PURPOSE: To reduce heating by an exhaust jet stream to the bottom surface of a rocket, by providing an auxiliary rocket, which is pivotally supported in its head part to the main rocket, in such a manner that the auxiliary rocket enables its tail part to be moved in a direction of separating from the main rocket in accordance with an increase of its altitude.

CONSTITUTION: When an auxiliary rocket 2 is mounted to a main rocket 1, the head part of the auxiliary rocket 2 is rotatably supported to the main rocket 1 by a mounting part A provided in the auxiliary rocket 2. While the auxiliary rocket 2 provides in its tail part a mounting part B to be connected to an actuator provided in the tail part of the main rocket 1. This actuator is drivingly controlled by a control unit in a main unit of the main rocket 1 so as to move the tail part of the auxiliary rocket 2 in a direction of separating from the main rocket 1 in accordance with a launching altitude of the rocket. In this way, the rocket, as its altitude increases, controls a jet stream 4 of the auxiliary rocket 2 so as to separate from a jet stream 3 of the main rocket 1, reducing heating by an exhaust jet stream to the bottom surface of the rocket.