Grant R. Cates

Modeling the space shuttle

We summarize our methodology for modeling space shuttle processing using discrete event simulation. Why the project was initiated, what the overall goals were, how it was funded, and who were the members of the project team are identified. We describe the flow of the space shuttle flight hardware through the supporting infrastructure and how the-model was created to accurately portray the space shuttle. The input analysis methodology that was used to populate the model elements with probability distributions for process durations is described in the paper. Verification, validation, and experimentation activities are briefly summarized.

Generic simulation models of reusable launch vehicles

Analyzing systems by means of simulation is necessarily a time consuming process. This becomes even more pronounced when models of multiple systems must be compared. In general, and even more so in todays fast-paced environment, competitive pressure does not allow for waiting on the results of a lengthy analysis. That competitive pressure also makes it more imperative that the processing performance of systems be seriously considered in the system design. Having a generic model allows one model to be applied to multiple systems in a given domain and provides a feedback mechanism to systems designers as to the operational impact of design decisions.

Supporting the vision for space with discrete event simulation

On January 14, 2004, President George W. Bush announced a new vision for space exploration. This vision called for NASA to complete the assembly of the international space station by 2010 and retire the space shuttle immediately thereafter. A discrete event simulation (DES) based tool has been built to assess the viability of NASA accomplishing all of the space shuttle missions required to assemble the space station by the end of the decade. This paper describes this DES tool i.e. the manifest assessment simulation tool (MAST)

A discrete event simulation model for assembling the international space station

In late 2002 and early 2003, a discrete event simulation (DES) model was used to help NASA assess the viability of achieving a politically important goal. The goal was to complete the assembly of the international space station through the milestone known as United States Core Complete by February 19th, 2004. The analysis provided by the DES model was subsequently shown to be consistent with NASAs official assessments regarding the completion date for U.S. Core Complete. The success of this DES model has led to further improvements in modeling NASAs project to assemble the international space station.

Low earth orbit rendezvous strategy for lunar missions

On January 14, 2004 President George W. Bush announced a new vision for space exploration calling for NASA to return humans to the moon. In 2005 NASA decided to use a low Earth orbit (LEO) rendezvous strategy for the lunar missions. A discrete event simulation (DES) based model of this strategy was constructed. Results of the model were then used for subsequent analysis to explore the ramifications of the LEO rendezvous strategy

Risk Analysis of On-Orbit Spacecraft Refueling Concepts

On-orbit refueling of spacecraft has been proposed as an alternative to the exclusive use of Heavy-lift Launch Vehicles to enable human exploration beyond Low Earth Orbit (LEO). In these scenarios, beyond LEO spacecraft are launched dry (without propellant) or partially dry into orbit, using smaller or fewer element launch vehicles. Propellant is then launched into LEO on separate launch vehicles and transferred to the spacecraft. Refueling concepts are potentially attractive because they reduce the maximum individual payload that must be placed in Earth orbit. However, these types of approaches add significant complexity to mission operations and introduce more uncertainty and opportunities for failure to the mission. In order to evaluate these complex scenarios, the authors developed a Monte Carlo based discrete-event model that simulates the operational risks involved with such strategies, including launch processing delays, transportation system failures, and onorbit element lifetimes. This paper describes the methodology used to simulate the mission risks for refueling concepts, the strategies that were evaluated, and the results of the investigation. The results of the investigation show that scenarios that employ refueling concepts will likely have to include long launch and assembly timelines, as well as the use of spare tanker launch vehicles, in order to achieve high levels of mission success through Trans Lunar Injection.

Simulation, modeling and analysis of Space Shuttle flight hardware processing

This paper describes key aspects of the history of the space shuttles flight rate and the uses of simulation for estimating and assessing flight rate. When initially proposed, the shuttle was to fly 50 to 150 times per year. The earliest simulation models supported these projects but were based upon faulty assumptions. As the shuttle has evolved so have simulation models. Todays simulation accurately models the shuttles flight rate of approximately 7 per year and is benefiting both the current shuttle and future shuttle replacement vehicles being considered by NASA.

Statistical and Probabilistic Extensions to Ground Operations' Discrete Event Simulation Modeling

NASAs human exploration initiatives will invest in technologies, public/private partnerships, and infrastructure, paving the way for the expansion of human civilization into the solar system and beyond. As it is has been for the past half century, the Kennedy Space Center will be the embarkation point for humankinds journey into the cosmos. Functioning as a next generation space launch complex, Kennedys launch pads, integration facilities, processing areas, launch and recovery ranges will bustle with the activities of the worlds space transportation providers. In developing this complex, KSC teams work through the potential operational scenarios: conducting trade studies, planning and budgeting for expensive and limited resources, and simulating alternative operational schemes. Numerous tools, among them discrete event simulation (DES), were matured during the Constellation Program to conduct such analyses with the purpose of optimizing the launch complex for maximum efficiency, safety, and flexibility while minimizing life cycle costs. Discrete event simulation is a computer-based modeling technique for complex and dynamic systems where the state of the system changes at discrete points in time and whose inputs may include random variables. DES is used to assess timelines and throughput, and to support operability studies and contingency analyses. It is applicable to any space launch campaign and informs decision-makers of the effects of varying numbers of expensive resources and the impact of off nominal scenarios on measures of performance. In order to develop representative DES models, methods were adopted, exploited, or created to extend traditional uses of DES. The Delphi method was adopted and utilized for task duration estimation. DES software was exploited for probabilistic event variation. A roll-up process was used, which was developed to reuse models and model elements in other less- detailed models. The DES team continues to innovate and expand DES capabilities to address KSCs planning needs.

NASA Constellation Program End-to-End Discrete Event Simulation System Analysis

In support of NASA’s programs, engineers are enhancing their analysis capability of an increasingly complex network of materials, people, and information, which spans from sources on Earth to destinations in space (International Space Station, the Moon, and beyond). NASA is utilizing new decision support tools and techniques to assess operational and cost performance, in addition to supportability and affordability, of the program for many years to come. Discrete event simulation (DES) is one of these tools being used. With this tool a computer model representing the system under study is first developed and verified. The model is then populated with authoritative (planning and historically analogous) data, validated, and executed over a long time span. Once executed, the simulation model generates results for a set of pre-defined metrics that convey the system’s end-to-end (from manufacturing and assembly to ground and launch operations to flight and return mission phases) performance over time. The analysis of a variety of alternative program level scenarios highlights underperforming and overly utilized areas and bottlenecks, which enables the analyst to make decisions on how improvements can be made to the system. The purpose of this paper is to present results of a DES model developed to help the NASA Constellation Program conduct analysis pertaining to the Ares I/Orion vehicle conducting ISS Missions. The model is scoped from first tier suppliers all the way to the space mission and the return of Orion and Ares I’s Solid Rocket Booster, as well as the refurbishment of reusable components. The output of the model includes cycle times, throughputs, launch rates, inventory behavior, delays, maintenance, and rollbacks. The model was developed with a commercial discrete event simulation software application and an easy to use graphical user interface for input and scenario management.