IX 



RECENT CONTRIBUTIONS TO BASIC HYDROBALLISTICS 



R. N. Cox and J. W. Maccoll 

 Armament Research and Development Establishment 



I. HYDROBALLISTICS AND THE BALLISTIC SCIENCES 



As the word implies, hydroballistics is primarily concerned with the motion of 

 projectiles through water or any other liquid medium. The subject is thus the hydro- 

 dynamic counterpart of aeroballistics which concerns the corresponding behavior in air. 

 As it is usual for missiles to have an air flight path before entering the water, water entry 

 is also included as an important section of hydroballistics. 



Hydroballistics has however connections with other branches of the ballistic 

 sciences. Thus certain problems of terminal ballistics are closely related to similar 

 problems in hydroballistics: the latter subject can in fact be regarded as the terminal 

 ballistic study of the ideal liquid-like medium. Terminal Ballistics is a field in which 

 the basic scientific background is still largely undeveloped and it may be useful to con- 

 sider certain terminal ballistic problems from an hydroballistic standpoint. This topic 

 will not be carried further here as the present symposium is concerned with hydrobal- 

 listics in its relation to naval problems. 



In the present paper we deal mainly with some of the basic researches carried 

 out during the last decade: most emphasis will be on work that has appeared during the 

 last five years. 



Our subject is nowhere dealt with extensively in the open literature, but Profes- 

 sor Garrett Birkhoff's volume on Hydrodynamics (1950) makes frequent reference to 

 selected topics of a hydroballistic character, and we shall assume some degree of famili- 

 arity with the matters touched upon there. Much of the literature on the subject, in 

 both the United States and in the United Kingdom, is in the form of Research Estab- 

 lishment reports. Such reports are not distributed very widely and are not always 

 readily obtainable. Because of this we trust we have not omitted from our review any 

 major contribution to our subject. 



II. The several sections of Hydroballistics 



The hydroballistic field divides conveniently into a number of phases which can 

 be associated with the typical trajectory. Thus, referring to Fig. 1, we have 



a. Shock phase which covers an initial, and extremely short, period of shock 

 when the water surface is hit (I). 



b. Water Entry phase which covers the subsequent cavity forming period (II). 



c. Closure phase which follows, and which includes either or both surface seal 

 and deep closure (III). 



d. Quasi-Steady Cavity phase in which the missile moves with an attached cav- 

 ity (IV). 



e. The Fully-Wetted condition which will exist at sufficiently great hydrostatic 

 pressures, when a cavity cannot be maintained. The conditions of motion are 

 here similar to those existing in subsonic motion in air at the corresponding 

 Reynolds number: due to the density ratio, the forces in water are about 800 

 times greater than those in air. 



215 



