German Explosives
X-4
General Description
The X-4 is a fin-stabilized guided missile with a proximity fuzed warhead developed specifically for use by fighter planes against enemy bomber formations.
Stabilization is obtained by four large fins fitted symmetrically to the body and steering is achieved by rake spoilers operating in four small tail fins which are interdigitated with respect to the large fins. In appearance, it is quite similar to "Fritz X" except that it is much smaller, the warhead weighing only 20 kg and the total weight being 60 kg. It is launched without assisted take-off from fighter planes. It can be either wire-controlled by an operator in the fighter plane or it can be used as a self-steering target seeker by the use of acoustic homing. If it is wire-controlled, some sort of rocket propulsion is to be used; while if acoustic homing is used, it is proposed to dispense with the rocket propulsion unit.
Operational Aspects
The principal operational use of the X-4 would be for fighter use against large enemy bomber formations. Of course, the wire-control version would necessitate the launching plane remaining in the vicinity in order to control the flight of the missile and this in turn seriously restricts its maneuverability. On the other hand, if acoustic homing were used, the X-4 could be launched and the plane could immediately execute an evasive maneuver.
With the 20kg warhead, it is extremely doubtful that much damage could be done to a four-motored bomber at a distance of 15 meters, the operating range of the "Kranich". The effectiveness would undoubtedly be increased if the power plant were dispensed with and the payload doubled, as would be the case if acoustic homing were used in conjunction with a plane of the ME 262 type, it would be quite a formidable weapon to counter enemy bomber formations, and could probably be used for other tactical purposes as well.
Details
Airframe: The airframe of the X-4 consists of a cylindrical body 190 cm in length and 22 cm maximum diameter on which are mounted four large plywood fins fitted to the midsection of the missile and four small tail fins in which the rake spoilers are mounted. The main wing span is 58 cm.
Rotation about the longitudinal axis achieved by small trim tabs attached to the four large stabilizing fins. These give a rotational speed of 1 and 1/2 r.p.s., which in turn is used to give greater stability in flight and also permits greater manufacturing tolerances in the entire assembly. The main fins are swept back at an angle of 45 degrees, which gives a maximum air speed of 270 m/sec. However, due tot he comparatively short range over which this weapon is to be used, it is not expected to attain a velocity of greater than 240 m/sec before it reaches the target.
Steering is effected by means of rake spoilers located in the tail fins. These spoilers vibrate at a rate of 5 cycles per second, control being effected by making the period during which the spoiler projects from one side longer than that during which it projects from the other. When the two periods are equal, no control is applied. This method of steering has the disadvantages of appreciable drag and a certain amount of delay, but has the advantage of simplicity and instantaneous mechanical response.
Design Data
Launching Weight: 60 kilograms
Weight less Fuel: 50 kilograms
Wing Surface: 0.56 square meters
Wing Loading: 200 kg/m2
Maximum Speed in Horizontal Flight at 6,500 m altitude: 270 meters per second
Lateral Acceleration in Horizontal Flight at 6,500 m altitude: 40 m/sec2
Total Energy available during 33 sec Burning Time: 1,600 kg/sec
Power Plant
The Bi-fuel liquid propellant motor is the BMW 109-548 rocket motor using a mixture of Salbei (98 to 100 percent nitric acid) and Tonka 250 (57 percent crude m-xylidine with 43 percent triethylamine). This motor is capable of delivering an initial thrust of 150 kilograms which drops to about 25 kilograms after 30 seconds.
The solid propellant which it was proposed to use was "Mixture 167" by Wasag. Initial thrust is 120 to 140 kg which is also reduced to about 25 kg after 30 seconds.
Of course, there were also plans under way to eliminate the propulsion unit entirely, in which case the payload could have been appreciably increased.
Intelligence and Control System
In all the proposed versions of X-4, the missile rotates and therefore only one gyro is needed. When wire control is used, the principal purpose of the gyro and commutator system is to translate the right-left and up-down signals into the proper pulses to feed to the rake spoilers in the tail fins. The gyro is spun up while the missile is still attached to the launching aircraft. During the flight of the missile, the gyro is not power driven. The gyro was made by Th. Horn. Leipzig.
