  We present the simplest, string-derivable, supergravity model and discuss its
experimental consequences. This model is a new string-inspired flipped $SU(5)$
which unifies at the string scale $M_U=10^{18}\GeV$ due to the introduction of
an additional pair of \r{10},\rb{10} flipped $SU(5)$ representations which
contain new intermediate scale `gap' particles. We study various model-building
issues which should be addressed in string-derived incarnations of this model.
We focus our study on the no-scale supergravity mechanism and explore
thoroughly the three-dimensional parameter space of the model ($m_{\tilde
g},m_t,\tan\beta$), thus obtaining several simple relationships among the
particle masses, such as: $m_{\tilde q}\approx m_{\tilde g}$, $m_{\tilde
e_L}\approx m_{\tilde\nu}\approx0.30 m_{\tilde g}$, $m_{\tilde
e_R}\approx0.18m_{\tilde g}$, and $m_{\chi^0_2}\approx2m_{\chi^0_1}\approx
m_{\chi^\pm_1}$. In a strict interpretation of the no-scale supergravity
scenario we solve for $\tan\beta$ as a function of $m_t$ and $m_{\tilde g}$,
and show that $m_t$ determines not only the sign of the Higgs mixing parameter
$\mu$ but also whether the lightest Higgs boson mass is above or below
$100\GeV$. We also find that throughout the parameter space the neutralino
relic abundance is within observational bounds ($\Omega_\chi h^2_0\lsim0.25$)
and may account for a significant portion of the dark matter in the Universe.
