Hxen.com's tips：Every day persisted can learn English well!-->■Learn English Everyday■
milestone – n. an event that reaches never before seen levels
supersonic - adj. faster than the speed of sound
efficient – n. working well and not wasting time or energy
emission – n. the act of sending gas, heat or light out into the air
carbon-neutral– adj. not producing carbon emissions
propulsion – n. the force produced by a system for moving, or propelling, a vehicle or other object
wind tunnel – n. a closed passage of room through which currents of air are forced in order to study the effects of moving air on aircraft and other vehicles
