US 3458197 A
Description (OCR text may contain errors)
July 29, 1969 w. c. WOCDWARD 3,458,197
CONSUMABLE INFRARED FLARE TOW TARGET Filed July 15, 1966 INVENTOR.
W ILMER C. WOODWARD TTORNEY United States Patent 3,458,197 CONSUMABLE INFRARED FLARE TOW TARGET Wilmer C. Woodward, Blue Bell, Pa., assignor to the United States of America as represented by the Secretary of the Navy Filed July 15, 1966, Ser. No. 565,498 Int. Cl. F41j 9/10; B64d 3/02 US. Cl. 273-1053 11 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates to aerial tow targets and more particularly to a consumable flare tow target which can simulate an infrared heat source of a target aircraft thereby enabling its use in missile firing exercises of infrared seeking missiles.
Aerial tow targets have long been employed in practice exercises for surface-to-air missiles and air-to-air missiles. In general, infrared (IR) flares are commonly used as one means of simulating the IR heat source of a target aircraft. The heat source may be carried on the wing tips of a powered target drone or installed within the fins of a tow target by means well known in the art. Such targets, however, are too expensive to be expendable and, accordingly, there is a great need for an economical tow target that is small in size and does not require complex launching requirements normally required for carrying and launching the relatively larger size tow targets. Additionally, some prior art towed targets employing the use of IR flares installed within the fin area of the tow target produce uneven radiation patterns (due to the fin surfaces) and hence do not properly simulate target aircraft and missiles.
The general purpose of the present invention is to provide a consumable IR flare tow target which can provide an economical IR source for missile firing exercises and because of its small size and compactness, does not require complex launching equipment normally required of tow targets. Additionally, by making the tow target a self-contained fin stabilized flare, so designed that the trailing portions of the fins are consumed by the intense heat of the burning flare, a full rear hemispherical IR radiation pattern is produced at all times during the burning period.
It is contemplated that the aerial tow target will have an aerodynamically stable configuration for flight throughout the burning period; that is, as a portion of the flare and fins is burned away, the center of gravity of the target remains forward of the aerodynamic center, thereby providing increased static stability to the tow target as well as a continual reduction in target pitch attitude.
An object of the present invention is therefore to provide a target which may be towed from an aircraft and provides a full rear hemispherical IR radiation pattern, and in which the target itself is consumable with the burning of the IR flare.
Another object is to provide a nose-towed consumable IR flare target which has aerodynamic flight stability in both the unconsumed and partially consumed conditions and in which unevenness in the burning is minimized.
A further object of the invention is to provide an economical infrared source for missile firing exercises which is small in size, does not require complex launching equipment and is fully expendable.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:
FIG. 1 is a side view of a partially consumed target being towed by an aircraft, the scale of the target being increased relative to the aircraft;
FIG. 2 is a side view of a towed IR target according to an embodiment of the invention;
FIG. 3 shows a section of the tow target taken on the line 3-3 of FIG. 2 looking in the direction of the arrows;
FIG. 4 illustrates an alternate fin arrangement for the target; and
FIG. 5 is a side view of an alternate fin configuration for the target.
Referring now to the drawing, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIG. 1 an embodiment of the invention in which an aircraft 11, having a tow line 12 extending therefrom, is attached to a consumable flare tow target 13 by means of a swivel mechanism 14. The consumable flare tow target 13, hereinafter referred to as a tow target, is shown in the partially consumed condition, with the consumed portion shown by the dashed lines. The tow target 13 has a slight positive angle-of-attack or pitch attitude with respect to its horizontal line of motion for providing aerodynamic lift and stability as will be described hereinafter.
The aircraft 11 may be either a manned or unmanned aircraft, the latter being referred to as a drone and may obviously contain more than one tow target to be released at selected intervals either by the pilot in the case of a manned aircraft or by remote control means in the case of a drone.
Referring now to FIG. 2, the tow cable 12 is attached to the tow target 13 at a tow line connection point 15 along the nose portion thereof. The swivel mechanism 14 may be of any variety well known in the art so long as it permits free movement of the target about its longitudinal axis in flight. Typical prior art swivels which may be employed for this purpose are disclosed in Humphrey et al., US. Patent No. 3,065,967, and Edwards US. Patent No. 3,075,726.
The tow target 13 consists of an IR flare 17 having an elongated tubularshaped housing with a nose cap 16 firmly attached thereto, to which the towline connection point 15 is secured. The IR flare 17 is encompassed within a skeletal structure containing triangular-shaped stabilizing fins 18 equally spaced along the longitudinal axis of the flare body. While the illustrated embodiment shows four triangular-shaped fins disposed axially about the flare, it is obvious that more or less differently shaped fins may be employed without departing from the spirit and scope of the invention.
