HS 117 "Schmetterling"
Design and Performance Data
Length Overall: 429 centimeters
Length of Fuselage: 369 centimeters
Span of Wings: 200 centimeters
Height of Vertical Rudder: 92 centimeters
Diameter of Fuselage: 35 centimeters
Weight (Empty): 150 kilograms
Launching Weight: 430 kilograms
Weight at Target: 175 kilograms
Weight of Assisted Take-off Units: 180 kilograms (90 kg per)
Main Power Fuel: 12.7 kilograms
Main Power Oxidizer: 59.2 kilograms
Compressed Air: 3 kilograms
Explosive: 25 kilograms
Maximum Velocity: 250 meters per second
Average Velocity: 240 meters per second
Maximum Range: 20 kilometers
Service Ceiling: 10.5 kilometers
Absolute Ceiling: 13 kilometers
Main Propulsion System
The main power unit is a liquid rocket with a pressurized fuel feed and a variable automatic thrust.
Main Power Fuel (Tonka): 12.7 kilograms
Main Power Oxidizer (Salbei): 59.2 kilograms
Compressed Air: 3 kilograms
Charged Weight: 159 kilograms
Weight (Empty): 80 kilograms
Total Launching Impulse: 14,000 kilograms per second
Main Thrust Unit Impulse: 12,500 kilograms per second
Total Impulse: 26,500 kilograms per second
Launching Thrust: 3,00 kilograms per second
Maximum Thrust of Main Unit: 380 kilograms
Minimum Thrust of Main Unit: 60 kilograms
Burning Time of Main Unit: 40 to 90 seconds
Maximum Combustion Chamber Pressure: 40 atmosphere
Average Thrust: 220 kilograms
Average Time of Burning: 57 seconds
Overall Length of Main Thrust Unit: 17 inches
Overall Diameter: 4 and 1/2 inches
Throat of Nozzle Diameter: 1 and 7/16 inches
Exit of Nozzle Diameter: 2 and 7/8 inches
Description: This missile, probably better known as the "Schmetterling", is a rocket propelled, radio-controlled, missile for use against bomber formations. Some versions are for ground-to-air and some for air-to-air operations. The fuselage is of the conventional type mounting a single vertical tail surface and a single horizontal surface. The horizontal surface is mounted well above the center line of the fuselage. The arrowhead wing is mounted on the center line of the fuselage. The forward section of the fuselage is non-symmetrical in order to accommodate a proximity fuze and a propeller-driven generator.
This missile is launched from the ground from a simple two armed zero length rail launcher. The two arms support the missile at the wing roots at about the center of gravity. Two assisted take-off units are used for ground-launching, one above and one below the fuselage. At the end of burning these ATO units are jettisoned by means of explosive bolts which throw them clear from the airframe. The air launched version does not use ATO units but is simply dropped from a standard bomb shackle.
Some of these models are automatically homed and some are remote controlled. A single gyro automatic pilot is used for stabilization.
Details
Airframe - Type and Description: The main units of the fuel system form the backbone of the fuselage. At the forward end is the steel air flask nested into the forward end of an aluminum Salbei tank. Next comes an aluminum casting through which passes the main wing spar, and which is also used to space the hydrocarbon tank further aft for the proper distribution of the C.G. of the fuel since it is desired to have the fuel C.G. coincide with the C.G. of the entire aircraft. On the after end of the hydrocarbon tank another aluminum casting is bolted which supports the main propulsion unit and the air stabilizer structure. All of these tanks and castings are securely bolted together, forming the entire backbone of the aircraft. This structure is covered with sheet aluminum. The nose section, also formed of aluminum, is screwed to the forward end.
The wing and tail are built-up sections consisting of a cast magnesium frame with an aluminum covering. The case magnesium wing frame is extremely light in weight and is rigid.
Airframe - Aerodynamic Characteristics or Peculiarities: This missile is controlled in roll by spoilers of the trailing edges of the wings. These spoilers work out of phase with one another. Yaw is controlled by the trailing edge spoilers on the wings.
Spoiler control was used for this model because, as in other missiles, it gave adequate and easy control as compared to other systems and produced less drag. The spoilers also present a much more simple method of control than other systems.
