  Two relativistic X-ray jets have been detected with the Chandra X-ray
observatory from the black hole X-ray transient XTE J1550-564. We report a full
analysis of the evolution of the two jets with a gamma-ray burst external shock
model. A plausible scenario suggests a cavity outside the central source and
the jets first travelled with constant velocity and then are slowed down by the
interactions between the jets and the interstellar medium (ISM). The best
fitted radius of the cavity is $\sim$0.36 pc on the eastern side and $\sim$0.46
pc on the western side, and the densities also show asymmetry, of $\sim$0.015
cm$^{-3}$ on the east to $\sim$0.21 cm$^{-3}$ on the west. A large scale low
density region is also found in another microquasar system, H 1743-322. These
results are consistent with previous suggestions that the environment of
microquasars should be rather vacuous, compared to the normal Galactic
environment. A generic scenario for microquasar jets is proposed, classifying
the observed jets into three main categories, with different jet morphologies
(and sizes) corresponding to different scales of vacuous environments
surrounding them.
