Mr Fowler and Mr Lock, The initial motion of a shell 311 



The Origin of the Disturbances in the Initial Motion of a Shell. 

 By R. H. FowLEE and C. N. H. Lock. 



[Read 28 February 1921.] 



(1) The following paper* is an attempt to throw some light on 

 one of the most obscure outstanding problems in gunnery, namely, 

 the precise cause of the initial angular oscillations of the axis of a 

 (stable) spinning shell. The factors that produce the initial oscil- 

 lations have hitherto only been guessed at, and design, which aims 

 at reducing these disturbances to a minimum, has been guided 

 solely by empirical results. It is therefore a matter of some im- 

 portance to analyse carefully the experimental evidence which has 

 recently been acquired, for the purpose of discriminating between 

 possible causes, and suggesting the proper lines of future research. 

 On the scientific rather than the technical side, we point out the 

 desirability for a solution, if possible, of the elastic vibrations of 

 the gun under its firing stresses. 



In experiments carried out by ourselves and others in January, 

 1919f, we succeeded in recording with reasonable accuracy the 

 initial oscillations of the axis of a series of shells, of four different 

 types, fired at a series of muzzle velocities from a pair of 3-inch 

 guns of two different twists of rifling. Specimens of these observa- 

 tions will be found in the paper quoted. By extrapolation of the 

 observed curves backwards to the neighbourhood of the muzzle of 

 the gun, it is possible to deduce with some confidence rough values 

 for the magnitude and direction of the initial angular velocity of 

 the axis of the shell. The extrapolation is not a serious one, for the 

 law of motion of the axis of the shell is well understood. By 

 analysis of these values and their variation with the varying circum- 

 stances of projection, it is possible to throw hght on the origin of 

 the disturbances themselves. 



The most obvious origin for these disturbances would appear to 

 he in random gas pressure variations during the last part of the 

 travel of the shell down the rifled bore, and in random asymmetry 

 of the blast which flows past the shell for the first few feet of its 

 motion outside the barrel. The only reasonable alternative sug- 

 gestion is that the initial angular velocity is primarily due to some 

 form of barrel vibration. It is important to discriminate between 



* This paper is published by permission of the Ordnance Committee, for whom 

 the experimental work was carried out. The authors also thank the Admiralty 

 Director of Scientific Research and Experiment, who recently propounded to one 

 of them a technical problem on initial motions. This problem suggested the possible 

 importance of these results. 



t "The Aerodynamics of a Spinning Shell," Trans. Roy. Soc. A, vol. ccxxi, 

 1920, p. 295. 



