Design of Nihka propellant liner trimming tool : final design report

Abstract

Bristol Aerospace Ltd. manufactures a stage of the Black Brant sounding rocket known as the Nihka rocket motor. After casting propellant into the motor casing, a two stage trim process must occur whereby first the excess propellant is trimmed away, followed by the removal of insulation tab which protrudes above the remaining propellant. This process is currently done by hand, and operator error has resulted in significant damage to the nearly completed motor. To avoid such an occurrence in the future, Bristol has requested a tool be designed which can remove the element of operator skill from the process. Research was conducted to obtain relevant information that may aid in developing solutions to the problem, as well as to determine if any patents pertaining to the issue exist. None were found. The information gathered was used by the design team through a concept development phase that resulted in the major geometry of the device being determined, as well as a framework for how the required motion of the device would be accomplished. Following approval by the client, this selected concept was developed further to ensure its technical viability, and to address factors such as safety, manufacturability, reparability, and cost. Detailed part drawings and manufacturing instructions are contained in the report. The device will locate onto the rocket via a custom made interface plate that will bolt into pre-existing radial holes in the rocket. The use of three or more holes will guarantee the tool coincides with the rocket axes. Rotational motion is accomplished with a commercially available bearing. A tool holder can translate across the diameter above the propellant by sliding on radial bars which are fixed to the inner surface of the bearing. Two separate blades for the different cutting processes were designed which can move up and down relative to the rocket by means of a rack and pinion gear system contained in the tool holder. These three motions can be used together to ensure that the blades can cover all required surfaces for both the propellant and insulation liner. The entire assembly is fabricated from SAE 300 series stainless steel. All necessary target specifications and constraints were adhered to in the solution of the problem. The final weight of the design is approximated at 60.7 pounds, and estimated budget, including machining costs, is $7461.31. As this budget is significantly less than the cost of repairing a single damaged motor, Team 14 recommends implementation of this device to the Nihka manufacturing process.