US3111088A

US3111088A - Target seeking missile

Info

Publication number: US3111088A
Application number: US176058A
Authority: US
Inventors: Newton H Fisk
Original assignee: Martin Marietta Corp
Current assignee: Martin Marietta Corp
Priority date: 1962-02-27
Filing date: 1962-02-27
Publication date: 1963-11-19
Anticipated expiration: 1980-11-19

Description

N. H. FISK 3,111,088

'Nov. 19, 1963 TARGET SEEKING MISSILE 3 Sheets-Sheet 1 Filed Feb. 27, 1962 INVENTOR.

NEWTON H. FISK Nov. 19, 1963 N. H. FISK 3,1

TARGET SEEKING MISSILE Filed Feb. 27, 1962 3 Sheets-Sheet 2 Nov. 19, 1963 N. H. FISK TARGET SEEKING MISSILE 3 Sheets-Sheet 3 Filed Feb. 27, 1962 United States Patent 3,111,088 TARGET SEEKING MISSHLE Newton H. Fisk, Orange County, Fla, assignor to Martin- Marietta Corporation, Middle River, Md, a corporation of Maryland Filed Feb. 27, 1962, Ser. No. 176,058 9 Claims. (Cl. 102-51) This invention relates to target seeking missiles, and more particularly to radiant energy responsive target seeking missiles capable of detecting the presence and relative posit-ion of a target and of altering course in flight as necessary to strike the target or come within effective range thereof. Target seeking missiles such as bombs, rockets, and the like, have been proposed heret0- fore comprising movable steering means such as rudders, vanes, or other air foils requiring linkages "and power actuators for positioning thereof to effect deviations in trajectory in response to signals from control means such as radiant energy responsive target scanning apparatus operating independently of the steering means. While such systems have achieved a certain degree of refinement, there remain problems relating to the provision of linkages and actuators which are small in size and light in weight and yet sufliciently powerful and reliable to accurately and rapidly posit-ion rudders and the like against the resistance of air at high speeds. Additionally, such systems have required relatively complex means for scanning the area ahead of the missile and for translating the relative position of the target into signals for operating the rudder positioning means.

-It is a primary object of this invention to overcome the foregoing disadvantages and shortcomings by providing an improved radiant energy responsive missile and guidance system wherein target scanning and missile steering or attitude control means are integrated to provide a particularly simple, compact and reliable system.

Another object of this invention is the provision of a target seeking missile and guidance system which is capable of looking on a target which is off the missile axis, and of continuously applying a course correcting force until the missile axis is brought into alignment with the target.

As another object this invention aims to provide an improved target seeking missile, such as a rocket powered explosive weapon or the like, to be propelled through the air toward an objective or target which may be distinguished from its surroundings by radiation such as ultra-violet, infra-red, or light energy reflected from or emanated by the target, the missile being of a canard configuration having an elongated body or airframe comprising forward and after sections independently rotatable in opposite directions about the longitudinal axis of the missile, one of the sections, for example the after section, having fin means which impose a rotational moment thereon in one direction of rotation with respect to a fixed frame of inertial reference, the other or forward section having cauards or other air foils for imposing a rotative moment thereon in the opposite direction and for creating :a net lift normal to the missile axis and constant in direction with respect to the forward section, so that when the latter is freely rotating the effect of the not lift is simply to impose a small oscillatory motion on the missile in its trajectory, and target position responsive means for actuating brake means between the sections for causing the rotation of the forward section to be overcome by the rotation of the after section and pass through a condition of zero rotational velocity with respect to the fixed frame of reference, whereby the net lift acts to efiect a deviation in the flight path of the missile in the form of a change of course or direction of travel toward a target.

Another object of this invention is the provision of a target seeking missile of the foregoing character wherein the forward section comprises optical means including an aperture rotatable therewith for scanning the target area and for causing an image of a target to fall on radiant energy sensitive means, such as a photomultiplier tube in the after section, when the target is off the longitudinal axis of the missile, the photomultiplier tube being responsive to presence of light energy in the form of the target image to send an electrical signal through suitable amplifying and rectifying means to actuate the brake means which preferably comprises an electrically energized magnetic particle brake. The aperture is so disposed with respect to the canard means that the target image falls on the photomultiplier tube and actuates the brake to cause the forward section to reverse direction and pass through zero rotational velocity with respect to the fixed frame of reference when the net lift or lateral effort of the canard means is acting on the missile in a direction tending to bring the missile axis toward alignment with the target.

