Fritz X



Fritz X was the most common name for a German guided anti-ship glide bomb used during World War II. Fritz X was a nickname used both by Allied and Luftwaffe personnel. Alternate names include Ruhrstahl SD 1400 X, Kramer X-1, PC 1400X or FX 1400 (the latter is also the origin for the name "Fritz X"). Along with the USAAF's similar Azon weapon of the same period in World War II, it is one of the precursors of today's anti-ship missiles and precision-guided weapons.

History
The Fritz X was a further development of the high-explosive bomb SD 1400 (Splitterbombe, dickwandig, 1400 kg ). It was a penetration weapon intended to be used against heavily protected 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 themselves contained the aerodynamic controls. The Luftwaffe recognized the difficulty of hitting moving ships during the Spanish Civil War. ''Dipl. engineer'' Max Kramer, who worked at the DVL, had been experimenting since 1938 with remote-controlled free-falling 250 kg bombs, and in 1939 fitted radio-controlled spoilers. In 1940, Ruhrstahl was invited to join the development, since they already had experience in the development and production of unguided bombs.

The dual-axis, single-joystick-equipped Funkgerät (FuG 203) Kehl series of radio-control transmitter sets on board the deploying aircraft, were used to send the control signals to the Fritz-X, with the ordnance itself picking up the signals through a Funkgerät (FuG 230) Straßburg receiver, named for the city, within it to send the signals on to the movable spoilers in the Fritz-X's thick vertical and horizontal tail fin surfaces, within the annular tailfin structure. The Straßburg receiver's antennas were aerodynamically integrated into the trailing edge of the annular surfaces of the tailfin, within a quartet of "bulged" sections in the trailing edge.

Control setup on the Fritz X
The Fritz X possessed a spoiler-based control setup on its tailfin unit, using three sets of aerodynamic control spoiler systems, with two of them giving the ordnance control in both the pitch and yaw axes, differentially operating and oscillating under direct control from the Kehl-Straßburg radio control link. The roll control setup, operating autonomously and not under control from the deploying aircraft, were located 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 own "aileron" control surfaces, commanded by an internal gyroscope in the tail's central housing 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 ordnance did not have. The yaw control spoilers housed in the vertical tailfin surfaces were also under control through the radio link, and had similar "fence" surfaces to guide airflow over them. All three spoiler surface sets, when deployed, 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 their rate of oscillation from side to side when a control input was sent to them.

Combat service
The only Luftwaffe unit to deploy the Fritz-X was Gruppe III of Kampfgeschwader 100 Wiking (Viking), designated III./KG 100, the bomber wing itself evolved as the larger-sized descendent of the earlier Kampfgruppe 100 unit in mid-December of 1941. This unit employed the medium range Dornier Do 217K-2 bomber on almost all of its attack missions, though in a few cases toward the end of its deployment history, Dornier Do 217K-3 and M-11 variants were also used. Fritz-X had been initially tested with a Heinkel He 111 bomber, although it was never taken into combat by this aircraft. A few special variants of the troublesome Heinkel He 177A Greif long-range bomber were equipped with the Kehl transmitter and proper bombracks to carry Fritz-X and it is thought that this combination might have seen limited combat service, at least with the combinations known to have been involved in test drops.

Fritz-X was first deployed on 21 July 1943 in a raid on Augusta harbor in Sicily. A number of additional attacks around Sicily and Messina followed, though no confirmed hits were made and it appears the Allies were unaware that the large bombs being dropped were radio-guided weapons.

On 9 September, the Luftwaffe achieved their greatest success with the weapon. After Pietro Badoglio publicly announced the Italian armistice with the Allies on September 8, 1943, the Italian fleet had steamed out from La Spezia and headed to Malta. To prevent the ships from falling into Allied hands, six Do 217K-2s from III. Gruppe of KG 100 (III/KG 100) took off, each carrying a single Fritz X. The Italian battleship Roma, flagship of the Italian fleet, received two hits and one near miss, and sank after her magazines exploded. 1,255 men, including Admiral Carlo Bergamini, died. Her sister ship, Italia, was also damaged but reached Malta.

