  Two ROSAT high resolution images separated by nearly five years have been
used to determine the expansion of the X-ray remnant of Tycho's supernova
(SN1572). The current expansion rate averaged over the entire remnant is
$0.124\pm0.011$ % per yr, which, when combined with the known age of the
remnant, determines the mean expansion parameter $m$, defined as $R\propto
t^m$, to be $0.54\pm0.05$. There are significant radial and azimuthal
variations of the X-ray expansion rate. The radial expansion in particular
shows highly significant evidence for the more rapid expansion of the forward
blast wave as compared to the reverse-shocked ejecta, an effect that has not
been seen previously. The expansion parameter varies from $m=0.71\pm0.06$ at
the outermost edge of Tycho's supernova remnant (SNR) to a value of
$m=0.34\pm0.10$ on the inside edge of the bright rim of emission. These values
are consistent with the rates expected for a remnant with constant density
ejecta evolving into a uniform interstellar medium during the ejecta-dominated
phase of evolution. Based on the size, age, and X-ray expansion rates, I obtain
values for the explosion energy and ambient density of $E\approx 4-5\times
10^{50} \rm ergs$ and $n_0 \approx 0.35-0.45 \rm cm^{-3}$. As is also the case
for Cas A and Kepler's SNR, the X-ray expansion rate of Tycho's SNR appears to
be significantly higher than the radio expansion rate. In the case of Tycho's
SNR, however, the difference between radio and X-ray expansion rates is clearly
associated with the motion of the forward shock.