The cruciform fins 18 have an L-shaped cross section as illustrated in FIG. 3 and are rigidly attached along the longitudinal axis of the flare body 17 by means of liquid solder, cement or other appropriate bonding means. The fins are also attached to the nose cap 16 by screws, rivets or the like at points 19 and to the flare body by a strap 20 at the rear section of the fins 18. A plurality of straps spaced at selected intervals along the longitudinal axis of the body member may also be employed if so desired, in preference to or in addition to the bonding material.
The flare 17 may be of any of a variety well known to those skilled in the art for providing an IR radiation pattern provided, however, that the heat generated by the flare be of such an intensity as to cause disintegration of the flare housing along its longitudinal axis at a rate at least equal to the combustible rate of propagation of the flare substance itself. This may be accomplished by selecting a housing material having a heat of combustion (liquification or vaporization) characteristic such that the intensity of the heat generated by the burning flare causes disintegration (such as by oxidation, liquification or vaporization) of the selected housing material. For example, referring to the Handbook of Chemistry and Physics, third edition, page 1900, it can be found that the melting point of aluminum is 659.7 C. and the boiling point (or point of vaporization) is 2450 C.; then if the flare housing were made of such aluminum, it would be necessary for the flare to generate a sufficiently high temperature to either melt or vaporize the metal housing in order to provide a completely consumable flare. Flare compositions for producing these high temperatures are well known in the art and may comprise magnesium, aluminum and Teflon in selected proportions to obtain the desired temperatures.
The Mark 37 flare has been found to have suflicient pyrotechnic characteristics that, when enclosed in an aluminum housing (using 60-61 (T6) aluminum) with fins and strap of the same material, the entire device was consumed as previously described within eight to ten minutes after ignition. By this technique then, as the flare burns from the aft portion to the forward portion thereof, no portion of the stabilizing fins or flare body will remain; therefore, there will be no interference with the IR radiation pattern presented by the flare and a full rear hemispherical radiation pattern will result.
Alternatively, the flare housing may be of a non-metallic substance having a low combustion temperature thereby requiring less heat from the flare for total disintegration. Obviously, many other flare housing materials may be employed by those skilled in the art in view of the above teachings.
Referring now to the aft section of the flare, there is illustrated in FIG. 2 an explosive squib 21 mounted in a disc 22 secured within the inner diameter of the flare 17. The squib 21 may be actuated by an electrical signal through a conductor 23 from a control circuit within the aircraft 11 at the time of deployment. The control circuit may typically be a pulse generator, relay controlled electrical source or a multivibrator. Since such control circuits are well known to those skilled in the art, they will not be described in any greater detail. By actuating the squib 21, the disc 22 and conductor 23 are blown clear of the flare and at the same time the pyrotechnic material 24 contained with the flare housing is ignited.
Considering now the aerodynamic stability problems involved in the design of the tow target, it can be seen that the fin configuration must be designed to possess adequate stability for the target in towed flight in both the unburned and partially burned conditions. This may be accomplished by employing triangular shaped fin panels with their leading edges extending from the forward end of the flare cylinder at such an angle so that the aerodynamic center of the fins is approximately at the same longitudinal position as the center of gravity of the unburned flare. This configuration may be best illustrated by the following example.
Assume that it is desired to maintain the tow target pitch attitude at or less in towed flight at an anticipated speed of Mach 0. 8 and at an altitude of 35,000 feet. Additionally, assume that the gross weight of the tow target is lbs. and that due to the homogeneous nature of the flare material contained within the flare 17, the center of gravity of the flare alone will be at its midpoint. Then if the overall length of the flare from point a to point b is 20 inches and the diameter thereof is 2.5 inches, it is therefore necessary to provide stabilizing fins of such dimensions as to provide aerodynamic lift and stability at the above-mentioned speed and altitude. The embodiment disclosed herein illustrates the use of triangular shaped fin panels 18 with their leading edges extending from point a to point 0 for a distance and at an angle with the flare body determined by the desired flare pitch attitude. Since it is desired to maintain the aerodynamic center of the tow target at the same position as the center of gravity of the flare (along the longitudinal axis), it is necessary to select a root chord length and an overall span characteristic to meet this criteria. Using static equations, it can be seen that a root chord of 15 inches ((1 to d) and a fin dimension (0 to d) of 3 inches will provide an aerodynamic center of the fins at approximately the same longitudinal position as the center of gravity of the unburned flare. By using trigonometric functions, the angle can be determined and is found to be approximately 11.3 degrees. The ratio of the flare diameter to the overall span (0 to e) is then equal to 0.294. Using this value and the angle of 11.3", it can be found from Table II in the US. Army Ordnance Missile Command Report No. -RFTR621 (revised) that the value of the normal force coeflicient, C,,, a force coeflicient normal to the flare body axis, is equal to 16.5 per radian or 0.288 degree.
