The relative merits and possible applications of liquid and solid propellant rocket motors have long been topics for discussion and controversy. The arguments for and against have been sharpened by the fact that the development of the two types of motor are usually vested in quite different teams of people. At the same time, a fair comparison has often been confused because the ultimate choice has been made for reasons which are really not technical at all, but instead depend on availability and the utilisation of already deployed capital resources.

Nevertheless, the merits and de-merits of the “conventional” (as opposed to the packaged) liquid motor and the solid motor can be simply defined with reasonable accuracy. The liquid motor is complicated, demanding at least some, and often a great deal, of attention before use. At the same time, it has a much greater performance potential, can be used repeatedly and can be designed to give variable thrust. The solid motor is simpler (although is getting less so), is cheaper (although this is getting less true) and demands less preparation. However, it has definite performance limitations, is inflexible in its output, and is often quite sensitive to its environment, whether during storage, carriage, or operation.