Base Bleed Projectiles
Within the field of artillery techniques there has been a continual striving to increase the range and precision of field guns. Increased range is achieved either by gun improvements, which even include such modifications to propellant charges that a redesign of gun parts is required due to for example increased gas pressure in the barrel, or by improvements in the projectile performance. The turnover time for gun parts in the meantime is long, and therefore it is more attractive to attempt to improve the performance of the projectile itself without altering the gun as the ammunition has a continual turnover time of a totally different character to that of the gun.
Improved projectile performance can be achieved in several different ways which to a certain extent can be combined in one and the same projectile. At present work is proceeding along three different lines, of which the first involves attempting to produce a low-resistance projectile where the air resistance is reduced to a minimum. This work has resulted in longer and slimmer projectiles. The second line involves equipping special projectiles with their own source of power in the form of a built-in rocket motor, so-called "reatiles" or Rocket Assisted Projectile (RAP). With regard to the third line, the work has been concentrated around reducing the base resistance of the projectile, caused by the stream of air round the projectile generating a lower pressure immediately behind the projectile base than in the surrounding air.
A conventional bullet in flight forms a partial vacuum immediately therebehind. This partial vacuum or low pressure area creates a force which acts on the projectile in a direction opposite its motion thereby lessening the flight velocity of the projectile. This force is commonly referred to as "base drag". Drag may be defined as that force acting on the projectile in a direction opposite its motion. Base drag is that drag acting on the projectile at its base, and this invention concerns itself primarily with base drag and the reduction thereof.
An artillery projectile in flight forms a low pressure area immediately behind the projectile which creates a force (commonly called "base drag") which lessens the velocity of the projectile. This method is a completely different approach to the problem, since no rocket assisting effect is achieved.
It is known that theoretically this base resistance can be reduced or even eliminated by allowing a stream of gas to flow out of the base surface of the projectile in a suitable manner, thereby increasing the base pressure. This base pressure can be further increased if the stream of gas is combined with the release of heat. The effect produced by this, the so-called base-bleed effect, differs from purely rocket power in as much as the flow generated is so low that the reaction force generated by the flow is practically negligible when compared with the change in pressure affecting the projectile base. The problem with producing a satisfactory base-bleed projectile has been predominently on the practical level. The necessity for a long burning time and a subdued gas outflow has caused attempts to be made to produce slow burning powder charges which ran towards the base surface of the projectile via a relatively large gas outlet opening. Consequently it has been a problem to produce sufficiently slow burning powder charges which in addition did not disintegrate under the aggregate influence of all the forces affecting the projectile. Slow burning powder charges for earlier actual base-bleed type projectiles even suffer from the fact that powder charges which are open to the surrounding atmosphere via a relatively large outlet opening will burn at varying speeds at different external pressures, i.e. the speed of burning will vary according to the trajectory height.
Base drag contributes generally to a relatively large part of the total drag and depends upon the fact that the base pressure due to the resulting wake flow in the base region is lower than the ambient air pressure. By ejecting a mass flow from the base region of the body into the near wake the flow pattern can be affected in such a way that the base pressure is increased and thereby the base drag is reduced. If the ejection of mass is combined with liberation of heat, e.g. by combustion, the base pressure can be further increased. The base flow/burning effect differs from rocket propulsion foremost therein that the reaction force caused by the base flow is very small as compared to the decrease in drag due to increased base pressure. In order to efficiently utilize the base flow effect the base mass flow should occur during a considerable part of the flight time.
Small caliber (e.g., about 30 mm. or less) projectiles have been equipped with tracer or fumer compositions which provide some relief to the base drag problems with these types of projectiles. These systems, however, burn relatively rapidly (usually for only a few seconds) and cannot be controlled to provide prolonged relief for longer times. Tracer or fumer compositions are generally solid, relatively inflexible compositions which also contain relatively high amounts of metals (e.g., about 35%) and thus produce a gas containing high amounts of solid combustion particles which do not provide relief to the base drag problem. Also, these systems are most often applicable to small caliber (e.g., about 30 mm or less), short range (e.g., about 4000 m., often about 300-400 m.) projectiles, although proposals for use with larger caliber projectiles have been made.
Conventional tracer bullets have reduced total drag by about 9.3 percent and base drag about 18.6 percent over comparable inert rounds, with attendant increases of velocity of about 4 percent, due to the rearward pressures exerted by the reaction gases formed during combustion of the tracer compositions. It would be desirable, however, if base drag could be further reduced or substantially eliminated, thus providing fighting forces with more effective weapons having greater striking power.
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