KRAMATORSK, Ukraine — Russia has made liberal use of one of its most fearsome conventional weapons in the fighting in eastern Ukraine, according to Ukrainian military commanders, medics, British officials and videos from the battlefields.
The weapon, a track-mounted rocket artillery system nicknamed Solntsepek, or the Heatwave, fires thermobaric warheads that explode with tremendous force, sending potentially lethal shock waves into bunkers or trenches where soldiers would otherwise be…