Elsevier

Acta Astronautica

Volume 181, April 2021, Pages 352-361
Acta Astronautica

Lunar human landing system architecture tradespace modeling

https://doi.org/10.1016/j.actaastro.2021.01.015Get rights and content

Highlights

  • We present parametric models evaluating human landing systems for lunar exploration.

  • The proposed models account for system reusability and key architectural decisions.

  • We propose an integrated technical and cost analysis of landing system architectures.

  • We discuss engineering tradeoffs in competing human landing system designs.

  • We identify the 1-stage LOX/LH2 system architecture as a preferred design baseline.

Abstract

A renewed interest in lunar exploration with the focus on establishing a constant human presence on the Moon calls for developing new lunar human landing systems (HLS) which would deliver the crew from the prospective Lunar Gateway station to the surface of the Moon and back. Over the years, different human lunar lander architectures were proposed and multiple architecture studies were performed. However, those studies are relevant to the specific assumptions and lunar architectures proposed at the time of conducting the study. Since the current vision for lunar exploration includes new features, such as having the Lunar Gateway and switching to reusable systems, there is a need for a new HLS architecture study. Such studies are being performed by private companies; however, those are rarely publicly available. The goal of this paper is to address this gap and provide a publicly available architectural analysis within the current views on the future human lunar exploration.

We assume the Lunar Gateway in an L2 near rectilinear halo orbit and a landing site at the lunar South Pole; the number of HLS crew of 4; the surface stay time of ~7 days, the payload mass delivered to the surface of 500 kg, and the payload mass returned from the surface of 250 kg. A set of parametric models including an HLS model and an HLS program cost model is developed for the analysis. 39 architectures with varying number of stages (1, 2, and 3 stages) and propellant combinations (LOX/LH2, LOX/CH4, and MMH/NTO) are explored. The Pareto analysis shows that there is a difference between typical performance trends for expendable and reusable architectures. For expendable architectures, the 2-stage option seems to be the most advantageous while, for reusable architectures, the 1- and 3-stage options are either comparable or win over the 2-stage option even for the number of system uses as low as 3. In terms of the propellant combinations, pure LOX/LH2 or combined LOX/LH2/LOX/CH4 architectures dominate the tradespace. Assuming that the inter-stage propellant compatibility is a preferred option for systems refueling from the Gateway, 1-stage and 3-stage all LOX/LH2 architectures are identified as the likeliest candidates to have lowest HLS-related production and launch costs. Further cost analysis of those two architectures shows that the 1-stage HLS wins over the 3-stage system in terms of the overall HLS program cost if a long-term exploration program (on the order of tens of missions) is assumed.

Introduction

In 2017, the United States government provided a clear direction for the US human spaceflight program for a return to the Moon by 2024 [1]. The focus of those efforts culminated in the development of the Artemis program [2], which forms the backbone of international collaboration for future human space exploration endeavors. The next steps for the US human spaceflight program, according to the policy at the time of writing this paper, will likely result in the establishment of a deep space Gateway and sortie missions at the lunar South Pole. Reusable systems will likely be employed in such architecture, to ensure sustainable operations over the long term. Architectural decisions such as system reusability need to be evaluated in the context of all other system-level engineering decisions, in order to develop an integrated plan for technology development. In a recent paper, we proposed a technology roadmap for lunar human landing systems, which are key building blocks for future exploration of the Moon [3].

Over the years, different human landing system architectures have been considered; multiple architecture studies were conducted. Those studies, however, are relevant to the specific assumptions and lunar exploration architectures proposed at the time of conducting the study. Since today we have new assumptions (such as having the Lunar Gateway in the lunar architecture and switching to reusable systems), there is a need for a new human landing system architecture study. Such studies are being performed by private companies – see, for example, the study by Aerojet Rocketdyne [4]; examples of other private companies that are likely concerned with architectural studies on human lunar landing systems are Blue Origin, Dynetics and SpaceX, as we can infer from NASA press releases [5]. Those studies, however, are usually either not publicly available or their public representation does not provide enough details on the respective modeling assumptions (as in case with [4]) which makes it difficult to fully understand the justification behind such study conclusions. Making well-informed system architecture decisions is important as they have a significant impact on the end outcomes of program cost and performance (as described in the well-known ‘80/20 rule’ in the context of conceptual system design [6]).

The goal of this paper is to address the gap of an open architectural study of human landing systems (HLS) for future Moon exploration. We develop parametric mathematical models to enable a comprehensive tradespace exploration of future (potentially reusable) HLSs. Open-source parametric models allow for a publicly available architectural analysis, within the framework of future lunar exploration programs. The mathematical models proposed in this paper can be reused and adapted for a range of planetary exploration systems, and are thus of general interest to the space systems engineering community. We achieve our goal in two stages:

  • First, we develop a set of figures of merit (FOMs) and a general HLS model applicable to each of the three architecture types (1-stage, 2-stage, or 3-stage). The HLS model is used to calculate the mass properties of a specific HLS which are then used to calculate the respective FOMs. Using those FOMs as preliminary proxies for the HLS program cost, we conduct a Pareto analysis on the overall HLS architecture tradespace and narrow down architecture options to a few promising candidates.

  • Second, we develop an HLS program cost model. This includes developing a refueling vehicle model to model the costs associated with delivering the HLS elements and propellant from Earth to the Gateway (the launch costs). The HLS program cost model is then used to perform the cost analysis on the HLS candidates identified at the first stage. As a result, one of the architectures is selected as having the lowest costs under the assumptions of this study.

The remainder of this paper is structured as follows. Section 2 of this paper describes the methodology used in this study. Section 2.1 provides an overview of HLS architectures; section 2.2 lists the main study assumptions; sections 2.3 defines the figures of merit used in the study; sections 2.4 and 2.5 develop the HLS and refueling vehicle models, respectively; section 2.6 is devoted to the HLS program cost model. Section 3 presents the results of the Pareto (section 3.1.) and cost (section 3.2) analyses. Section 4 summarizes the findings of the study and identifies opportunities for future work.

Section snippets

Methodology

We adopt a comprehensive architecting approach to tradespace exploration in this paper. We have discussed our approach in previous papers [7,8], including previous human spaceflight assessments [9,10]. The methodology can be briefly described as follows. We first provide a brief overview of the system under investigation – human landing system. We then narrow the focus of the study with a set of assumptions, which also set the limitations of our investigations and point out avenues for future

Pareto analysis

HLS architectures with different numbers of stages (1, 2, or 3 stages) and different types of propellant for each of the stages (LOX/LH2, LOX/CH4, or MMH/NTO) were explored. Pareto charts for different numbers of HLS system uses are shown in Fig. 2. The two axes of the upper-left chart are the total HLS wet mass in the Gateway orbit (the horizontal axis) and the total HLS dry mass (the vertical axis). Those masses are used here as proxies for the launch and production costs associated either

Conclusion

In this study, we performed a human landing system architecture tradespace analysis relevant to the current vision of lunar exploration which includes the Lunar Gateway in an L2 near rectilinear halo orbit and tasks a reusable HLS with delivering the crew from the Gateway to a landing site at the lunar South Pole and back to the Gateway. A set of parametric models including an HLS model applicable to the 1-, 2-, and 3-stage cases and an HLS program cost model were developed for the analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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