Orbital Refueling of the Starship Architecture: Operational Mechanics, Feasibility Analysis, and Strategic Timeline

Key Takeaways

• Settling is Critical: Propellant transfer relies on continuous milli-g acceleration to manage fluid dynamics.
• Cadence Beats Boil-off: Success depends on rapid launch turnaround to outpace cryogenic thermal losses.
• 2026-2027 Critical Path: Operational demonstrations in LEO are requisite before Artemis III can proceed.

Introduction

The transition of spaceflight from expendable, single-mission architectures to reusable, multi-mission logistics networks hinges on a single, largely unproven capability: on-orbit cryogenic propellant transfer. While the SpaceX Starship system represents a paradigm shift in launch capacity and reusability, its ability to execute missions beyond Low Earth Orbit (LEO) – specifically the Human Landing System (HLS) requirements for NASA’s Artemis program – is entirely predicated on the feasibility of refilling the spacecraft in space. This article provides an exhaustive analysis of the technical steps required to achieve this capability, the physics governing cryogenic fluid management in microgravity, the operational risks associated with rapid launch campaigns, and a forecasted timeline for operational readiness.

The analysis indicates that while the fundamental physics of propellant transfer are understood, the engineering implementation at the scale of hundreds of tons of liquid methane and liquid oxygen introduces novel challenges regarding “ullage settling,” thermal management, and orbital rendezvous. Furthermore, the operational cadence required to aggregate sufficient propellant before significant “boil-off” occurs necessitates a launch frequency unprecedented in the history of heavy-lift rocketry. Current trajectories suggest that while a propellant transfer demonstration is feasible by 2026, the fully integrated depot-and-tanker architecture required for a lunar landing faces significant schedule pressure, with operational readiness likely converging in the 2027–2028 timeframe.

This article provides a comprehensive technical and programmatic review, synthesizing data from flight tests, government audits, and engineering literature to give a definitive outlook on the future of orbital refueling.

The Paradigm of On-Orbit Aggregation

The concept of orbital refueling, or propellant aggregation, fundamentally alters the tyranny of the rocket equation. Historically, launch vehicles have been constrained by the necessity to lift their entire mission propellant load from the surface of the Earth in a single event. This constraint exponentially increases the size of the rocket required for deep space missions, as the vehicle must lift the fuel required to lift the fuel. By breaking the mission into two distinct phases – launch to Low Earth Orbit (LEO) and refueling in LEO – the Starship architecture decouples the payload capacity of the launch vehicle from the delta-v capability of the spacecraft.

This architectural shift allows a vehicle as massive as Starship to act as both a heavy-lift launcher and a long-duration deep space transport. However, this capability comes at the cost of operational complexity. Instead of a single “all-up” launch like the Saturn V, the Artemis III mission profile requires a synchronized campaign of potentially a dozen launches or more, all converging on a single point in orbit within a tight temporal window. The success of this model depends not just on the performance of the Raptor engines or the heat shield, but on the ability to manage cryogenic fluids in microgravity at an industrial scale – a feat never before attempted.

The Physics of Cryogenic Fluid Management in Microgravity

Refueling a spacecraft in orbit is fundamentally different from fueling a rocket on the launchpad. On Earth, gravity settles liquid propellant at the bottom of the tank, ensuring that pumps ingest liquid rather than gas. In the microgravity environment of orbit, the behavior of fluids is governed by surface tension rather than gravity, leading to phenomena that complicate the transfer process.

Microgravity Fluid Dynamics and Propellant Settling

The primary technical hurdle for Starship refueling is ensuring that the donor tank (the Tanker) feeds liquid propellant to the receiver (the Depot or HLS) without ingesting ullage gas. Ullage gas is the pressurizing gas that fills the void as propellant is drained. If gas enters the transfer lines, it can cause pump cavitation or “vapor lock,” stalling the transfer and potentially damaging hardware.

