Hohmann Transfer Orbits

Lore & Background

In the early days of the Kerbal space program, before the KSC flight computers could plot multi-body trajectories, a young engineer would sketch a single ellipse on a napkin and say, 'Bump it up here, settle it there.' That napkin was a Hohmann transfer. The concept became so central to Kerbal orbital culture that the phrase 'do a Hohmann' is used colloquially at the KSC to mean any planned, two-burn altitude change—whether the target is a few kilometers higher or an entire planet away.

The elegance of the maneuver is what Kerbal flight directors love: no complex phasing, no multi-burn spiral, just a clean ellipse that touches both worlds. For interplanetary missions the geometry extends naturally—the transfer ellipse's focus sits at the Sun (or, in the case of Kerbin-centric transfers, at Kerbin itself), and the two burns become the departure and arrival Δv. The Kerbal Space Program's physics engine, built on Newtonian gravitation, makes the Hohmann transfer analytically exact, so the numbers a pilot calculates by hand will match the game's simulation to the decimal.

There is a quiet pride in the Kerbal community about the Hohmann. It is the maneuver that separates a tourist who bounces off the Mun's surface from an engineer who can park a station in a stable orbit around Jool. Veterans will tell you that every great Kerbal mission—every crewed Duna landing, every Eeloo flyby—began with the same two small prograde and retrograde taps on the throttle.

In Their Own Story

The cockpit smelled of recycled air and cold coffee. Jannan Voss pressed her thumb against the throttle lever and watched the prograde marker creep toward the tiny blue dot marking periapsis. The Kerbin below was a pale blue marble, already too small to read the city lights. She exhaled, squeezed. The engine's low growl vibrated through her boots, and the apoapsis marker—had it been visible—would have been climbing, inch by inch, toward the rust-colored smudge of Duna hanging in the black.

Forty-three minutes of coast. She watched the navball, counted the seconds, and let the ellipse do its quiet work. Then the retrograde marker aligned, the Duna atmosphere bloomed into a thin orange crescent, and she tapped the throttle again. The velocity readout ticked down. The orbit closed. Duna was home.

She killed the engine, floated back in her seat, and whispered the words every Kerbal says after a clean burn: 'Textbook Hohmann.' Then she poured the cold coffee and drank it in one long pull, because the Mun was still out there, and she'd have to do it all again next week.

Reader's Guide

Rule: A Hohmann transfer is a single elliptical orbit tangent to your current circular orbit at periapsis and to your target orbit at apoapsis (or vice-versa for a lowering maneuver). You execute it with exactly two engine burns: a prograde burn at periapsis to raise apoapsis, or a retrograde burn at apoapsis to lower periapsis.

Why it matters: In KSP's Newtonian physics, this is the minimum-Δv path between two coplanar circular orbits. Wasting fuel on a non-Hohmann path means you need more tanks, more staging, or a shorter mission. For interplanetary transfers the same geometry applies with the Sun as the central body.

Common failure modes: Burning at the wrong point in the orbit (you need to be at periapsis or apoapsis, not mid-arc); under- or over-burning, which leaves you in a non-circular ellipse that never matches the target; forgetting to account for the target body's gravity well when doing interplanetary transfers (you need a periapsis inside the sphere of influence, not just a close flyby).

Pro tips: Use the maneuver node to preview the ellipse before committing. For interplanetary transfers, aim your periapsis about 50–100 km above the target's surface to leave room for a capture burn. Check your prograde/retrograde markers against the navball, not just the attitude indicator. And if your transfer window is off by a few minutes, a small phasing orbit (one extra revolution) is cheaper than trying to correct mid-transfer.

Did You Know?

• The Hohmann transfer is named after Walter Hohmann, a German engineer who published the geometric description in 1925—decades before any spacecraft existed.
• In KSP, the transfer time equals exactly half the orbital period of the transfer ellipse, so a Kerbin-to-Duna Hohmann takes roughly 185 days of in-game time regardless of where you start in the orbit.
• The maneuver is only truly optimal between two circular, coplanar orbits; for highly elliptical or inclined targets, a multi-burn or bi-elliptic transfer can actually save Δv.
• Kerbal flight veterans often call the two-burn sequence 'the squeeze and the set,' a nod to the prograde push at periapsis and the retrograde settle at apoapsis.