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What is Power per Unit Mass?

Lockheed Martin F-22 Raptor fighter aircraft

Power per unit mass, also known as specific power, is the power output for a given mass of a substance or machine. In the case of an engine or vehicle this would be referred to as the power to weight ratio.

The power to weight ratio may be measured for the engine in order to make comparisons between different designs or models, or may be calculated for the entire vehicle in order to obtain an indication of its performance. For an aircraft, for example, the power to weight ratio is a critical factor in determining its maximum rate of climb.

Power to weight ratio is of varying importance depending on the type of vehicle. For land-based power generation this is unimportant and the design of engines is usually mostly focussed on maximising fuel efficiency and controlling emissions. For marine propulsion the mass of the vehicle is usually so large that the power to weight ratio is not as important as in say a railway engine or, to a greater degree, a car. For aircraft it is critical and will determine the types of engine that are suitable. In the case of military fighter aircraft, power to weight ratio is of paramount importance and will result in devices such as afterburners that will produce large amounts of peak power output while sacrificing fuel efficiency.

A family car will have a power to weight ratio of between 50 and 100 W/kg, high performance sports cars will be double this. Racing cars and passenger aircraft are of the order of 1000 W/kg, while fighter aricraft reach about 10,000 W/kg. The NASA space shuttle during launch (with rocket boosters) reaches about 100,000 W/kg.

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