Radiographic Inspection of loaded Rocket Motors JACK BUCHANAN
AND
8. DEANE HERBERT
Thiokol Chemical Corp., Redsfone Division, Huntsville, Ala.
Solid propellant rocket motors using a propellant grain formed directly in the rocket case are successfully inspected for critical voids and irregularities b y modern radiographic equipment. Improvements have been made in standard industrial techniques to obtain a 270 sensitivity in the radiographs of a plastic material where the radiation must necessarily pass through a metal case. Fluoroscopic equipment was used with success for inspection of aluminum-cased rocket motors whose sizes were up to 3 inches in diameter. Gamma ray sources, such as cobalt-60, produced film with adequate sharpness and contrast for inspection of large rocket motors.
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I T H the development of solid propellant rocket motors utilizing propellants which are cast directly into the rocket case, a method for the inspection of propellant grains after forming into the rocket case became desirable as an aid in the process development. I n high performance rounds 1%-heremetal parts are light and highly stressed, safety factors are reduced so that the reliability may not be ensured without some nondestructive test. Fortunately, radiography presented a solution. Development of Radiographic Technique
For our investigation of x-ray techniques, two 140-lrv. portable industrial x-ray machines capable of continuous operation a t 10 milliamperes were used. Several types of rockets were radiographed; adjustments from initial settings were made as needed t o determine which gave the best definition and contrast. Variations in performance of machines, even those of the same tj-pe and manufacturer, required different settings for the same results. The definition and penetration were checked using rockets with known voids and penetrometers. The penetrometers were made by drilling holes of known sizes through a slab of propellant whose thickness was 2% of the thickest section being inspected. TTThen the size of the rocket permitted, x-ray photographs were made by exposing the film with the x-rays passing through the diameter of the rocket normal t o ita longitudinal axis. When this was not possible, film cassettes were cut t o fit into the giain cavity and radiographs made a t several positions for coverage of the grain. After some experience was gained and usable settings were made, various intensifiers and filters w-ere tried for their effect on the films produced. The intensifier used was a wrapping of 0,005-inch lead foil placed over the films to absorb the scatter radiation. This resulted in a sharper image on the film. -4 filter was placed over the aperture in the tube head t o reduce the scatter radiation reaching the film and t o partially absorb the longer wave-length (soft rays) component of the primary beam. The resultant x-ray beam has a higher proportion of more penetrating wave lengths vhich do not generate so much scatter in the object being radiographed as does an unfiltered beam. Such a beam is desirable when x-raying objects which are of different materials and thicknesses. Because of scatter, the longer wave lengths will penetrate the thinner sections and expose the film i n those areas before the thicker sections are fully penetrated. Several thicknesses of each filter material were tiied, and the film giving the best definition and contrast a t the shortest expo-
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sure time, aithout a change in kv. and ma. settings, was determined. An aluminum filter, 0.080 inch, produced good film definition and contrast and also reduced the exposure time required when denser materials were used. Kodak Type A and Ansco Type Superay A x-ray films were compared for use in the radiographing of multithickness rocket propellant grains. Both of these films are nonscreen, or fine grain, and are for use where high definition and contrast are of great importance. We found the Superay A film gave the better definition and, more important, reduced the necessary exposure time by as much as 50%. X-raying through internal burning rocket grains results in a considerable range of film density due to different thicknesses of material through which the x-ray beam must pass. Therefore, exposure time must necessarily be a compromise, and sharpness of detail is a necessity if the film readers are t o find the defccts and to locate their position. To obtain sharp definition, x-ray tubes having small focal spot sizes should be used. Satisfactory results were obtained using a focal spot size of 2.3 mm., but smaller sizes would be an improvement. Small focal spot tubes have been used for medical fluoroscopy for some time, but only in the last few years have such tubes been available for industrial use. The use of tubes having focal spots in the range of 1 t o 1.5 mm. in the 150-kv. size results in much sharper film images. Because of the difficulty of dissipating the heat generated when the electron beam strikes the target, focal spots are larger with higher kv. equipment. Thus with a 250-kv. machine, which mould be necessary for inspection of rocket motors whose sizes run from 6 t o 15 inches in diameter, a focal spot of 2.3 mm. is the smallest available in the latest standard types of equipment.