The reverse rotation of the forward section and oper ture through a small degree will cause the target image to be intercepted and the resulting loss of light energy on the photomultiplier tube to interrupt the brake energizing signal so that the forward section is caused by its canard fins to again reverse and rotate through a small degree until the aperture once more allows the target image light energy to fall on the photomultiplier tube and a signal to be passed to energize the brake and repeat the reversing sequence of the forward section. Since the foregoing sequence of events results in rapidly changing rotative direction or oscillation of the forward section within a small increment of rotation, the net of the canard means will provide a lateral efiort which substantially constantly urges the missile to assume a course in alignment with the target. Thus, missiles embodying this invention may be said to lock on the target. in one practical embodiment the canard means comprises four vanes or canards extending from the forward section at intervals, and three of which are oriented to provide the rotating movement which is utilized in scanning while.

the fourth is of reverse hand and cooperates with the canard opposite thereto to produce the net lift or lateral effort utilized for steering the missile.

Other objects and advantages of this invention will become apparent from the following detailed description of a preferred embodiment thereof read in conjunction with the accompanying sheets of drawings forming a part of this specification, and in which FIG. 1 is a side elevational view of a rocket powered target seeking missile embodying the invention;

FIG. 2 is an enlarged front view of the missile of FIG. I viewed along line 2--2 thereof;

FIG. 3 is a tail view of the missile taken along line 3-3 of FIG. 1;

FIG. 4 is an enlarged view of the missile, partly in section and with the mid portion broken out;

FIGS. 5A through 5E are a series of schematic illustrations showing different phases of operation of the inventiOn;

FIG. 6 is a schematic illustration showing the circuit missile is responsive to light energy in the infra-red band of the spectrum such as may be emanated or reflected from a target such as an armored vehicle or tank. Of course, missiles embodying the invention may be responsive to other forms of radiant energy such as ultraviolet, electrom-agnetic, or the like either reflected from or emanated by a target or objective in non-radiating surroundings.

The missile 10 comprises an elongated airframe or body including an after section 11 and a coaxial nose or forward section 12 which are independently rotatable in opposite directions about the longtiudinal axis of the missile. The after section 11 has a plurability of missile stabilizing tail fins 13 mounted thereon, there being four fins in this instance spaced at 90 intervals. The trailing edge of each fin 13 is bent as at 13a to provide a rolling moment to the after section of the missile airframe as it passes through the air in flight. This moment produces clockwise rotation of the after section of the missile as viewed in FIG. 2 and with respect to a fixed frame of inertial reference. This missile 10 is adapted to be propelled by a conventional rocket motor carried in airframe after section 11 and having a nozzle 14- which opens through the tail end of the missile as shown in FIG. 3.

Missile 10 may be considered to be of basic canard configuration with the fins 13 serving as wings and steering being effected by four

canards

16a, 16b, 16c, and 16d extending from, and forming part of, the rotatable forward airframe section 12. Three of the canards, 16a, 16b, and Ida are oriented to provide a rotational moment, indicated by arrows 17, to forward section. 12 as the missile passes through the air, which moment tends to effect rotation of the forward section in a counterclockwise direction as viewed from the nose in FIG. 2, and with respect to the fixed frame of reference. Canard 16d, however, is of the reverse hand and cooperates with canard 16b opposite thereto to provide a net lift or lateral eifort on the forward section of the missile as shown by vector arrows 18. The after section 11 is characterized by greater rotational inertia than is the forward section 12 and the rotational moment imposed on the after section by fins 13 is somewhat greater than the oppositely directed rotational moment imposed on the forward section by

canards

16a, 16b, and 16c.

It will be recognized that the net lift or lateral effort 18 is substantially normal to the longitudinal axis of the missile and is constant in direction with respect to the rotatatable forward airframe section 12. That is to say, the direction of the lateral effort rotates with the forward section. This lateral effort is utilized in a manner which will become apparent as the description proceeds to cause the missile to be accelerated from its normal trajectory and to curve toward a target.

Canards 16 are preferably formed of molded plastic material formed integrally with a reflector member 21 presenting an annular, forwardly directed parabolic reflecting surface 21 surrounding a recess a. As shown in FIG. 4, reflector member 24) is supported for rotation about the longtiudinal axis of the missile by ball bearing means, of which the outer race 23 is conveniently partially imbedded in the reflector member and the inner race 24 is carried on an annular boss 25 defining a central opening 26 in an end wall 27 after missile section 11.

