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.