of the target, allowing the gunner to maintain continuous tracking while the sight computes the correct firing solution.

K-3 computing gun sight

Gun Sight in Turrret

NORDEN BOMBSIGHT

The Norden M‑series was a bombsight used by the United States Army Air Forces (USAAF) and the United States Navy during World War II, and later by the United States Air Force in the Korean and Vietnam Wars. It was an early tachometric design that directly measured the aircraft’s ground speed and direction—parameters that earlier bombsights could only estimate through lengthy manual procedures. The Norden improved upon previous designs by incorporating an analog computer that continuously recalculated the bomb’s projected impact point as flight conditions changed, along with an autopilot interface that could rapidly and accurately correct for wind and other disturbances.

Under combat conditions, the Norden bombsight did not achieve its expected level of precision. In 1943, it yielded an average circular error probable (CEP) of approximately 1,200 feet (370 m)—meaning that 50 percent of bombs fell within 1,200 feet of the intended target. This performance was comparable to that of other Allied and German bombing systems of the period. As a result, both the U.S. Navy and the Army Air Forces abandoned the concept of routine pinpoint bombing. The Navy shifted to dive bombing and skip bombing for attacks against ships, while the Army Air Forces developed the lead bomber formation method to improve overall accuracy and increasingly adopted area bombing tactics using large bomber formations.