🧑‍🔧 Plan an Orbital Maneuver

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Every calculator used here is tested against independent reference values · how PhDino checks its numbers

Work out the delta-v an orbital transfer needs, then how much of your rocket's mass has to be propellant to deliver it, and how that changes with the engine.

Every orbital maneuver starts with the same question: how much delta-v (total velocity change) does the maneuver actually require? Once that number exists, the rocket equation translates it into a hard mass requirement: for a chosen engine efficiency and starting mass, exactly how much of that mass has to be propellant.

This guide runs those two steps in the order a real mission plan does: figure out what the maneuver costs in delta-v first, then check whether a given vehicle can actually deliver it, then ask how the answer moves when you change the engine or the geometry.

Delta-v is the currency of spaceflight

In orbit you cannot steer with a wheel; you change orbit by changing your velocity, and the total velocity change a mission needs is its delta-v budget. Every burn adds to the bill. What makes the budget matter is the rocket equation, which converts delta-v into mass: the more velocity you must add, the larger the share of your starting mass that has to be propellant, and that share grows exponentially rather than in a straight line.

That exponential is why the same mission looks affordable or hopeless depending on the engine. The rocket equation links the delta-v to the engine's specific impulse (a measure of how much velocity change a kilogram of propellant delivers) and to the ratio of the starting mass to the final mass, so it answers questions in both directions.

What a Hohmann transfer assumes

A Hohmann transfer moves a spacecraft between two circular orbits in the same plane with two burns. The first burn, along the direction of motion, stretches the orbit into an ellipse whose lowest point is the starting orbit and whose highest point touches the target orbit. The second burn, at that high point, raises the low point and circularizes. It is the cheapest two-burn transfer between coplanar circular orbits, and it takes half an orbit of the transfer ellipse.

It also assumes that each burn is instantaneous, that only one body's gravity acts, and that the two orbits share a plane. Real burns take time and lose a little efficiency to gravity, other bodies perturb the orbit, and the orbits usually differ in inclination, which the example handles with a plane change.

Steps

1. 1🌍Hohmann Transfer Orbit
   Pick a central body and the initial and final orbit altitudes for the transfer. The classic case is Earth low orbit to geostationary. This returns the delta-v for each of the two burns and the total. Hit "📋 Copy Values" once you have a result: the total delta-v carries directly into the rocket equation next.
   🧮 Open the Hohmann Transfer Orbit calculator
2. 2🚀Rocket Equation
   "📥 Paste Values" fills in Delta-V with the total from Step 1. Enter your engine's specific impulse and the vehicle's starting (wet) mass, and leave Final (Dry) Mass at 0 to solve for it. The result is the mass left after the burns, so the starting mass minus that is exactly the propellant the maneuver takes. Run it again with a different specific impulse, or with a larger delta-v, to see how the answer moves.
   🧮 Open the Rocket Equation calculator

Worked example: a 10 tonne spacecraft from a 400 km orbit to geostationary orbit

A spacecraft with a fully fueled mass of 10,000 kg sits in a circular orbit 400 km above the Earth. It must move to a geostationary orbit 35,786 km up, using a chemical engine. The first question is the delta-v, the second is the propellant, and the third is how the answers move when the engine or the geometry changes.

What it adds up to

Two changes of similar size matter very differently. The plane change adds about a tenth to the delta-v and costs 3.2 percentage points of the vehicle's mass in extra propellant. Moving from a 320 s to a 450 s engine removes 12.5 points of propellant. The exponential in the rocket equation is the reason: every additional kilometer per second costs more mass than the one before it, and better efficiency is the lever that acts on all of it at once.

Notes & limitations

• The transfer assumes both orbits share the same plane and that each burn is instantaneous. A real transfer between differently inclined orbits needs a plane change with its own delta-v cost, which the worked example adds by hand (see the Hohmann Transfer article).
• Leaving a different field at 0 lets the Rocket Equation solve for something else entirely. For instance, leaving Specific Impulse at 0 (with wet mass, dry mass and delta-v all filled in) answers "what engine efficiency would I need" instead.

Where Hohmann numbers stop being true

The budget above is a clean starting figure, and real missions add to it.

• Finite burns. A real engine burns for minutes, not instantly, and gravity works against it during the burn, so the actual delta-v is somewhat higher.
• Low-thrust propulsion. Electric engines have very high specific impulse but tiny thrust, so they spiral out over weeks instead of following an ellipse, and the delta-v of a spiral is larger than a Hohmann transfer's.
• Margins and reserves. Real budgets add delta-v for navigation corrections, phasing and, in low orbit, end-of-life disposal, often a few percent or more.
• Staging. The final mass includes every tank and engine you carry to the end. Dropping empty tanks along the way, staging, is how vehicles reach the high delta-v values of a launch.
• Other bodies. Interplanetary transfers need departure and arrival hyperbolas at each planet as well as the transfer ellipse, which this calculator does not include.

What the rocket equation does not know

The equation treats the final mass as one number. It does not know how much of it is structure and how much is payload, which is why mission planners work backward from a structural fraction, the share of a stage that is tanks and engine, and check that the answer is buildable at all.

Frequently asked questions