Miniature pyrotechnic squib switch, single pole, normally open
BACKGROUND OF INVENTION 
     In the patented art, the U.S. Pat. No. 3,111,808 to Fritz discloses a single-stroke actuator in which a piston is driven by detonation of an explosive charge, from a retracted position to a projected position in which it is held against return movement by penetration of a sharp edge at the trailing end of the piston into the cylinder wall under the reactive load (e.g. air pressure) tending to thrust the piston back into the cylinder. An actuator or propelling device having an explosive charge-propelled piston is also disclosed in Barr U.S. Pat. No. 3,119,302. Other patents disclosing devices powered by explosive charges are: Gross U.S. Pat. No. 2,742,697; Stupian U.S. Pat. No. 2,897,799; Bohl U.S. Pat. No. 2,924,147; and Stott U.S. Pat. No. 2,942,818. 
     RESUME OF THE INVENTION 
     This invention provides a miniature actuator (of which the case may be as small as 1/2 inch in length and 1/8 inch in diameter). It is of single-stroke requirement, powered by detonation of an explosive charge. It is normally open switch, closing an operative circuit when detonated. 
     The miniature size and weight of the switch make it especially suitable for single-stroke actuation of stage operations in the flight of a missile or outer space vehicle, and its relative simplicity of construction makes it ideally suitable for operation with minor possibility of failure. Its detonative action provides for split-second timing in stage actuation of the various functions of such a vehicle. 
     The provision of an actuator having such capabilities is the general object of the invention. Other objects will be apparent in the ensuing specifications, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial sectional view of a switch embodying the invention, on a scale enlarged many times; and 
     FIG. 2 is a cross-sectional view thereof, taken on the line 2--2 of FIG. 1; and 
     FIG. 3 is a cross-sectional view of the switch after firing thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     OBJECT OF THE INVENTION 
     Referring now to the drawing in detail, all operative parts of the device shown therein as an example of one form in which the invention may be embodied, are enclosed within a cylindrical shell 10 closed at its rear end by a header 12 and at its forward end by a header 14. Each of these headers is composed of a cylindrical ring 16 fitted snugly into a respective end of the shell, sealed thereto by a solder ring 18, and enclosing a cylindrical core 20 of ceramic material. At the rear end of the switch, a pair of lead wires 22 extend through the core 20 of header 12 and into a flat pancake charge of explosive material 24, and are flattened at their ends, as at 26, for secure attachment thereto of a fusible bridge wire 28. At the forward end of the switch, a pair of open-circuit wires 30 extend through the core 20 of header 14 and terminate in penetrative points 32 adapted, upon forward propulsion of the piston and conductor-pellet assembly 34, to penetrate through an insulator cup 36 on the forward end of assembly 34, and into a conductor pellet 38 enclosed within the cup 36, thus establishing an electrical circuit through pellet 36, across the wires 30. 
     Pellet assembly 34 further includes a piston unit including a head 40 having a flat rearward face and a cylindrical periphery, head 40 being of resilient, compressible material such as to be expanded into pressure-sealing contact with the internal wall of cylinder 10 when subjected to pressure by a disc 42 of polyethylene material, of flat pancake form, with a cylindrical periphery fitted to the internal wall of cylinder 10, and having opposed flat faces in contact respectively with the flat rear face of piston unit 40 and with the flat forward face of explosive charge 24. Piston unit 40 further includes an integral forward plug portion 44 which is snugly fitted into the rearward portion of insulator cup 36, thus holding the pellet assembly 34 and piston unit 40 in assembly. Explosive charge 24 is of flat, pancake form, filling the space between the opposed, parallel flat faces of header 12 and disc 42 within cylinder 10. 
     Bridge wire 28 has its respective end portions secured, by welding, to the flattened ends of lead wires 22. 
     The drawing discloses the switch in greatly magnified scale. The length of case 10 may actually be as little as 1/2 inch, and its diameter only 1/8 inch. 
     OPERATION 
     Upon application of electric current to the leads 22 at the rear end of the switch, the bridge wire 28 will be heated to a temperature such as to ignite the powder charge 24. The resulting gases of combustion will drive polyethylene disc 42, piston 44, conductor pellet 38 and insulator cup 36 forwardly with sufficient force to drive the points 32 of wires 30 through the bottom of the insulator cup 36 and to embed points 32 in the conductor pellet 38, thus completing the circuit through conductor wires 30. 
     In the commercial embodiment of the invention, the flattened ends 26 of leadwires 22 have eliminated one cause of occasional previous failures. Another feature to eliminate occasional rupture of case 10, was to lower the powder charge from 4.0 mg to 2.0 mg. Still another feature of the invention is the addition of polyethylene disc 42, improving the sealing in front of the powder charge.