This missile is rather unique in that assisted take-off rockets are place both above and below the fuselage mounted in the vertical plane of the center line of the fuselage. It is also interesting to note the extent of asymmetry of the forward portion of the fuselage. It was considered by the designer, Prof. Wagner, that horizontal asymmetry would be less harmful than vertical asymmetry, the asymmetry being necessary to correctly place the generator propeller and the fuze.
For reasons of stability, the missile, upon ground launching, makes at least one complete roll about its longitudinal axis. If at the end of this roll the air speed is not great enough for adequate stability, it will make a second complete roll. Seldom, however, is the second roll necessary. This roll event is built into the control equipment and is entirely automatic. During launching, the acceleration is about 8g.
Operation
Because aerodynamic characteristics at high speed change rapidly with small changes in speed, to make the control problems as simple as possible, it was decided that the speed of the missile should be held as nearly constant throughout the flight as was practicable. This was accomplished by automatically regulating the thrust output of the main jet in relation to the velocity of the missile, 240m/s, being the arbitrary average velocity. This regulation was accomplished by balancing the two pressures taken from a pitot static tube across two opposed aneroid barometric elements which, in turn, by means of electrical contacts, actuate an electric motor which operates a fuel control valve limiting the flow of fuel to the combustion chamber. A constant proportion of the two reactants entering the combustion chamber must be maintained at all times at a ration of 2 parts nitric acid to 1 part hydrocarbon. If one or the other of the reactants is allowed to collect in the combustion chamber an explosion will occur. The nitric acid is also used to cool the nozzle. It flows into the cooling jacket at the after end of the nozzle and then flows forward and enters the combustion chamber where it meets the hydrocarbon. No ignition apparatus is necessary as these two fuels are self-igniting.
Professor Wagner was not satisfied with the motors which were developed by Dr. Sbyrowsky of BMW, feeling that they were too heavy. Dr. Conrad, of the Berlin Technical High School, had been obtained by BMW to improve the motor design. Dr. Conrad was experimenting with un-cooled liquid fuel motors for this project and at least two forms of combustion chamber material had been tried; one a form of graphite and the other a material built up of many layers of a very pure ceramic material such as silica or alumina. These showed considerable promise and might have been incorporated in later designs. Other liquid fuels were also being considered. By means of various refinements it was hoped that the missile speed could be raised from a Mach No. 0.75 to 0.8.
Both reactant tanks are built of aluminum and are carefully machined. The outside casting is bored so that a closely fitting piston (without rings) will slide in it. In operation the liquid side of the tank is initially full, forcing the piston against the head. As air enters the head, the piston travels to the right, forcing liquid out of the tank. The object of this design is to insure that when the tanks are less than half full no slugs of compressed air are fed down the reactant pipes as the aircraft performs tight maneuvers. Although the motor would re-ignite after stopping, a collection of one of the two reactants would cause an explosion. On the forward side of the air bottle is a connection to which is T'ed a filling plug and pressure gauge which reads up to 250kg/sq.cm. The bottle is charged with air to 220 atms. On the aft connection to the bottle is a diaphragm which is punctured by an electrically fired squib. The outlet from this connection leads to a regulator which delivers air to the two reactant tanks at 40 atms. (590 lbs/sq.in.) On each side of each reactant tank there is a diaphragm valve which is blown when the 40 atms. pressure appears at that point. This is done to prevent any possible contact of the reactants during storage or handling, and also to insure that both fuels enter the combustion chamber at the same time in order to prevent an explosion. At full thrust, nitric acid enters the combustion chamber through 16 holes, and the hydrocarbon through 8 holes of equal size. The thrust regulation is obtained by the aforementioned regulator motor, operating (through a gear train) a rotary valve plate which blocks off the holes limiting the amount of fuel which can enter the combustion chamber (fuel pressure being constant) and also maintaining the fuel ratio of 2 to 1.
Control System
This missile at first mounted a Friesecke & Hoptner receiver which operated at about 6 m. The equipment was considered as too complicated and heavy for the missile and consequently was superseded by a further development of the "Staru" radio receiver for the Hs 293. This new receiver was of the super-regenerative type having a much smaller form factor than the previous Hs 293 receiver, and was designated the E230-3.