Based upon this data, the flare pitch attitude or angle of attack of the tow target necessary to balance the 10 lb. weight of the target is found to be:
Where W is the weight of the target, q is the dynamic pressure, A is the tow target frontal area and C is the normal force coeflicient. At the selected speed and altitude of Mach 0.8 and 35,000 feet, the dynamic pressure, q, is 222 lbs. per square foot, A is 0.341 square feet and C is 0.288 per degree; therefore, a is approximately equal to 4.6. Thus, the pitch attitude or angle of attack of the nose-towed target at Mach 0.8 and altitude of 35,000 feet will be 4.6 for the unburned flare tow target. As the flare burns, the center of gravity will continuously move forward while the aerodynamic center of gravity of the fins remains essentially fixed until the fins themselves commence to be consumed. As the fins are being consumed, the center of gravity of the tow target will remain forward of the aerodynamic center, providing increased static stability to the target as well as a continual reduction in pitch attitude.
To allow for the unevenness in the trailing edges and possible warpage of the fins due to the intense heat and burning action of the flare, the tow target is provided with the swivel mechanism 14 for permitting the target to rotate about its longitudinal axis. This rotation, however, will in no way reduce the effectiveness of the rear hemispherical radiation pattern since the pitch attitude and stability of the tow target is still maintained.
As can be seen from the previous equation, the angle of attack or pitch attitude of the unburned IR tow target is inversely proportional to the flight dynamic pressure. For example, at Mach 0.8 and an altitude of 10,000 feet, the angle of attack would be 1.6. By using this equation, a plot of dynamic pressure as a function of equivalent airspeed can be constructed with the angle of attack as the dependent variable, thereby providing data on the angle of attack for various attitudes.
The preceding discussion has accordingly illustarted a technique for determining the dimensions of a typical tow target for use in the disclosed embodiment; however, this information has been presented for purposes of illustration only and is not to be considered by way of limitation.
FIG. 3 illustrates a cross sectional view of the tow target looking in the direction of the arrows 3-3. The
thickness of the cruciform fins 18 is illustrated as being uniform in cross sectional area. However, it is anticipated that the cross sectional area may of a variable nature, in particular it may be similar to that illustrated in FIG. 4 in which the cross sectional area of the fins 18:: decreases with increasing distance from the flare body. This latter configuration could be most advantageously employed where the tow target is flown at high rates of speed such that the heat generated by the burning of the IR flare is quickly dissipated and the outer portions of the fins are not evenly burned. Therefore, by tapering the fins as shown, less heat is required to burn the outer areas than is required for the inner areas. Accordingly, uniform burning can be maintained even though there is a varying temperature gradient extending outwardly from the flare axis.
An additional means for insuring even burning of the fins is illustrated in FIG. 5 wherein slots 25 are selectively spaced along the length of the fins 18b so that as the flare burns from the aft section to the forward section, any unevenness in the burning will be minimized. This segmented type fin construction may also be achieved by doubling the number of fins and alternately segmenting the adjacent fin so that the same total lift area is main-' tained while providing burning of only one alternate fin at a time. Obviously many other segmented fin arrangements can be employed for achieving the same purpose without departing from the spirit and scope of the present invention.
As a result of the small size and weight of the consumable IR tow target, it is necessary to carefully consider the stability problems encountered by the tow target and the towline combination in towed flight. In particular, for the tow target described above, it is estimated that about 5 lbs. of drag is exhibited by the tow target in a flight condition of Mach 0.8 and 35,000 feet altitude. By employing the target-towline stability criterion which is applicable to long towlines, it is found that the true airspeed must not exceed the velocity of wave propagation downstream of the towline by an empirically determined factor of approximately 1.5. At the above-mentioned airspeed and altitude, the true airspeed of the tow target is 674 feet per second. The tension, T, in the towline is the x/D -l-W where D is equal to the drag force and W is equal to the net weight of the tow target. In this case, where the drag force is 5 lbs. and the net weight 10 lbs., the tension, T, is approximately equal to 11.2 lbs. for the unburned target.