Outer race 23 has a ring gear 36 formed thereon which is in meshing engagement with a pinion 31 fixed on the common shaft 32 of a magnetic particle brake 33 and a permanent magnet electric generator 34 mounted behind wall 27 in after section 11. Rotation of forward section 12 effects operation of permanent magnet generator 34- through pinion 31 and shaft 32, while brake 33 is adapted to be energized to impede or halt rotation of the forward section with respect to the rafter section 11. Brake 33 and generator 34 form part of a missile steering control circuit including a radiant energy sensitive element such as a photo-multiplier tube 35 mounted in alignment with opening 26, and adapted to convert radiant energy from a target into electrical signals for controlling brake 33 so as to effect steering of the missile toward the target. Magnetic particle brake 33 is conveniently of a type defined as Model 3547A manufactured by Lear, Incorporated of Santa Monica, California, though other forms of electrically energized braking means may be used.

A dome 44 which is transparent to infra-red light energ, is secured to reflector member 2%} and supports a second reflector member 41 having a reflecting surface 42 facing reflecting surface 21. Reflecting

surfaces

21 and 42 are coaxial with the longitudinal axis of missile 1t) and with opening 26 in wall 27 of after section 11. A wedge shaped aperture 43, including an angle of about 40, is formed in reflector member 20 between recess 26:: thereof and opening 26, the apex of the aperture terminating short of the longtiudinal axis of the missile as is best shown in FIGS. 5A-5E. Aperture 43 is disposed at approximately from the lift producing canard 16d and extends away from the missile axis in a direction opposite to the net lift vector 18. Preferably, the aperture is provided with a suitable infra-red filter 44.

Reflecting

surfaces

21 and 42 are arranged to gather radiant energy photons or rays from a field of view of 15 from the missile axis to focus the energy rays toward photomultiplier tube 35. When the forward section 12 is rotating in flight, aperture 43 rotates therewith and causes the target area to be scanned and an image I of target T, which is off the missile axis but within the field of view, is formed on photomultiplier tube 35 as shown in FIG. 4. It should here be noted that in the present example the image is formed on the opposite side of the missile axis from that occupied by the target T, because of the reflective principle used.

Photomultiplier tube 35, which may be tube type IP28 such as is manufactured by the Radio Corporation of America, is capable of varying electron flow between cathode and anode means in accordance with the intensity of light energy falling on the tube. Thus, photomultiplier tube 35 may be said to convert photons or light energy into an amplified electrical signal. The anode means are impressed with a suitable electrical potential by means of a battery 47 and a suitable power supply as as is best shown in FIG. 6. Battery 47 is preferably a silver-zinc type such as is manufactured by the Eagle- Picher Company of Cincinnati, Ohio, while the power supply 48 is preferably a conventional DC. to A.C. inverter of the transistor oscillator type, including A.C. voltage step-up means, the power supply being comparable to that identified as Model 591H-C of Arnold Magnetic Corporation of Los Angeles, California. Photornultiplier tube 35 is connected to pass its output signals through a band pass filter 49, these signals being applied as one of two inputs to a conventional transistor amplifier 50 which is in turn connected through rectifier 51 to brake 33.

The band pass filter 49' and amplifier 50 may be of any types suited to the signal frequencies concerned, and in the present embodiment amplifier 50 is similar to type 466A made by Hewlett-Packard Company of Palo Alto, California. Rectifier 51 is preferably a silicon diode semiconductor type 1N599.

The foregoing components have been found to fulfill the required functions in the embodiment being described, although other comparable components, well known to those skilled in the art to which the invention pertains, may be used in practicing the invention. The battery 47, power supply 48, band pass filter 49, amplifier 50, and rectifier 51 are conveniently assembled in an annular package 52 mounted in after section 11.

The application of light energy to photomultiplier tube 35, such as by formation of an image I thereon, causes the tube to pass a signal through filter 49 to amplifier 50, which signal is there amplified by power received from 5. battery 47. The amplified signal is passed through rectifier 51 for conversion to direct current which is utilized to energize magnetic particle brake 33. Brake 33 is there by actuated to oppose rotation of forward section 12 with respect to after section 11 so long as light energy, such as image I, remains focused on the photomultiplier tube.

Conversely, whenever little or no light of the wavelength concerned falls on photomultiplier tube 35, such as when aperture 43 is not positioned in alignment with image I, there is no signal passing from the photomultiplier tube through amplifier 50 and rectifier 51 to brake 33. In this condition the forward section is allowed to rotate under the influence of

canards

16a, 16b, and 160 in one direction with respect to the inertial reference frame, while the after section 11 is rotating in the opposite direction under the influence of fins 13.

