Base boost is a method for extending the range of artillery (or naval gunfire) shells. To augment the kinetic energy provided the projectile by the combustion of propellant in the gun barrel (which stops as soon as the projectile leaves the barrel), a small rocket motor is attached to the base of the shell. This motor is ignited by the firing of the projectile, and during the first few seconds of the shell's flight, burns to produce a sharp if brief acceleration beyond the gun. This boost translates into additional range, as the shell continues to accelerate beyond the normal barrel travel.

This method provides advantages in the range extension it offers. It may also be used to compensate for weaker metallurgy, by providing more of the initial boost after the barrel travel; this allows the pressure in the barrel and breech of the gun to be reduced. However, it also brings penalties; for one, the shell is more complex, with a new potential failure point. For another, it increases the cost and fragility of the munition, and perhaps most problematic, it decreases the accuracy of the shell. This is not only due to the normal enlarging of aiming error produced by a longer travel distance, but also due to the additional forces imposed on the projectile during flight which are usually less predictable than those within the confines of the gun.

Also known as rocket assisted projectiles (RAP), these shells are most profitably employed if their payload is guided or navigated, which will compensate for the additional error induced by the addition of the booster. The method should not be confused with base bleed or base burn projectiles, even though the latter also contains a rear-mounted pyrotechnic.

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