The stability factor for a towed target is defined by the following equation:
where K is the stability factor, V is the true airspeed of the target, T is the tension on the towline and p. is equal to the mass density per linear foot of the towline. Using this equation andconsidering various type cables such as -inch steel, A and -inch nylon cable, it is possible to determine the cable diameter necessary to meet the stability criterion of 1.5 over a range of anticipated flight conditions.
An additional factor to consider in the selection of the tow cable is the method of deployment of the tow target. For example, when employing the unrestricted free-fall release method, the cable is subjected to a high impact force which may be of suflicient magnitude as to cause the cable to break. To alleviate this situation, larger size cables may be employed or restricted free-fall release mechanisms, such as described by E. J. McQuillen in US. Patent No. 3,211,396, may be used for reducing the impact forces.
In operation then, an aircraft 11 containing one or a plurality of IR tow targets either within the aircraft or attached to the outer structure thereof is flown at a parrticular altitude and speed in accordance with the capabilities of an IR seeking missile. Upon a command to deploy the tow target, a latch mechanism (not shown) within the aircraft releases the target and at the same instant an electrical signal is conducted to the squib 21, causing it to explode and ignite the IR flare 17. The force of the explosionalso causes the disc 22 and associated conduc tor 23 to be blown clear of the target, thereby allowing the target to fall freely until the total length of the cable 12 has been expended. At this time, the target, being towed from the nose cap .16, will orient itself as shown in FIG.-1 and will continue to burn, thereby emitting a full rear hemispherical IR pattern for the IR seeking missile. As described previously, in the event of uneven burning or warpage, the tow target will rotate along its longitudinal axis while maintaining the same pitch attitude.
After the tow target has been consumed, and it is necessary to deploy a second tow target, the cable 12 maybe released from the aircraft and the second tow target may be deployed from a target done and that as Although the above description has made reference to an aircraft towed target, it is anticipated that the tow target'may be deployed from a target drome and that as a result of the small size, weight and drag characteristics of the tow target, it is ideally suited for multiple installations on a single target drone since the drag caused by the tow target is so small that it will not degrade the performance of the drone itself. After deployment and consummation of the first tow target, the towline may be released therefrom by electrically operated cutting means (not shown) actuated by a signal from the drone control system and a second tow target released in a manner similar to the first.
Accordingly, there is described herein a completely consumable flare tow target which provides an unobstructed radiation pattern and which remains aerodynamically stable throughout the burning period and in which unevenness in the target burning is minimized.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. A completely consumable tow target comprising:
completely combustible flare means adapted to be towed by an aircraft; and
completely combustible means disposed on said flare means for providing aerodynamic stability throughout the entire burning period.
2. A consumable flare tow target as recited in claim 1 wherein said flare means comprises:
an elongated housing member; and
a pyrotechnic material contained in said housing member for causing disintegration thereof along its longitudinal axis.
3. A consumable flare tow target as recited in claim 2 wherein said flare means further comprises:
an igniter on the aft end of said housing operatively connected to said material for igniting said material.
4. A consumable flare tow target as recited in claim 3 wherein:
said completely combustible means projects radially from said housing and is made of a material which disintegrates at a rate substantially equal to that of said housing.
-5. A consumable flare tow target as recited in claim 4 wherein said stabilizing means comprises:
a plurality of fins spaced about the longitudinal axis of said housing providing an aerodynamically stable configuration.
'6. A consumable flare tow target as recited in claim 5 wherein said flare means further comprises:
a swivel means operatively connected to said housing for permitting rotation about the longitudinal axis thereof.
7. A consumable flare tow target as recited in claim 5 wherein said pyrotechnic material emits infrared radiation signals.
8. A consumable flare tow target as recited in claim 5 wherein said plurality of fins are slotted along their length, whereby unevenness in their disintegration is minimized.
9. A consumable flare tow target as recited in claim 5 wherein said plurality of fins are tapered transversely along their length.
10. A consumable flare tow target comprising:
a disintegratable housing having an opening at the downstream end thereof;
completely combustible means disposed on said housing for providing aerodynamic stability throughout the entire burning period; and
a pyrotechnic material contained in said housing and having an exposed surface at said opening, said material upon ignition at said surface progressively burning and simultaneously disintegrating said housing and said complete combustible means toward the upstream housing end. 11. A consumable flare tow target as defined in claim 10 wherein:
References Cited UNITED STATES PATENTS Lloyd. Humphrey et al. Edwards. Hopper et al. Norman et a].
ANTON O. OECHSLE, Primary Examiner MAX R. PAGE, Assistant Examiner US. Cl. X.R.