The rate of scanning rotation by forward section 12 is limited by applying the electrical output of permanent magnet generator 34, which is proportional to the angular velocity of shaft 32, as a second input signal to amplifier 50 Where it is amplified and then transmitted to brake 33 through rectifier 51. It will be recognized that the effect of generator 34 is to cause brake 33 to dampen the rotational moment of forward section 12, this being desirable to stabilize the seeking characteristics of the missile. In the present example, the after section 11 has a steady roll rate of about 15 radians per second in a clockwise direction as the missile is viewed in FIG. 2, while the steady state roll rate of the lighter forward section is limited by the dampening effect of generator 34 to about 32 radians per second in the opposite direction.

Because of the greater rotational inertia of the after section and the large rotational moment of fins 13, energization of brake 33 to oppose rotation of forward section 12 with respect to after section 11 in response to formation of a target image on photomultiplier tube 35, will result in the rotation of the forward section being overcome by rotation of the after section so that the forward section passes from its normal counterclockwise rotation through zero rotational velocity with respect to the fixed inertial frame of reference and into clockwise rotation with the after section.

Referring to FIGS. A through SE, in which the reflector member 20 and aperture 43 are viewed from a position forward of the missile and looking aft toward the nose thereof, the sequence of events in target acquisition and steering will be described. In FIG. 5A forward section 12 is rotating counterclockwise under the influence of

canards

16a, 16b, and 16c, so that the optical system including aperture 43 is scanning an area ahead of the missile. At this time a target, to the right of the missile axis as viewed in FIG. 5A, is represented by a radiant energy image I intercepted by member 20 which prevents the image energy from reaching the photomultiplier tube 35.

As aperture 43 rotates to the FIG. 5B position, the image I is formed on the photomultiplier tube so as to fully energize brake 33. Energization of brake 33 in response to the formation of the target image on photomultiplier tube 35 halts rotation of the forward section 11 with respect to the after section and, because of the greater rotational inertia of the after section and the large rotational moment of fins 13, the forward section will pass from its counterclockwise rotating condition through zero rotational velocity with respect to the fixed inertial frame of reference and into clockwise rotation with the after section. The finite inertia of the forward section requires some time (on the order of .01 second) for the brake 33 to reverse rotation of the forward section and during this time the image I is brought deeper into the sensitive area defined by aperture 43 as shown in the brake on view of FIG. 5B.

Thereafter, the forward section 12 and aperture 43 will rotate clockwise with after section 11 for a few degrees until the image I is intercepted by member 20 and the light energy thereof is lost to photomultiplier tube 35 as shown in FIG. 5C. The result of loss of light energy on the tube 35 is that the signal ceases to pass therefrom through filter 49, amplifier 50, and rectifier 51 to brake 33, and the forward section is released to resume its counterclockwise rotation under the influence of

canards

16a, 16b, and 160. Again, the forward section passes through zero rotational velocity as it reverses from clockwise to counterclockwise rotation. This counterclockwise rotation of forward section 12 will continue only for a few degrees until the image I is regained as shown in the subsequent brake on view of FIG. 5D at which time the rotation is again reversed.

The forward section 12 is thereby oscillated between clockwise and counterclockwise rotation, with the amplitude of oscillation being limited to a few degrees between zero rotational positions with respect to the fixed inertial frame of reference. Because of the positional relationship of the aperture 43 to

canards

16b and 16d, the net lift produced thereby exerts a lateral effort 18 which acts throughout the period of oscillation to veer the missile to a course more directly toward the target T.

The relative movement path of image I with respect to forward section member 20 and aperture 43 is represented by a dotted line 53 in FIGS. 5A5E, by which it may be seen that the image moves progressively toward the center of member 20 as the longitudinal axis of the missile 10 is brought into alignment with the target.

When image I reaches the FIG. 5B position, the missile is on target and the image is occluded because of the blunted shape of the aperture 43. The occlusion of the image interrupts the photomultiplier output signal and brake 33 releases the forward section 12 for scanning rotation which continues until the target is hit or the image moves out of the FIG. 5E position because of target movement, windage, or the like, in which case the steering sequence will be resumed as necessary to return the missile to an on target course.

It will be recognized that although the forward section 12 and the direction of canard net lift or lateral efiort 18 oscillate slightly during the seeking or steering phase, the resulting effect is that the missile is locked on the target and the major force components of the net lift act continuously to veer the missile 10 toward alignment with the target.