The wire control system consists essentially of a small joystick control unit mounted in the aircraft, a pair of control wires and a receiving unit in the missile consisting of a gyroscope and a pair of relays. The control unit contains two drums which revolve at a rate of r.p.s. one drum controlling azimuth and the other elevation. The control wires consist of 2 insulated single strand Swedish spring-steel wires of 6,000m length and 0.22mm diameter. The receiving unit in the missile is quite simple consisting primarily of a polarized relay for azimuth control and unpolarized marginal relay for elevation control. The polarized relay responds only to polarity changes in the direction of the current flow through the wires while the unpolarized marginal relay responds only to changes in the value of the current, regardless of its polarity. In this way, both azimuth and elevation control signals can be transmitted simultaneously over the same pair of wires.
The relays are connected to the spoiler solenoids in the tail fins, through the gyro commutator system. This arrangement converts the left-right and up-down signals into the proper pulses which are to be fed to the solenoids actuating the spoilers. The power supply is a small 9-volt dry battery located in the afterbody of the missile.
During the flight tests, there were no detrimental effects from static electrical charges accumulating on the wires and the mechanical difficulties had been solved by paying out the wire from the bobbins on the missile and similar bobbins on the parent plane simultaneously. Wire control was selected primarily because, compared to radio remote control, it is practically jamproof.
In March 1945, there was also under development a plan to dispense with the power plant and wire control and to use an acoustic homing system "Pudel" as the control device. This system is based on the same fundamental principles as the acoustic proximity fuze "Kranich". It consists essentially of a mica and 0.03 mm aluminum foil diaphragm connected to a carbon microphone, the output of which is fed to a single stage amplifier and relay output. This assembly is mounted at an angle of approximately 60 degrees to the longitudinal axis of the body and the sound passes into the diaphragm through a series of wire mesh screens which serve to attenuate differences of air pressure due to rotation but not the sound of motors and propellers of enemy aircraft. A small lyre arrangement is attached to the vibrating system in such a way as to broaden out the mechanical resonance curves of the individual components of the system. The composite curve of the acoustic system with the 15-wire lyre attached is practically flat from 200 to 400 cycles. The wire mesh screens attenuate the slow air pressure variations about 40 or 50 db, whereas the sound vibrations of 200 to 400 cycles are attenuated only 1 db.
As the X-4 rotates, if the missile is homing directly on the target, the output of the microphone will be constant and there will be no modulation output and consequently no steering corrections. If the missile is not aimed directly at the target, there will be generated a modulation frequency of 1 and 1/2 cycles per second, the rotating speed of the X-4. This modulation frequency of 1 and 1/2 cycles per second is used to transmit information to the spoilers through the gyrocommutator system. For this purpose, the normal gyrocommutator system has been modified to some extent. Only 8 models of "Pudel" have been built and so far no flight tests have been made.
The modification of X-4 with "Pudel" and also equipped with the "Kranich" acoustic proximity fuze when launched from a fighter plane of the ME 262 type appears to be a weapon of great promise, since after launching, the fighter pilot can immediately take evasive action. The range of the acoustic homing device is expected to be about 1,000 meters, so that if it were launched at a range of 2,000 meters, the first 1,000 meters of its flight would be uncontrolled.
Warhead and Fuze
When X-4 was originally conceived, it was expected to use a 20-kg cast-steel warhead enclosing the explosive material. However, due possibly to the shortage of steel in Germany at the beginning of 1945, some thought was being given to the use of an uncased molded plastic type of warhead which was to be attached to the afterbody by wood screws and had an adaptor for the nose fuze attached to the nose in the same way.
The fuzing system consisted of the acoustic proximity fuze "Kranich" which forms the nose of the missile, an impact and graze fuze, and a self-destroying fuze which operates 35 seconds after release. The acoustic and impact fuzes are armed 7 seconds after release. The acoustic fuze consists of a light diaphragm actuated mechanism which responds to the sound of aircraft propellers at a range of 15 meters. This fuze has a time delay of 1/50 second to make it more effective.
Launching Devices
Originally the X-4 was being carried only by the Focke-Wolf 190. However, it was later planned to use it also in connection with ME 262 and DO 335. The missile is suspended from the parent craft by means of ETC70AI, which is a modified 70kg bomb rack with a 7-prong umbilical cord and provision for two control wires. The X-4 is released electrically when the pilot presses the bomb release switch.
Next Time: Rockets (Part 8)
Source: German Explosive Ordnance Vol. 1: Bombs, Rockets, Grenades, Mines, Fuzes & Igniters
