Self-propelled artillery includes those artillery pieces, such as howitzers and multiple rocket launchers, which are an integral part of a motor vehicle and can move significant distances without being towed. Tactical guided missile launchers also fall into this category. Weapons of this type are indirect fire and are expected to fire from well behind a front line, so their armor protection is not that of an armored fighting vehicle, but sufficient to stop machine gun fire and fragments from artillery.
They usually can be sealed from outside air, which can be pumped through filters when a supply is needed. While the purpose of such sealing is principally to protect against chemical, biological and radioactive contamination, sealing may be necessary due to the weapon itself: the exhaust from large rockets and missiles often is both toxic and superheated.
One of the motivations for highly mobile artillery is the danger of counterbattery fire, which, with modern technique, can have enemy projectiles in the air, aimed at the point of origin of a shell or rocket, before that "friendly" projectile has even hit the ground. Current doctrine against an opponent with modern counterbattery capabilities is "shoot and scoot": fire, retract any bracing legs or other impediments to moving, and drive to a new location, outside the danger zone for plausible return fire. Unless the artillery piece can fire multiple rounds within seconds, it will not attempt a second firing from the same position.