US3309044A

US3309044A - Method and apparatus for arresting a missile

Info

Publication number: US3309044A
Application number: US475164A
Authority: US
Inventors: John S Strance; Arthur G Condodina; Silver Floyd
Original assignee: EW Bliss Co Inc
Current assignee: EW Bliss Co Inc
Priority date: 1965-07-27
Filing date: 1965-07-27
Publication date: 1967-03-14
Anticipated expiration: 1984-03-14
Also published as: CH441054A; GB1141222A

Description

March 1967 J. s. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 27, 1965 4 Sheets-Sheet 1 STROKE FT.

$-22? m ITILIFII'FILILIL INVENTORS JOHN s. STRANGE BY ARTHUR G.CONDOD|NA FLOYD SILVER 7M 41 M, 1%? J. 5. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 27, 1965 4 Sheets-Sheet 2 I INVENTORS JOHN S.STRANCE BY ARTHUR G.CONDODINA FLOYD SILVER THEM/ 3! W March 14, 1967 J. 5. STRANGE ETAL METHOD AND APPARATUS FOR ARRESTING A MISSILE 4 Sheets-Sheet 5 Filed July 2'7, 1965 HAN W AW! Fig 70 INVENTORS JOHN SSTRANCE BY ARTHUR G.CONDODINA FLOYD SlLVER March 14, 1967 J. 5. STRANGE ETAL 3,309,044

METHOD AND APPARATUS FOR ARRESTING A MISSILE Filed July 2'7, 1965 4 Sheets-Sheet 4 V v Q, (L P 3 ;L\Q I l 17* M 1 v I N VEN TORS JOHN $.5TRANCE y ARTHUR G.CONDOD|NA United States Patent Ofilice 33%,044 Patented Mar. 14, 1967 3,309,044 METHOD AND APPARATUS FOR ARRESTING A MISSILE John S. Strance, Drexel Hill, Arthur G. Condodiua, Philadelphia, and Floyd Silver, Secane, Pa, assignors to E. W. Bliss Company, Canton, ()liio, a corporation of Delaware Filed .Iuly 27, 1965, Ser. No. 475,164 7 Claims. (Cl. 244-4110) This invention relates to a missile arresting system and method for absorbing the kinetic energy of a missile while in flight so as to bring it to a controlled stop within a limited vertical distance.

More specifically, the invention will be described with reference to the arrestment of a missile in free flight by means of a rotary, reel-type, arresting engine similar to that disclosed in US. patent application Ser. No. 441,560 filed Mar. 22, 1965, which employs hydraulically actuated friction disc reel brakes as the energy absorbing medium, however it will be appreciated that any suitable energy absorbing device may be employed so long as it has a quick response performance capability.

In test firing missiles it is customary to obtain tele- Inetry data beginning with the initial lift-01f and continuing for a specified period depending on the type of flight information to be obtained. Where the telemetric data desired is primarily available only in the first several feet after firing, the remainder of the flight is merely an exercise in ballistics with the missile impacting at a specified point down range. In the absence of complicated soft landing recovery equipment, this results in the destruction of the missile shell and any telemet-ric equipment on board. Obviously therefore, such one-shot testing has been a substantial factor in the soaring costs of missile testing programs.

Another problem is that advanced clearance is required from government authorities when the trajectory of a missile will take it above a certain altitude. Merely scheduling the test thus becomes a difficulty in itself.

These and other problems are overcome with the present invention which has as a primary purpose the provision of a missile arresting method and apparatus capable of engaging the missile after a period of sustained free flight and then bringing it to a controlled stop in a restricted space.

In accordance with the invention, a missile is fired upon a course at a given speed and after a period of free flight is restrained by an arresting gear including a rotatable reel and a linear purchase tape considerably wider than it is thick so as to permit coiling upon itself about the reel in ever increasing convolutions. The running end of the tape is engaged by the missile at the end of its free flight period and carried aloft by it. The tape is woven of synthetic yarns having a sufiiciently low modulus of elasticity so that the portion paid-out between the reel and missile will stretch longitudinally and maintain engagement with the missile until the reel can accelerate to supply the tape needed to follow the missile. Rotary friction brakes on the reel reduce the rate of tape payout thus slowing the missile to a stalling speed. And a speed sensing control is arranged to program the brake pressure so that energy stored in the tape as stretch is relieved prior to bringing the missile to a controlled stop at its apogee.

Further in accordance with the invention, the missile is guided vertically and a plurality of reel-type arresting engines are positioned equidistant from the missile launcher, the arrangement being such that the missile is launched with each tape attached to its shell by means of a loose bridle, the length of which equals the desired free flight distance and the running end of each purchase tape is releasably anchored adjacent the launcher so that after launch each tape and bridle make an oblique angle with the vertical when tensioned at the moment of missile impact on the system.

