Abstract:
A solid rocket motor having a liner surrounding a propellant includes an array of transmitter elements and receiver elements disposed within the liner. Transmitter electronics provide the transmitter elements with a transmit signal and receiver electronics receive the outputs of the receiver elements. The outputs are analyzed to determine any defects in the propellant.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     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. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to testing apparatus and methods, and, in particular, to apparatus and methods for detecting defects in solid rocket motor propellant. 
     BACKGROUND OF THE INVENTION 
     Solid rocket motors may be utilized in weapons such as air-to-air and air-to-ground missiles, as well as in model rockets and boosters for satellite launchers. Air pockets within the propellant grain or fractures of the grain may produce an instantaneous increase in burn surface area. The increase in burn surface area may produce a corresponding increase in exhaust gas and pressure, which may result in a rupture of the casing containing the propellant. 
     Aging of the propellant may lead to significant degradation in weapon performance and catastrophic failure. Tactical missiles may be kept for extended periods of time and, accordingly, there is a dire need to know if there is any defect in the propellant prior to use. 
     An aging model may be used to predict and detect material degradation, given an assumed or measured environmental history. Other approaches may use non-destructive testing, such as ultrasound and X-rays. All of these approaches, as currently practiced, may be inadequate to meet the needs of a real-time self-sensing monitoring system for full-up rounds. In addition, the rounds may not be able to be examined in-situ, but must be removed from their storage location and brought to an examining machine. 
     In addition to the above approaches, other testing methods may include the use of embedded sensors, such as fiber optics or electrical strain gages. Drawbacks to these methods include fragility, difficulty in placement during manufacture, and the need for expensive and bulky interrogators. 
     SUMMARY OF THE INVENTION 
     It is an aspect of the invention to provide an apparatus that may image flaws in a solid rocket motor propellant in-situ, without moving the propellant from its storage location. 
     An apparatus for detecting defects in a solid rocket motor propellant contained within a rocket casing may include a liner disposed between the propellant and the casing. An array of radio frequency (RF) transmitter elements and RF receiver elements may be disposed within the liner and around the propellant. Transmitter electronics may be provided to supply the transmitter elements with a transmit signal, and receiver electronics may be provided to receive the transmitted signals. The output signals of the receiver elements may be analyzed by a microprocessor to determine if there are defects in the propellant. 
     A method of detecting defects in a solid rocket motor propellant contained within a rocket casing may include providing an array of RF transmitter elements and RF receiver elements disposed within a liner that surrounds the solid rocket motor propellant. The transmitter elements may transmit RF signals through the rocket motor propellant. The receiver elements may receive the RF signals. Defects in the solid rocket motor propellant may be detected by analyzing the received RF signals. 
     The invention will be better understood, and further objects, features and advantages thereof will become more apparent from the following description of the exemplary embodiment, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIG. 1  illustrates the transmission of RF energy through a structure having a defect. 
         FIG. 2  is an axial cross section through a rocket motor illustrating an embodiment of the invention. 
         FIG. 3  illustrates an array of transmitter and receiver elements, together with transmitter and receiver electronics. 
         FIG. 4  is a partially cut away sectional view of a solid rocket motor in accordance with one aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the Figures, transmitter elements are shown as circles and receiver elements are shown as triangles. In  FIG. 1 , an RF transmitter element  10  projects RF energy through a structure  12  having an internal defect  14 . On the other side of structure  12  a receiver element  16 , moving in the direction of arrow  18 , receives the projected RF energy and produces a corresponding output, indicated by waveform  20 . As receiver element  16  scans past defect  14 , there is an increase in receiver element output, as indicated by pulse  22 . The invention may use an array of transmitter elements and an array of receiver elements to pinpoint, precisely, defects in the propellant of a solid rocket motor. 
       FIG. 2  is an axial cross-sectional view of a solid rocket motor  24 . Solid rocket motor  24  includes an outer casing  26 , made of, for example, metal. Disposed inside of casing  26  is propellant  28 . An internal central bore  30  in the propellant  28  allows the propellant  28  to burn. Positioned between the casing  26  and propellant  28  is a protective liner  32 , made of, for example, an elastomeric material, such as rubber. 
     Embedded within the liner  32  are an array of transmitter elements  34 , and an array of receiver elements  36  alternately arranged with the transmitter elements  34 . The transmitter elements  34  and receiver elements  36  are carried by a flexible sheet  38 , which divides the liner  32  into inner and outer halves  32   a  and  32   b , respectively. 
     In operation, the transmitter elements  34  are energized, either all together or in a predetermined sequence, to transmit RF energy into the propellant  28 . Upon encountering a defect  40 , the RF signal is altered and then received at various receiver elements  38 . By analyzing the output of the receiver elements  38 , the defect  40  may be detected. 
       FIG. 3  illustrates the transmitter elements  34  and receiver elements  36  contained in or on sheet  38 . In one exemplary embodiment, the sheet  38  may be of a plastic material, such as Mylar. The transmitter elements  34  and the receiver elements  36  may be, for example, adhesively attached to the sheet  38 , or the elements may be deposited on the sheet  38  by a chemical vapor deposition process. The transmitter electronics  44  supply transmit signals via wiring bundles  42  to the transmitter elements  34 . Similarly, wiring bundles  46  conduct receiver signals from the receiver elements  36  to the receiver electronics  48 . 
     A microprocessor  50  controls the transmission sequence and is also operable to perform an analysis of the received signals by any one of a number of well-known tomographic image reconstruction algorithms. The output  52  of microprocessor  50  is provided to a utilization device (not illustrated) which will display or otherwise pinpoint the defect. 
       FIG. 4  is a partially cut away sectional view, illustrating a solid rocket motor assembly  60 . Sheet  38  may be sandwiched between liner halves  32   a  and  32   b . Transmitter and receiver electronics  44  and  48  may be disposed within the liner  32  at the lower portion of the casing  26 . When testing for defects, the microprocessor  50  may be brought to the location of the stored rocket motor assembly  60 , plugged into the transmitter and receiver electronics  44 ,  48 , and a test commenced. The solid rocket motor assembly  60  need not be removed from its stored location. Alternatively, each solid rocket motor assembly  60  may include its own microprocessor  50 . 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 
     Any numerical parameters set forth in the specification and attached claims are approximations (fore example, by using the term “about”) that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of significant digits and by applying ordinary rounding.