The Bond Number and Surface Tension

In the absence of gravity, the fluid behavior is defined by the Bond number, a dimensionless number representing the ratio of gravitational forces to surface tension forces. In orbit, the Bond number is near zero, meaning surface tension dominates. This causes propellants to float in amorphous “blobs” or coat the tank walls (wetting), leaving the ullage gas to potentially settle over the drain valve. This random distribution makes reliable fluid transfer impossible without intervention.

The Necessity of Settling Thrust

To overcome this, SpaceX employs a technique known as “settling.” This involves applying a small, continuous acceleration to the vehicle stack. By firing auxiliary thrusters, the spacecraft generates a “milli-g” force (approximately to g), which acts as artificial gravity.

This acceleration forces the denser liquid propellants to the “bottom” of the tank (relative to the thrust vector), separating them from the lighter ullage gas. This creates a stable liquid-gas interface, allowing the transfer lines located at the aft of the tank to draw pure liquid.

• Operational ConOps: The current Concept of Operations (ConOps) suggests that once docked, the combined Starship stack will use hot-gas reaction control system (RCS) thrusters or vented ullage gas to maintain this settling thrust throughout the duration of the transfer.
• Fuel Consumption: Analysis indicates that maintaining this settling force is propellant-efficient. For a 100-ton transfer, the mass of propellant consumed to maintain settling is negligible compared to the volume transferred, provided the transfer rate is sufficiently high. The theoretical consumption for settling a 100-ton stack is measured in kilograms per hour, a fraction of the total capacity.

Ullage Collapse and Geysering Risks

A critical failure mode during this process is “ullage collapse.” This occurs if the liquid propellant sloshes violently and mixes with the ullage gas. In cryogenic systems using autogenous pressurization (where the tank is pressurized by the gas form of the propellant itself), mixing cold liquid with warm gas can cause the gas to rapidly condense (collapse). This results in a sudden, catastrophic drop in tank pressure, potentially compromising the vehicle’s structural integrity or causing engines/pumps to starve.

Autogenous Pressurization Challenges

Starship uses autogenous pressurization, meaning it taps a small amount of liquid propellant, heats it (likely using the engine heat exchangers), and feeds the resulting gas back into the tank to maintain pressure. This system saves weight by eliminating heavy helium tanks but introduces the risk of ullage collapse.

• Thermodynamic Instability: The gas in the ullage space is significantly warmer than the liquid propellant. If a slosh event increases the surface area interaction between the gas and liquid, the heat transfer accelerates, causing the gas to condense back into liquid. This rapid phase change creates a vacuum effect, collapsing the tank pressure.
• Mitigation History: SpaceX has experienced this failure mode in early atmospheric prototypes (e.g., SN10), leading to the temporary implementation of helium pressurization backups in some subsystems. However, the flight design for HLS aims to rely on pure autogenous pressurization to avoid the logistical complexity of transporting helium to the Moon.

Geysering

Another related phenomenon is geysering. In long vertical feedlines, liquid propellant can warm up, boil, and form a large bubble. As this bubble rises, it pushes liquid above it, reducing pressure on the liquid below, which then flash-boils. This chain reaction creates a violent “geyser” of liquid and gas that can damage internal plumbing or create pressure spikes. In microgravity, where buoyancy doesn’t naturally clear bubbles, geysering prevention relies heavily on thermal management and active circulation or settling.

The Transfer Mechanism: Passive vs. Active

Once the propellant is settled, it must be moved from the Tanker to the Receiver. There are two primary methods for this: active pumping and passive pressure differential.

Passive Pressure Differential

The simplest method, and the one SpaceX appears to be prioritizing for early tests, relies on thermodynamics rather than mechanics.

1. High Pressure Source: The donor tank (Tanker) is maintained at a higher pressure. This can be achieved by allowing a controlled amount of boil-off to pressurize the tank or by using onboard heaters.
2. Low Pressure Receiver: The receiving tank (HLS/Depot) is vented to lower its internal pressure, essentially opening a valve to the vacuum of space (controlled venting).
3. Flow: When the valves connecting the two ships open, the pressure gradient drives the liquid from the high-pressure Tanker to the low-pressure Receiver.

