what is the name of the rocket engine that is manufactured by npo energomash, and have an isp (vacuum) bigger than 248.6?
(AND spaceflight.rocket_engine (AND (JOIN spaceflight.rocket_engine.manufactured_by m.02p_3s6) (ge spaceflight.rocket_engine.isp_vacuum 248.6^^http://www.w3.org/2001/XMLSchema#float)))