what bipropellant rocket engine use the fuel unsymmetrical dimethylhydrazine and has a dry mass less than 168.5?
(AND spaceflight.bipropellant_rocket_engine (AND (JOIN spaceflight.bipropellant_rocket_engine.fuel m.017q1y) (lt spaceflight.rocket_engine.dry_mass 168.5^^http://www.w3.org/2001/XMLSchema#float)))