Editing Working mass (section)

==Rockets and rocket-like reaction engines==
In rockets, the total velocity change can be calculated (using the [[Tsiolkovsky rocket equation]]) as follows:

<math>\Delta\,v  = u\,\ln\left(\frac{m + M}{M}\right)</math>

Where:
* ''v'' = ship velocity.
* ''u'' = exhaust velocity.
* ''M'' = ship mass, not including the working mass.
* ''m'' = total mass ejected from the ship (working mass).

The term working mass is used primarily in the [[aerospace]] field. In more "down to earth" examples, the working mass is typically provided by the Earth, which contains so much momentum in comparison to most vehicles that the amount it gains or loses can be ignored. However, in the case of an [[aircraft]] the working mass is the air, and in the case of a [[rocket]], it is the rocket fuel itself. Most rocket engines use light-weight fuels (liquid [[hydrogen]], [[oxygen]], or [[kerosene]]) accelerated to supersonic speeds. However, [[ion engine]]s often use heavier elements like [[xenon]] as the reaction mass, accelerated to much higher speeds using electric fields.

In many cases, the working mass is separate from the [[energy]] used to accelerate it. In a car, the engine provides power to the wheels, which then accelerates the Earth backward to make the car move forward. This is not the case for most rockets, however, where the rocket propellant is the working mass, as well as the energy source. This means that rockets stop accelerating as soon as they run out of fuel, regardless of other power sources they may have. This can be a problem for satellites that need to be repositioned often, as it limits their useful life. In general, the exhaust velocity should be close to the ship velocity for optimum [[Efficient energy use|energy efficiency]].  This limitation of rocket propulsion is one of the main motivations for the ongoing interest in [[field propulsion]] technology.