This method eliminates the need for heavy, complex cryogenic pumps in the transfer circuit, which are prone to failure and require significant electrical power. However, it requires venting valuable ullage gas from the receiver to space to maintain the pressure drop, which represents a consumable loss of propellant mass.

Active Pumping

For larger scale transfers where speed is critical to minimize loiter time and boil-off, active pumps may be employed. Pumps can achieve higher flow rates and ensure more complete tank draining. However, they introduce mechanical complexity and localized heat input into the cryogenic fluid, which can induce local boiling. Current flight tests (IFT-3) successfully demonstrated internal fluid transfer, likely using a pressure-fed system to validate the basic settling-and-flow physics before committing to a full pumping architecture.

The Starship Vehicle Architecture

The execution of orbital refueling requires a fleet of specialized vehicles. While they share the common Starship airframe and Raptor propulsion, their internal systems and mission profiles differ significantly.

The Starship Propellant Depot

To decouple the launch of the HLS from the launch of the tankers, SpaceX utilizes a “Depot” variant. This vehicle effectively acts as a buffer in the supply chain.

• Function: The Depot acts as an orbital gas station. It is launched first and remains in Low Earth Orbit (LEO). Its primary role is to aggregate fuel from multiple tanker flights and store it until the HLS is ready to launch.
• Design Optimization: The Depot is a stretched Starship with extended tanks to maximize volume. Crucially, because it is not designed to return to Earth, it lacks the heavy thermal protection system (TPS) tiles, flaps, and header tanks required for reentry. This mass savings is converted into increased insulation and propellant capacity.
• Thermal Protection: To minimize boil-off during the weeks or months of loiter time, the Depot will feature enhanced thermal control. This likely includes Multi-Layer Insulation (MLI) blankets and potentially deployable sunshades to block solar radiation and Earth albedo.

The Starship Tanker

The Tanker is the logistics hauler of the system. It is a standard Starship aerodynamically but optimized for carrying propellant as payload.

• Capacity: While the Starship stack has a gross propellant capacity of ~1,200 to 1,500 tons, a Tanker launching to orbit burns most of this to reach orbital velocity. The “payload” delivered is the remaining propellant in the main tanks plus any additional header tanks or internal “payload” tanks.
• Payload Estimates: Current estimates suggest a Tanker can deliver between 100 and 150 tons of propellant to the Depot per launch. This payload capacity is the single most critical variable in the entire Artemis architecture; a drop in Tanker payload capacity linearly increases the number of launches required, compounding risk.

The Human Landing System (HLS)

The HLS is the end-user of the propellant. It is a specialized variant designed solely for operations between lunar orbit and the lunar surface.

• Modifications: Like the Depot, the HLS lacks heat shield tiles and flaps for Earth reentry. It features landing legs, a crew elevator, and specific life support systems.
• Propellant Demand: A fully fueled Starship HLS requires approximately 1,200 tons of propellant to execute the Trans-Lunar Injection (TLI), lunar orbit insertion, landing, and ascent. It must launch from Earth, dock with the Depot in LEO, fill its tanks rapidly, and depart for the Moon before boil-off reduces its margins.

Interface and Docking Configuration

The physical connection between vehicles is the mechanical linchpin of the architecture. Early concepts depicted “butt-to-butt” docking, where ships would connect at the engine section. However, the evolution of the Starship design and the installation of “Quick Disconnect” (QD) panels on the ship’s leeward side suggest a different approach.

Side-to-Side (Parallel) Docking

The prevailing engineering consensus and recent inputs point to a side-to-side docking configuration.

• Mechanism: Both Starships (Tanker and Depot) are equipped with identical docking ports located on the body (likely near the payload bay or mid-body).
• Procedure: The vehicles rendezvous and align parallel to each other. They clamp together using a mechanism derived from the Dragon 2 docking system or a specialized grapple.
• Advantages: This configuration allows the propellant lines to be shorter and avoids the complexity of routing plumbing through the engine heat shield, which is necessary for the “butt-to-butt” concept. It also aligns with the location of the ground-side filling ports, allowing a unified plumbing architecture for both ground and orbit filling.

