AeroAstro Magazine Highlight

Special delivery: MIT tackles interplanetary transportation of supplies, equipment, and humans

By Olivier L. de Weck

Between 1969 and 1972, Project Apollo achieved six lunar landings, enabling a total stay of 300 crew hours (12.5 days) on the lunar surface. Today, President Bush's Vision for Space Exploration charges NASA with a return to the Moon before 2020, and developing both lunar technologies and operational experiences as a springboard for human missions to Mars and beyond. The new challenge is how to accomplish this under a constrained budget and in a sustainable manner, using the Moon as a stepping stone to Mars. NASA is conducting studies, and has engaged industry and academia in this pursuit. Among academic institutions, MIT is playing a central research and educational role in the Vision for Space Exploration.

Aero-Astro Professor and former Shuttle astronaut Jeffrey Hoffman tests a Mars concept space suit at the Haughton Crater base on Devon Island in the Canadian Arctic during a 2005 visit by MIT researchers. The base offers some conditions similar to those that may be experienced on the moon and Mars. (Jessica Marquez photograph)

A number of large projects are underway at MIT, spearheaded by the Department of Aeronautics and Astronautics, to help the nation meet this challenge. The first of these was the Concept Evaluation & Refinement (CE&R) study, conducted jointly with the Charles Stark Draper Laboratory in 2004-2005. This involved a team of eight faculty members led by Professor Edward Crawley, and about 30 graduate students, mainly from Aero-Astro. The project gave us a first opportunity to specify what is required to achieve sustainability in space exploration: value delivery, affordability, risk/safety, and policy robustness. A focus on value delivery ensures that new discoveries are continuously generated and that public interest and a consistent level of funding are maintained over time. Affordability minimizes lifecycle costs, not just development costs, and upfront consideration of risk and safety helps avoid hazards from the start. Policy robustness ensures that the exploration program will survive changes in objectives, leadership and funding through a modular, step-like approach. As part of the CE&R study, we also developed specific recommendations for lunar transportation architectures, the design of the Crew Exploration Vehicle, and other mission elements.

One of the keys to affordability is a high degree of commonality and modularity among mission elements. This is true even if some performance penalties and inefficiencies in individual elements are encountered along the way. Commonality in the design of propulsion systems, landing gears, tankage and structural interfaces, avionics, and software was identified as a high priority. We also developed methods and tools for quantifying both the benefits and penalties of commonality for sets of future exploration missions and elements with applicability to NASA and other industries. However, this requires a paradigm shift by decision makers to not evaluate space missions one-at-a-time, but to consider the effects over an entire program. The other key idea we developed is designing systems so that they are easily evolved over time and adaptable for new missions. We pioneered the "Mars-back" approach, where hardware is primarily designed for the long-term objective - Mars - and accommodations for lunar use are only made where absolutely necessary.

Paradigm shift: space logistics

It is becoming clear that to be sustainable over the long term, a fundamental paradigm shift has to take place to include operational considerations early in the design of space hardware. Both the Space Shuttle program and the International Space Station demonstrated that the majority of funds are consumed during operations, not during the design or implementation phases. Therefore, a new way to view space exploration is to treat it primarily as a logistics problem. This is the underpinning of the project on Interplanetary Supply Chain Management where we are teaming with Professor David Simchi-Levi, of MIT's Engineering Systems Division and Department of Civil and Environmental Engineering.

In Project Apollo, each mission was self-contained; none of the missions left anything behind for another mission's use. Future space exploration will use more complex supply networks, on the ground and in space. For example, several orbits and Lagrange points might be employed, and the missions will be intertwined. Lagrange points are stable libration points where the gravitational pull of two bodies such as the Earth and the Moon are in equilibrium. As an example, consider a human mission to the Moon preceded by several robotic and perhaps other human missions. Rather than shipping all supply with the human crew, it might be beneficial to pre-position some of the supply either at the Moon, or at one of the orbits or Lagrange points. Specifically, we are investigating the interplay of the following three fundamental supply chain strategies on space exploration:

pre-positioning - transporting elements and cargo ahead of human crews based on forecasts (push)
carry-along - transporting human crews and supplies together
resupply - sending supplies to human explorers at remote sites based on actual demand (pull)

Component commonality and modularity, as shown here in these MIT-designed crewed lander and surface habitat concepts, is a key to the affordability of future interplanetary exploration.

The optimal strategy for remote exploration beyond Earth might be a carefully balanced mix of robotic pre-deployments, carry-along as well as on-demand resupply. This translates directly into the terrestrial concept of push/pull boundaries in supply chains were some items are manufactured based on forecasts, while others are only assembled and shipped once actual orders have been received. This is a paradigm shift for NASA and ways to simulate and optimize missions and campaigns in this way are currently beyond developed in our NASA-funded research project.

Other, yet to be explored, strategies by NASA are in-space refueling and in-situ resource utilization. ISRU focuses on obtaining resources directly from the planetary surface or atmosphere to produce propellants or breathable oxygen. While the necessary technologies have not yet fully matured to perform these functions, their benefits may be significant. An analytical tool to assess such options is necessary much like analytical tools are used in terrestrial supply chains to understand the impact of various supply chain strategies. The primary goal of the Interplanetary Supply Chain Management project is to develop such a comprehensive framework and planning methodology for space logistics.

SpaceNet - a systems approach

To evaluate the effect of different supply chain strategies, transportation architectures and vehicle technologies, we developed a software environment called SpaceNet. The basic building blocks are an integrated database of elements (vehicles) and their capabilities, astrodynamical trajectories using both chemical (impulsive) and electrical continuous-thrust trajectories, as well as nodes in the interplanetary system. We distinguish between surface, in-space and Lagrangian nodes. Surface nodes exist on the Earth (such as the spaceport and launch site at the Kennedy Space Center), on the Moon and Mars as well as other bodies of interest such as near-earth-objects. Orbital nodes represent stable orbits around a central body and are characterized by their apoapsis, periapsis and inclination. Lagrangian nodes might lend themselves well for in-space depots and cargo transfer points.

