Rockets, Missiles and Computers Anti Tank Rocket Experience

Transcription

1 GSRS/MLRS Rockets, Missiles and Computers Anti Tank Rocket Experience While the author was still working the artillery rocket project, he was asked, by the head of ballistics and ignition, to examine a request for information (RFI), from a Michigan prime contractor, concerning an anti tank rocket motor. The RFI specified that the solid propellant rocket motor would have a six point star port (grain) design. All the literature he had studied, regarding combustion instability (destructive sound waves inside the motor), indicated that a six point star design was quite susceptible to combustion instability. A formal reply to the RFI included pricing, schedules, performance predictions and a recommendation against the six point star design. The next time the Virginia rocket company personnel heard of the anti tank rocket motor project, it was from a prime contractor in Texas. An old friend of the author was made the Project Engineer. The revival of the anti tank project occurred after the crunch phase of the underwater rocket motor program but while a continuing stream of data analysis was being conducted. He had hired a data technician to run the SFA computer code and was training him, at that time. For several weeks after the project was transferred to Texas, numerous speculative designs were made while waiting for the actual specification from the US Army. In this interim period, An Arkansas man was named Program Manager for the Virginia rocket company. Other members of the design team were not identified during that period but he continued on as the interior ballistician. The Specification When the specification was received from the Army, it was drastically different from that which the prime contractor and the rest of the industry had anticipated. As soon as the Army specification was issued and the Virginia rocket company had a copy of it to review, the author began to study it. First impressions from everyone at the Virginia rocket company were that it was a routine rocket motor design. The author noted several differences. First, the burn time was cut in half, compared to previous requirements. Secondly, the missile was to be spin stabilized, as opposed to guided. Third, the missile was to be launched from a rifled tube with no access to the forward end of the rocket motor. Fourth, the primary criterion was lowest cost consistent with meeting the performance requirements. The specification itself stated that cost would be the major criteria by which the winner would be selected. The rocket motor would be purchased in large quantities. He noted that, contrary to previous Army 1

2 designs, no safe-arm device was specified. He noted that the missile, to be propelled by the rocket motor, would be spin stabilized. When he brought up, to program manager, the fact that there was no safe-arm requirements specified, program manager assumed it was an oversight by the Army. The author wasn t convinced. Secretly (the author was not allowed to contact the customer - directly), he called his friend and asked him what that meant. The question had not been posed prior to that time. The friend sent a FAX of the question to the Army. The friend called the author back later and said the Army s reply was read the specification. Their reply was, simply read the spec. This reply caused everyone to critically examine the specification. It was realized that there would be no safe-arm device (reducing the cost by about a thousand dollars), there would be no access to the head end of the motor (necessitating a nozzle end igniter or wires running up to the head end of the motor through the nozzle) and everything must be done to reduce the cost of the motor. The Anti Tank Rocket Motor Design Team The critical study of the specification jolted the Virginia rocket company management into assembling a veteran team of engineers to work this program. At that point, a Program Manager, a Project Engineer, a Design Engineer, a Thermal Engineer, and a Nozzle Design Engineer were named; also, the author was named the Interior Ballistics Engineer. The Nozzle Design Not recognizing the cost impact of a complex nozzle design and not realizing that such a design was not necessary to meet the anti tank rocket motor requirements, nozzle design engineer had designed a typical 23 piece nozzle. One of the first problems addressed was the nozzle design engineer s 23 piece nozzle design. Had the anti tank rocket motor been of the ordinary, run-ofthe-mill, variety, the nozzle design engineer s design would have been acceptable. However, the severe cost constraint dictated by the Army s specification demanded a more unique design. Just prior to the beginning of the anti tank rocket motor program, the author had conducted (his part of) a nozzle materials study. He discovered that a material called DUREZ, a wood flour phenolic, was an excellent nozzle material with a highly reproducible but moderate erosion rate. Understanding that the interior ballistics need of the anti tank rocket motor did not demand the low erosion rate of the graphite nozzle material, proposed by the nozzle design engineer, but could tolerate the moderate and highly reproducible erosion rate of the DUREZ led to a reevaluation of the nozzle design. Therefore, a single piece nozzle of DUREZ could be incorporated into the design. The nozzle design engineer s nozzle design would have cost around $1000 while the DUREZ nozzle would cost only 2