Further in accordance with the invention, each arresting engine includes a stator member, a reel rotatably mounted on the stator member, a plurality of friction disc elements carried by the reel and extending radially in interleaved fashion with stationary friction disc elements carried by the stator member and hydraulic control means for applying braking pressure from opposite sides of the reel in accordance with a missile braking program, the reel, rotary friction disc elements, and tape stack on the reel constituting a low inertia system capable of quick response to the longitudinal stress waves generated at the running end of the tape.

The invention also contemplates a method for arresting a missile comprising the steps of restraining the missile after a period of free flight with a woven purchase tape made of synthetic yarn placed in tension by means of oblique impact with the missile, paying out additional tape from a reel upon which the tape is coiled at a rate suflicient to prevent excessive tape strain, reducing the rate of tape payout after the reel has accelerated to missile velocity and thereby slowing the missile to a stalling speed, increasing the rate of tape payout as the missile approaches zero velocity to relieve tape strain, and halting tape payout after the strain has been relieved and as the missile comes to rest.

The principal object of the invention is to provide an arresting gear and method for arresting a missile tethered by means of a purchase member to an energy absorbing means in such a fashion that after an initial period of free flight the missile impacts with the arresting gear and is brought to a controlled stop by the energy absorbing means within a limited distance.

Another object is to provide an arresting gear employing a woven tape of synthetic fibers as the purchase member characterized by having a low modulus of elasticity so that the tape is capable of suflicient elongation at oblique impact angles with the missile whereby its ulti- 'mate stress is not exceeded before the tape payout means can be set in motion to feed additional tape into the system.

Another object is to provide an energy absorbing and purchase tape payout gear having a relatively low effective mass with its complement of tape stored thereon so as to readily respond to the longitudinal stress wave impulses transmitted from the running end of the tape.

These and other objects will become apparent by reference to the following description and drawings wherein:

FIGURE 1 is a front elevation schematic illustration of a missile launching installation utilizing diametrically opposed reel-type arresting engines as contemplated by the invention;

FIGURE 2 is a side view of the installation shown in FIGURE 1 illustrating the missile position at the apogee of its flight path;

FIGURE 3 is a diagram showing a family of curves representing the relationship of oblique impact stress on a nylon tape system;

FIGURE 4 is a diagram showing the relationship between tape tension, missile velocity and payout rate at various missile heights during the dynamic period of the arrestment;

FIGURE 5 is a diagram depicting arresting performance during the entire arrestrnent stroke;

FIGURE 6 is a perspective view of an arresting engine of the type used in the missile arresting system shown in FIGURE 1;

FIGURE 7 is a partial cross-sectional view of the arresting engine in FIGURE 6 taken approximately along line 7-7 of FIGURE 6;

FIGURES 8 and 8a are enlarged views showing the arrangement of the tape to tape connector and tape to missile connector for the missile arresting system of FIGURE 1; p

FIGURE 9 is a diagram of the speed. responsive hydraulic control system for operating the rotary friction brakes of the arresting engine in FIGURE 7;

FIGURES l and 10a are enlargements of the braking program control cam and valve of the hydraulic system shown in FIGURE 9 and a cross-section of the pressure control valve element showing the pressure relief ports therein; I

FIGURES l1 and 11a are schematic views representing a modification of the invention as a below ground missile launching installation in which the missile engages parallel horizontal pendants at ground level; and

FIGURE 12 is another modification of the invention showing the use of a single arresting engine with vertical payout of the purchase tape.

Referring now to the figures where the showings are for the purpose of illustrating a preferred embodiment of the invention only and not for the purpose of limiting same, FIGURES 1 and 2 show in more or less schematic form a missile launching and testing installation 10 including a launching tube 12 housing a missile 13 in a position ready for firing along a vertical flight path 15. The missile 13 reaches the apogee of its flight path 15 at the completion of the test as shown in FIGURE 2.

Cable guides

16, 17 are suspended at their upper ends from a boom 18 by means of a winch and cable mechanism generally indicated at 19 and are anchored at their lower ends within the launching tube 12 in any known manner. The missile 13 carries diametrically opposed, one-way

cable grip devices

22, 23 which ride on the

cables

16,17 respectively as the missile is propelled aloft. The

cable grippers

22, 23 may be of any known type which permit the cable to slip in one direction yet lock and. hold against movement in the opposite direction.

As will be appreciated by those familiar with missile testing procedures, the installation shown in FIGURES 1 and 2 is primarily useful in the situation where the telemetric data desired is available during the first instant after liftoff which means the remainder of the missile flight is normally useless so far as taking readings on missile flight behavior are concerned. In the example chosen for illustrating the invention, the test is compieted after th missile has been propelled aloft a free flight distance of sixty-seven feet as indicated by the elevated missile position in FIGURE 1. This corresponds to the end of the test and the beginning of the arrestment period designed to bring the missile 13 to a controlled stop at its apogee (FIGURE 2). Throughout the test the missile is tethered to a missile arresting gear 24 which is designed to not interfere with the period of free flight as will be explained hereinafter.

