Wiktionary
n. (''aerospace'') The total mass of the burnt fuel etc. ejected from a rocket to provide thrust
Wikipedia
Working mass, also referred to as reaction mass, is a mass against which a system operates in order to produce acceleration. In the case of a rocket, for example, the reaction mass is the fuel shot backwards to provide propulsion. All acceleration requires an exchange of momentum, which can be thought of as the "unit of movement". Momentum is related to mass and velocity, as given by the formula P = mv, where P is the momentum, m the mass, and v the velocity. The velocity of a body is easily changeable, but in most cases the mass is not, which makes it important.
Usage examples of "working mass".
Your working mass, your exhaust, is air and vaporized rocket motor, hotter than hell, with a terrific exhaust velocity.
On long voyages, standard practice was to spray water into the alcohol flame as working mass.
A fusion motor radiates visible light because it is hot, or because working mass has been added to increase thrust.
He kept scooping debris up against it, and eventually it became a simple heap of working mass again.