The only Allied version of a radio-controlled missile appeared late in the war. This was the American Azon guided missile. It had an octagonal assembly of fins at the tail, allowing its navigational position above the ground — its azimuth — to be remotely controlled (the code name was derived from AZimuth ONly; its official designation was the VB-1, standing for ‘vertical bomb’) and it carried a 1,000lb (450kg) bomb. As in the case of the Fritz-X, a bright flare was fixed to the tail so that its trajectory could be followed by the crew of the delivery aircraft. Others followed; the range extended through to the VB-13 Tarzon which was a sophisticated glide bomb that was being constructed as World War II ended, and went on to be used in the Korean War (1950–53).
The design was originally proposed by two American engineers, Major Henry Rand and Thomas O’Donnell of the United States 458th Bombardment Group, as a means of attacking the Burma railway and was dropped from a B-24 Liberator which was specially modified for the weapon. Few were used during the war. It was the German bombs that proved the principle, and the Allied version came too late to make a difference. The version produced by the Germans established that a radio-controlled glide bomb was a viable weapon, and it has remained a mainstay of present-day warfare.
Simpler rocket weapons were also produced by the Germans. Their R4M missiles were air-to-air weapons that could be fired from pods beneath the wings of Fw-190 and Me-262 aircraft; they were used to break up groups of American bombers in the closing months of the conflict.
The United States began developing sophisticated guided rockets late in the war, and some of their designs were remarkably futuristic. The Consolidated-Vultee Aircraft Corporation designed and built the Lark missile, a surface-to-air device designed to be launched from the decks of ships with solid propellant booster rockets. Its range was up to 40 miles (65km) and it delivered an explosive payload weighing 100lb (45kg). Work on the project began in 1944, but it was not ready for use in World War II. During 1946–50 it was used to refine missile systems and was the first American surface-to-air missile to bring down a test drone in flight.
From the start of the war, the Rheinmetall-Borsig Company had carried out design work on the Hecht (Pike) anti-aircraft missile. The Hecht-2700 was conceived as an 8ft (2.5m) long missile weighing 300lb (136kg), fitted with four stabilizing fins, and designed to fly at about 500mph (800km/h). Within two years the project was ended and no missiles of this type were ever constructed.
But the ideas lived on and gave rise to the Feuerlilie (Fire Lily), on which Göring’s German Research Institute for Aviation (Deutsche Forschungsanstalt für Luftfahrt) began work in 1940. The first version attempted was the Feuerlilie (4.4 F), a scaled-down model designed to prove the principle. From this arose the F-25, produced jointly by the German Gliding Research Institute (Deutsche Forschungsanstalt für Segelflug) and the Post Office Research Department (Reichspost-Forschungsamt). Initial problems with the remote-control system were overcome by early 1943 and wind-tunnel tests proceeded according to plan. Further difficulties were experienced in developing the propulsion system, and it was not until 1943 that the first test firing was attempted at Leba, Poland, on the Baltic Sea. Within a year there had been four tests, though none of them was truly satisfactory.
By now, interest was focusing on the successor — the F-55. This was to be a radio-controlled two-stage device with a solid-fuel first stage and a supersonic liquid-fuel second stage. It would be launched somewhat like a plane, with a sloping ramp for take-off. The Ardelt Company in Breslau was given the contract for the production of five test rockets in January 1943, but problems with both the propulsion unit and the remote-control system persisted. The initial order for 25 missiles was reduced to just 11 late in 1944, and early the following year it was agreed to use the control units designed for the successful Hs-293 which were known to be reliable. Decision making continued to be unpredictable, however, and early in 1945 the entire project was scrapped.
No sooner had Professor Herbert Wagner seen his design for the Henschel Hs-293 successfully realized, than he proposed a new missile concept to the German Air Ministry. Wagner could see a tremendous future for guided missiles, and his new invention was the Hs-117 — the Schmetterling (Butterfly). This would be the next stage after the Hs-293. The idea was that two people would fly what was, in essence, a miniature exploding aircraft. The pilot would fly a Junkers Ju-188, Ju-388 or Dornier Do-217 that would be specially modified to launch the missile, while the other crewman would be solely concerned with steering it towards its target.
Schmetterling was a great advance; rather than being steered as it glided down to its target, this missile could fly wherever the controller desired and would attack an enemy 3 miles (5km) away, even if it was at an altitude far higher than the delivery aircraft. Today’s cruise missiles exploit exactly this kind of technology to the full. It was a brilliant insight, and history has shown how timely Wagner’s ingenious concept was.
Even so, it did not impress the Nazis. Hitler was convinced that he would be victorious, and he felt that his government had invested more than enough in the development of novel guided missiles. Work was scaled down and development soon stopped. The High Command changed its mind in 1943, when the large-scale attacks on Germany began to turn the tide. Hitler now wanted everything he could throw back at the Allies, so the Schmetterling was suddenly revived after all, and with the greatest urgency. During 1944 many of the test firings of prototypes revealed design problems that were methodically rectified, and by the end of the year production was set to start. The order was placed in December, with the missiles scheduled to enter service in March 1945, but by February it was clear that time was running out, and the orders were finally cancelled for good.
This futuristic Wasserfall Ferngelenkte Flakrakete surface-to-air guided missile — in English: the waterfall remote-controlled anti-aircraft rocket — was designed and developed at Peenemünde on the Baltic coast. In some ways it was similar to the V-2 — for example, it was designed to have four tail fins but was also fitted with four near the middle-point of the body to aid control in flight. However, it was one-quarter the size, measuring 26ft (7.9m) tall, with an effective range of 17 miles (27km). And, unlike its famous larger cousin, it could be radio-controlled throughout its flight so it was not a ballistic missile. Another crucial difference was that the V-2, like modern space rockets, was charged with liquid oxygen immediately prior to launch, whereas the Wasserfall was intended to rely on fuels held in store until needed in action, and could then be launched with little notice. In reality, the Wasserfall was intended to stand ready for periods of up to a month and fire on command, so the fuel chosen for the new rocket motor was to be a mixture of vinyl isobutyl ether and SV-Stoff which comprised 94 per cent fuming nitric acid and 6 per cent dinitrogen tetroxide. These were stored in tanks alongside the missile, and were forced in at speed by nitrogen under pressure.
It was the missile’s Rheinland guidance system that was particularly ingenious. This used a simple radar unit to track the target, and a transponder on the missile that was tracked by a direction finder at the launch pad. The operator would see the two spots on the screen and home the missile on the target so that the two spots merged. The rocket exhaust could be diverted from side to side by four graphite vanes placed within the exhaust gas — the same system eventually used for the V-2 — in order to establish it in controllable flight. From then on, as the graphite burned away, four ailerons on the tail fins were used to control the trajectory. Like the earlier examples we have already encountered, the radio control was achieved using the proven 18-channel Kehl/Strassburg system. A later modification was designed, in which the missile would itself home in on a radar beam that was pointed at the target (by the operator) and picked up by transponders on the Wasserfall.