Two common measurements of the performance of a rocket engine are thrust-to-weight ratio and specific impulse.

Thrust-to-weight ratio (T/W ratio) This is a straight-forward ratio of the maximum thrust of the engine to the weight of the engine. T/W ratio is often assumed to be a percentage, which means that a T/W ratio of two would be reported as 200. Since both thrust and weight can be measured in the same units, the units of T/W ratio are not specified, and are irrelevant.

Specific impulse (Isp) This is a measurement of engine efficiency. It is measured in seconds, and represents the number of seconds that one pound of propellant can produce one pound of thrust. An engine with an Isp of 200 can maintain one pound of thrust for 200 seconds.

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