Starship Moon Plan Faces Estimates of Six to 22 Refuelings

SpaceX and NASA have a wide disagreement to resolve before Starship can carry astronauts toward a Moon landing: a calculation presented by NASA’s Aerospace Safety Advisory Panel puts the requirement at 22 in-orbit refuelings, while Elon Musk says the actual number would be six to eight. Either estimate leaves the mission dependent on repeatedly completing an operation no rocket has performed before.

The reported figure of 22 covers the refueling needed for one Moon-bound Starship. Responding to that estimate, Musk wrote, “Actual number will be more like 6 to 8.” The gap is more than a dispute over launch count. It reflects uncertainty about how much propellant each tanker can deliver, how efficiently it can be transferred and how much will be lost while the cryogenic liquids are stored in orbit.

NASA’s current lander development plan describes a chain resembling an orbital gas station. SpaceX would launch a depot version of Starship into low Earth orbit, followed by multiple tanker Starships carrying propellant. Those tankers would fill the depot, which would then fuel the Starship Human Landing System for its trip to the Moon.

The lander is intended to connect with NASA’s Orion spacecraft and move two crew members to the lunar surface before returning them to Orion. NASA is targeting a crewed lunar landing in 2028, but it has not assigned either SpaceX or Blue Origin to a specific landing mission under its realigned contracts. Both companies are now participating in the revised program, and NASA says lander readiness will determine which provider receives the first 2028 landing opportunity.

Why the estimates can diverge so sharply

Starship uses liquid oxygen and liquid methane, both of which must remain extremely cold. Heat entering a tank can vaporize some of its contents, raising pressure and forcing a spacecraft to manage or vent the gas. Propellant can also be consumed while cooling transfer lines and the receiving tank before a transfer begins.

Those losses make time and efficiency central to the launch estimate. A tanker does not merely need to reach orbit; it must deliver a useful quantity of propellant after accounting for its own mission requirements and losses during storage and transfer. More boil-off or less efficient transfer means more tanker flights. Better payload performance and lower losses can push the count down.

NASA research explains why this is not equivalent to connecting two fuel hoses on Earth. In microgravity, liquid and vapor do not settle predictably at opposite ends of a tank. Engineers must control tank pressure, position liquid for withdrawal and transfer it without allowing unwanted vapor into the flow. NASA has conducted smaller-scale microgravity research into tank pressurization, mixing and cooling, but full cryogenic tank-to-tank rocket refueling has never been demonstrated in space.

SpaceX has not yet attempted the required orbital operation. That leaves the six-to-22 range anchored in assumptions rather than flight results from the complete system. Even a six-flight sequence would require the depot, tanker launches, rendezvous operations, storage period and transfers to work together before the lunar vehicle departs Earth orbit. A 22-transfer sequence would multiply the launch scheduling and hardware demands substantially.

The demonstration must validate a chain, not one component

NASA requires both lander providers to conduct uncrewed demonstration missions before carrying astronauts. For SpaceX, NASA says that demonstration will include the propellant-aggregation approach. The test must establish more than whether liquid can move between two tanks: mission planners need measured transfer rates, losses, thermal behavior and repeatability to determine how many tanker flights are actually required.

Blue Origin is now also competing for the first 2028 landing opportunity. For its accelerated mission concept, the company plans to defer orbital propellant transfer and instead use three transfer stages that separate as their propellant is depleted. That does not settle which lander will fly first, but it gives NASA an architecture that does not rely on the same initial orbital-refueling chain.

The disputed launch count will remain fluid until Starship demonstrates cryogenic propellant storage and transfer at relevant scale in orbit. The first such test will therefore measure more than a new spacecraft capability: it will begin replacing a range of six to 22 refuelings with the flight data needed to plan an actual Moon mission.

By David Whitaker — Associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into accessible technical stories.