214 



NATURE 



[October 14, 1920 



The Behaviour of Time Fuzes. 

 By Prof. A. V. Hill. F.R.S. 



THE time fuze is a device for exploding^ a shell 

 at any desired interval after it is fired. 

 Before the late war the time fuze was used mainly 

 with shrapnel shell, to burst the shell in the air 

 and so propel the bullets down on to the objective. 

 For this purpose the ordinary " pyowder-train " 

 fuze gave — considering its simplicity — remarkably 

 good and consistent results ; at any rate so good 

 that no serious impetus had been given to a proper 

 scientific study of its properties under a variety of 

 conditions. The development of anti-aircraft 

 gunnery, however, in which the employment of a 

 percussion fuze was useless, and in which the 

 target moved so fast that no preliminary "rang- 

 ing " on it was possible, not only required a much 

 greater reliance to be placed on the accuracy of 

 the time fuze, but also subjected it to much more 

 severe conditions than had ever occurred before. 

 The conditions referred to were those set up by 

 variations of velocity, air pressure, spin, and tem- 

 perature. Moreover, the enormous quantity of 

 powder suddenly required for military use made it 

 difficult for the manufacturers to produce it with 

 the same quality and consistency as of old. All 

 these factors led to a series of extraordinary diffi- 

 culties in connection with time fuzes, such as 

 irregular burning and a wholesale failure to burn 

 at all ; these difficulties were never completely 

 overcome in practice, but they stimulated a much 

 fuller investigation of the factors governing them, 

 and have resulted in a far greater understanding 

 of the physical behaviour of fuzes. As so often 

 happens in the history of knowledge, urgent 

 practical need led to scientific discovery. 



The powder train fuze consists of one or more 

 rings of highly compressed gunpowder forced into 

 a metal groove. The ring is fired by a detonator 

 at the moment the shell is accelerated in the 

 barrel, and after a certain amount of it, adjustable 

 beforehand, has been burnt it ignites a pellet 

 which fires a second detonator which explodes 

 the charge. The "fuze-setting," determining the 

 length of powder to be burnt, and therefore the 

 time of burning, is adjusted by turning the ring 

 round an axis parallel to that of the shell. The 

 gases produced by the combustion escape from 

 a hole in the fuze, usually at the side, but some- 

 times in the nose. The position of this hole is 

 of great importance, as w'ill be shown below. 



In a fuze at rest the time of burning is propor- 

 tional to the length of powder burnt, and it has 

 long been known that the rate of burning is a 

 function of the atmospheric pressure. Very exact 

 relations have been established between the pres- 

 sure and the rate of burning under a variety of 

 conditions, though their explanation is by no 

 means clear, and some very interesting problems 

 in the physical chemistry of combustion are pro- 

 vided by them. The gunpowder, of course, burns 

 inside a closed ring, supplying its own oxygen, 

 so that the effect of pressure is simply one of 

 NO. 2659, VOL. I06J 



pressure as such. In the fuze fired in a shell from 

 a gun the time of burning is by no means pro- 

 portional to the length of powder bprnt ; usually 

 the rate of burning is greatest at first («.e» when 

 the velocity of the shell is highest), decreasing 

 gradually as the shell slows up until a more or 

 less constant value is attained. In some fuzes, 

 however, the rate of burning is least at first, 

 increasing later on. Indeed, in some cases the 

 same fuze may show one phenomenon when 

 fired in one shell, and the opposite when fired 

 in another. This complex relation between 

 length of powder burnt and time of burning has 

 received a complete explanation in the theory of 

 the "dynamic pressure" at the escape holes. 

 When a body moves rapidly through the air the 

 pressure at any given point varies with the speed, 

 and at any given speed varies from point to point 

 of the shell. So completely does this theory ex- 

 plain the phenomena that an observed relation 

 between "fuze-setting" and time of burning has 

 been used even in the converse way to determine 

 the pressure at a variety of points on the head of 

 a shell moving at various speeds up to 1600 ft. 

 per second. It is possible, of course, for the 

 "dynamic pressure " to be a negative one — i.e. to 

 be a "suction" — in which case, if it be sufficiently 

 large, the powder may refuse to burn at all, and 

 the shell will be "blind." This will be the case if 

 the escape hole be too far back from the nose of 

 the shell, or be under the lee of a projection on 

 the fuze. It is necessary to take particular 

 account of these factors in the design of the fuze 

 body. 



The scientific development of the theory of fuze 

 burning dates largely from a trial carried out in 

 the winter of 191 6-1 7 at Portsmouth, in which a 

 large number of fuzes of the same type and " lot " 

 was fired to various heights up to 20.000 ft. in 

 exactly similar shells, from five different 3-in. 

 guns differing only in respect of their muzzle 

 velocities. The results were very peculiar, and at 

 first almost incredible; it was found that the effect 

 of a given fall of atmospheric pressure in the 

 upper air, whether in lengthening the time of 

 burning or in producing a liability to irregularity 

 and "blinds," was far greater in the case of a 

 shell fired from a high-velocity gun than it was in 

 the case of one fired from a low-velocity gun — 

 quite independently of what its actual velocity 

 might be at the moment considered. A given fuze 

 in a given shell, moving at a given velocity, at a 

 given reduced atmospheric pressure in the upper 

 air, might be expected to burn at a definite fixed 

 rate. It did not ! The rate of burning depended 

 on the previous history of the shell — viz. on the 

 velocity with which it had left the muzzle of the 

 gun. What effect could this previous velocity 

 have left upon it? The mystery was so com- 

 plete that one was clearly on the eve of a 

 discovery. Various theories were put forward to 



