which bi-propellant rocket engines use unsymmetrical dimethylhydrazine and have a dry mass of less than 980.0?
(AND spaceflight.bipropellant_rocket_engine (AND (JOIN spaceflight.bipropellant_rocket_engine.fuel m.017q1y) (lt spaceflight.rocket_engine.dry_mass 980.0^^http://www.w3.org/2001/XMLSchema#float)))