by Brian Ford
FLYING MISSILES
Carrying artillery and ammunition to the war front is a time-consuming and tedious business. Far better, the Germans realized, for the weapons to take themselves to the front. From the start of the war — and indeed in the Spanish Civil War, which was a prelude to World War II — the Germans began to look at ways of carrying explosives by plane, and dive-bombing soon became an early strategy in planning an attack. But the bombers were vulnerable, and losses were soon rising fast. So the Germans turned to designs for planes without pilots. These ideas could be far-reaching, because — since there was no crew whose lives could be put at risk — the planes themselves would be expendable. Nothing need be omitted in the search for an answer.
Rhine Maiden and Rhine Messenger
By 1942, Rheinmetall-Borsig AG had risen to the challenge, and announced the design for their Rheintochter (Rhine Maiden) surface-to-air missile. It was a remarkable device, a two-stage surface-to-air vehicle which was named from Wagner’s famed Ring cycle. The Rheintochter was designed with a cylindrical fuselage bearing four rounded steering fins operated by servo-mechanisms. Four large swept-back fins on the first stage kept the flight of this solid-fuel rocket-powered device stable in flight. A later modification substituted a liquid fuel engine, but even this did not provide the desired performance and — although many were launched — the project was never fully operational, and it was finally cancelled in December 1944.
In 1943 development was announced of the successor to the Rheintochter — it was the Rheinbote (Rhine Messenger) and was designed by the Rheinmetall-Borsig Company. This was a design for a slender 37ft (11.4m) rocket that could deliver a modest payload over distances up to 125 miles (200km). The solid propellant was to be diglycol dinitrate and the missile would be a four-stage rocket: the first stage would launch the main rocket from the ground before being discarded; the second and third stages would fire in succession, carrying the payload aloft, and the final fourth stage would fire it into its maximum altitude where it was set on its course to the target.
However, there was a major problem with accuracy. Each of the four stages was stabilized by four fins at the aft end of the rocket, and the stages were ignited in turn as the fuel charge from the previous stage reached the end of its burn. This was clearly a rocket of limited appeal, for it consumed 2 tons of steel in manufacture, with all the concomitant requirement for energy, it demanded more than half a ton of fuel propellant, yet it could deliver no more than a 44lb (20kg) explosive to its target. It produced no fragment damage and could make a crater no more than 5ft (1.5m) across. Other projects — like Herbert Alois Wagner’s design for the Schmetterling (Butterfly) — seemed to offer far more promise and informed opinion was that the rocket was militarily valueless. This cut no ice with the High Command; the weapon could be simply understood and a four-stage device was simply too good to miss. Hitler and General Hans Kammler (who reported to Reichsführer Heinrich Himmler directly) immediately ordered production of this worthless missile. Tests were carried out, but it proved impossible to calculate the accuracy because the impact craters were so small that they could not be found.
The single advantage of the design was that stages could be removed, if the distance to be covered was reduced. Rheinbote looked spectacular, and over 200 were used against the strategically important Belgian port of Antwerp. They caused limited pockets of damage in unpredictable areas of the city, but this missile was of little use to anyone — and existed only because of the Führer’s capricious decision.
Era of the glide missile
We are all familiar with glide missiles. Although the term has the ring of modernity about it, and sounds like a state-of-the-art weapon of the twenty-first century, it is a concept that was in fact born back in World War I. It was in October 1914 that Wilhelm von Siemens proposed a revolutionary new concept that was to become the torpedo glider. In principle, it was a conventional torpedo with a primitive unmanned glider fixed above. The glider was fitted with flares to enable its course to be tracked by the attacker, and was controlled by fine wires spooled out by the controller. Siemens-Schuckertwerke had already experimented with radio-controlled attack boats, the Fernlenkboote (FL-boats) and flight testing of the proposed guided aerial torpedo began in 1915. It was intended to glide the device on course towards the target, where the glider would become detached on a signal from the operator, and the torpedo would be dropped into the sea to home in on its target. The device was just ready for production at the very end of World War I, but was not used in warfare.
With the re-emergence of pilotless planes in World War II, a reliable guidance system was now urgently needed. Infrared, the heat radiation given off by an engine, could be detected and this offered the best way for a missile to home in on an enemy aircraft. Like light, infrared travels for immense distances and in straight lines. The Germans soon realized that a steering system that homed in on the infrared given off by an engine could follow an enemy aircarft for miles.
The first in the world to use infrared tracking equipment was a missile named the Enzian (named after Gentiana clusii, the gentian flower). As we have discovered the first rocket fighter, the Messerschmitt Me-163 Komet, posed practical problems for the pilot. It had a short flight time and high speed up to 596mph (959km/h) at 39,000ft (12,000m) which made it difficult for the crew to find and attack their target in time. Designers at Messerschmitt decided to build a similar aircraft that could carry a huge payload to its target, and would dispense with the need for a pilot aboard. The Enzian would be launched from a sloping ramp with the aid of four booster rockets to attain a maximum speed of 600mph (almost 1,000km/h). It would be 12ft (4m) long and weigh 4,350lb (about 2,000kg) with a range of some 18 miles (30km).
