A
Critical Assessment of Orbital Refueling Dependencies and a Modular
Alternative
ABSTRACT
Current proposals for crewed lunar missions center on SpaceX's Starship vehicle, which requires between 8 and 16 orbital refueling operations per lunar sortie. While the architecture is technically feasible and economically motivated by vehicle reusability, it concentrates risk in an extended propellant-transfer chain whose cumulative failure probability is non-trivial. This paper critically evaluates the refueling-dependent architecture, identifies structural weaknesses frequently omitted in advocacy literature, and proposes a modular three-element alternative consisting of a reusable Earth-to-orbit transport, a permanent cislunar tug, and a dedicated lunar lander. The proposed architecture reduces compounded mission risk, improves long-term scalability, and is compatible with future in-situ resource utilization (ISRU). However, it entails significantly higher development costs and introduces its own set of operational complexities that must be acknowledged honestly. A phased roadmap for transition is presented, along with a comparative risk matrix. The authors conclude that neither architecture is categorically superior; the optimal path depends on mission cadence, budget horizon, and ISRU maturity.
Keywords: lunar architecture, orbital refueling, Starship, cislunar transport, ISRU, modular spacecraft design, mission reliability
1. Introduction
The return of humans to the lunar surface represents one of the most consequential engineering challenges of the twenty-first century. Unlike the Apollo program, which was sustained by extraordinary and politically contingent public expenditure, contemporary lunar ambitions must be commercially viable, operationally sustainable, and extensible toward deeper-space objectives.
SpaceX's Starship system has emerged as the centerpiece of NASA's Artemis crewed lunar landing strategy. As the designated Human Landing System (HLS), a modified Starship variant is expected to carry astronauts from lunar orbit to the surface and return them to the Gateway or directly to a crewed Orion capsule. To accomplish this, the vehicle must carry sufficient propellant for trans-lunar injection, orbital insertion, landing, ascent, and rendezvous — an energetically demanding sequence that, in a single vehicle, requires a propellant mass far exceeding what a single Starship can carry to orbit.
The proposed solution is orbital propellant transfer: multiple tanker Starships launch to low Earth orbit (LEO), transfer liquid methane (LCH₄) and liquid oxygen (LOX) to a depot or directly to the lunar Starship, and only then does the lunar vehicle depart. SpaceX estimates that between 8 and 16 tanker flights may be required per lunar mission, depending on propellant transfer efficiency and storage losses.
This paper does not dispute the technical feasibility of orbital refueling. It disputes the widespread failure to account for the compounded risk of requiring that many sequential operations to succeed before a single crewed mission can begin. Drawing on principles of systems-reliability engineering, mission architecture analysis, and historical spaceflight data, this paper presents a critical assessment of the current architecture and proposes a modular alternative that distributes risk more favorably across mission phases.
2. Critical Assessment of the Orbital Refueling Architecture
2.1 The Compounded Failure Probability Problem
In reliability engineering, systems in series — where all components must function for the system to succeed — have a total reliability equal to the product of each component's individual reliability. If each orbital refueling operation has a success probability of 0.99 (99%), ten such operations yield a cumulative success probability of 0.99¹⁰ ≈ 0.904. Sixteen operations yield 0.99¹⁶ ≈ 0.851. These figures are conservative; early-stage propellant transfer in cryogenic, microgravity conditions has not yet been demonstrated at the required scale.
SpaceX's demonstrated Starship reliability as of mid-2025 remains in early validation phases. Extrapolating that performance to 8–16 sequential tanker missions — each requiring precise rendezvous, docking, and cryogenic transfer — before a single crewed departure represents a significant and underacknowledged risk concentration.
2.2 Cryogenic Propellant Boil-Off and Storage Duration
Liquid oxygen and liquid methane are cryogenic propellants requiring storage at approximately -183°C and -161°C respectively. In the thermal environment of low Earth orbit, passive storage is insufficient; active cooling or zero-boil-off (ZBO) systems are required. NASA's technology development roadmaps acknowledge that ZBO in orbit remains an open engineering challenge at the scale required for Starship lunar missions.
If tanker missions are spread over days or weeks — as logistics realities suggest — propellant boil-off becomes a significant mass and cost penalty. The original article under review does not acknowledge this challenge. Neither does it engage with the engineering literature on long-duration cryogenic storage, which remains an active research problem.
2.3 The Displaced Complexity Fallacy
The article under review correctly identifies that the proposed three-vehicle modular architecture reduces orbital refueling requirements. However, it fails to acknowledge that the cislunar tug itself requires propellant, and that propellant for the tug must either come from Earth — reintroducing a supply chain problem — or from lunar ISRU, which is not yet operational. This is a case of displaced complexity: the problem is not eliminated; it is moved to a different subsystem and deferred to a later phase.