The steering characteristics of the missile 10 may be altered by modifying the leading edge of the aperture 43 to provide proportional control of the brake means. For example, if it is desired to reduce the oscillation of for: ward section 12 to effect a still more constant force for veering of the missile toward an on target course, the aperture may be modified by providing the leading edge thereof with shading means 55 as shown in FIG. 7. The shading means 55 is substantially opaque at 55a to the radiant energy concerned and becomes gradually more transparent thereto going towards edge 55b.

When the target image is acquired in the lightly shaded or open portion of the aperture 43-, the brake will be applied as before, causing the forward section 12 to roll with the after section and the image I to progress into the deeper shaded portion of the aperture. The intensity of the radiant energy reaching the photomultiplier tube 35 will be reduced until brake 33 is allowed to slip at a rate which will maintain the image I under a substantially constant degree of shading. This, of course, will maintain the forward section 12 so that the net lift will be continuously and fully acting to direct the Thissile toward the target. The image I will therefore follow the more direct relative movement path shown in FIG. 7 until it reaches the occluded central portion of the reflector 20 corresponding to the longitudinal axis of the missile, at which time scanning will 'be resumed.

Other modifications of aperture 43 may be made to achieve different flight characteristics, for example the shape of the leading edge could be curved to provide increasing or decreasing amounts of course changes as the target image approaches the center.

From the foregoing description it will be apparent that both the scanning function and the steering function are effected by use of the canard means 15, and that this integration of functions eliminates the necessity of providing and operating independent rudders, impulse means, or the like. It will also be apparent that missiles embodying the invention may be made responsive to other forms of energy such as visible light, ultraviolet, or sound waves, and that other forms of braking means, canards, and energy focusing elements, than those described, may be used.

Accordingly, although the invention has been described in considerable detail, and with reference to a specific target seeking missile embodying the invention, it will be understood that the invention is not limited thereto, but rather the invention includes all those modifications, adaptations, substitutions, and uses as are reasonably embraced by the scope of the claims hereof.

Having described my invention, I claim:

1. A target seeking missile comprising:

(a) an elongated airframe having first and second coaxial sections rotatable relative to one another about the longitudinal axis of the airframe,

(b) first fin means on said first section for effecting rotation thereof in one direction about said axis and for imposing a net lift normal to said axis and acting in a direction which is rotatable with said first section,

(c) second fin means on said second section for effecting rotation thereof in a direction opposite to said one direction,

(a?) brake means between said sections and actuable to oppose relative rotation of said sections,

(e) said rotation of said second section being effective to overcome said rotation of said first section upon actuation of said brake means so that said first section passes through a condition of zero rotational velocity and undergoes a reversal of rotative direction,

(f) radiant energy sensitive means adapted to produce a brake actuating signal corresponding to energy falling thereon,

and

(g) scanning means in one of said sections for causing radiant energy from a target which is off said axis to fall on said sensitive means and actuate said brake means only when said net lift is acting in the direction in which said target is off said axis.

2. A target seeking missile as defined in claim 1 and wherein said scanning means comprises aperture means rotatable with said one section so that said reversal of rotative direction of said first section interrupts said energy falling on said sensitive means, and said brake means is alternately actuated and released to repeatedly reverse rotative direction of said first section so that said net lift provides continuous lateral effort for veering said missile toward said target.

3. A target seeking missile comprising:

(a) an elongated airframe having forward and after coaxial sections rotatable relative to one another about the longitudinal axis of the airframe,

(b) canard means on said [first section for effecting rotation thereof in one direction about said axis and for imposing a net lift normal to said axis and acting in a direction which is rotatable with said forward section,

() fin means on said after section for effecting rotation thereof in a direction opposite to said one direction,

(d) brake means between said sections and actuable to oppose relative rotation of said sections,

(2) said rotation of said after section being effective to overcome said rotation of said forward section upon actuation of said brake means so that said forward section passes through a condition of zero rotational velocity,

(1) radiant energy sensitive means mounted in said airframe and adapted to produce a brake actuating signal corresponding to energy falling thereon,

and

g) scanning means in said forward section for causing radiant energy from a target which is off said axis to fall on said sensitive means and actuate said brake means only when said net lift is acting in the direction in which said target is off said axis.

4. A missile as defined in claim 3 wherein said canard means comprises a plurality of air foils distributed about said forward section, a majority of said air foils being of like hand so as to effect said rotation and at least one of said air foils being of reverse hand and cooperable with one of said majority to produce said lateral effort.

