For the "8-117" Hs.117 "Schmetterling", Professor Wagner tasked J.J.Henrici to assemble a production team. The Henschel works in Berlin was collaborating with a whole raft of other German design and manufacturing organisations: Walterwerke, BMW and Rheinmetall-Borsig and Schmidding for motors, but also Opta Radio, Siemens, Askania, AEG, Telefunken and Horn for other electronic and mechanical control systems, and the test and research institutes at DVL, AVA, DFS and others.
With a wealth of experience from the Hs.293 Glide Bomb, Wagner's team had decided on a line-of-sight controlled missile, small enough to be manhandled by a ground team, but carrying a warhead capable of rendering a B.17 unserviceable from a distance of an approximately 8 yard proximity burst. The Henschel Hs.117 was designed for production with electronic receiver E232 a/b "Colmar", and proximity fuses "Kakadu" (from Donag), "Marabu" (Siemens) or "Fox" (AEG). Although an electronic guidance system would have been preferred, it was recognised that development time, and current system accuracy would render the project unfeasable. So an optical guidance system which was proven to work was chosen for the first production series, knowing that this could be upgraded to a more sophisticated guidance system when available.
To enable the best performance, the Hs.117 was designed to fly under, but as close to the speed of sound as possible whilst being able to retain good control and manouverability. Initially Henschel promised a speed of 75% of the speed of sound to the RLM, with the intention of increasing this speed during development. Depending on the ability of the missile's aimer, Henschel were predicting an initially oscillating flight path whilst the target was acquired, with a final straight flight to target. A manouvering limiter was included to keep acceleration to about 7.5G. Control was effected by oscillating spoilers or "Wagnervators" (shown here on the left), which during flight, oscillated evenly at the trailing edge of the wing, until a control signal from the ground caused a greater deflection above or below the wing to induce a roll in the appropriate direction and a so a course correction.
In order to ensure predictibility of flight close to the speed of sound, the airframe was designed to be as symmetrical as possible. However, one asymmetry was the "double" nose with the electrical generator on one aspect and the aerial for the proximity fuse the other. The second asymmetry was the shape of the fuselage at the tail. Tests were made in the high speed wind tunnel of the DVL up to 90% of the speed of sound and the initial design of a square tail was found to give lack of control at higher speeds. As the manufacturing of control components was already committed, a thinner tail could not be used, so interestingly, a tapered tail was used for achieving greater speeds. The production missiles were also carefully examined to remove sources of surface defect, for best speed.
As previously mentioned, the Hs.117 project was designed to be manhandled from storage to launch. An Hs.117 battery consisted of two sets, each with six launching stands. Both sets had one aiming stand with one observer and one aimer seated on a special gimble-mounted frame, who could follow the tracks of missiles through the air with special telescopes. Under command, the observer directed his telescope onto the target, which was linked to the aimer's telescope. When ready, the aimer would launch the missile, and with the help of the observer continuing to track the enemy, would direct the missile to the specified target until the proximity fuse exploded the warhead.
The first Schmetterling tests, from the ground and from air-dropped launches, began in May 1944, with the first rocket motor test flights in August 1944. Tested at Peenemunde, approximately sixty launches were made - around twenty reached speeds above Mach 0.90, without noticeable deterioration in performance. At first it was hoped that the missile could be seen by the light of the motor alone for up to ten miles. But even when tested with coloured dyes in the fuel, or dropped into the rocket efflux, the flare from the rocket motor alone was insufficient for directing the missile. Therefore, the rear fuselage was modified to carry flares, as were used in the Hs.293.
The plan was to have started production in February 1945, with a production output of 3,000 units a month by October 1945. However, the project was experiencing significant delays. The most serious of the delays was caused by the BMW motor. Motor production was late, numbers produced were small and the thrust available was not to design specification. In order to move forwards, Dr Schmidt of Walterwerke proposed a motor design with a different regulator, much improved performance and a new, lighter, uncooled combustion chamber. There were still concerns about the delivery of fuel from the tanks, and the introduction of air into the fuel lines during manouvering, but it was hoped that these would be resolved during testing. For a discussion of the design issues of the 109-558 and 109-729 motors for "Schmetterling", follow this discussion link.
In the end, the testing process was not completed before the war came to an end, and so "Schmetterling" never managed production to any degree, and never saw active service.