KSP Semi Major Axis Calculator

Advanced Orbit Calculator

Choose a body, select a method, and enter your values. Results appear above this form after submission.

Central body

Calculation method

Distance unit

Custom μ m³/s²

Custom body radius Uses selected distance unit.

Output precision

Apoapsis altitude

Periapsis altitude

Orbital period

Period unit

Assumed eccentricity

Current altitude

Current speed

Speed unit

Flight path angle Degrees. Zero means tangential motion.

Inclination note Shown in results. It does not change axis size.

Optional rotation period

Rotation period unit

Formula Used

From apoapsis and periapsis: a = (rₐ + rₚ) / 2

Orbital radius: r = body radius + altitude

Eccentricity: e = (rₐ - rₚ) / (rₐ + rₚ)

From period: a = ∛( μ × (T / 2π)² )

From state vector: ε = v² / 2 - μ / r and a = -μ / (2ε)

Vis-viva speed: v = √( μ × (2 / r - 1 / a) )

Orbital period: T = 2π × √(a³ / μ)

How to Use This Calculator

Select a KSP body or choose a custom body.
Pick the method that matches your available data.
Enter apoapsis and periapsis, period, or current state values.
Choose kilometers or meters for distance fields.
Add an optional rotation period for synchronous altitude comparison.
Press the calculate button.
Review the result above the form.
Download the CSV or PDF for mission notes.

Example Data Table

Body	Periapsis Altitude	Apoapsis Altitude	Expected Orbit Type	Use Case
Kerbin	80 km	100 km	Low elliptical orbit	Early parking orbit
Mun	15 km	50 km	Elliptical capture	Landing preparation
Minmus	10 km	30 km	Small moon orbit	Science mission
Duna	60 km	250 km	Capture orbit	Aerobrake planning

KSP Semi Major Axis Planning Guide

Why the Axis Matters

The semi major axis is the central size value of an orbit. In KSP, it shows orbit width. It helps judge the path around a body. It is measured from the ellipse center to the longest edge. It is not the same as altitude. Altitude starts from the surface. The semi major axis starts from the body center.

Supported Planning Methods

This calculator supports three planning methods. You can enter apoapsis and periapsis altitudes. You can enter orbital period with gravity. You can also enter speed and altitude at one point. The tool then estimates the axis, eccentricity, period, speeds, and orbital energy.

Body Presets

Use body presets when you work with common Kerbol system worlds. Kerbin, Mun, Minmus, Duna, Eve, and Jool are included. Custom gravity and radius fields support modded systems. Always check the game map view before burning. Mods and patched conics can change final results.

Orbit Shape

A larger semi major axis means a wider and slower average orbit. A smaller axis means a tighter and faster orbit. Eccentricity shows how stretched the path is. A value near zero is close to circular. A value near one is very stretched. Negative periapsis altitude means the orbit hits the body.

Visual Check

The chart gives a quick visual check. It draws the ellipse with the selected body at a focus. This is useful when testing transfer orbits. It can show whether an orbit is almost circular or highly eccentric.

Delta-v Notes

Delta-v estimates are guidance values. Circularization changes depend on burn timing, direction, and steering. The calculator uses ideal two body equations. It ignores drag, thrust arcs, terrain, and third body effects. That is good for planning high orbits. It is weaker for atmosphere passes.

Best Practice

For best results, keep all inputs realistic. Use meters for precise mission math. Use kilometers for quick planning. Compare the output with map nodes. Save the CSV when you need a mission record. Export the PDF for notes, tutorials, or design logs. The result helps you understand the orbit before committing fuel.

Pilots can use this number when building relay shells. Matching semi major axes keeps satellites spaced more reliably. Small errors grow over many orbits. Recheck after every burn and correction later.

FAQs

1. What is a semi major axis in KSP?

It is half the longest width of an elliptical orbit. In practical KSP planning, it shows the orbit’s central size from the ellipse center, not from the surface.

2. Is semi major axis the same as altitude?

No. Altitude starts from the body surface. Semi major axis is measured from the orbit ellipse center and uses radii from the body center.

3. Why does the calculator add body radius?

Orbital formulas use distance from the body center. KSP altitude is usually measured above the surface, so radius must be added first.

4. Can I use this for modded planets?

Yes. Select Custom Body, then enter the gravitational parameter and radius. Use values from your planet pack or configuration files.

5. Why is my periapsis below the surface?

That means the calculated orbit intersects the body. Raise periapsis or reduce eccentricity before expecting a stable full orbit.

6. Does inclination change semi major axis?

No. Inclination changes the tilt of the orbital plane. It does not change the ellipse size in a two body calculation.

7. Are delta-v estimates exact?

They are ideal estimates. Real burns can differ because of steering loss, thrust duration, atmosphere, gravity losses, and timing errors.

8. Why use the orbital period method?

It is useful for relay networks and synchronized orbits. Enter a target period, and the calculator estimates the needed semi major axis.