Fritz X

Fritz X was a German guided anti-ship glide bomb used during World War II. Developed alongside the Henschel Hs 293, Fritz X was one of the first precision guided weapons deployed in combat. Fritz X was a nickname used both by Allied and Luftwaffe personnel. Alternative names include Ruhrstahl SD 1400 X, Kramer X-1, PC 1400X or FX 1400 (the latter, along with the unguided PC 1400 Fritz nickname, is the origin for the name "Fritz X").

Fritz X was a further development of the PC 1400 (Panzersprengbombe, Cylindrisch 1,400 kg) armour-piercing high-explosive bomb, itself bearing the nickname Fritz. It was a penetration weapon intended to be used against armoured targets such as heavy cruisers and battleships. It was given a more aerodynamic nose, four stub wings, and a box-shaped tail unit consisting of a roughly 12-sided annular set of fixed surfaces and a cruciform tail with thick surfaces within the annulus, which contained the Fritz X's aerodynamic controls.

The Luftwaffe recognised the difficulty of hitting moving ships during the Spanish Civil War. German engineer Max Kramer, who worked at the Deutsche Versuchsanstalt für Luftfahrt (DVL), had been experimenting since 1938 with remote-controlled free-falling 250 kg (550 lb) bombs and in 1939 fitted radio-controlled spoilers. In 1940, Ruhrstahl AG [de] was invited to join the development, since they already had experience in the development and production of unguided bombs.

Fritz X was guided by a Kehl-Straßburg radio control link, which sent signals to the movable spoilers in the thick vertical and horizontal tail fin surfaces, within the annular tail fin structure. This control system was also used for the unarmoured, rocket-boosted Henschel Hs 293 anti-ship ordnance, first deployed on 25 August 1943. The Straßburg receiver antenna installations on the Fritz X were aerodynamically integrated into the trailing edge of the annular surfaces of the tail fin, non-metallically encapsulated within four "bulged" sections in the trailing edge. This design feature of the FuG 230 Straßburg receiver installation is like the Azon (US contemporary guided bomb), which had its receiving antennas placed in the four diagonal struts bracing the fixed sections of its tail fins.

Minimum launch height was 4,000 m (13,000 ft) – although 5,500 m (18,000 ft) was preferred – and a range of 5 km (3.1 mi; 2.7 nmi) was necessary. As it was an MCLOS-guidance ordnance design, the operator had to keep the bomb in sight at all times (a tail flare was provided, as with the Azon, to assist the operator in tracking the weapon) and the control aircraft had to hold course, which made evading gunfire or fighters impossible. Approximately 1,400 examples, including trial models, were produced.

The Fritz X possessed a spoiler-based control setup on its tail fin unit, using three sets of aerodynamic control spoiler systems, with two of them giving control in the pitch and yaw axes, differentially operating and constantly oscillating rapidly under direct control from the Kehl-Straßburg radio control link. The roll control setup, operating autonomously and not under control from the deploying aircraft, oscillated similarly to those under the externally controlled sets, and were on the outboard sections of the horizontal tailfin surfaces within the annular set of outer tailfin surfaces. These were like the American Azon ordnance's aileron control surfaces in their purpose, commanded by an internal gyroscope in the tail's central housing in both the Azon and Fritz X, to keep the ordnance level during its trajectory. The inboard set of spoiler surfaces in the tailfin's horizontal surfaces, which used a set of wing fence-like flat surfaces for airflow separation from the autonomous roll control spoilers, controlled the pitch angle after release and were controlled by the radio control link, giving the Fritz X's bombardier in the deploying aircraft the ability to control the range of the drop, a capability that the Azon did not have. The yaw control spoilers housed in the vertical tail fin surfaces were also under control through the radio link, and had similar "fence" surfaces to guide airflow over them. All three spoiler surface sets barely protruded from the surface during operation, with the pair of spoiler systems under external control having a degree of "proportionality" in their operation by varying the "dwell time" spent on one side or the other during their rapid rate of oscillation from side to side when a control input was sent to them.

The Fritz X was steered by the bombardier in the launching aircraft over a radio link between the aircraft's Kehl transmitter and the weapon's Straßburg receiver. The bombardier had to be able to see the target at all times, and like the Azon, the Fritz X had a flare in the tail so it could be seen from the controlling aircraft for its MCLOS-form guidance to control it properly. The disadvantage with this — in comparison to fully autonomous-guidance glide bombs like the operational U.S. Navy's Bat radar-homing glide bomb, used against Japan in 1944–45 — were that the aircraft had to be flown towards the target on a steady course and that as the missile neared its target it became possible to misguide it by jamming its radio channel.

Unlike the Hs 293, which was deployed against merchant ships and light escort warships, the Fritz X was intended to be used against armoured ships such as heavy cruisers and battleships. The Fritz X had to be released at least 5 kilometres (3 mi) from the target. The plane had to decelerate immediately after bomb release so the bombardier could see the bomb and guide it; this deceleration was achieved by making a steep climb and then levelling out. The bombardier could make a maximum correction of 500 metres (1,600 ft) in range and 350 metres (1,150 ft) in bearing. The bomber was vulnerable to fighter attack and ship-based air defence weapons while maintaining a slow, steady course so the bombardier could maintain visual contact to guide the bomb. When working properly, the missile was able to pierce 130 mm (5.1 in) of armor.