SpaceNet visualization: planetary surface and orbital nodes are connected by arcs representing launch, in-space trajectories as well as entry-descent-and-landing operations. Elements containing human crews, robotic agents, propellant, collected samples as well as various other supply items, are traveling on these arcs

Additionally, we created a new system of supply classification for exploration. The theoretical basis of SpaceNet is the concept of time-expanded networks, which allow simulating and optimizing the flow of elements, crews and associated supplies through the interplanetary supply chain, while taking into account the time-varying nature of launch windows. A time-expanded network is one where static nodes are copied at discrete time steps to capture the four-dimensional space-time underlying space exploration missions and campaigns.

Our focus is on analyzing the requirements for a human return to the Moon, specifically the sortie missions and buildup of a lunar outpost in the 2018-2023 timeframe. However, SpaceNet has proven general enough, so that we are also analyzing alternative strategies for resupply of the International Space Station after the retirement of the Space Shuttle in 2010, as well as opportunities for commercial services for refueling spacecraft in low Earth orbit. An important part of the systems approach underlying SpaceNet is the ability to evaluate the impact of new architectures and technologies on overall measures of effectiveness:

Measures of Effectiveness for Space Logistics

Benefits
- Crew Surface Days [d]
- Exploration Mass Delivered [kg]
- Total Exploration Capability [kg-d]
Costs
- Total Launch Mass [kg]
- Total Scenario Costs [$]
Risk
- Total Scenario Risk (scales with the number of required launches, rendezvous-and-dockings, and entry-descent and landing operations in a scenario.)

Additionally, we compute a Relative Exploration Capability (REC) metric, which is essentially the product of crew days at a planetary surface and exploration mass delivered, divided by the required total launch mass from Earth's surface. This is a measure of interplanetary supply chain efficiency, a quantity also known as the "divisia" index by traditional economists. When we normalize this ratio by what was achieved in Apollo 17, mankind's last manned mission to the Moon, we can asses the relative impact of new approaches and technologies such as:

in-situ resource utilization (extraction of oxygen from the lunar regolith, or methane from the Martian atmosphere)
refueling of propulsion stages in low Earth orbit with propellant shipped separately by cheaper, potentially less reliable, commercial consumables launchers
reconfigurability and commonality of subsystems and components so that they can be used for various functions and scavenged from idle elements as needed
closing loops in environmental control and life support systems to reduce the amount of crew consumables needed for long interplanetary flights and extended surface stays
trading off relative amounts of consumables, spares, and exploration equipment for build-up and resupply of a lunar or Martian base

The ability to think through and model these complex interactions in a systematic way all the way from subsystem technologies to high level operational scenarios is one of our unique strengths. Moreover, we believe that costs can be reduced by testing many technologies and approaches in Moon- and Mars-analogue environments on Earth, representing a rich area of supporting research.

Terrestrial planetary analogs

Such an effort was the 2005 expedition to the Haughton-Mars Project research station on Devon Island. A team of nine MIT researchers went to the Canadian Arctic to participate in the annual HMP field campaign from July 8 to August 12. We investigated the applicability of the HMP research station as an analogue for Moon and Mars planetary macro- and micro-logistics, and collected data for modeling purposes.

At HMP, we also tested new technologies, such as Radio Frequency Identification, and procedures to enhance the ability of humans and robots to jointly explore remote environments. Effective logistics is as much about information management and real-time awareness of system health and inventory levels as it is about transportation. Additionally, a complete inventory of the HMP research station was compiled for future modeling and to create a complete picture of the current state of operations and this was compared against parametric demand models for a lunar base. In a wider sense our model helped establish a benchmark model for efficient operation of a multinational, multi-organizational research base in a remote environment.

Making an impact

NASA has adopted SpaceNet as a major component of its Integrated Modeling & Simulation infrastructure in support of a strategic analysis capability in the Exploration Systems Mission Directorate. Project Constellation, a major NASA program component that uses this capability, is charged with designing and procuring the next generation of manned spacecraft that will return mankind to the Moon and enable our voyages to Mars and beyond.

Research in interplanetary supply chain management and exploration technologies offers our Aero-Astro faculty, students, and research staff a unique opportunity to take a systems approach in designing sustainable exploration elements and operations. This requires a combination of theoretical modeling, simulation, benchmarking against data from NASA's past and current operations, dedicated field research and cooperation with our partners at Draper, NASA, JPL and industrial companies like Payload Systems Inc. and United Space Alliance LLC. Our students embrace this opportunity. We have combined these research activities with our educational efforts and systems engineering courses at both the graduate (Subject 16.89) and undergraduate levels (Subject 16.83). MIT Aero-Astro is continuing to lead in this new area by developing unique methods and tools for space systems design and logistics, while making a real impact on decisions and technologies that will determine mankind's future in space.

More information about the project is available at: http://spacelogistics.mit.edu

Olivier L. de Weck is an Assistant Professor of Aeronautics and Astronautics and Engineering Systems. He has more than 15 years of military, commercial, and academic experience working on a variety of systems such as the Northrop F-5, F/A-18, Next Generation Space Telescope, Space Interferometry Mission, and various commercial communications satellites. His research focuses on strategic aspects of systems engineering and multidisciplinary design optimization. His web site is http://deweck.mit.edu and he may be reached at deweck@mit.edu.