Delta-v is the currency of spaceflight in KSP. Understanding it is the difference between planning missions confidently and guessing. This guide explains it clearly — no physics degree required.
Delta-v (Δv) literally means "change in velocity." It's the total amount of speed your rocket can add or subtract from itself using fuel. Every manoeuvre in KSP costs a certain amount of delta-v. When you run out, you're coasting — which is either exactly what you want, or a very bad situation.
The KSP delta-v map (search "KSP delta-v map" — print it out) lists the delta-v cost of every journey in the Kerbol system. Kerbin surface to low orbit: 3,400 m/s. Low orbit to the Mun surface and back: about 3,600 m/s. Duna and back: roughly 4,800 m/s from LKO.
THE ROCKET EQUATION
Delta-v is calculated by the Tsiolkovsky rocket equation:
Δv = Isp × g₀ × ln(m₀ / m₁)
Where:
• Isp = specific impulse of your engine (efficiency rating — higher is better)
• g₀ = 9.82 m/s² (standard gravity)
• m₀ = wet mass (rocket + all fuel)
• m₁ = dry mass (rocket with no fuel)
• ln = natural logarithm
You don't need to calculate this by hand. The VAB shows it automatically once you have the KER mod, or you can read it from the staging display in later versions.
WHY STAGING MATTERS
Carrying empty fuel tanks costs delta-v because you're hauling dead weight. Staging — dropping empty stages — is how you cheat the rocket equation. A two-stage rocket with the same total fuel as a single-stage rocket will always have more delta-v. This is why real rockets (and KSP rockets) shed stages as they go.
A practical rule: design your upper stages as light as possible. The Terrier (LV-909) engine weighs 0.5t and has Isp 345s in vacuum — it's the workhorse of upper stages precisely because it's light and efficient.
Isp matters more in vacuum, TWR matters more in atmosphere. For your launch stage, use high-TWR engines (Mainsail, Skipper). For your upper stages and interplanetary transfers, prioritise Isp (Poodle, Terrier, Ion for probes).
PRACTICAL DELTA-V TARGETS
Here's a quick reference for common missions (from Kerbin surface):
• Low Kerbin Orbit (LKO): 3,400 m/s
• Mun orbit: 5,000 m/s
• Mun surface + return: 6,800 m/s
• Minmus surface + return: 6,200 m/s (lower gravity makes it easier than Mun!)
• Duna orbit: 8,000 m/s
• Duna surface + return: 10,500 m/s
• Eve orbit: 9,500 m/s (landing on Eve is a one-way trip without a purpose-built ascent vehicle)
Always add a 10–15% reserve. Gravity losses, bad manoeuvres, and course corrections eat into your budget.