Delta-V (ΔV)

Understanding Delta-V

How much delta-v a spacecraft has: the rocket equation

A spacecraft's available delta-v is set by the Tsiolkovsky rocket equation, Δv = I_sp · g₀ · ln(m₀ / m_f), where g₀ is standard gravity (9.81 m/s²). It hinges on just two factors: the engine's specific impulse — how efficiently it converts propellant into thrust — and the ratio of fuelled mass to dry mass. Because that mass ratio sits inside a natural logarithm, delta-v grows only slowly as propellant is added, so wringing out extra velocity means carrying exponentially more fuel. This 'tyranny of the rocket equation' is why an orbital launcher is roughly 90% propellant by mass, why rockets use staging to shed empty tanks, and why raw chemical propulsion struggles to travel far beyond Earth without a gravity assist.

A delta-v map of the Solar System

Every manoeuvre has a price, and because delta-v is additive you can plan an entire mission by summing the legs. The approximate figures below assume efficient Hohmann transfers; real missions trim them with gravity assists and aerobraking, or pay extra for speed. Treat them as order-of-magnitude values — each shifts with altitude, timing and mission design.

Chemical vs electric: trading thrust for delta-v

The same delta-v can be bought in two very different ways. Chemical propulsion delivers enormous thrust but a specific impulse of only about 300–450 s, so a chemical stage holds a modest delta-v budget and spends it in minutes. Electric propulsion — such as an ion thruster — reaches 1,500–4,000 s, yielding several times more delta-v from the same propellant, but at gram-scale thrust that must fire for weeks or months. Many modern satellites carry both: chemical engines for rapid orbit raising, then electric thrusters for efficient station-keeping. Real orbit-adjust burns show up on the satellite manoeuvre tracker.