What is delta-v and why is it the fundamental currency of spaceflight?

Delta-v (change in velocity) measures the impulse per unit mass needed for a maneuver, expressed in m/s. It is the fundamental measure of propulsion capability because: It directly relates to propellant mass through the rocket equation (Tsiolkovsky): delta-v = Isp * g0 * ln(mi/mf); It is independent of vehicle mass (unlike thrust or fuel mass); It can be budgeted across mission phases. Applications: Launch to orbit (~9.4 km/s for LEO), Orbit raising/lowering, Plane changes (expensive), and Interplanetary transfers. Mission planning budgets delta-v for all maneuvers with margins. Higher Isp propulsion (electric vs chemical) achieves more delta-v from same propellant mass but with lower thrust.