Solution
= Solution
Without dust collection, $dM=-dm$. The <rocket equation> reduces to
$$
M\,dv=u\,dm=-u\,dM.
$$
Integrating from $v=0$, $M=M_0$ gives
$$
\boxed{v=u\log\frac{M_0}{M}.}
$$
This is the usual <rocket equation> with constant relative exhaust speed, valid while the mass remains positive and the prescribed ejection continues.