After numerous test failures of the new solid-fuel Bulava submarine launched ballistic missile (SLBM) intended for the new Borei-class submarines, there are reports that it could be replaced by the liquid fueled Sineva missile, which is currently deployed on older submarines.

My guess is that it would take less time and effort to make the Bulava work than it would to replace it, since it is derived from a land-based system which already works, and the rework of the Borei’s to do this would be involved, but I am speaking from a position of profound ignorance here.