The American light cruiser Savannah was hit by Fritz-Xs at 10:00 on 11 September 1943 during the invasion of Salerno, and was forced to retire to the United States for repairs. A single Fritz-X passed through the roof of "C" turret and killed the turret crew and a damage control party when it exploded in the lower ammunition handling room. The blast tore a large hole in the ship's bottom, opened a seam in her side, and blew out all fires in her boiler rooms. Savannah lay dead in the water with the forecastle nearly awash and took eight hours to relight boilers and get underway for Malta.

Savannah's sister ship, Philadelphia, had been targeted earlier that same morning. While it is often believed the ship was hit by a Fritz X, in fact the bomb just missed the ship, exploding about 15 meters away. Damage was minimal.

The light cruiser HMS Uganda was hit by a Fritz-X off Salerno at 1440 on 13 September. The Fritz X passed through seven decks and straight through her keel, exploding underwater just under the keel. The concussive shock of the Fritz X's underwater detonation close to Uganda's hull extinguished all her boiler fires, and resulted in sixteen men being killed, with Uganda taking on 1,300 tons of water. The Uganda was towed to Malta for repairs.

Two merchant ships may have been hit by Fritz X bombs at Salerno, though the evidence is uncertain. SS Bushrod Washington was hit by a glide bomb, either a Fritz-X or a Hs 293, on 14 September while offloading a cargo of gasoline. SS James W. Marshall was set afire by a conventional bomb, Hs 293 or Fritz-X on 15 September. As with the Bushrod Washington, the nature of the weapon that damaged James W. Marshall is uncertain. A witness aboard a ship nearby, Joseph A. Yannacci, attributes the attack to Ju 87 "Stuka" dive bombers, which were too small to carry glide bombs. While an attack with a Fritz-X cannot be ruled out, there is at least an equal case to suggest that, if a glide bomb was involved, the culprit was actually a Hs 293 from II./KG 100; Luftwaffe records show that II./KG 100, armed only with Hs 293 glide bombs, was active over Salerno that day.



KG 100 scored another success with Fritz-X while the British battleship Warspite was providing gunfire support at Salerno on 16 September. One bomb penetrated six decks before exploding in number 4 boiler room. This explosion put out all fires and blew out the double bottom. A second Fritz-X near-missed Warspite, holing her at the waterline. She took on a total of 5,000 tonnes of water and lost steam (and thus all power, both to the ship herself and to all her systems), but casualties were few. She was towed to Malta by tugs Hopi and Moreno, then returned to Britain via Gibraltar and was out of action for near 9 months; she was never completely repaired, but returned to action to bombard Normandy during Operation Overlord.

The last Fritz-X attack at Salerno again lightly damaged the light cruiser Philadelphia with two near misses on 17 September. This attack is sometimes reported as taking place on 18 September. However, according to US Navy records, the cruiser Philadelphia departed Salerno the night of 17/18 September. Moreover, according to Luftwaffe records, III./KG 100, the Luftwaffe unit armed with the Fritz-X, flew its last mission on 17 September. Other ships damaged by Fritz-X included Dutch sloop Flores and destroyer Loyal.

The control system used for Fritz-X, known as Kehl-Straßburg and named for Kehl, a German suburb of Strasbourg, and Strasbourg, the French/German city on the Rhine, was also used by the Hs 293. It relied on radio contact between the bomb and the guidance unit, and was susceptible to electronic countermeasures. After the initial attacks in August 1943 the Allies went to considerable effort to develop devices which jammed the 48.2 MHz to 49.9 MHz low-VHF band radio link between the Kehl transmitter aboard the launching aircraft and the Straßburg receiver embedded in the Fritz-X ordnance. Early efforts by the U.S. Naval Research Laboratory produced the XCJ jamming transmitter installed aboard the destroyer escorts USS Herbert C. Jones and Frederick C. Davis in late September 1943, too late for Salerno. The XCJ was ineffective because the frequencies selected for jamming were incorrect. This was updated in time for combat at Anzio with the XCJ-1 system, installed aboard two destroyer escorts above as well as destroyers USS Woolsey, Madison, Hilary P. Jones and Lansdale. These six ships rotated service at Anzio, with three deployed at any time. This manually operated system met with some success, though cumbersome and easily overwhelmed if large numbers of weapons were deployed simultaneously.

