Setback acceleration

Typical setback accelerations experienced by projectiles are of the order of 30,000 times that of gravity (Ref 8). The acceleration increases from zero to its max value (see Fig 1) in a few milliseconds... 

The Setback Mechanism. The duration of the setback acceleration period is long compared with the transit time for a compression wave in the material, but short compared with the time required for significant heat transfer. Thus the compression that results may be considered to be essentially adiabatic. As soon as the physical mechanism of setback is understood to be adiabatic compression, it becomes clear how explosives can be initiated by setback. Hot spots are caused by the adiabatic compression of minute air spaces within the explosive (See also under Hot Spots in Vol 7, HI 70 ff)... 

Setback Acceleration. Setback is the relative rearward force of component parts in a projectile, missile, or fuze undergoing forward acceleration during its firing or launching. This tendency to move is caused by the setback force, the rearward force of inertia that is created by the forward acceleration of the projectile or missile. The force is directly proportional to the acceleration and mass of the parts being accelerated... 

Each structure shall be rated for the following applicable loading conditions. The structures shall be designed to meet or exceed these conditions in accordance with the applicable specifications set forth herein. The following ratings do not include any allowance for impact. Acceleration, impact, setback, and wind loads will reduce the rated static hook load capacity. 

Setback Force. The rearward force of inertia which is created by a forward acceleration of a projectile or missile during its launching phase. This force causes arming and eventual functioning of fuzes... 

Upon firing a proplnt chge in a gun, the projectile starts to move with acceleration and rotation. Acceleration will cause setback, while rotation will produce centrifugal force. Both forces will act upon movable safety parts inside the fuze and if it is a time fuze, the setback force alone will be sufficient for arming. If the fuze is impact-type, a combination of setback and centrifugal force will be required to achieve the same purpose. 

As the projectile leaves the muzzle of the weapon, the acceleration (and hence setback) ceases, while rotation (and hence centrifugal force) continue to act, although at gradually reduced speed. Fuzes using centrifugal force for arming must be so designed that they will not be unarmed as the rotational velocity... 

In armed position, flat section of segment shaft permits lever of setback device and its shaft to rotate clockwise once acceleration of projectile ceases. During acceleration of projectile in rifle tube, friction between rotor... 

When the projectile is fired, sustained acceleration creates setback forces which act upon the sequential leaf arming mechanism located in the release mechanism assembly. The leaves are sequentially displaced by setback forces until leaf No 3 is locked in its rear or armed position. 

S) Spin-Setback Simulator Test. Accdg to Ref 39, pIIB-43, equipment simulating spin velocity, angular acceleration and setback exerted on an arilllery fuze as experienced in combination during the phase of internal ballistics was supposed to be constructed at the end of 1966 by the Artillery Ammunition Laboratory, Ammunition Engineering Directorate, PicArsn,... 

In addition to this very specific acceleration that causes setback, there are several other accelerations of similar magnitude to which this discussion applies. The axial force in the direction opposite to setback has been designated as setforward. It is the forward force of inertia that is created when a projectile, missile, or bomb decelerates. Deceleration occurs on water entry and target impact. Setforward also occurs when projectiles are rammed into an automatic weapon. Present point-detonating, time, and proximity fuzes will withstand about 1000 g setforward. While weapon designers would like to double or triple the ram velocity, present fuzes cannot survive this force (Ref 8)... 

To simulate the conditions experienced by the filler of a projectile during acceleration in a gun, the apparatus shown in Fig 2 (Ref 1) was developed. By the action of the propellant, a pressure pulse is transmitted to an expl specimen thru the piston system that closely resembles setback. The criterion for each expl tested is the max pressure at which the expl cannot be initiated, when at an initial temp of 125°F, in 25 or more trials. Setback sensitivity data so obtained for various expls are listed below ... 

The mode of extraction for PAHs is highly dependent on the matrix. For solid-based matrices such as food samples, sediments, soil, marine organisms, etc. extraction methods such as Soxhlet extraction with nonpolar solvent [35 6], hollow fiber membrane solvent microextraction (HFMSME) [10], pressimzed hquid extraction (PLE) [37,38], sonication extraction [3], microwave-assisted extraction (MAE) [3], supercritical fluid extraction, (SEE) [39], accelerated solvent extraction (ASE) [40], cold extraction [41], soxtec extraction [42], microwave-assisted alkaline saponification (MAAS) [43], dynamic microwave-assisted extraction (DMAE) [44], add-induced cloud point extraction (ACPE) [45], methanolic saponification extraction (MSE) [7], etc. are employed. Of all these, Soxhlet extraction is the most common for solid samples and has achieved excellent extraction with high-level recovery but its setback is the high consmnption of solvent and time associated with it. 

Inertial or setback activators are devices that are activated by a large-magnitude shock or rapid acceleration, as is generated upon firing of a morfar or artillery round. They are designed to react to a predetermined combination of g force and its duration. Inertial activators are typically firmly mounted inside the battery structure in order to withstand severe dynamic environments. 

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