The pulse detonation engine (PDE) has recently emerged as an aerospace propulsion system of the future. In the PDE system, self-sustaining detonation waves propagate in tubes and gases burning behind the detonation front are exhausted backward repeatedly. The PDE system momentum is equal in absolute value to the exhausted gas momentum, but in the opposite direction. The PDE system has high thermal efficiency because of its constant-volume combustion, and it has a simple structure composed of tubes. In this paper we analyze the net momentum obtained by using a detonation tube as a ballistic pendulum, and we estimate precisely the PDE's specific impulses by using room-temperature 101kPa 2H2 + O2 mixtures. The specific impulses obtained by the ballistic pendulum method were 137.6 ± 5.2 sec at stoichiometric hydrogen-oxygen mixtures of 101 kPa. The specific impulses obtained in the detonation tubes were almost constant when the diaphragm thickness changed from 12 to 48 mm. The effective specific impulse reached its maximum value, 247.4 sec, at a 0.437 partial filling rate. The effective specific impulse of the case in which xd=200 mm does not depend on the open area ratios of the orifices. When the tube length divided by the tube diameter (L/D) is less than 18, the effective specific impulse is larger than 130 sec, which is almost identical to that predicted by the Endo-Fujiwara theory.