The arresting gear 24 generally includes two arresting engines 25, as each spaced at equal distances from the launching tube 12 and. 180 degrees apart facing the missile. Each arresting

engine

25, 25 includes a

rotatable reel

27, 28 adapted to store a continuous coil of

purchase tape

29, 30 to be paid out over

sheaves

31, 32 during the arrestment stroke. Prior to launch. each

tape

29, 30 is releasably anchored at the periphery of the launching tube 12 in a manner to be described and is connected to the missile 13 by the means of a

loose bridle

34, 35 constructed in the same manner as the

tapes

29, 30. The length of the

bridles

34, 35 is suflicient to allow for the period of free flight before the arresting gear is impacted. At the battery position or moment of impact, the anchored ends of

purchase tapes

29, 30 are simultaneously jerked. free as the

bridles

34, 35 are tensioned as shown by the dotted lines in FIGURE 1. The

purchase tapes

29, 30 are then paid out from the

reels

25, 26 at a programmed controlled braking rate so that the missile 1'3 is brought to a stop (FIGURE 2) whereupon it is held by the boom 18 from falling back to the ground. The boom Winch mechanism 19 may then be operated to lower the missile and the

guide cables

16, 17 thus saving it and the telemetric equipment on board for another test.

To better understand the present invention, a more detailed consideration of the theoretical concepts involved should be considered. By way of analogy, the arrestment of an aircraft on short runways provides a background for the arrestment of a missile. For a thorough deveio'pment of the principles involved in arresting an aircraft, reference is made to United States Reissue Patent No. 25,406 issued June 25, 1963, which discusses in considerable detail the arresting properties of arresting gear employing a woven tape of synthetic yarn such as nylon as a purchase member. Briefly, these benefits arise primarily from the fact that such synthetic yarns have a low modulus of elasticity as compared to that of steel. It will be readily recognized that a steel cable stretched across a runway must immediately acceierate from Zero to the speed of the aircraft or fail in tension. There is a finite time before the stress Wave front traveling at the speed of sound can reach and actuate the payout mechanism and provide the necessary extra cable at the center span demanded by the movement of the aircraft down the runway thus the cable must either be susceptible to continuous stretching (strain) at some rate in feet per second until the system physically moves to provide the fed in, or the arresting gear simply must fail in tension due to separation of the purchase member. Otherwise stated, there is an upper limit to longitudinal stress which can be applied. to a steel cable in excess of which the cable will fail in tension. This upper limit condition may be expressed by the equation:

where S equals the stress in p.s.i.; E is the modulus of elasticity of the cable in p.s.i.; V is the velocity of impact on the cable in feet per second; and C is the speed of sound in the cable in feet per second. An accepted modulus of elasticity for steel is about 12 l0 p.s.i., a C value of 10,000 feet per second and a maximum stress value of 240,000 p.s.i. Substituting these values in the equation it will be seen that the maximum velocity of impact sustainable by the cable cannot exceed 200 feet per second. As disclosed in the above-mentioned patent, the stretch necessary in the pendant, while not available from a steel cable alone, can be provided by the use of a nylon tape connected at the opposite ends of the steel pendant. The nylon tape provides substantially all the stretch needed while at the same time transmitting the initial longitudinal stress wave to the payout means putting it into motion before the maximum stress capacity of the nylon tape can be exceeded. Referring once again to the above equation, it may be seen that with nylon having a maximum strength capacity of about 50,000 psi, 21 modulus of elasticity of around 300,000 psi. and a speed of sound of 5,000 feet per second; the maximum allowable impact velocity is increased. from 200 feet per second to approximately 833 feet per second. By comparison, there is approximately a fourfold advantage in the use of a nylon tape primarily because of the much lower modulus of elasticity and While the maximum stress of 240,000 p.s.i. for steel and the speed of sound at 10,000 feet per second are Ofisetting factors in favor of the use of steel, they do not compensate for the fact that the modulus of elasticity of steel is about 40 times greater than that of nylon.

T-hees same principles as applied to the arrestment of an aircraft also hold true for the arrestment of a missile, however, an additional consideration is required in the case of a missile due to the geometry of the system at the instant of impact or at the battery position. That is, in the normal case of aircraft arrestment, the pendant is stretched taut across the runway so as to extend perpendicular to the glide path of the aircraft in the battery position. While in the case of arrestment of a missile, the missile initially engages the purchase member which extends at an oblique angle to the flight path known as the angle beta.

