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KSP · BEGINNER HELPER

Kerbol System · Mission Planning · Reentry · Descent

SYSTEM ONLINE
DIJKSTRA ACTIVE
ES
Route Planner
Rocket Builder
Capsule, crew, batteries, instruments, rover, etc. Do not include engines or tank mass — those go in each stage below.
KSP Aerocalc
To calculate aerobraking precisely — how much your apoapsis will drop each pass, what periapsis you need, and how many passes to take — there's a dedicated community tool:

KSP Aerocalc by alterbaron
Recommended on the official game wiki · Runs in browser

Enter your current orbit, the target body, and your desired apoapsis — it tells you exactly what periapsis to set.
Reentry calculator — Kerbin
Descent guide by body

Juan S.G. Castellanos

Fan-made tool · Not official