For controlling the missile, the controlling ground station sends out a high-frequency signal which, by modulation with a lower frequency and appropriate keying, conveys to the missile directions for altitude, or elevation, and azimuth. By referring to its gyro the missile is able to interpret and follow these signals. On the ground, the signals are manually initiated by means of a "joy-stick" control which sets the pitch and yaw directions for the missile to follow, and which the missile will follow until the position of the stick is changed.
On the ground, the courses of the target and the missile are followed by means of an optical sighting device. From observations with this device, missile course corrections are set which will bring it to the target. No homing devices had been applied to this missile, although they had been discussed and were under development in several firms. Professor Peterson of AEG had been working for some time on a universal radar ground computer for guiding flak missiles to their target. RIM wanted Wager to incorporate this in the Hs 117 control, but he wished to keep it in its original simple state. In the future he intended to develop homing devices to supplement his visual control for use in thick weather. It is reported that Wagner would have liked to have this missile beam-guided to a radar followed target, but he felt that German radar was too inaccurate for the job.
Various developments for the radio control of this missile were being carried out be several firms. Some work was being done in the range from 20 to 40 centimeters. These frequencies were desirable since they could be beamed and were interfered with less by the jet than the longer wave lengths. Shorter wave lengths were not considered, since suitable tubes were not developed. In some cases, jet interference could be reduced by changing reactants. In all cases solid powder gave the most trouble. It was considered that Telefunken was doing the best work in radio control. They were concentrating on 40 cm.
The electrical power for this missile was delivered by a propeller-driven generator located in one of the two noses of the missile. On the ground, before launching, this same generator was driven by a motor in an outside power source. During this period the propeller did not turn as it was provided with a three ball free-wheeling cam device.
The missile is stabilized about its longitudinal axis only by one gyro.
Limitations of the Control System
The only apparent limitation of this missile lies in the fact that it cannot be seen and consequently followed in overcast weather. If homing devices, other than optical, were incorporated, this limitation would no longer exist.
Warhead and Fuze
Warhead: Considerable divergence of opinion existed in the RIM on the effectiveness of warheads with blast effect only as against fragmentation or incendiary pellet filling, but when the tests of the Hs 117 had been completed in the summer of 1944, the first series was ordered in August of that year with a blast effect warhead weighing 25kg filled with "Trialen" which was manufactured by Wasag at Reinsdorf near Wittenberg.
Fuze: It was intended to use a proximity fuze whenever developed and available. The performance specifications for the fuze required operation between 6 to 10 meters. It was intended that a small clock work arming device would arm the missile about 10 seconds after take-off. Another device is incorporated, probably working off of the control gyro, which explodes the main charge if the missile rolls over on its back in flight, in which case all controls would be revered, making the missile uncontrollable from the ground by an operator who would not be aware of this condition. Another timing device was incorporated which operated 120 seconds after launching to destroy the missile.
Auxiliary Equipment
This missile is equipped with two assisted take-off units, one above and one below the fuselage, each provided with nozzles offset at an angle such that the lines of thrust intersected at about the C.G. of the aircraft. In launching, the lower booster is fired first, thus forcing the missile upward and forward off the launcher which is a simple two-armed cradle supporting the missile at the wing roots (zero length rails). There is a 2 and 1/2-meter firing lanyard, which fired the top unit automatically. Both units then burn until their powder is burned out by which time the main jet has been ignited and takes over. Each assisted take-off unit has a total weight of 90 kg and contains 40 kg of powder charge. Burning takes place from the inside out only since the outside is treated with "Polygon", a plastic preparation which prevents burning on the surfaces to which it is applied.
The booster thrust totals 3,500 kg lasting for about 4 seconds. It burns at a rate of about 5.8 grams per second. This thrust brings the missile up to a speed of about 240 m/sec. These boosters units are known as SG (Schmidding Geraet) 33. They were developed by Schmidding in Bodenbach, and used powder made by Wasag, located near Wittenberg. At the end of the booster burning time, the boosters are jettisoned by means of a powder charge and piston device. Also under discussion as a method of dropping the boosters was a device which depended upon the reduction of the pressure within the booster unit itself.
Launching Device
This missile is launched from a simple two-armed cradle which supports each wing at its root in such a way that an upward and forward motion of the projectile will carry it free from the launcher. There is no movable dolly on this launcher. The launcher is manually aimed in elevation and azimuth in response to signals received from a fire-control point. Sighting devices for the launcher were contemplated but were never incorporated.