3 about $25 to $50. It was not an easy task but he eventually convinced the program manager, with the help of the project engineer, to accept the DUREZ nozzle design. Note: In production, a steel ring was used to trap the DUREZ piece into the motor. The Igniter Design After having arrived at a nozzle design, the Virginia rocket motor company was faced with a sticky igniter design problem. The lack of a need for a safearm device and the lack of access to the forward end of the motor dictated either wires running through the nozzle, along the grain, to an igniter at the head end of the motor or a highly unconventional nozzle end igniter design. If a head end igniter was proposed, it was almost guaranteed to cause reliability problems. Vibration and/or shock could lead to either broken wires or inadvertent auto ignition of the grain due to friction heat, generated by the wires during vibration. A nozzle mounted igniter would require some original and unconventional thinking. The author s own knowledge of ignition material characteristics paid off, in this case. He was quite familiar with the hot gas producing characteristics of Boron Potassium Nitrate (BKNO3), including all the different sized pellets and granules. He had used the materials in ignition design for years. He was also familiar with the low gas but hot particle producing characteristics of Magnesium Teflon. However, he had never heard of a case where they were used in combination. What he envisioned was a device somewhat like a shotgun shell, with which he was quite familiar. In the shotgun shell analogy, the BKNO3 acted as the gunpowder and the Magnesium Teflon acted as the shot. He envisioned a small charge of BKNO3 granules at the base of the device that would be ignited with one or more squibs. This small, gas producing charge would simultaneously light and expel the much larger mass of Magnesium Teflon to the head end of the motor. The burning Magnesium Teflon particles were expected to sweep the grain while going forward then sweep the grain again, after being reflected off the head end of the motor, impinging on the grain in both directions. Because of the length of the grain bore (about five feet) and the complexity of the grain bore ( five point star or a four spoke wagon wheel), he estimated a five gram charge of BKNO3 and a forty gram charge of Magnesium Teflon, in an igniter body, that was two inches in diameter and four inches long. After considerable urging, the program manager authorized the project engineer and the author to conduct some witness board tests. These tests confirmed his reasoning and that configuration was adopted. Once the igniter and nozzle configurations were defined, the design engineer designed a nozzle mounted igniter retention device that would seal the 3

4 nozzle, during storage and handling, until motor ignition and would be expelled by the motor ignition gases. The igniter/nozzle closure combination proved to be the ideal answer to the anti tank rocket motor ignition question. The Coolie Hat Design Since it was determined that there would be no safe-arm device, there would be no head end igniter and since there would be no access to the head end of the motor, it was decided there would be a minimal opening at the head end of the motor. The opening at the head end of the motor would be just large enough to anchor the casting mandrel forward end. Weight and cost were critical and a fully open head end case was much heavier and much more expensive than an almost closed head end case. This decision created a problem for manufacturing, regarding the removal of the bore forming (casting) mandrel. Bonding of the grain to the head end of the motor, a natural consequence of the blind closed head end casting, created a grain stress situation that necessitated a stress relief groove in the grain, near the head end of the motor. All efforts to devise a groove or slot forming mandrel that could be removed after grain cure failed. The author had just been involved in an experiment where a Styrofoam mandrel was left in a motor during ignition. The experiment was a booming success. Therefore, he proposed that a Styrofoam leave-in-place piece of casting mandrel be used. The purpose of the Styrofoam piece was to form a radial slot in the grain that would relieve grain stress under low temperature conditions. Because the slot s toroidal outer contour was closer to the case wall, more insulation had to be put on the case wall under the slot. Again, it was a hard sell but the program manager authorized the project engineer and the author to make some six inch diameter motors with leave-in-place foam mandrels, vibrate them and fire them with the mandrels in place. They worked perfectly. A foam mandrel piece, called a coolie hat, was born and became a part of the anti tank design. The sketch of Figure 1 shows, schematically, the coolie hat configuration. Figure 1 Coolie Hat Schematic 4

