Feasibility study of a miniaturized magnetorhological grease timing trigger as safety and arming device for spinning projectile

Abstract

A safety and arming (S&A) device keeps the fuze of a projectile unarmed during shipping, handling and storage, while arming the firing section at a proper time by sensing external conditions such as pressure, position, etc. With the increasing need for smaller S&A devices, a miniature design with a compact configuration and high reliability is in demand. This paper proposes a miniaturized timing trigger as the S&A device for a spinning projectile by utilizing the ‘locking’ and ‘unlocking’ properties of magnetorheological (MR) grease with/without the presence of a magnetic field. First, the design and arming mechanism of the timing trigger are introduced, in which the MR grease is locked by a magnetic field generated by two permanent magnets (PMs). Under sufficient firing acceleration, the PMs disengage to unlock the contraction flow of the MR grease, which enables its triggering function. A theoretical analysis was conducted to interpolate the delay time against the geometry of the device, the shear/extensional characteristics of MR grease and the spinning rate of a projectile. A series of tests have been conducted to measure the delay times by tuning the physical parameters, including particle concentration, spinning rate and orifice diameter etc. The experimental results showed that this theoretical model is capable of calculating well the delay time of a MR grease timing trigger.