The Orbital Mechanics of Refueling

Orbital refueling adds a layer of orbital mechanics complexity that does not exist for single-launch missions. The Depot and Tankers must not only meet in space but must do so with high precision and within specific timing constraints.

Orbital Phasing and Launch Windows

For a Tanker to rendezvous with a Depot, it must launch at a precise time when the Depot’s orbital plane passes over the launch site.

• Plane Alignment: Changing the inclination of an orbit is extremely propellant-expensive. Therefore, the Tanker must launch directly into the plane of the Depot.
• Phasing: Even if the plane is correct, the Depot might be on the other side of the Earth. The Tanker must launch into a slightly different orbit (lower or higher) to “catch up” (phase) with the Depot. This phasing maneuver takes time – hours to days – during which boil-off is occurring.
• Launch Windows: The requirement to match planes restricts launch windows to once per day (or twice if the orbit geometry allows). This constraint drives the need for high reliability; a scrubbed launch means a 24-hour delay, extending the loiter time of the Depot and increasing boil-off losses.

Rendezvous and Docking Sensors

SpaceX utilizes a suite of sensors for autonomous docking, evolved from the Dragon 2 program.

• Lidar and Optical: The vehicles use LiDAR (Light Detection and Ranging) and optical cameras to determine relative position and velocity.
• Starlink Integration: Enhanced by inter-satellite links, Starship utilizes high-bandwidth data sharing to coordinate maneuvers.
• Automation: The entire docking sequence is autonomous. The operational altitude for these transfers is LEO, where communication latency is low, but the precision required demands onboard processing.

Operational Logistics and Ground Infrastructure

The physics of refueling are challenging, but the logistics are daunting. The requirement to launch 10–20 tankers in rapid succession creates a stress test for ground infrastructure that has no parallel in history.

The Launch Cadence Equation

The viability of the architecture depends on the “Launch Ratio” – the number of Tanker flights required to fully fuel one HLS/Depot.

• Total Demand: A fully fueled Starship HLS requires approximately 1,200 tons of propellant.
• The Calculation: If each Tanker delivers 100 tons, a minimum of 12 launches is required (1 Depot launch + 11 Tanker refills + 1 HLS launch).
• Boil-off Factor: This math assumes zero propellant loss during the aggregation phase. In reality, cryogenic propellants absorb heat. If the aggregation campaign takes months, the Depot must carry extra fuel to compensate for these losses. NASA and GAO estimates have ranged from “high single digits” to “high teens” (e.g., 15–18 launches) depending on the efficiency of the transfer and the boil-off rate.

Ground Infrastructure Requirements

To support a rapid launch cadence, SpaceX is developing massive infrastructure at Starbase (Boca Chica, Texas) and Kennedy Space Center (Florida).

• Multiple Towers: A single launch pad likely cannot support the daily or weekly cadence required due to the need for inspections and maintenance between launches. SpaceX is constructing a second tower at Starbase and a tower at LC-39A in Florida to allow simultaneous or alternating launch operations.
• Propellant Farm: The ground propellant farm must be capable of chilling and loading thousands of tons of methane and oxygen daily. The “Tank Farm” at Starbase has been significantly upgraded with horizontal storage tanks and massive chillers to support this throughput.

The “Mechazilla” Catch

A critical component of the rapid reuse model is the ability to catch the Super Heavy booster and the Starship ship using the “chopstick” arms of the launch tower.

• Turnaround Time: By catching the booster directly on the launch mount, SpaceX eliminates the time required to transport the booster from a landing pad or barge back to the launch site. This theoretically allows a booster to be inspected, refueled, and relaunched in a matter of hours or days.
• Status: The successful catch of a Super Heavy booster on IFT-5 validated the mechanical feasibility of this concept. However, consistently executing this maneuver without damaging the launch infrastructure is a key operational risk.

