Abstract:
An impulsive safety-arming device provides handling safety in a rocket  mo. Prior to arming, a piston is blocked in position to contain a squib, thereby preventing inadvertent ignition. After arming, the piston block is removed allowing high temperature, high pressure gas from the squib to force the piston past vent ports and to escape and ignite the motor.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to safety-arming devices for rocket motors wherein an electro-initiator device (EID) is used to ignite a booster charge of pyrotechnic material which in turn ignites the rocket propellant. More particularly, this invention relates to such devices which will prevent rocket motor ignition by containing premature EID combustion, and which utilize energy from the properly ignited EID to remove an ignition barrier between the EID and the booster charge. 
     Rocket motors of the type used in guided missiles which are launched from surface or airborne platforms use a variety of devices for preventing unintentional rocket motor ignition. One class of such devices uses an EID aligned with a booster charge in the rocket motor and rendered safe by electric switches, electronic filters, electrically insensitive bridgewires or by a combination of such elements. Another class of similar devices utilizes a barrier between the EID and the booster charge. When rocket motor ignition is likely to be desired, the barrier is removed manually by external means. 
     These designs are not sufficiently fail-safe to be stored in quantity in the assembled condition and thus must have the EID installed just prior to use. Also it is possible for inexperienced personnel to improperly arm the rockets thereby making them impossible to launch, or to unintentionally store rockets in the armed condition with other unarmed rockets and thus create an unreasonable danger of explosion. Further, freedom of missile design is somewhat restricted if a provision must be made for installation of an EID prior to use. The missile must have an access port for EID installation and therefore will result in a more complex structure than if the EID is permanently installed deep within the missile. 
     SUMMARY OF THE INVENTION 
     The safety-arming device of the present invention uses a sealed piston sliding within a vented outer housing. An electrically initiated squib device (EID) is placed within the piston and is sealed against leakage so that when the squib is ignited, it produces a large quantity of high temperature, high pressure gas which drives the piston to the opposite end of the outer housing. The EID, piston and seal remain intact after ignition to prevent leakage so that the gas pressure may act on the piston rather than escape through the piston to the interior of the safety-arming device. The piston uncovers a plurality of vents as it travels. High temperature, high pressure gas confined within the housing by the piston escapes through the uncovered vents and ignites a pyrotechnic booster charge which ignites the rocket motor propellant. 
     A mechanical blocking member is positioned to forcibly retain the piston in its initial position covering all housing vents. This blocking member has sufficient strength to withstand the force on the piston caused by gas pressure from the squib if the squib is ignited unintentionally or prematurely. 
     A step motor aligned axially with the blocking member rotates the blocking member ninety degrees from a safety position in which piston travel is prevented, to an armed position in which piston travel is possible. This motor is controlled by externally generated signals which precede an intentional launch. 
     If the squib is ignited when the blocking member is in the safety position the high pressure gas produced by the squib is contained by the outer housing and piston, and ignition of the rocket motor is prevented. An added safeguard against premature or unintentional rocket motor ignition is the electric current supply circuit to the squib. This circuit employs electrical contacts which are only closed when the blocking member is rotated to the armed position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the present invention will emerge from a description which follows of a possible embodiment of a safety-arming device according to the invention, given with reference to the accompanying drawings, in which: 
     FIG. 1 is a longitudinal sectional view of a safety-arming device in the safety position; 
     FIG. 2 is a perspective exploded view of the blocking member and piston base according to the invention; 
     FIG. 3 is a partially cut away sectional view of the invention within a rocket motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numerals correspond to like parts and elements throughout the several figures there is shown in FIG. 1 elongated cylindrical housing 11 equipped with a plurality of vent ports 12. Within housing 11 resides hollow piston 13 and piston base 55. Piston 13 is sealed against the inner surface of housing 11 in housing cavity 16 by resilient o-rings 14 which encircle piston 13. O-rings 14 are separated by resilient separator 15. Hollow piston 13 has a centrally bored stepped hole 71 which retains electro-initiator squib 21 against threaded end 72 of piston base 55. Electrical lead-in wires 22 and 23 from squib 21 lead through central cavity 73 of piston base 55 and are firmly attached to electrical contacts 24 and 25 on piston base isolation member 32. Elongated cylindrical housing 11 is firmly attached to main housing 51 by any appropriate means. FIG. 1 illustrates elongated cylindrical housing 11 being threaded into main housing 51, but it is to be understood that other fastening means will work as well. Piston base isolation member 32 is retained to piston base 55 by being cemented to piston base flange 31. Piston base isolation member 32 has projections 33, 33&#39; which penetrate piston base flange 31 and engage shallow holes 75, 75&#39; in housing 51, thereby preventing relative rotation between piston base isolation member 32 and main housing 51. 