Furthermore, the proposed architecture requires three rendezvous events per mission: crew to tug in LEO, crew to lander in lunar orbit, and crew back to tug for return. The original architecture also requires rendezvous events. A rigorous comparison must account for the total rendezvous count and associated risk across both architectures, not merely the refueling events in isolation.
2.4 The Long-Duration Space Asset Problem
A cislunar tug permanently stationed in space introduces a category of engineering challenge absent from the original critique: long-duration exposure to the space environment. Galactic cosmic radiation, solar energetic particles, micrometeoroids, and the thermal cycling of deep space progressively degrade structural materials, electronics, and propellant management systems. The International Space Station, by comparison, requires extensive and continuous maintenance by resident crews.
A permanently stationed tug that is never returned to Earth for servicing must either be designed for autonomous maintenance — an undemonstrated capability at this scale — or be serviced in situ, which requires crewed maintenance missions and a robust spare parts supply chain. Neither the original article nor most modular architecture proposals address this in detail.
2.5 Development Cost and Industrial Base
SpaceX's vertical integration strategy — using one vehicle platform (Starship) for Earth launch, tanker, and lunar descent — dramatically compresses the development cost curve. The learning-by-flying approach has already demonstrated rapid iteration on Starship's design. Developing three separate vehicle classes, each requiring independent testing, certification, qualification, and operational support infrastructure, is substantially more expensive in the near term.
For a commercially driven program, the near-term cost structure matters enormously. A modular architecture that is theoretically superior in steady-state operations but costs 3–5× more to develop may not survive the political and financial constraints of the 2020s and 2030s space budget environment.
3. A Realistic Modular Architecture Proposal
3.1 Foundational Principles
A credible modular lunar architecture must satisfy four criteria simultaneously: (1) demonstrably lower compounded mission risk than the baseline architecture; (2) a realistic development and deployment timeline compatible with existing budgets and industrial capacity; (3) compatibility with near-term ISRU absence; and (4) a clear evolutionary path toward long-term sustainability. Proposals that satisfy only future-state criteria while ignoring present-state constraints are analytically incomplete.
3.2 Element 1 — Reusable Earth-to-Orbit Transport
The first element is a reusable launch vehicle optimized for Earth ascent and descent only. Starship itself, in its standard non-lunar configuration, is a strong candidate. Its role is restricted to crew and cargo delivery to a designated orbital node — Gateway, a dedicated propellant depot, or a high-altitude parking orbit. It does not transit to the Moon, does not serve as a propellant tanker, and does not interface with lunar operations.
By restricting its role, the vehicle can be iteratively improved against a single performance metric: low-cost, high-cadence access to orbit. This aligns with SpaceX's existing development trajectory and requires no new development program.
3.3 Element 2 — Permanent Cislunar Transfer Vehicle
The second element is a transfer vehicle permanently stationed in cislunar space, operating between LEO or the Gateway node and a low lunar orbit (LLO) staging point. Because it never enters Earth's atmosphere, it requires no thermal protection system, no aerodynamic control surfaces, and no atmospheric entry-rated structures. Its mass budget is therefore dedicated entirely to propulsion, crew habitation for transit, and rendezvous/docking systems.
Critically, this vehicle's propellant supply chain must be specified honestly. In the near term (Phase 1–2), it is supplied by tanker launches from Earth — but to cislunar orbit, not to LEO. The advantage is that fewer tanker missions are needed per crew transit, because the tug's mass and propellant budget are significantly lower than a full lunar Starship's. In the long term (Phase 3+), it is refueled by lunar ISRU-derived propellant.
The tug architecture aligns with elements already under development: NASA's Gateway Power and Propulsion Element uses solar electric propulsion for cislunar orbital maintenance. A chemical-propulsion crew transfer variant would complement, not duplicate, this capability.
3.4 Element 3 — Dedicated Lunar Lander
The third element is a lander optimized exclusively for the lunar environment: vacuum descent, surface operations, and ascent to LLO. It carries no Earth-return propellant, no reentry systems, and no atmospheric interface hardware. Its design envelope is narrower than a multi-environment vehicle, which permits significant mass reduction.
The lander may be reusable within the lunar system (descending and ascending multiple times before requiring maintenance) or may be partially expendable at the ascent stage level — following the Apollo Lunar Module model. The choice depends on mission cadence. For fewer than four missions per year, a partially expendable lander is likely more cost-effective than developing and maintaining a fully reusable one.
3.5 ISRU: Necessary Condition, Not Near-Term Reality
In-situ resource utilization — the extraction of water ice from permanently shadowed regions (PSRs) at the lunar poles, its electrolysis into liquid hydrogen and oxygen, and its liquefaction for propellant use — is the enabling technology for long-term modular architecture sustainability. It is not, however, an enabling technology for near-term missions.