Fluoroscopic Techniques Considerable savings in time and in material costs can be made if fluoroscopic inspection is used instead of x-ray photography. Successful methods had been developed for rocket grains alone, but had not been reported for the loaded rocket motor. Various fluoroscopic screens were tried directly and had very limited success. The contrast was so low that the operator required a well darkened inspection room and about 15 to 30 minutes were necded for his eyes to become sensitive enough to distinguish defects. With the Radelin-F screen, which was superior t o others tried, some limited success was obtained, but not enough t o warrant its adoption as a standard inspection method.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 4
ROCKET PROPELLANTS I n an attempt to develop a satisfactory method, the following equipment was purchased and installed: a 150-kv. constant potential x-ray machine with a Machlett Super Dynamax x-ray tube, and a Westinghouse Fluorex image amplifier. Considerable improvement was expected because of the special nature of the x-ray tube and the greater light intensities obtainable with the Fluorex. (See photo page 729.) The Super Dynamax is a special, oil-cooled, rotating anode tube with a focal spot of 1.0 mm. For continuous service with this small focal spot, the heat generated during the production of x-ray is dissipated by rotating the anode t o give more surface and by circulating cooling oil through the x-ray head. The Fluorex image amplifier is designed with an image amplifier tube and an optical system. The image amplifier tube is selfcontained and operates an follows : the x-rays, passing through the object being inspected, impinge upon an input fluorescent screen about 5 inches in diameter on the inside face of the glass envelope. I n contact with this fluorescent screen, there is a photoelectric surface which converts the light image into an electron image. The electrons are accelerated by a potential of 30,000 volts and focused on an aluminized phosphor layer which is 1 inch in diameter. The resulting image on the output phosphor is about 200 times brighter than that on the fluoroscopic screen. The optical system magnifies the small image on the output screen and enables the operator to view this image in its true orientation with respect to the object being viewed. Two different magnifications are provided. The operator is protected from radiation by viewing the image through a mirror so that his head is not on a line with the x-ray beam. Daylight viewing is possible without the use of specially darkened rooms, but the light level should not be above 10 foot-candles. This equipment was mounted so that the image amplifier could be located as near to the object to be inspected as possible and a lead-lined cubicle was constructed for personnel protection. Within this cubicle, both the x-ray head and a holding carriage for the rocket motor were installed. The carriage permitted remote movement of rocket motors longitudinally and in rotation. All controls were mounted outside the cubicle a t the inspector's station. A spot-film device was installed so the operator could obtain a radiograph of any questionable defects or when special study of a defect was required. I n the initial use of this equipment, it was found that the cooling oil pressure was too high and that the inner glass envelope around the anode was being broken. This tube would not stand more than 20 pounds per square inch presaure in the cooling system where the factory setting on the x-ray machine was 35. A 3-inch-outside-diameter aluminum-cased rocket was selected so that a comparison could be made between x-ray film and fluoroscopy. An initial x-ray setting of 100 kv. and 9 mm. was made. The x-ray head was 22 inches from the face of the image amplifier with the rocket motor 4 inches from the screen. At this setting, the image was fairly fuzzy. By trial and error, the setting for the best definition was found to be 80 kv. and 6 ma. where the penetration was required for 0.921 inch of aluminum. Aluminum penetrometers equaling 2 , 4 , 8, and 16'% of the thickness were fastened t o the outside of the rocket case. Only the 8 and 16% could easily be seen in all positions. Better definition could be obtained with the ma. reduced t o 4.5 and the x-ray head moved to a position 16 inches from the fluoroscope screen. Furthermore, sharper detail was obtained by moving the head so the x-ray beam was 4" from a line normal t o the object being viewed, since less back scatter occurred in this position. With these changes the 2% penetrometer became visible. The 4% was clearly seen except a t some positions during rotation of the motor. Excellent definition was obtained with the 8% penetrometer. It was assumed that with this motor, a sensitivity of 6% could be expected with the fluoroscope as compared to 2% obtained with x-ray film. April 1956
Some 1600 motors were inspected by both x-ray film and the fluoroscope. Eleven motors out of 245 rejectable motors could not be resolved between the two methods. On rechecking these eleven rounds, only one remained in which the fluoroscopic inspection would not show the defect. The void was very shallow and when tested by static firing the round operated satisfactorily. -4t the time of this comparison, the spot-film device was not in operation, and it is believed that reinspection would not have been necessary if it had been in service. For confidence in this inspection, a spot check should be made with the spot-film device wherever a sizable change in cross section occura. The inspection reports obtained with the two methods were not identical as the x-ray inspection reported six times as many voids inch or smaller than the fluoroscopic inspection. However, with critical defects, agreement was excellent. Man-hours used in the two methods were about the same. Further improvements are needed if more general use is to be made of fluoroscopic inspection of rockets; the range of sizes that can be inspected is very limited. Contrast could be improved, and easier viewing is necessary. New equipment is now available which can correct some of these faults. An image intensifier is distributed by North American Phillips Corp. for which it is claimed that image brightness is increased 1000 times. With this equipment, industrial television viewing is possible, giving the advantages of complete operator protection, convenient viewing by more than one inspector, and adjustable brightness and contrast controls. Also this image intensifier can operate satisfactorily with x-ray fields up t o 300 kv., whereas the Fluorex t h a t we have used is not usable a t sett.ings over 120 kv. because of t h e type of fluoroscopic screen in the amplifier tube. Now both Bendix and General Electric are developing light amplifiers which may have application to this field.
Gamma Ray Inspeetion For loaded rocket motors whose size makes x-raying impractical or the equipment too costly, gamma ray sources can be used. The simplest method is to suspend the source in the grain cavity and to wrap the film circumferentially around the rocket case. The images on the film are somewhat distorted, but the film reader makes allowance for this. Inspection of smaller sizes of rocket motors showed that gamma rays were not advantageous because of longer exposures and the greater scatter radiation ae compared to x-rays. Radium and cobalt-60 were compared by radiographing one type of larger rocket motor and determining which source gave t h e better results. The films exposed with the cobalt-60 showed sharper images and better contrast because of less scatter radiation and better penetration. With proper protective precautions and the use of common sense in its handling, cobalt-60 is very easy t o use. It is fairly cheap and readily available. With the procedures and equipment required by the Atomic Energy Commission (available t o the user), higher energy sources are available with which exposure times can be reduced if necessary. A dummy rocket section with built-in defects was inspected radiographically using a million-volt x-ray machine, a 22,000 betatron, and cobalt-60. The resulting film using the betatron and using cobalt-60 was nearly equal in quality and better than the film from the million-volt machine. A prime requirement for the use of radiographic inspection is trained and experienced inspectors t o interpret the x-ray images. Inspectors must learn t o differentiate between critical and minor defects. They must learn t o identify tool marks in the metal parts, scratches in paint or plating, discontinuities in welds, and metal porosity, as opposed to true defects in the propellant grain. Two years of experience was accumulated before any true degree of confidence was placed in this inspection method. I n one type of rocket alone, 500 static firings were made to check critical and noncritical defects shown by radiographic inspection. RBCEIVED for review September 23, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTED March 9, 1956.
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