US 3063345 A
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NOV. 1962 R. N. HARMON ETAL 3,063,345
METHOD AND APPARATUS FOR EXPLODING BOMBS Filed May 25, 1943 jhznsmiffer on /7/'rcra1f 5 fig J. 23
? fiece/l er on Bomb WITNESSES: INVENTOR W fia/p/i A! flar/rm and ATTO United States 3,053,345 Patented Nov. 13, 1962 3,063,345 METHOD AND APPARATUS FOR EXPLODING BOMBS Ralph N. Harmon and John R. Boyltin, Baltimore, Md
assignors to Westinghouse Electric Corporation, a corporation of Pennsylvania Filed May 25, 1943, Ser. No. 488,346 Claims. (Cl. 89-15) Our invention relates to explosive bombs to be dropped from airplanes and, in particular, to electrical arrange ments for timing the detonation of such bombs.
Bombs to be dropped from airplanes are now known which embody within the bomb shell a small radio transmitter and receiver. The transmitter generates high frequency waves as the bomb falls and these waves are reflected from the ground and the reflected wave picked up by the receiver. The received wave then, for example, by interaction with the transmitted wave, causes the output current of the receiver to actuate a detonator which explodes the bomb. While such arrangements have a field of usefulness they possess many disadvantages which it would be desirable to eliminate. For example, it is both expensive and ditficult to build a transmitter which is sutficiently compact so that it may be incorporated in the limited space available within a bomb, and, of course, both the transmitter and receiver are destroyed when the bomb explodes. Furthermore, in their conventional form such arrangements make it ditficult to regulate the height above earth at which the bomb will explode, inasmuch as the points of detonation are determined by the strength of the receiver output current. This is a fixed quantity determined in part by the sensitivity of the receiver at the time the receiver is installed in the bomb, and come quently it is impossible in the field later to vary the height at which the explosion takes place. The ability to vary this height to fit the circumstances of different military situations would be highly desirable.
Our invention avoids the foregoing ditficulties by making it possible to use a transmitter carried by the airplane from which the bomb is dropped, the bomb, therefore, in- :orporating only the radio receiver. In this way the Jomb can be made much cheaper and more compact, and he expense of destroying a transmitter with every bomb s avoided. Furthermore, as will hereinafter appear, it is )ossible by our arrangement to vary at will the height above ground at which the bomb explodes by making iroper adjustments in the transmitter on the airplane.
It is accordingly one object of our invention to produce t system for detonating explosive bombs or the like by adio waves emanating from a transmitter mounted inlependently of the bomb structure.
Another object of our invention is to provide an an angement for detonating bombs or the like in which the me of explosion of the bomb can be regulated at will.
Other objects of our invention will become apparent pon reading the following description taken in connecon with the drawings, in which:
FIGURE 1 is a diagrammatic illustration of an airlane carrying out bombing operations in accordance ith our invention;
FIG. 2 shows one form of circuit adapted for use as a ansmitter of radio waves utilized in our invention; and
FIG. 3 shows the circuit of a receiver suitable for inarporation in the bomb utilizing our invention.
One of the principal novel features which distinguishes 1r invention over the prior art is the employment of a dio transmitter mounted on an airplane from which )mbs are to be dropped, and which projects short groups 'pulses of high frequency radio waves toward the ground the time that the bombs are dropped.
These groups or pulses are separated from each other by intervals which preferably are considerably longer than the groups themselves and also longer than twice the height from which bombs will be dropped. It will be apparent that under such circumstances a jet or group of waves will travel to the earth at substantially the speed of light, and will be reflected therefrom. During the time intervening between the instant at which the first wave of the group strikes the reflecting earth and the instant at which the last wave of the group strikes it, there will be an interference between the incident and reflected waves in a region closely adjacent to the earth. The interference of such incident and reflected waves may, in fact, be thought of as producing standing waves for the short interval in question in the region immediately above the point of incidence on the ground. A moments consideration will show that the height of the zone in which both incident and reflected waves are present and interfering will vary with time, starting at zero at the instant the foremost wave is reflected, rising to a maximum equal to one-half the length of the pulse or group at the time the foremost wave of the group in its upward course passes the last wave of the group in the latters downward course, and reaching zero again at the time the last wave of the group strikes the earth. Thereafter for an interval equal to the time between groups there will be no interference and hence no standing waves adjacent the earths surface; but on the arrival of a second jet or group at the earths surface the establishment of the standing waves in the manner just described will occur again.