     Within main housing 51 is contained stationary blocking member 41 which is rigidly retained within housing 51 and braced against relative rotation by pins 45, 45&#39; as shown in FIG. 2, which project into holes in main housing 51. Within stationary blocking member 41 is rotational blocking member 36 which is free to rotate through a predetermined angular displacement within central bore 76 of stationary blocking member 41. Rotational blocking member 36 has central receiving cavity 35 shown in FIG. 1 positioned ninety degrees to the longitudinal direction of piston base isolation member 32. This relative orientation corresponds to the safety-arming device being in the safety position. Rotational blocking member 36 is retained within stationary blocking member 41 by ears 53, 53&#39; which abut sliding surfaces 47, 47&#39; on stationary blocking member 41. Recesses 48, 48&#39; in the side of stationary blocking member 41 provide room for ears 53, 53&#39; to enter when piston base 55 and piston base isolation member 32 are forced by gas pressure in housing cavity 16 to enter central receiving cavity 35 in rotational blocking member 36. 
     When rotational blocking member 36 has been rotated to the armed position by shaft 39 which is powered by electric motor 81, in response to a first externally generated signal, piston base isolation member 32 no longer bears against blocking surfaces 34, 34&#39; and is able to enter central receiving cavity 35. When squib 21 is ignited, piston base isolation member 32 bottoms in receiving cavity 35 and forces rotational blocking member 36 forward on splines 37 of shaft 39. Rotational blocking member 36 disengages splines 37 and frees shaft 39 for other functions. Splines 37 then enter enlarged hole 38. 
     Rotational blocking member 36 is also restrained by a supporting member, not shown, which bears against tang 54 on rotational blocking member 36 and prevents axial motion when rotational blocking member 36 is in the safety position. This supporting member allows axial motion only when rotational blocking member 36 is rotated to the armed position, thereby aligning tang 54 with a receiving cavity, not shown, in the supporting member. This supporting member is rigidly attached to housing 51 and occupies space adjacent blocking members 36 and 41 in the region forward of central bore 76. 
     Torsional coil spring 42 engages both stationary and rotational blocking members 41 and 36 and produces a relative torque which urges ear contact surfaces 52, 52&#39; against stop contact surfaces 46, 46&#39; on stationary blocking member 41 thereby yieldingly retaining rotational blocking member 36 in the safety position. 
     Stationary blocking member 41 has electrodes 26 and 27 embedded in stops 65, 65&#39; which are supplied with electric current by lead-in wires 28 and 29. Lead-in wires 28 and 29 follow lead-in paths 43 and 44 to electrodes 26 and 27. As shown in FIG. 1, electrodes 26 and 27 are positioned adjacent electrical contacts 24 and 25 on piston base isolation member 32. Electrical contact bridging members 49, 49&#39; on ears 53, 53&#39; electrically bridge between electrodes 26 and 27, and electrical contacts 24 and 25, respectively when rotational blocking member 36 is rotated to the armed position. When rotational blocking member 36 is rotated to the safety position electrical contact bridging members 49, 49&#39; are separated from electrodes 26 and 27 and electrical contacts 24 and 25 so that no continuity exists in the circuit supplying firing energy to electro-initiator squib 21. 
     Piston base 55 and piston 13 are connected by screw threads at 72 to allow disassembly so that piston base 55 may be inserted through main housing 51 at close fitting hole 19. During the assembly process, piston base 55 is cemented to piston 13 over the area of threads at 72 with an epoxy sealant to prevent piston base 55 from disengaging piston 13 and also to prevent possible gas leakage past the threaded area at 72. 
     Main housing 51 and attached elongated cylindrical housing 11, together making up safety-arming assembly 10, are installed in missile 61 near the head end of combustion chamber 62 as shown in FIG. 3. In normal operation within a missile, when missile launch is desired, rotational blocking member 36 is rotated to the armed position and squib 21 is electrically ignited. Gas pressure in housing cavity 16 presses against piston 13 and forces piston base 55 into central receiving cavity 35 in rotational blocking member 36. At this time shoulder 17 moves through space 74 and compresses o-ring 18 against main housing shoulder 76. Gas escaping from vent ports 12 ignites pyrotechnic material in canister 64 and in a very short period of time completely fills combustion chamber 62 with a high temperature high pressure gas which uniformly ignites the exposed surfaces of solid propellant 63. As pressure in combustion chamber 62 increases, piston 13 at shoulder 17 compresses o-ring 18 with still greater force thereby producing an even tighter obturating seal. 
     Obviously many modifications and variations of the present invention are possible in the 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.