Shell

A shell comprising:

• a shell body;
• a steering wing that includes a drive shaft and that is rotatably mounted on an external surface of the shell body;
• an auxiliary wing that includes a shaft connection portion which is connected to the drive shaft in such a manner that the auxiliary wing is able to move in the lengthwise direction of the drive shaft within the drive shaft, and that is installed in such a manner that the auxiliary wing is able to be inserted into and be spread outward from within the steering wing;
• an auxiliary-wing holding unit that includes a holding protrusion which is fixedly arranged in a direction of intersecting the shaft connection portion in such a manner that the holding protrusion is engaged with or disengaged with the shaft connection portion depending on an angle at which the drive shaft rotates, and that selectively holds the auxiliary wing in place;
• and
• an auxiliary-wing spreading unit that is installed within the drive shaft, and that provides driving force for spreading the auxiliary wing outward from within the steering wing when the holding protrusion is disengaged with the shaft connection portion.