The angle beta at which the

purchase tape

29, 30 and

bridle extensions

34, 35 first become taut after being carried aloft by the missile is determined by the spacing of

sheaves

31, 32 and the requirements of the test as to the vertical free flight distance. In the example selected to illustrate the invention, the vertical free flight distance is 67 feet from launch and the sheave to sheave distance is 200 feet measuring on the horizontal between the

sheaves

31, 32. Thus the angle beta formed with the vertical by the

taut purchase members

29, 30, 34, 35 at the instant of impact is approximately 5 6 degrees (dotted line position in FIGURE 1) and the straight line distance from each

sheave

31, 32 to the missile is approximately 120 feet. This is the beginning of the arrestment stroke and is analogous to the battery position of a cross decked pendant in an aircraft arresting installation only in that case beta in 90 degrees. The main difference comes from the obliquity of the engagement. The eifect of oblique engagement may be more fully understood by referring to FIGURE 3 which shows a family of curves for angles of beta between zero and 90 degrees for oblique impact stress on a nylon tape. The two extremes are direct longitudinal engagement at which beta is zero degrees and perpendicular engagement where beta is 90 degrees. The curves show the relationship of the ratio of stress to modulus of elasticity plotted on the ordinate versus the ratio of the engagement velocity to speed of sound in nylon plotted on the abscissa. These values may be obtained by referring to the above formula EV S- C For the purposes of illustration it will be assumed that the missile velocity at battery position is in the order of 270 feet per second, thus a value of V/ C is obtained of 5.4 and by following the beta curve of 56 degrees, we find that the value of S/E is 3.5 and since for nylon E equals 300,000 p.s.i.; the impact stress is 10,500 p.s.i. For a tape 7 inches wide and 0.19 inch thick this is a tension of 14,000 pounds. By comparison, with an angle of beta at 90 degrees and assuming the same engaging velocity, we find that the value for S/E is approximately 1.7. Again multiplying by the modulus of elasticity of 300,000 p.s.i. this gives an impact stress of about 5,100 p.s.i. or approximately half the stress that was obtained on the oblique engagement. Thus the strain on the purchase tape varies inversely with the size of the angle beta, i.e., for lower angles of beta higher longitudinal stress waves will be generated in the purchase tape. Thus the requirements for tape stretch at oblique engagement will greatly exceed the requirements where the engagement is head on.

The dynamic period of missile arrestment is the period of time it takes the

reels

27, 28 to accelerate to a payout speed equaling the speed of the missile. The performance of the arresting gear 24 during the dynamic period may best be understood by reference to the chart shown in FIGURE 4 which breaks down the dynamic period into four increments beginning with the initial impact or battery position. Actually the increments selected are an arbitrary choice to facilitate discussion and the actual curve plots would be smoothed out versions of those shown in FIGURE 4. With the ordinate of the chart representing velocity in feet per second and tape tension in thousands of pounds and the abscissa missile height above ground in feet and using the previous example of a free flight distance of 67 feet, it is seen from the middle curve that at the battery position, tape tension goes from approximately zero to 14,000 pounds with a missile velocity of 270 feet per second (upper curve). During the time it takes for the longitudinal stress wave due to initial impact to travel down the

purchase members

29, 30, 34, 35, and back to the arresting

engines

25, 26, the missile will have traveled approximately 17 feet vertically which means a change in length of the purchase members of approximately 12 feet, all of which must come from stretch within the tape itself. The second increment of the chart is selected to begin at the instant the longitudinal stress wave reaches the arresting

engines

25, 26 which represents a time lag of about 0.064 second. The

reels

27, 28 now begin to accelerate under the influence of the 14,000 pound initial impact force and assuming a 40 percent tension increase at the reel due to reflection of the stress wave, reel acceelration is determined by the formula:

where A is acceleration in feet per second squared; T is the tape tension in pounds and Me is the effective mass of the rotatable reel and its complement of stored tape giving an acceleration of 1565 feet per second squared. Assuming an increment of time of 0.05 second, the reels will be playing tape out at the end of the second increment at about 78 feet per second for an average velocity of about 40 feet per second (bottom curve). This then is a payout length of about 2 feet. However, during this time increment, the missile will have traveled another 13 feet for a total of about 97 feet above ground bringing the total tape stretch requirements, by virtue of geometry of the system, to 20 feet. Since now the payout reels are rotating and have dumped approximately two feet of additional tape in the system, the net requirement for stretch in the tape itself is 18 feet or an additional stretch over that imposed by initial impact of approximately 6 feet. The tape stress is now up to about 15,100 p.s.i. (20,000 pounds tension) or approximately half of the allowable stress for nylon of 30,000 p.s.i. Thus with the missile having traveled 30 feet from the battery position, the tape stress has increased to approximately half its maximum allowable stress and the velocity of the missile has decreased from 270 feet per second to about 260 feet per second.

In contrast, it will be readily apparent that under the same conditions a purchase member having a high modulus of elasticity such as steel would fail before the

reels

27, 28 could be accelerated sufficiently to supply the needed feed in.

For the example selected, at the end of the dynamic period the missile velocity will be slowed to 250 feet per second at a stroke height of about 123 feet. By this time the

payout reels

27, 23 will have accelerated to the point where they are dumping tape into the system at the same velocity as the missile or approximately 250 feet per sec- 0nd and the tape tension will level off at 24,000 pounds. At this point and to prevent overspeed of the reels, the reel brakes are engaged in a programmed fashion to bring the missile to a controlled stop (FIGURE 2) within about 200 feet. The total time from launch to stop at the apogee is less than 1 second.