Cryogenic Fluid Management: The Boil-off Challenge

Cryogenic Fluid Management (CFM) is the technical long-pole of the program. Liquid oxygen (LOX) boils at 90 K (-183°C) and liquid methane at 111 K (-161°C). In the vacuum of space, particularly in LEO where the spacecraft is exposed to direct sunlight and Earth’s albedo (reflected heat), maintaining these temperatures is difficult.

Passive Thermal Control

For short-duration missions, passive control is sufficient.

• Insulation: The Depot and HLS will utilize advanced Multi-Layer Insulation (MLI) blankets. Unlike the standard Starship, which relies on stainless steel’s thermal properties, the HLS/Depot variants will likely be wrapped in specialized white or aluminized insulation to reflect solar radiation.
• Orientation: “Barbecue rolls” (spinning the ship) or specific orientations can keep the tanks shadowed by the engine section or nose cone.
• Sunshades: High-reflectivity sunshades may be deployed to block solar radiation. Renders of the HLS show white paint or tiles to improve thermal rejection compared to the standard stainless steel.

Active Thermal Control (Cryocoolers)

For the Depot, which may sit in orbit for weeks or months accumulating fuel, passive measures may be insufficient.

• Zero Boil-Off (ZBO): NASA has invested heavily in ZBO technologies, which utilize powered cryocoolers (essentially space-rated refrigerators) to remove heat from the tanks and re-condense boiled vapor back into liquid.
• Implementation: It is highly probable that the Starship Depot will incorporate active cryocoolers powered by large solar arrays. This adds mass and complexity but is essential to prevent the propellant from boiling away before the HLS arrives.

Operational Implications of Boil-off

Boil-off drives the launch cadence.

• The Race Against Time: If the Depot loses 1% of its propellant per day to boil-off, and it takes 10 days between Tanker flights, the Depot loses 10% of its aggregated fuel between launches. This creates a “Zeno’s Paradox” where the Depot might never get full if the launch rate is too slow.
• Threshold: To outpace boil-off without active ZBO systems, SpaceX would need a launch cadence of one Tanker every few days. With ZBO, this constraint relaxes to weeks or months.

Progress Analysis: Flight Tests and Demonstrations

SpaceX is following an iterative “fly-test-fail-fix” methodology. The roadmap to operational refueling is visible through the specific objectives of the integrated flight tests (IFT).

IFT-3: The Propellant Transfer Demo

On March 14, 2024, during the IFT-3 mission, SpaceX successfully executed an internal propellant transfer demonstration.

• Objective: Transfer liquid oxygen from a header tank to the main tank during the coast phase.
• Result: NASA and SpaceX confirmed the test was successful. This provided the first flight data on fluid dynamics during a settling burn and the behavior of the fluid management systems in a real orbital environment.
• Significance: While this was an intra-ship transfer (within one vehicle), it validated the physics models for ullage settling and fluid flow that are applicable to inter-ship (ship-to-ship) transfer.

IFT-4 and IFT-5: Reusability Milestones

Subsequent flights (IFT-4 and IFT-5) focused on the other half of the refueling equation: getting the Tankers back.

• IFT-5 Success: The successful “catch” of the Super Heavy booster on IFT-5 demonstrated the feasibility of rapid reuse. Without the ability to rapidly turn around and relaunch boosters, the “15 launch” campaign would be economically and logistically impossible.

Future Milestones (2025–2026)

• Ship-to-Ship Transfer: The next critical milestone is an actual docking and fluid transfer between two Starships in orbit. This is targeted for 2025 or 2026. This test will likely involve two ships launching in rapid succession to demonstrate rendezvous, docking, and the transfer of a significant quantity of propellant (10+ tons).
• Long-Duration Loiter: A Starship will be launched to loiter in orbit for an extended period to validate boil-off models and thermal control systems. This “Depot Proto-flight” is essential to prove the ZBO or passive thermal management strategies.

