Future human space exploration to distant locations including the Moon, Mars, and beyond carries many logistical challenges. Imagine planning a year-long road trip for a family of four. All consumables, food, water, and oxygen, must be a part of the logistics plan. However, there are also no gas or service stations along the way. No garbage or waste disposal either. Oh, and the environment outside is a freezing vacuum bombarded with radiation. Some of the key questions to answer include: How much cargo space is needed and how much is available for non-essential items including your favorite science experiment? Are there intermediate locations where cargo can be stored and retrieved? What is the right tradeoff between bringing all possible spares versus having a chance to break down on the way? How much investment should be made in resource recycling to reduce the overall burden?
From this perspective, the Apollo missions were similar to a two week backpacking trip and ISS expeditions are similar to visiting a permanently-staffed remote wilderness outpost. Both carry significant logistical problems and issues; however, they are substantially less complex than future human space exploration missions.
Network-based simulation models can help plan logistics for space exploration missions. SpaceNet is a discrete event network simulation which models the flow of people, vehicles, and resources between planetary surfaces and stable orbits. Each of these nodes are linked by a series of edges which represent valid trajectories – launches, landings, and in-space maneuvers. During a simulation, vehicles and resources are generated, moved, and consumed to satisfy needs of human crew members. Architects can specify a baseline plan (mission concept) and, through simulation, evaluate a series of performance metrics to determine how good of a plan it is relative to others.
SpaceNet leverages two key contributions to understanding and modeling space logistics. First, it builds on a ten main classes of supply which help to distinguish resources from one another. These include:
- Propellants and Fuels: used for propulsion
- Crew Provisions: food, water, oxygen, and other essentials
- Crew Operations: health, sanitary, and communications equipment
- Maintenance and Upkeep: tools for repair and spare parts
- Stowage and Restraint: packaging materials and containers
- Exploration and Research: tools for exploration and scientific experiments
- Waste and Disposal: garbage, human waste, and processing equipment
- Habitation and Infrastructure: vehicles with inhabitable environments for crew
- Transportation and Carriers: vehicles capable of transporting items
- Miscellaneous: anything else!
In addition to the 10 main classes of supply, there are numerous subclasses to help refine definitions to finer and finer levels of detail. For example, there are several types of propellants and fuels which are completely incompatible with each other (liquid oxygen, kerosene, hypergolic, solid fuels). Classes of supply help to identify what resources are compatible with which demands and enable advanced logistics strategies such as in-situ resource production and caching at an intermediate depot.
Second, SpaceNet identifies a core set of seven mission events which occur in the exploration network. Events are the atomic mission units which generate state transitions simulation model. Create, Reconfigure, Move, or Destroy elements events operate on vehicles, crew, or other persistent objects. Generate, Move, or Consume resources events operate on ephemeral substances ultimately consumed by demands.
More complex processes such as launches or explorations compose these seven core events in many new ways. For example, a launch process consumes resources (propellant), destroys elements (expended stages), and finally moves elements to the intended orbit. Similarly, an exploration process consumes resources (crew metabolism), reconfigures elements (crew don EVA suits), moves elements (crew exit the habitat), generates resources (crew collect surface samples), and moves resources (crew store samples in return vehicle).
Simulation alone cannot answer all of the questions about future space exploration. Humans are still required to code all of the important behaviors in a simulation model and come up with the various mission concepts to evaluate. Future research may improve the ability of simulation models to automatically adapt mission concepts and improve performance metrics. Recent work by other researchers including Dr. Takuto Ishimatsu and Dr. Koki Ho contribute to these goals; however, more work is still necessary to help plan for future space exploration missions.
For more information, please see:
- MIT Space Logistics Project
- SpaceNet Project and GitHub Repository
- P.T. Grogan, A flexible, modular approach to integrated space exploration campaign logistics modeling, simulation, and analysis, S.M. Thesis, Massachusetts Institute of Technology, 2010. (link)