FIGURE 5 is a curve which represents the overall performance of the arresting gear showing the retarding force in pounds imposed on the missile at the various heights. It should be noted that after the dynamic period which ends at about 123 feet, a relatively flat curve is shown dropping off rapidly near the end of the arrestment stroke. The significance of this portion of the curve will become more apparent from the following discussion.

- Referring now to FIGURES 6 and 7, the arresting engines '25, 26 are reel-type units similar to those disclosed in the aforementioned United States patent application Ser. No. 441,560. Since the

engines

25, 26 are identical, a description with respect to one will be understood as applying equallyto the other. With respect to engine 25, the purchase tape 29 is a woven construction of synthetic yarns considerably wider than it is thick so as to permit coiling upon the reel 27 of the engine 25. For details of such a Woven tape reference is made to US.

patent application Ser. No. 461,123 filed lune 3, 1965. The tape 29 is stored on the reel 27 and is paid out through a tape duct 37 and the sheave 31. After arrestment the tape can be detached from the missile and the etraction motor 38 clutched in to recycle the reel 27 and rewind it to a position ready for the next ar'restment. The tape 29 is connected to the bridle extension 34 by means of a tape to tape connector 41) shown in FIGURE 8 which is merely a lightweight metal block having transverse slots 42 in each end adapted to receive the ends of tape 29 and bridle extension 34, each of which is molded to the configuration of the slots 42 so as to slide laterally into the slots 42 thus holding the ends securely against longitudinal movement. Since the tapes are of a woven nylon material it is preferable to form the contoured ends by means of molding a hard, plastic material around them as disclosed in US. patent application Ser. No. 438,459 filed Feb. 19,1965 and now Patent No. 3,263,289. The tape to tape connector 40 has a rod 45 extending laterally from opposite sides thereof which rests in ears 4-7 projecting outwardly from the lip of the launching tube 12. All of the slack is taken out of the tape 29 by a slight pretensioning against the cars 47. The bridle extension 34 is not under any tension and passes over the lip of the launching tube 12 for attachment with the missile 13 by tape to missile connector 423. The connector 48 is pivotally mounted at 49 to a pair of laterally projecting lugs 51) mounted on a circular band 51 girding the missile. The swivel arrangement permits the connector 43 to swing as the missile is launched.

It is important to note in FIGURE 7 that the reel 27 is rotatably mounted at 53 upon a stationary drum 54 to provide a low inertia construction. The reel 27 is radially spaced from the drum 54 to define a brake chamber 56 sealed at opposite ends by rotary seals 57. A rotary friction brake 58 is mounted within the brake chamber 56 and comprises an annular array of hydraulic brake actuators 60 mounted on opposite ends of the drum 54 and arranged in diametrically opposed relationship. A plurality of brake stator members 61 are guided in keyways on the drum 54-. Rotary disc brake elements 62 are attached to the reel 27 in a similar fashion and the stators 61 and rotors 62 are radially interleaved with each other across brake chamber 56. Hydraulic lines 63 attached to the brake cylinders 60 connect with an hydraulic system (FIGURE 9) provided for operating the brakes in accordance with the requirements of the missile arresting application.

Attention is now directed particularly to the hydraulic system for operating the disc brakes shown schematically in FIGURE 9 and it will be understood that while the description is with respect to the system associated with arresting engine 25, an identical system would be provided for arresting engine 26 and the two engines would be operated in unison. In FEGURE 9 a static oil reservoir 113 is maintainable under pressure in any known manner. Oil is directed through line 117 to shuttle valve 119. Pressure in branch 117A shifts piston 121 to the right thereby connecting branch line 117B to brake actuator feeder line 121F which is connected to hydraulic line 63 of the brake 58. Static brake pressure tank 113 maintains slight pressure on brake 58 which allows tape 29 to be tensioned so as to take up any slack between the reel 27 and the tape to tape connector 40. The gear is now in readiness for the launch of the missile (FIGURE 8). After launch and at the moment of impact (FIG- URE 1), the

bridles

34, 35 become taut and a longitudinal stress wave travels down the

purchase members

29, 30, 34, 35 in a finite time which, in the previous example, is 0.064 second for nylon tape. When this wave reaches the reels they immediately begin to rotate due to their low inertial force. Chain driven by each

reel

27, 28, is a hydraulic pump 126. As pump 126 is operated hydraulic fluid is drawn from pump fluid reservoir 129 through line 131 connected on the low pressure side of the pump through pump 126 and then outwardly on the high pressure side of the pump through line 133. Line 137 is tapped from line 133 at junction 135 and is conected to a valve 139 which is opened at some predetermined value to control the return of fluid through the line 141 to the brake fluid tank 129. The valve opening is calculated to satisfy the performance requirements and geometrical configuration of each application as indicated by parameters such as allowable decelerating loads, sheave span, missile weight, available runout, etc. For example, if missile weight is high in relation to available runout, the valve 139 will be more closed at the beginning of arrestment; or likewise if the sheave span is wide so as to increase braking torque to compensate for a lower tape tension due to the geometry of the system. The fluid also is carried in line 143 through check valve 145 to the right side of the spool valve 119. As fluid pressure builds in the system corresponding to the increase of velocity of the reel, the static pressure side of the valve 119 applied by line 117A is overcome and the spool piston 121 is shifted to connect feeder lines 134F and 121F whereupon the pressure from the pump 126 replaces the minimal static pressure from the static brake pressure tank 113. 7

