# Rockets and Other Thermal Jets Using Nuclear EnergyWith a General Discussion on the Use of Porous Pile Materials^{*}

To bring out the crucial problem in rocket propulsion, consider the motion of a rocket in a force-free space, i. e., a space without a gravitational field. Let *m*_{1} denote the mass of the rocket at any time and d*m*_{2} the mass ejected from the rocket in a small time interval. The absolute velocities of the rocket and the ejected mass are *u*_{1} and - *u*_{2} respectively. The relative velocity *w* of the ejected mass with respect to the rocket is then *u*_{1} + *u*_{2} (Fig. 1).

## 1. Simple Theory of Space Rockets

To bring out the crucial problem in rocket propulsion, consider the motion of a rocket in a force-free space, i. e., a space without ...