If a moving object carrying an antenna moves across a set of standing electromagnetic waves, voltages will be generated in the antenna which will have a time period equal to the time required by the moving object to pass from the crest of one standing Wave to the crest of the next wave. To illustrate, if the standing Waves are produced by reflection of radio waves one meter long, the period of the voltages generated in the antenna will be the time required by the moving object for traveling one meter. If, for example, the moving object is a falling bomb, the period of the voltage generated in the antenna will correspond to the time required for the bomb to move one meter; and since the velocity of a falling bomb is always extremely small compared with the velocity of propagation of radio waves, it is evident that the frequency of the voltage generated in the bomb antenna would be correspondingly low compared with the ultra high frequency characteristic of one meter radio waves.
it will be noted that the region in which such standing waves are produced is limited to a height above the earth which cannot be greater than one-half the length of the wave group being transmitted. Hence if a bomb equipped with an antenna and radio receiver tuned to respond to the frequency characteristic of the cutting of the abovementioned standing waves, but substantially insensitive to the vastly higher frequency of the traveling radio waves themselves, is dropped downward from an airplane carrying the above-mentioned pulse transmitter, the output circuit of the radio receiver will remain substantially unenergized as long as the bomb remains at a height above the earth greater than one-half the length of the pulse or group of waves sent out by the transmitter. Hence by adjusting the length of the pulse or group of Waves sent out by the transmitter, it is possible to vary at will the height above ground at which the receiver on the bomb will be energized, and the bomb thereby exploded.
While the foregoing refers to the effects on the antenna 0 as produced by movement of the latter through standing electric waves, it may alternatively be considered that the effects of the incident and reflected waves on the antenna is due to a Doppler effect in virtue of which the frequency actually generated in the antenna of an object moving in the direction of propagation of the waves is less than the frequency that would be produced by the same waves if the antenna were stationary; and the frequency produced in an antenna by waves moving in an opposite direction to the motion of the antenna is greater than that which the same waves would produce on a stationary antenna. Thus in the present instance the antenna of a falling bomb is moving in the same direction as the incident waves and the frequency generated is less than the wave frequency while the antenna is moving in the opposite direction to the wave reflected from the earth, and so the antenna has generated in it a frequency greater than that in reflected waves. These two frequencies, one slightly less and the other slightly greater than that of the radio wave generated at the transmitter may be considered to interfere and beat" with each other in the radio receiver on the falling bomb, thereby producing a lower frequency to which the receiver circuit is made sensitive.
To take a specific example of the operation of the above described arrangement, suppose that the transmitter on the airplane sends out waves of one meter length in a group or pulse lasting for 4x10 seconds, and that the succeeding pulse follows the first after an interval of seconds. Since the time intervening between the incidence of the foremost wave of the group on the earths surface and the incidence of the last wave of the group is 4x10 seconds, and the velocity of radio waves is approximately 3 10 meters per second, the length of the pulse or group is approximately 120 meters. The maximum height above the earth in which standing waves capable of energizing the explosion of the bomb can exist is, accordingly, 60 meters. It is thus impossible for the receiver on the falling bomb to be energized at any height higher than 60 meters.
It can be readily shown as follows that by reason of the small velocity of the falling bomb relative to the velocity of radio wave movement, the receiver on the bomb will be energized with certainty at a distance not substantially less than 60 meters above ground. A simple calculation shows that, neglecting air resistance, a bomb dropped from a height of 10,000 meters would be falling at a velocity slightly under 300 meters per second when it reached a point 60 meters above the earth. Air resistance would, in an actual case, reduce the velocity to a considerably smaller figure. The maximum time which would elapse between one establishment of a standing- Wave region at the height of 60 meters and a second establishment of the same standing waves at that height would be the time between the pulses of the transmitter; namely, 10- seconds. The falling bomb must, therefore, encounter a standing-wave region at the height of 60 meters at some time during an interval not greater than 10- seconds. The distance moved by the falling bomb in 10 seconds is only 0.09 meter. Hence it is seen both that the radio receiver on the bomb will not traverse standing waves above 60 meters and that it will certainly meet them at a point which is less than 0.09 meter below 60 meters.
The bomb falling at 300 meters per second will, of course, traverse 300 standing waves of 1 meter length per second, and so the beat frequency generated at the receiver will be 300 cycles per second. This is obviously, as predicted above, a value widely different from the 3 10 cycle frequency of the transmitter.