It is important that the reels begin to rotate as quickly as possible when they sense that the system has been impacted by the missile. Thus the static pressure is very slight and while technically speaking the reels are not free wheeling, the static pressure can be disregarded as a load on the system when compared to the inertial forces resisting rotation by virtue of the reel masses and the complement of purchase tape. After the reels are accelerated to a tape payout speed equal to the velocity of the missile, the pump 126 will be delivering pressure at a value that exceeds that from tank 113 which signals the approaching end of the dynamic period and the beginning of the programmed braking period. This is the point of transition between the initial erratic and the flattened portion on the performance curve shown in FIGURE 5. The arrangement is such that as the reels 25, 2e rotate, the length of moment arm due to the tape Wrappings is decreasing. This tends to increase tape tension because tension is equal to torque divided by moment arm. Thus, to compensate for this decrease in moment arm, it is necessary to decrease the torque by reducing brake pressure. This is accomplished automatically by slower rotation of the

reels

25, 26 and pump 126 due to the progressive decrease in missile velocity the net result is that the tension of the tape during the programmed braking remains substantially constant. Thus, due to the speed sensing characteristic of the hydraulic systems, it is unnecessary to synchronize the pump drives of

reels

25 and 25 for, if reel 25 overspeeds for example, it will apply its brakes harder causing it to slow down, while on the other hand, if it underspeeds, then the pressure on the brakes will be relieved permitting the reel to speed up a compensating amount.

It is now necessary to consider the programmed braking cycle in more detail. The programming means comprises a gear box 149 chain driven from the reel to rotate a cam 151 which in turn operates a cam follower 153 to open and close the valve 139 which thereby acts as a secondary control On the pressure in the hydraulic system. Due to the inherent speed sensing characteristics of the system a constant tension is maintained without having to manipulate the valve 139 any great extent during the braking cycle. As a result the strain (stretch) placed in the tape during the dynamic period has had no opportunity to be relieved and as the missile approaches a stalling speed near its apogee, this strain will be unloaded through the missile and guide

cables

16, 17 possibly with sufficient force to collapse the boom 18.

In accordance with the invention, this is prevented by a brake programming especially tailored for missile arresting applications. Cam 151 has a profile as shown in 9 FIGURE 10 divided into arcuate sections A, B, C. A reciprocal valve element 160 in the valve 139 restricts the flow in line 137 back to the reservoir 129 and during the programmed braking period corresponding to cam section A is maintained at a predetermined value as previously mentioned with the pump 12.6 varying the brake pressure as explained above. As the missile is slowed to a stalling speed, the cam profile exhibits a sharp drop at the section B winch permits the cam follower 153 to rise under the influence of spring 161 so that the valve element 16!) becomes wide open releasing the pressure on the reel brakes. With this type of valve there is a chamber 162 behind the valve element 160 which contains hydraulic fluid under pressure from line 137 delivered through longitudinal ports 164 to partially balance the valve element 164 and increase its responsiveness. A plurality of these ports are required so that the valve element 160 is quickly responsive to the change in cam profile at section B, however any other means which insures that the cam follower 153 will track on the cam may be used. The effect of the cam profile drop at section B is to instantly release or dump substantially all of the brake pressure so that the reel accelerates rapidly to feed additional tape into the system during the period covered by cam section C. Thus all the stress is taken out of the

purchase members

29, 30, 34, 35 so that as the missile reaches zero velocity, there will be no downward force acting on it. Points on the cam profile are related to exact locations in runout via the direct drive from the reel. By this means, it is possible to maintain pressure for the proper length of time during portion A of the cam profile and to precisely control the pressure dump location between A and C. The missile is now slowed to a stalling velocity which means that without any additional braking it will come to a stop under the influence of gravitational forces alone. As the missile reaches zero velocity, the cam 151 has rotated beyond section C to the abrupt rise at 165 where it closes the valve element 160 causing the pressure from pump 126 to rise, thus reapplying the reel brakes so as to prevent tape backlash which would result by the reel continuing to rotate while the missile had come to a stop. As the reels stop so also does the pump 126 whereupon the missile can be lowered by winch and cable mechanism 19 and the tapes rewound by the reels by means of the retraction engine 38. In order to return the system to the initial condition, normally closed manual shutoff valve 155 (FZGURE 9) is opened to relieve the operating pressure on the one side of the spool valve 119 and thereby permit the pressure from the static brake pressure tank 113 to again shift the piston 121 to reestablish static pressure on the reel brakes. Line 143 is provided with a relief valve 157 for overpressure protection.