5 Thickening the Case The slot formed by the coolie hat created a case overheating problem for the thermal design engineer. The thermal design engineer thought that the case would have to be insulated under the slot. Trying to install and bond a ring of insulation material onto a blind ended case five feet away would be a manufacturing nightmare. That insulation ring was to protect the case from hot combustion gases after the propellant in the region of the slot burned out. When presented with this problem, the author suggested that no insulation be used but that the steel case be thickened in that region. The thermal design engineer s first reaction was that that would not work. However, at the program manager s urging, the thermal design engineer did the thermal analysis and found that a 0.05 inch added thickness would work. The increased case wall thickness under the coolie hat slot was adopted. The Teaming Deal Several weeks had passed during this design effort but the Virginia rocket company s feeling was that they had a unique and superior design. As the day approached for the Texas prime contractor to select a rocket motor supplier, the Virginia rocket motor company management conceived the idea that a teaming arrangement with the Texas prime contractor could significantly reduce the cost of the missile to the Army and result in a win for both the Texas prime contractor and the Virginia rocket motor company. A rocket motor team went to the prime contractor s Bidder s Conference prepared to make that proposal to them. During the morning of the conference, all the rocket motor manufacturers presented their technical and cost proposals (in secret - separate from each other), including the Virginia rocket motor company. The afternoon was reserved for final presentations by all competitors. The Texas prime contractor proposed to make their final selection the following week. The Virginia rocket motor company had asked for, and received, permission to make a special lunch break presentation. The Virginia rocket company had lunch catered for the meeting. During the lunch break, the Virginia rocket company learned that their motor design had greatly impressed the prime contractor s technical personnel. When the rocket motor company laid out the teaming arrangement, the prime contractor was sold. They cancelled the afternoon meetings and told the other competitors that they had selected the Virginia rocket motor company s design. 5

6 The Eight Motor Shoot-Off After the teaming arrangement was completed, the question as to whether to use the five point star design, proposed by the head of ballistics, or the four spoke wagon wheel design, that the author proposed, should be the selected design. The ballistics head s recommendation for the anti tank design was a five slot star design. The author s initial design for the anti tank rocket motor was also a five slot star design. He quickly realized that a five slot star design pushed the propellant burn rate technology to its highest edge and ran up the cost of the motor. He evaluated a four slot wagon wheel design and found that it would do the job with a cheaper propellant. The head of ballistics favored the five slot star design. There was considerable debate within the Virginia rocket motor company as to which should be the selected design. The company personnel could not come to a definitive decision. The question was posed to the Texas prime contractor. They could not make a clear cut decision, either, because of internal debate. They proposed to fund an eight motor firing competition in which four heavy wall motors of each configuration was to be made and tested. Whichever motor was the easiest and cheapest to make and could deliver the highest performance would be the selected design. The star turned out to be the hardest to make, was the most expensive and delivered slightly less performance. The decision was made to select the four spoke wagon wheel design. The Missile Competition The means by which the US Army would select the winner of the anti tank missile competition was to have a shoot off competition. By the time of the shoot off competition, all the prime contractors had teamed with a rocket motor manufacturer. Over a period of about four weeks, four prime contractors and their rocket motor manufacturing partners were to field missiles for the shoot off. The Texas/Virginia team fired four missiles and hit the target every time. Of the other three competitors, only one team managed to have one successful launch. The contract was awarded to the Texas/Virginia team. The Arkansas Plant A part of the original proposal was to build a production facility on an Army reservation in Arkansas. The many hundreds of thousands of rocket motors to be built far exceeded the capacity of any of the Virginia rocket motor company s facilities. Once the development effort was complete, a team was moved to Arkansas to set up the production facility and get the motor under production. 6