📘 Definition
Delta-v (ΔV) is the change in velocity a spacecraft must achieve to perform an orbital manoeuvre. It is the fundamental "currency" of spaceflight — every manoeuvre (launch, orbit raising, plane change, deorbit) has a delta-v cost, and a spacecraft's total available delta-v (determined by its propulsion system and fuel load via the Tsiolkovsky rocket equation) dictates what it can accomplish. For example, reaching LEO from Earth's surface requires approximately 9.4 km/s of delta-v. A Hohmann transfer from LEO to GEO requires about 3.9 km/s additional.
9.4 km/s
Surface → LEO
2.5 km/s
LEO → GTO
1.5 km/s
GTO → GEO
3.2 km/s
LEO → Moon
Understanding Delta-V
The Tsiolkovsky Rocket Equation
The amount of delta-v a spacecraft can achieve depends on its exhaust velocity (specific impulse) and its mass ratio (full mass / empty mass): ΔV = Isp × g₀ × ln(m₀/mf). This exponential relationship means that carrying more fuel has diminishing returns — doubling fuel doesn't double delta-v.
Delta-V Budget
Mission planners create a "delta-v budget" — a ledger of every manoeuvre required and its velocity cost. For a GEO communications satellite launched by Falcon 9, the budget might be: launch to LEO (provided by rocket), LEO to GTO (2.5 km/s, also rocket), GTO circularisation (1.5 km/s, satellite engine), station-keeping over 15 years (50 m/s/year), and final graveyard orbit boost (10 m/s).