In early 1944 the UK began to deploy its Type 650 transmitter, which employed a different approach. This system jammed the Straßburg receiver's intermediate frequency section, which operated at a 3 MHz frequency and appears to have been quite successful, especially as the operator did not have to attempt to find which of the 18 Kehl/Straßburg command frequencies were in use and then manually tune the jamming transmitter to one of those frequencies. This system automatically defeated the ordnance's receiver regardless of which radio frequency had been selected for an individual Luftwaffe missile.

Following several intelligence coups, including a capture of an intact Hs 293 at Anzio and recovery of important Kehl transmitter components from a crashed Heinkel He 177 on Corsica, the Allies were able to develop far more effective countermeasures in time for the invasions at Normandy and Southern France. This included an updated XCJ-2 system from the U.S. Naval Research Laboratory (produced as the TX), the modified airborne AN/ARQ-8 Dinamate system from Harvard's Radio Research Laboratory, NRL's improved XCJ-3 model (produced as the CXGE), the Types MAS system produced by the Airborne Instruments Laboratory (at the time affiliated with the Radio Research laboratory), the British Type 651 and the Canadian Naval Jammer. Even more sophisticated jammers from NRL, designated XCK (to be produced as TY and designated TEA when combined with the upgraded XCJ-4) and XCL, were under development but were never deployed as the threat had evaporated before they could be put into service.

By the time of Normandy landings, a combination of Allied air supremacy, keeping bombers at bay, and ship-mounted jammers meant the Fritz-X had no significant effect on the invasion fleet. Some accounts say the Norwegian destroyer Svenner was hit by Fritz-X at dawn on D-Day. This is highly unlikely as III./KG 100, the unit which carried the Fritz-X into combat, had largely been re-equipped with the Hs 293 missile by that time for its anti-ship missions, and the attack on Svenner occurred before the first glide bombers launched their assaults on the Normandy beaches.

Fritz-X is often incorrectly listed as having been responsible for the loss of the hospital ship HMHS Newfoundland at Salerno as well as the destroyer HMS Janus and the light cruiser HMS Spartan at Anzio. However, these ships were hit by Hs 293s, as clearly demonstrated by a careful analysis of Luftwaffe records regarding the deployment of III./KG 100, the nature of the damage inflicted, as well as reports from witnesses. (In the case of Janus, either an Hs 293 or a conventional torpedo was responsible.)

The closest Allied equivalent to Fritz-X was Azon.

Combat procedure
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 ordnance, the Fritz-X bomb 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 self-contained 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 toward the target on a steady course and that as the missile neared its target it became possible to misguide by jamming its radio channel.

Unlike the Hs 293, which was deployed against merchant ships and light escorting warships, Fritz X was intended to be used against armoured ships such as heavy cruisers and battleships. The minimum release height was 4000 m and a release height of 5500 m was preferred assuming adequate visibility. The Fritz X had to be released at least 5 km from the target. The plane had to decelerate upon bomb release so momentum would carry the bomb in front of the aircraft where the bombardier could see and guide it. This deceleration was achieved by making a steep climb and then level out. The bombardier could make a maximum correction of 500 m in range and 350 m in bearing. The bomber was vulnerable to fighter attack as well as ship-based air defense 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.

Accuracy is the main reason for developing a weapon system of this kind, rather than continuing to use so-called "dumb bombs". A skilled bombardier could manage to guide 50% of the bombs to within a 15 m (50 ft) radius of the aiming point, and about 90% hit within a 30 m (100 ft) radius. (Other sources say 60% hits within 4.6 meters radius.)

Operators

 * 🇩🇪 Germany
 * Luftwaffe