The critical periods in missile arrestment are the dynamic period while the reels are being accelerated and at the conclusion of the braking cycle when the reel brakes are released to dissipate the stresses which have built up in the purchase tape returning the system to a neutral condition. Relative to the dynamic period there are three parameters which govern the maximum loads in this area:

(1) Sheave to sheave span which controls, in conjunction with the free flight distance, the oblique impact angle beta (2) the length of purchase tape between missile and arresting engine, and (3) the cross-sectional area of the tape. Depending upon the maximum permissible load which the missile can withstand, and the imposed nominal stroke distance, the final arrangement is of course flexible between a range of sheave span and purchase length configurations which may be selected as a matter of practical compromise in order to maintain the arresting loads below those which the missile can withstand. A narrower sheave span could be accommodated with a longer purchase length and vice versa; otherwise stated, the narrower the sheave span, the more oblique will be the angle of engagement of the missile. Consequently a lower angle beta results in a higher tension which must be accommodated by longer purchase tape lengths to supply the necessary feed in during the dynamic period. Also, any increase in the load carrying capability of the missile would be reflected in a more compact installation. For example, if the limiting factor is vehicle load, then the sheave to sheave span must be wider so that the maxi-mum load on the missile is not exceeded, however if the missile can take additional load then the sheave span can be brought in closer and the limiting factor then becomes the permissible strain on the purchase tape. As a general rule of practice for a missile stroke of 100 to 200 feet above ground, the sheave spacing should be such as to provide an angle of beta of between 20 and 60 degrees for nylon tape and preferably around 60 degrees.

Another important factor is the inertia of the reel and its stored tape complement during the dynamic period. The tape tension generated during this period is a function of accelerating the reel and tape masses. As soon as the reel is accelerated, the hydraulic system will program pressure to the friction brakes and maintain desired loads until the end of the arrestment. However, during the dynamic period it can be easily realized that if the reel inertia is high, then other compensating factors must be made, such as spreading out the sheaves resulting in a less compact system. Thus, it may be appreciated that there are a number of variables which combine to affect the actual engineering of a missile arresting installation and it is not the province of the present invention to describe all of the possible situations which could occur, however, by visualizing what may be termed the extreme opposite conditions, the in between arrangements can be readily accepted.

For example, two missile arresting installations are illustrated in FIGURES 11 and 12. FIGURES 11 and 11A represent what may be called the degree beta situation in which a missile 13A is launched below ground for a free flight distance before engaging with a horizontal pendant P stretched at ground level in the path of the missile 13A. The arrangement as shown in FIG- URE 11A is such that the missile 13A passes between two parallel strands of the pendant and has diametrically opposed hooks L adapted to engage each strand upon impact. A guide cable and boom arrangement may be provided similar to that shown in FIGURE 1 to prevent the missile from falling back to the ground.

FIGURE 12 represents what might be termed the zero beta situation in which a missile 13B is launched vertically for a free flight period determined by the length of slack bridle extension S and carries aloft with it a single purchase tape T paid out vertically directly behind the missile over a sheave 31' from a single arresting engine 25. The tape T is releasably anchored by the tape to tape connector 4d in a manner similar to that described in reference to FIGURE 8.

Having now described the invention and the preferred embodiment thereof, it should be appreciated by those skilled in the arts that obvious variations are intended to fall within the scope of the claims and be covered thereby except insofar as limited by the prior art.

We claim:

1. An apparatus for arresting a missile while in free flight within a prescribed distance comprising a rotatable reel,

a linear purchase tape considerably wider than it is thick so as to permit coiling upon itself about the reel in ever increasing convolutions having its running end releasably anchored adjacent the path of flight of the missile,

means carried by the missile for engaging said running end of the purchase tape after a period of sustained free flight carrying it aloft with the missile,

the tape being woven of synthetic yarns having a sufficiently low modulus of elasticity so that the portion paid out between the reel and missile will stretch longitudinally and maintain engagement with the 11 missile until the reel can accelerate at the demand rate per second required in order to keep the imposed stress and strain Within safe limits in said tape,

braking means attached to the reel for reducing the rate of tape payout thus slowing the missile to a stalling speed, and

speed sensing control means arranged to program the retarding force of the braking means so that energy stored in the tape as stretch, is relieved prior to bringing the missile to a controlled stop at its apogee.

2. An apparatus for arresting a missile as set forth in claim 1 including a plurality of rotatable reel and purchase tape units each positioned equidistant from the missile launch site and the running end of each purchase tape being releasably anchored adjacent the point of launch,

a loose bridle extension the length of which substantially equals said free flight distance of the missile connecting the running end of each purchase tape thereto, and

a plurality of sheaves positioned equidistant from the missile launch site through which the payout of each purchase tape occurs as it is carried aloft by the missile, the running end of each purchase tape being lifted from its anchored position when the missile has reached the limit of free flight imposed on it by said bridle extensions and being tensioned at an oblique angle to the missile flight path.