The foregoing being the principles of our invention, reference will now be made to the drawings in which FIG. 2 shows a suitable circuit for a pulse generator to be carried on the airplane. While pulse generators suitable for this purpose are described in the literature, the particular circuit which we choose to illustrate in FIG. 2 comprises a pair of electron tubes 1 and 2 having anodes connected to each other through a tank circuit comprising a capacitor 3 and an inductor 4, plate current for the tubes being supplied through a tap through a midpoint of the inductor 4 to a positive terminal 5 of a direct current source. The negative terminal of this source is grounded and hence connected to the cathodes of the tubes 1 and 2. The control electrodes of the tubes 1 and 2 are, respectively, connected to each other through a resonant circuit comprising a capacitor 6 and an inductor 7. The midpoint of the inductor 7 is connected to ground through a grid-leak circuit comprising a variable capacitor 8' and a variable resistor 9. The resonant circuit embodying reactors 3, 4 and 6, 7 are tuned to the frequency which it is desired to generate and are linked to an output antenna ll through coupling to a secondary 12 of an output transformer.
Such an arrangement will produce a succession of groups or groups of waves, the wave length corresponding to the adjustment of the reactances previously mentioned and the length of the group depending upon the value of the capacitor 8. By varying the magnitude of the capacitor 8, the length of the pulse can, accordingly, be set within wide limits. The period which intervenes between successive pulses or groups depends upon the value of the resistor 9 and adjustment of the later can be used to vary the group periodicity.
Referring in detail to FIG. 3 the receiver mounted on the bomb comprises an antenna 21 coupled in conventional fashion to a tuned radio frequency amplifier 22 which is of conventional type, the output of which passes through a band pass filter 23 which is transparent to currents having a period corresponding to the time required by the falling bomb to move at ground level a distance equal t the wave length sent out by the transmitter of FIG. 1, but substantially opaque to currents of the frequency of that transmitter. Since the time periods just mentioned are so widely different, the filter need not be critical as to the lower frequency, it being sufficient that it is a low pass filter capable of rejecting the transmitter frequency and of rejecting the pulserate frequency. The output of the band pass filter may be amplified in conventional fashion by an amplifier 24 in the output circuit of which is provided a relay 25 capable of responding to currents above a critical magnitude. When the relay 25 responds it actuates a detonator 26 which explodes the bomb in a manner too well known to require description here.
In the foregoing we have described a specific embodiment of our invention the principles thereof are of a broader application which in many ways will be evident to those skilled in the art.
We claim as our invention:
1. The method of detonating moving explosives which comprises directing an intermittent beam of electromagnetic radiation from a radiator along the path of movement of said explosive, reflecting said radiant energy at a point in said path, and detonating said explosive in response to the interaction at said explosive of the transmitted and reflected energy.
2. In combination, an aircraft means to release a bomb from said aircraft, said bomb embodying a radio receiver tuned to frequencies within a predetermined band, a transmitter on said aircraft having a frequency substantially outside said band, and provided with means for projecting intermittent pulses of radiant energy, the band to which said receiver is tuned containing the beat-frequency generated between the incident wave and the reflected wave of said radiation at the earths surface, and means responsive to the output of said receiver for exploding said bomb.
3. In combination, an aircraft means to release a bomt from said aircraft, said bomb embodying a radio re ceiver tuned to frequencies within a predetermined band a transmitter on said aircraft having a frequency sub stantially outside said band, and provided with mean:
for projecting intermittent pulses of radiant energy, means for varying the length of said pulses, the band to which said receiver is tuned containing the beat-frequency generated between the incident wave and the reflected wave of said radiation at the earths surface, and means responsive to the output of said receiver for exploding said bomb.
4. In combination, an aircraft means to release a bomb from said aircraft, said bomb embodying a radio receiver tuned to frequencies within a predetermined band, a transmitter on said aircraft having a frequency substantially outside said band, and provided with means for projecting intermittent pulses of radiant energy, the length of said pulses being small compared with the intervals between them, the band to which said receiver is tuned containing the beat-frequency generated between the incident wave and the reflected wave of said radiation at the earths surface, and means responsive to the output of said receiver for exploding said bomb.
5. In combination, an aircraft, means to release a bomb from said aircraft, said bomb embodying a radio receiver tuned to frequencies within a predetermined band,
a transmitter on said aircraft having a frequency substantially outside said band, and provided with means for projecting intermittent pulses of radiant energy, the length of said pulses being small compared with the intervals between them, means for varying the length of said pulses, the band to which said receiver is tuned containing the beat-frequency generated between the incident wave and the reflected wave of said radiation at the earth's surface and means responsive to the output of said receiver for exploding said bomb.
References Cited in the file of this patent UNITED STATES PATENTS 1,506,785 Sperry Sept. 2, 1924 1,769,203 Buckley July 1, 1930 2,165,800 Koch July 11, 1939 2,176,469 Moueix Oct. 17, 1939 2,193,361 Rice Mar. 12, 1940 FOREIGN PATENTS 91,592 Sweden Feb. 24,1938 524,876 Great Britain Aug. 16, 1940