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 energy efficiency. This limitation of rocket propulsion is one of the main motivations for the ongoing interest in field propulsion technology.
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 energy efficiency this limitation of rocket propulsion is one of the main motivations for the ongoing interest in field propulsion technology