3. An apparatus for arresting a missile as set forth in claim 2 wherein the missile is launched vertically and said oblique angle in between and 60 degrees.

4. An apparatus as set forth in claim 1 wherein the purchase tape is woven of yarn made of polytetrafluorethylene.

5. A missile arresting gear adapted for use in arresting a missile while in flight comprising a stator member,

a rotatable reel journaled adjacent its opposite ends on the stator member and being radially spaced therefrom to define a brake chamber therebetween,

a plurality of radially extending rotary friction disc elements in the brake chamber attached to the reel,

a plurality of stationary friction disc elements interleaved with the rotary friction disc elements to provide a rotary friction disc brake,

hydraulic acautor means at the opposite ends of said brake chamber for applying braking pressure upon said friction disc elements so as to retard rotation of said reel,

a purchase tape considerably wider than it is thick so as to permit coiling upon itself about the reel in ever increasing convolutions having its running end releasably anchored adjacent the point of launch of said missile,

a loose bridle extension the length of which substantially equals a predetermined free flight distance of the missile connecting the running end of the purchase tape onto the missile whereby as the bridle extension becomes taut after launching the missile the purchase tape is tensioned until the reel can be accelerated to feed additional tape at the demand rate per second required in order to keep imposed stresses and strains on the tape within safe limits, and

an hydraulic control system including cam and valve means providing a programmed braking pressure after the reel has been accelerated so as to retard payout of said purchase tape slowing the missile to a stalling velocity and thereafter reducing the braking pressure so as to permit rapid tape feed in relieving the tape stress and strain just prior to the missile coming to rest at its apogee.

6. A missile arresting gear as set forth in claim 5 wherein said cam and valve means comprise a rotatable cam lobe driven at a speed proportional to the speed of said reel,

a cam follower biased against said cam lobe, and

a valve element arranged to increase or decrease braking pressure in accordance with the position of the cam follower on the cam lobe, said cam lobe having a ZOne of rapid pressure drop as the missile reaches a stalling speed followed by rapid pressure increase just as the missile comes to rest under the influence of gravitational forces.

7. A method for arresting a missile comprising the steps of restraining the missile after a period of free flight with a woven purchase tape made of synthetic yarn placed in tension by means of oblique impact with the missile,

paying out additional tape from a reel upon which the tape is coiled at a rate sufficient to prevent excessive tape strain,

reducing the rate of tape payout after the reel has accelerated to 'missile velocity and thereby slowing the missile to a stalling speed,

increasing the rate of tape payout as the missile approaches zero velocity to relieve tape stress, and

halting tape payout after the strain has been relieved and as the missile comes to rest at its apogee.

References Cited by the Examiner UNITED STATES PATENTS 9/1962 Siegel et al. 73-167 3/1963 Shaller 73l67

Claims

1. AN APPARATUS FOR ARRESTING A MISSILE WHILE IN FREE FLIGHT WITHIN A PRESCRIBED DISTANCE COMPRISING A ROTATABLE REEL, A LINEAR PURCHASE TAPE CONSIDERABLY WIDER THAN IT IS THICK SO AS TO PERMIT COILING UPON ITSELF ABOUT THE REEL IN EVER INCREASING CONVOLUTIONS HAVING ITS RUNNING END RELEASABLY ANCHORED ADJACENT THE PATH OF FLIGHT OF THE MISSILE, MEANS CARRIED BY THE MISSILE FOR ENGAGING SAID RUNNING END OF THE PURCHASE TAPE AFTER A PERIOD OF SUSTAINED FREE FLIGHT CARRYING IT ALOFT WITH THE MISSILE, THE TAPE BEING WOVEN OF SYNTHETIC YARNS HAVING A SUFFICIENTLY LOW MODULUS OF ELASTICITY SO THAT THE PORTION PAID OUT BETWEEN THE REEL AND MISSILE WILL STRETCH LONGITUDINALLY AND MAINTAIN ENGAGEMENT WITH THE MISSILE UNTIL THE REEL CAN ACCELERATE AT THE DEMAND RATE PER SECOND REQUIRED IN ORDER TO KEEP THE IMPOSED STRESS AND STRAIN WITHIN SAFE LIMITS IN SAID TAPE, BRAKING MEANS ATTACHED TO THE REEL FOR REDUCING THE RATE OF TAPE PAYOUT THUS SLOWING THE MISSILE TO A STALLING SPEED, AND SPEED SENSING CONTROL MEANS ARRANGED TO PROGRAM THE RETARDING FORCE OF THE BRAKING MEANS SO THAT ENERGY STORED IN THE TAPE AS STRETCH, IS RELIEVED PRIOR TO BRINGING THE MISSILE TO A CONTROLLED STOP AT ITS APOGEE.