Technical Risks and Failure Modes

Despite progress, the path to a reliable 1,200-ton transfer capability is fraught with high-consequence risks.

Docking Dynamics

Docking two 1,000+ ton vehicles (when loaded) is unprecedented.

• Collision Risk: A hard dock or collision could rupture tanks or damage the delicate heat shield tiles required for the Tanker’s return. The mass of these vehicles means their inertia is enormous; stopping or correcting a drift requires significant thrust, which interacts with the docking sensors.
• Seal Integrity: The fluid couplers must seal perfectly in a vacuum at cryogenic temperatures. Even a microscopic leak of methane could create a propulsive jet that disturbs the attitude of the stack or creates an explosion hazard.

Propellant Stratification and Geysering

In large tanks, propellant can stratify, with warmer liquid rising to the top (or center in microgravity). If this warm liquid is rapidly depressurized or moved, it can flash-boil, causing a “geyser” of liquid and gas that can hammer valves and sensors. Managing the temperature homogeneity of the propellant during the transfer is a subtle but critical challenge.

Software and Automation

The entire sequence – launch, rendezvous, dock, settle, transfer, undock, land – must be automated. The latency of communication with Earth prohibits real-time joystick control. A software error in the station-keeping algorithms during the transfer could lead to structural loads that snap the docking mechanism.

Regulatory and Environmental Constraints

Launching 15+ huge rockets in a short window raises regulatory hurdles.

• FAA Licensing: SpaceX must obtain launch licenses for this high cadence.
• Environmental Impact: The noise, sonic booms, and thermal output of daily Starship launches may face opposition or regulatory caps at both Boca Chica and Cape Canaveral.

Timeline and Feasibility Forecast

Integrating the technical progress with the programmatic requirements of the Artemis program allows for a forecasted timeline.

NASA’s Requirements

For Artemis III, NASA requires:

1. An uncrewed HLS demo landing on the Moon (requires full refueling).
2. The crewed Artemis III landing.Both missions rely on the mature functioning of the Depot and Tanker fleet.

Forecasted Timeline

• 2025: Continuation of Block 2/3 testing. First attempt at Ship-to-Ship docking and transfer (small scale). Launch of the first “Depot-like” prototype for loiter testing.
• 2026: Operational validation of the Depot concept. Uncrewed HLS demo mission attempt (potential slippage to 2027). This year is critical for proving the “rapid relaunch” capability of the boosters.
• 2027–2028: Likely timeframe for the first crewed Artemis III landing. This assumes that the 2026 demo missions are successful and that the propellant aggregation campaign can be executed without major ground system failures.

Risk of Delay

The NASA Office of Inspector General (OIG) and GAO have consistently flagged the HLS schedule as “aggressive” and “unlikely” to meet the 2026 date. The sheer volume of launches required for a single mission makes the architecture sensitive to any grounding of the Starship fleet (e.g., due to a mishap investigation). A single failure during a 15-launch campaign could reset the entire aggregation process if the depot cannot wait for the investigation to conclude.

Strategic Implications and Future Outlook

The mastery of orbital refueling is the key that unlocks the solar system.

Mars Mission Architecture

The primary driver for Starship’s design is not the Moon, but Mars. A Mars mission requires refilling the ship in LEO to provide the delta-v for the trans-Mars injection. The techniques refined for Artemis – settling, high-volume transfer, boil-off management – are directly applicable to Mars logistics. The Depot concept may evolve into permanent orbital infrastructure, serving as a staging point for fleets of Mars-bound ships.

Commercial and Military Applications

Once operational, orbital refueling creates a new market.

• Satellite Life Extension: Tugs could refuel operational satellites.
• Deep Space Logistics: Commercial stations and lunar bases will require regular resupply, which is only economically feasible with reusable, refuelable transport.
• Military Utility: The US Space Force has expressed interest in “dynamic space operations,” which implies maneuverable satellites that can be refueled to change orbits.

