Transfer-window equations
aₜ = (r₁ + r₂) / 2 Tₜ = π √(aₜ³ / μ) φ = 180° − n₂Tₜ T_syn = 2π / |n₂ − n₁| v = √[μ(2/r − 1/a)] Δv_escape = √(v∞² + 2μ/rp) − v_parking
The calculator treats the main transfer as coplanar and approximately circular. Eccentricity and inclination are represented by warnings, modest window variation, and an optional plane-change estimate.
Mission planning steps
- Select the departure and destination bodies.
- Enter the current KSP year, day, and clock.
- Choose parking and capture orbit altitudes.
- Enter spacecraft mass, thrust, and vacuum ISP.
- Calculate, then copy the burn summary into your flight plan.
- Use the diagram and window table to compare opportunities.
Common stock mission presets
| Route | Suggested parking orbit | Arrival plan | Typical use | Load |
|---|---|---|---|---|
| Kerbin → Duna | 80 km | 50 km with optional aerobraking | First interplanetary mission | |
| Kerbin → Eve | 80 km | 100 km or atmospheric entry | High-energy return planning | |
| Kerbin → Jool | 100 km | 200 km or moon encounter | Outer-system expedition | |
| Kerbin → Eeloo | 100 km | 40 km capture | Long-range mission | |
| Kerbin → Mun | 80 km | 20 km capture | Local moon transfer |
Understanding KSP transfer windows
A transfer window occurs when two orbiting bodies reach useful relative positions. The correct phase angle lets a spacecraft and destination reach the same point together. Missing that geometry usually increases correction burns and mission time.
A Hohmann transfer uses half of an ellipse between two near-circular orbits. It is usually efficient, predictable, and practical for early planning. Real KSP encounters still need small node adjustments because actual orbits may be inclined or eccentric.
The ejection angle describes where to begin the escape burn around the departure body. The burn direction is usually prograde for outward transfers and retrograde for inward transfers. The displayed angle is an approximation, so refine the maneuver node while watching the projected solar orbit.
The departure delta-v combines local parking-orbit speed with the required hyperbolic excess speed. Arrival delta-v estimates the burn needed to enter the chosen orbit. Aerobraking can reduce that cost when the destination has an atmosphere, but heat and terrain risks remain.
The porkchop-style chart compares dates around the nominal solution. Lower-cost areas represent departures and durations closer to the selected transfer geometry. A true Lambert solver would be more exact, while this standalone version favors fast, transparent calculations.
Use future-window rows for long-term mission calendars. Every row repeats after the route’s synodic period. Build margin for launch delays, inclination corrections, capture changes, and mid-course navigation.
KSP transfer-window questions
How accurate is this calculator?
It provides a strong patched-conics and Hohmann baseline. Final maneuver nodes should be adjusted inside the game.
Does it support moons?
Yes. Transfers between moons sharing a parent are calculated directly. Cross-system moon routes use a simplified parent-orbit approximation.
Why can my in-game delta-v differ?
Parking-orbit shape, inclination, eccentricity, node timing, Oberth effects, and encounter geometry all change the final burn.
What is a phase angle?
It is the angular separation between the destination and departure bodies around their shared parent at departure.
What is the ejection angle?
It indicates the approximate parking-orbit location where the escape burn should begin.
Should I include plane-change cost?
Include it for conservative budgeting. In practice, combining burns or correcting near nodes can reduce the cost.
How does aerobraking change capture cost?
The calculator applies a broad reduction estimate. Actual results depend on periapsis, vehicle drag, heating, lift, and atmospheric depth.
Can I use modded bodies?
Yes. Add one custom body through JSON. More extensive systems can be added by extending the PHP body data array.
What does the window-quality label mean?
It compares current phase alignment and a small modeled cost variation. It is a planning indicator rather than a simulation result.