Next to be attacked was the British cruiser HMS Uganda which was struck by a Fritz-X near Salerno on 13 September 1943. The guided bomb hit at full speed and penetrated through seven decks before exploding, blowing out a section of keel. Later the battleship HMS Warspite was hit, the Fritz-X penetrating six decks and detonating in a boiler room.
At first, the attacks were thought to be administered by conventional weapons but the angular trajectory revealed by the trailing smoke soon revealed the fact that the bombs were radio-controlled. The German system involved a Kehl transmitter, operated by the attacking pilot, and a receiver aboard the bomb. This system had been designed for use on the Hs-293 (addressed below) and there was a choice of 18 Kehl/Strassburg frequencies from which to choose the command connection. As soon as the radio control had been recognized by the Allies, radio-frequency jamming began. It was not a success. The frequency was rarely selected correctly; also other simultaneously attacking aircraft would chose disparate frequencies and the defences could use only one at a time. They were quickly overwhelmed.
In time, examples of unexploded Fritz-X missiles were obtained by the Allied scientists and the control mechanism was examined closely and became better understood. After several months, the British designed and constructed their Type 650 transmitter, which worked on the common frequency of 3MHz which was subsequently used as the basic communications frequency for all transmissions to the bombs. This worked for all attacks, and did not rely on finding the command frequency for each individual weapon. Due to the Allies’ increasingly efficient counter-measures, the Fritz-X missile was no longer of use to the Germans by the time of the Normandy landings — though its career earlier in World War II was highly successful.
The Fritz-X needed adequate height to function as a weapon capable of piercing a ship’s steel decks. Its minimum release altitude was 13,000ft (4,000m) and the minimum flight distance was about 3 miles (5km) from the target. In practice, the chosen altitude was 18,000ft (5,500m). This posed a problem for the pilot of the bomber that delivered it: the plane could easily fly beyond the flight-path of the missile, losing visual contact. Pilots tended to go into a slow climb, thus reducing speed over the ground, so that they remained within sight of the missile and could steer it on its way. The Allies discovered this, of course; it meant that the bomber was now a sitting target for the anti-aircraft gunners. When this ruse worked, it worked well; pilots had a very high rate of success and the attacks on bombers that had launched a Fritz-X bomb became easier with time.
In order to provide the launching bomber with a safe distance at which to operate, German secret weapon scientists pioneered the deployment of extremely narrow wings, which offered an extended glide path. This was the Blohm & Voss Hagelkorn (Hailstone) glide bomb. It was designed by Dr Richard Vogt and the prototypes were designated the BV-226. In December 1943 the weapon went into series production as the BV-246.
Vogt’s idea was that this secret weapon could glide for extended distances, directed by radio control, thus allowing the mother ship which delivered the weapon to remain safely out of range. In the event, the control system was abandoned. The final design was for a conventionally tapered body fitted with a twin fin and rudder. The shoulder-mounted wings were revolutionary — they were extremely long and thin. To obtain strength, the main rib was constructed from steel and the profile was then laid on to produce the desired aerodynamic shape. In an era when plastics were not yet readily available, the moulded wing was made … from concrete. The Hagelkorn missile was 11ft 6in (3.53m) long with a wingspan of 21ft (6.4m). Its top speed would have been 560mph (900km/h) and it carried a warhead weighing just under 1,000lb (435kg).
The glide ratio was 1:25. It was intended that the glider would be launched from a Heinkel He-111 or Ju-88 at a cruising height of about 23,000ft (7,000m). From that altitude the glide-bomb could travel for over 100 miles (175km). Anti-aircraft fire could not touch the bomber at that distance.
In 1945 there was an urgent redesign to include an ultra short wave radio-detection device known as Radieschen (Radish). This was intended to detect enemy radar transmissions, so the weapon could home use Allied radar stations as a target. Prototypes of these homing glide bombs were constructed with a gyroscope to stabilise the flight path and with the detector built into a re-configured nose assembly. Ten were flown on a test range, eight of which failed. The two that succeeded had impacted within 6ft (2m) of the target.
About 1,000 were produced, though none were used operationally. The success of the V-1 meant that the BV-246 was superseded — though the design concept went on to find a home in high-altitude reconnaissance aircraft. The American U-2 spy plane was built to a similar design, and the novel construction of the Hagelkorn bequeathed a legacy to post-war engineers.
Since the vulnerability of the pilot had been recognized early in the war, the German engineers had set out to find an answer. And, as the Fritz-X was being developed as a controlled missile, research was directed to a flying weapon that could regulate itself rather than being steered by a pilot. In 1939 the Gustav Schwartz Propellerwerke produced designs for a glide bomb. It did not have real-time radio control, so the bomber that delivered it did not have to stay on location. Instead, it had its own onboard autopilot that flew it straight and level towards the target. This was the brainchild of Professor Herbert Wagner, chief designer at the Henschel Company, who immediately took up the project. Wagner was an Austrian aeronautical designer who was awarded his doctorate by the University of Berlin when aged 23. He decided to fit the production version with a HWK-109 rocket that could provide 1,320lb (600kg) of thrust for 10 seconds. Unpowered glider versions were first dropped from He-111 aircraft, and powered test runs had been successfully completed before the end of 1940. The finished version was the Hs-293, designed to carry a 1,100lb (500kg) bomb.
A course set by a fixed autopilot proved to be a limitation, and so a radio-controlled system was also tried. This was the highly successful 18-channel Kehl/Strassburg that had also been fitted to the Fritz-X bomb discussed earlier. Unlike the Fritz-X, the shell was contained in a conventional steel housing — this was not a steel-piercing missile. But the Hs-293 was susceptible to the increasingly sophisticated jamming techniques of the Allies, conducted not only by the British but by the Americans and Canadians as well. A particularly successful example was the MAS jammer which could intercept the signals and take over control of the Hs-293 and send it crashing into the sea. Even so, from 1942 to the end of the war, more than 1,000 Hs-293s missiles were manufactured and these glide bombs were a remarkably successful weapon of war.