It is usual to consider equilibrium between the forces and accelerations associated with individual fluid elements when deriving thrust formulae containing momentum terms. Thermodynamic energy equations for open systems, however, only rely on the premise that, as far as energy is concerned, what goes into a system must either come out or else build up inside.

In thermodynamic textbooks a distinction is usually drawn between steady flow, involving no change of energy with time inside a system, and non-steady flow, which implies a rate of change of energy within the thermodynamic system under consideration.

It is interesting to find that the thrust formula for a simple system, as shown in Fig. 1, may be derived directly from these thermodynamic energy equations by two alternative methods.