The Enzian (named for a genus of mountain flower, in English the Gentian) was a German WWII surface-to-air anti-aircraft missile that was the first to use an infrared guidance system. During the missile's development in the late stages of the war, it was plagued by organisational problems and was cancelled before becoming operational.
As early as 1943 it was becoming clear that Messerschmitt's super-interceptor, the Me 163, would be difficult to use in combat. After flying to the 25,000 ft altitude of the US bombers, it had only a few minutes to track them down before running out of fuel. Even if it did manage to find the bombers, it had the equally prickly problem of lacking a weapon that could be aimed effectively while closing on the target at some 400 mph difference in speeds.
Dr Wurster of Messerschmitt suggested the answer in the form of Flak Rakete 1 in 1943. Instead of tracking down the planes, the rocket would fly just in front of the target and then detonate a gigantic 500 kg warhead. With no human pilot on board, and thus no need to limit takeoff acceleration, the rocket could use solid fuel boosters for added thrust, reducing the amount of fuel needed for the rest of the climb. The result, even with this heavy warhead, was that a much smaller airframe was needed to carry the required fuel - so small that the design could be portable and launched from a modified 88 mm gun gun mounting.
The design made as much use of wood as possible, due to the need to conserve other "strategic" materials in the rapidly deteriorating war situation. For the same reasons a new type of Walter rocket was envisioned, a modification of the engine in the Me 163 that would burn coal-gas (benzine) instead of the hydrogen peroxide used in other Walter designs (although a small amount of peroxide was used to drive the fuel pumps).
The missile would be guided primarily under radio control from the ground. The operator would fly the missile into the vicinity of the bombers, then cut the engine and let it glide. This presented a real problem in the Enzian. Other German missiles were high-speed designs that could be flown directly at their target along the line of sight, easy enough to do even from the ground. The Enzian would instead be approaching its target from somewhere in front, which is considerably harder for the operator. Many experiments with radio and wire-guided missiles had demonstrated real problems with last-minute terminal guidance corrections.
The initial plans for solving this problem were rather advanced. The large airframe left plenty of room in the nose, which they intended to fill with a self-contained radar unit called Elsass. In the short term it was planned to use some sort of proximity fuze while flying the missile through the bomber stream. The warhead, of which several were studied, was to have a nominal lethal radius of 45 metres.
Several studies on the basic design were carried out, resulting in the FR-1 through FR-5. The FR-5 was considered to be a reasonable starting point, so development commenced on the newly-named Enzian E.1 (and its engine) in September 1943. By May 1944 60 airframes were complete, awaiting their engines. In order to gain flight test data they were fitted with RATO units instead.
A series of 38 flight tests commenced with generally favourable results, but the engine still lagged. Finally Dr. Konrad, designing the engine for the Rheintochter missile, was asked to modify his design for the Enzian. After study, it appeared this was a much better (and cheaper) solution anyway, and after January 1945 there were no plans to use the Walter design. The resulting E.4 version with the Konrad engine was considered the production version.
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