Summary

The feasibility of Starship refueling in orbit is no longer a question of if, but when and how efficiently. The physics of settling and pressure-fed transfer have been validated on a small scale. The infrastructure for rapid launch cadence is being built physically (towers) and tested operationally (catches). However, the scaling problem is immense. Moving from transferring 10 tons internally to aggregating 1,200 tons externally across 15 launches requires a synchronous reliability that has never been achieved in spaceflight.

The successful operationalization of this capability will likely occur between 2026 and 2027. Once proven, it unlocks not just the Moon, but the entire solar system, as the ability to reset the “rocket equation” in orbit allows for massive payloads to Mars and beyond. The primary risks remain the reliability of the thermal control systems (boil-off) and the logistical ability to sustain the high launch cadence necessary to outrun thermodynamic losses.

Appendix: Top 10 Questions Answered in This Article

Why does Starship need to refuel in orbit?

Starship is too heavy to reach the Moon with a meaningful payload on a single tank of fuel; it must refill in Low Earth Orbit to regain the energy (Delta-v) required for trans-lunar injection.

How do they move fuel without gravity?

SpaceX uses “settling burns” – firing small thrusters to create milli-g acceleration that pushes liquid fuel to the bottom of the tank, allowing it to be transferred via pressure differentials or pumps.

What is the “Depot” variant?

The Depot is a specialized Starship designed to stay in orbit, featuring extra insulation and sunshades to store fuel long-term. It acts as a gas station for the Human Landing System.

How many launches are needed for one Moon mission?

Current estimates range from 10 to nearly 20 launches, depending on the payload capacity of the Tankers and the rate of propellant boil-off during the aggregation phase.

What is “boil-off”?

Boil-off is the loss of cryogenic propellant (liquid oxygen/methane) as it warms up and turns into gas due to solar and Earth radiation. It is a major time constraint for the refueling campaign.

Has SpaceX tested this yet?

Yes, partially. IFT-3 successfully demonstrated internal propellant transfer between tanks within a single ship, validating the fluid dynamics models.

What happens if the fuel sloshes?

Violent sloshing can cause “ullage collapse,” where pressurizing gas mixes with cold liquid and rapidly condenses, causing a dangerous drop in tank pressure. Settling thrust prevents this.

Will they use pumps or pressure?

While active pumps are faster, early indications and simple physics favor using pressure differentials (higher pressure in the donor, lower in the receiver) to drive the flow to reduce hardware weight.

When will the first crewed Starship land on the Moon?

While officially targeted for late 2026, independent analyses and OIG reports suggest 2027 or 2028 is a more realistic timeframe given the complexity of the refueling campaign.

How do the ships connect?

The current architecture points to a side-by-side docking configuration using “Quick Disconnect” ports similar to those used on the launch pad, rather than a nose-to-tail connection.

Appendix: Top 10 Frequently Searched Questions Answered in This Article

Starship refueling timeline?

Demonstration expected in 2025-2026; operational readiness for Artemis likely in 2027.

How many tanker flights for Artemis III?

Estimates are between 12 and 18 flights to fill the depot.

Starship HLS fuel capacity?

Approximately 1,200 to 1,500 metric tons of Methalox.

What is propellant settling?

Using small thrusters to create artificial gravity so liquid fuel can be pumped in space.

Starship boil-off rate?

Without active cooling, rates can be significant (approx. 1% per day or less with shielding), necessitating rapid launch campaigns.

SpaceX propellant depot concept?

A stripped-down Starship without heat shields or flaps, optimized for long-term storage in orbit.

Starship IFT-3 transfer results?

Successful demonstration of internal liquid oxygen transfer in microgravity.

Butt-to-butt vs side-to-side docking?

SpaceX has moved toward side-to-side docking to utilize simplified plumbing and existing port designs.

Artemis III delay risks?

High risk of delay to 2027+ due to the complexity of qualifying the tanker/depot system.

Starship cryogenic fluid management?

Involves passive insulation (MLI) and potentially active cryocoolers (ZBO) to preserve fuel in orbit.