Rather than risk losing a pilot, it was proposed to control the flight of Enzian from the ground. The operator would fly the Enzian in front of an enemy bomber and it would then detonate with great destructive force. The plan was for a bomb with a lethal radius of about 150ft (45m) which could be detonated by means of a proximity fuse. Work began in September 1943 and by May 1944 some 60 airframes had been manufactured. The remaining problem was the lack of a suitable rocket motor. Since work on the Rheintochter missile was proceeding smoothly, this engine was selected for the Enzian and modification began to produce a series of the motors for test flights. These trials proceeded well, though the proximity fuse proved problematic. At this point a remarkably simple new device, code named Madrid, was developed. It featured a light-sensitive photoelectric cell fixed in front of a steerable mirror; a series of vanes masked the cell and the signal from the target — a shadow — was always kept in the middle. The steering system in the Enzian followed the shadow in the mirror and made corresponding adjustments to the trajectory, so the target was bound to be followed. As the tide of war turned increasingly against the Germans, it was realized that there was no time to perfect the system, and for this reason the device never came into use. After the war, with the developers transported to the United States under Operation Paperclip, the work proceeded and the design was eventually perfected. It was put in use by the United States Navy as the guidance system for their AIM-9 Sidewinder missile. This is the most widely used air-to-air missile in the West, and it is said that it will remain in use for many decades to come — yet it arose from German technology in World War II.
Flying Fritz
During the Spanish Civil War of 1936–39, bombs were designed to penetrate steel which proved effective against shipping, but the Luftwaffe soon discovered how difficult it was to hit a moving vessel squarely. The idea began to form of a radio-controlled bomb that could be steered on course during its free fall, and the first experiments began as early as 1938. In 1939 the first experimental bombs were designed with tail-fins and guided with radio-controlled spoilers. These could allow the bomb aimer to control the trajectory and maximize the chances of hitting the target. The Ruhrstahl Company, already expert at design and production of bombs, was brought in
to move development towards the production stage. The result was the successful Fritz-X bomb, which was controlled by spoilers fitted to the four tail fins. It was tested in various configurations, and the cruciform tail unit proved to be most adaptable, and was eventually used for other controllable weapons of war.
In the early tests of the Fritz-X the carrier was a Heinkel He-111 and some of the He-177 aircraft were adapted to carry the weapon, though they never became operational. When the Fritz-X entered operational service it was aboard the Dornier Do-217 bomber. In July 1943 the first Fritz-X was launched in a raid against Augusta in Sicily. The following month, six of the bombers attacked the Italian fleet, which was sailing across the Mediterranean towards Malta as the Italians had signed their armistice with the Allies. This infamous Armistizio di Cassibile had been signed after the Allied successes in North Africa in 1943, after which the Allies landed in Italy, occupied Sicily and even bombed Rome. It was agreed that the Italian naval ships would transfer to Malta and the Germans became determined that they should not become available for use by the Allies. And so, on 9 September 1943, the battleship Roma was attacked by Fritz-X guided bombs dropped by the Dornier bombers. Her magazines exploded in a catastrophic blast with the death of 1,255 crew. Among them was Admiral Carlo Bergamini. Although Roma’s sister ship Italia was hit she managed to limp into port in Valetta, Malta.
Two days later a German Fritz-X attack was directed against a convoy of United States Navy vessels including the USS Savannah, one of America’s top light cruisers. Observers spotted a Dornier bomber flying towards the USS Philadelphia. A bomb aimed at the ship narrowly missed, and exploded about 50ft (15m) away. The Savannah immediately increased her speed to 20 knots (37km/h) and then saw a second Do-217 K-2 attacking out of the sun from an altitude of 18,700ft (5,700m). The gunners opened fire, but the plane was not hit and the Fritz-X could be seen flying towards the American ship, leaving a trail of smoke from its flares as it flew. Its steel-piercing design ensured the bomb struck the ship and passed straight through three decks before exploding deep inside the vessel. The blast tore a hole in the keel and ripped along the port side of the ship. Fires started in the magazines and for half an hour a continuous series of explosions prevented fire-fighters from tackling the blaze. Nearly 200 sailors were killed in the attack. The crew responded brilliantly, sealing off flooded compartments and correcting the ship’s list to port. After 8 hours of frantic activity, her boilers were relit and the ship set off to steam to Malta for emergency repairs. Four days later, four sailors were found to have survived trapped behind water-tight doors and sealed inside. After returning to the United States, it took eight months to repair the damage caused by this single guided bomb.
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.
KEEPING A SAFE DISTANCE
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.
Hs-293
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. I
nstead, 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.