5. A target seeking missile =as defined in claim 3 and wherein said scanning means comprises aperture means rotatable with said one section so that said reversal of rotative direction of said first section interrupts said energy falling on said sensitive means, and said brake means is alternately actuated and released to repeatedly reverse rotative direction of said first section so that said not lift provides continuous lateral effort for veering said missile toward said target.

6. A target seeking missile as defined in claim 5 and wherein said aperture means comprises energy shading means of graduated transparency whereby said brake means allows said forward section to rotate as necessary to maintain an image of predetermined intensity on said sensitive means and to maintain said lateral effort acting toward said target.

7. A target seeking missile comprising:

(a) an elongated airframe having an after section and a forward section rotatable relative thereto about the longitudinal axis of said airframe,

(b) stabilizer means on said after section for producing missile roll in one direction about said axis,

(0) canard means comprising a plurality of air foils distributed about said forward section,

(d) a majority of said air foils being of like hand for effecting rotation of said forward section opposite to said roll,

(2) at least one of said air foils being of reverse hand and cooperable with one of said majority to produce a lateral effort normal to said axis and constant in direction with respect to said forward section,

(1) radiant energy sensitive means in said after section,

(g) said rotatable forward section comprising optical scanning means for focusing a radiant energy image of a target which is off said axis, with said image having a position off said axis in a direction determined by the position of said target,

(it) said scanning means comprising an aperture ro tatable with said forward section and disposed on one side of said axis in a predetermined position so as to pass said image to said sensitive means only when said lateral elfort is acting in said target direction,

and

(i) brake means between said forward and after sections and responsive to the presence of said image on said energy sensitive means to oppose rotation of said forward section with respect to said after section whereby rotation of said forward section is overcome by said after section and said lateral effort acts to veer said missile toward said target.

8. A target seeking missile as defined in claim 7 and wherein said aperture means comprises image shading means having graduated transparency, said shading means being adapted to increase and decrease the image intensity so as to change the effect of said braking means to tend to maintain said fori ivard seciion yvit zh said lateral References Cited in file of this patent gliiolrtt ;;:tt111nsgaie5 rg1;1g1;ct ns1y to veer said missile into alrgn- UNITED STATES PATENTS 9. A tar-get seeking missile as defined in claim 7 and 2,911,167 Null et a1 1959 wherein said canard means comprises four air foils and 5 said majority comprises three air foils of like hand.

Claims

1. A TARGET SEEKING MISSILE COMPRISING: (A) AN ELONGATED AIRFRAME HAVING FIRST AND SECOND COAXIAL SECTIONS ROTATABLE RELATIVE TO ONE ANOTHER ABOUT THE LONGITUDINAL AXIS OF THE AIRFRAME, (B) FIRST FIN MEANS ON SAID FIRST SECTION FOR EFFECTING ROTATION THEREOF IN ONE DIRECTION ABOUT SAID AXIS AND FOR IMPOSING A NET LIFT NORMAL TO SAID AXIS AND ACTING IN A DIRECTION WHICH IS ROTATABLE WITH SAID FIRST SECTION, (C) SECOND FIN MEANS ON SAID SECOND SECTION FOR EFFECTING ROTATION THEREOF IN A DIRECTION OPPOSITE TO SAID ONE DIRECTION, (D) BRAKE MEANS BETWEEN SAID SECTIONS AND ACTUABLE TO OPPOSE RELATIVE ROTATION OF SAID SECTIONS, (E) SAID ROTATION OF SAID SECOND SECTION BEING EFFECTIVE TO OVERCOME SAID ROTATION OF SAID FIRST SECTION UPON ACTUATION OF SAID BRAKE MEANS SO THAT SAID FIRST SECTION PASSES THROUGH A CONDITION OF ZERO ROTATIONAL VELOCITY AND UNDERGOES A REVERSAL OF ROTATIVE DIRECTION, (F) RADIANT ENERGY SENSITIVE MEANS ADAPTED TO PRODUCE A BRAKE ACTUATING SIGNAL CORRESPONDING TO ENERGY FALLING THEREON, AND (G) SCANNING MEANS IN ONE OF SAID SECTIONS FOR CAUSING RADIANT ENERGY FROM A TARGET WHICH IS OFF SAID AXIS TO FALL ON SAID SENSITIVE MEANS AND ACTUATE SAID BRAKE MEANS ONLY WHEN SAID NET LIFT IS ACTING IN THE DIRECTION IN WHICH SAID TARGET IS OFF SAID AXIS.