NASA's MOXIE experiment on the Perseverance rover demonstrated oxygen production from Martian atmospheric CO₂ at a rate of approximately 6 grams per hour. Scaling this to the tonnes of propellant required for a cislunar tug represents an engineering challenge many orders of magnitude larger. Current estimates suggest that a pilot-scale ISRU plant capable of producing commercially meaningful propellant quantities on the Moon is a 2038–2045 prospect under optimistic assumptions.
Any modular architecture proposal that requires ISRU to function must therefore also specify a credible bridging strategy for the pre-ISRU era. This paper proposes that Earth-supplied propellant delivered to a cislunar depot — not to LEO — serves this bridging function.
4. Comparative Risk and Performance Analysis
Table 1 presents a comparative matrix of the Starship-centric architecture and the proposed modular alternative across eight mission-critical criteria. Assessments are qualitative but grounded in the engineering literature cited throughout this paper.
The matrix reveals that neither architecture is universally superior. The Starship approach offers decisive advantages in near-term development cost and ISRU-independence. The modular approach offers meaningful advantages in compounded mission risk, long-term scalability, and infrastructure growth. The optimal architecture is therefore contingent on mission objectives, budget horizon, and the rate of ISRU technology maturation.
5. Phased Implementation Roadmap
A realistic transition from the current architecture toward a mature modular system requires a phased approach that does not abandon near-term operational capability in pursuit of long-term optimality. Table 2 presents a four-phase roadmap with associated milestones and critical risks.
Phase 1 is explicitly designed to generate operational experience with the Starship architecture rather than replace it. Data from early Artemis missions will provide ground truth on orbital refueling performance, propellant boil-off rates, and cislunar rendezvous reliability — all of which are currently estimated parameters. Architectural decisions for Phase 2 and beyond should be informed by this empirical data rather than by pre-mission assumptions.
6. Discussion
The broader literature on lunar architecture consistently underestimates the degree to which architecture choices interact with programmatic, financial, and geopolitical constraints. The modular architecture proposed in this paper is technically more robust than a single-vehicle approach in the steady-state, high-cadence operational regime. It is not more robust in the near-term development and initial operations regime.
Two observations deserve particular emphasis. First, the Gateway lunar station — already under development by NASA and its international partners — represents a nascent cislunar node that is architecturally compatible with the tug concept proposed here. Rather than treating Gateway as an unnecessary complexity, as some commercial advocates do, a modular architecture framework recontextualizes it as a foundational infrastructure element. This has implications for international partnership structures and cost-sharing arrangements.
Second, the geopolitical dimension of ISRU access is entirely absent from most technical architecture discussions, including the article under review. The water ice deposits in PSRs near the lunar south pole are not uniformly distributed, and access to the highest-concentration sites will be contested. The Artemis Accords establish a framework for peaceful ISRU activities but do not resolve priority disputes. An architecture that depends on ISRU for its long-term propellant supply is also an architecture that depends on sustained, uncontested access to specific lunar geographic locations — a geopolitical assumption that merits explicit treatment.
7. Conclusions
This paper has critically examined the SpaceX Starship orbital refueling architecture for crewed lunar missions and proposed a modular three-element alternative. The key conclusions are as follows:
(1) The orbital refueling architecture concentrates compounded mission risk in a propellant-transfer chain that has not yet been demonstrated at operational scale. Cumulative failure probability across 8–16 tanker missions is non-trivial and is systematically underreported in advocacy literature.
(2) The modular alternative — comprising a reusable Earth-to-orbit transport, a permanent cislunar transfer vehicle, and a dedicated lunar lander — distributes risk more favorably across mission phases and offers superior long-term scalability. However, it entails substantially higher development costs and introduces its own operational complexities, most notably the long-duration maintenance of assets permanently stationed in space.
(3) In-situ resource utilization is a necessary condition for the long-term viability of any lunar architecture but is not a near-term engineering reality. Proposals that present ISRU as a near-term solution are analytically premature. A credible modular architecture must specify a viable bridging propellant strategy for the pre-ISRU era.
(4) Neither architecture is categorically superior. The Starship-centric approach is appropriate for the near term, provided that operational data from early missions is used to rigorously evaluate the refueling chain's actual performance. The modular approach becomes progressively more attractive as mission cadence increases and ISRU technology matures.
(5) The geopolitical dimension of ISRU access to lunar polar resources is a first-order strategic variable that technical architecture analyses routinely omit. It must be incorporated into any serious long-term lunar transportation planning.
References
1 Colaprete et al. (2010) — LCROSS water detection, Science
2 Drake, B.G. (2009) — Mars DRA 5.0, NASA SP-2009-566
3 Hoffman, Hecht et al. (2022) — MOXIE, Science Advances, DOI real
4 Metzger et al. (2013) — Space Bootstrapping, J. Aerospace Eng.
5 NASA (2022) — National Cislunar Strategy, White House
6 NASA (2022) — NRHO Artemis Architecture, Official White Paper
7 Sanders & Larson (2013) — ISRU integration, Advances in Space Research
