AlAA Space 2003 Conference and Exposition AIAA2003-6370 Long Beach,CA,Sept.23-25,2003. UNDERSTANDING THE ORBITAL TRANSFER VEHICLE TRADE SPACE Hugh L.McManus Metis Design,46 Second St.,Cambridge,MA 02140 and Todd E.Schuman Massachusetts Institute of Technology,Cambridge MA 02139 ABSTRACT INTRODUCTION This study uses new methods to explore the theoretical An orbital transfer vehicle,or"space tug,"is one performance of over a hundred possible orbital transfer instance of a broad class of vehicles that can perform a vehicle designs.The designs have varying propulsion variety of on-orbit servicing functions.The simplest types,fuel mass fractions,and grappling/observation function of such a vehicle would be to observe space equipment capabilities.Simple sizing rules are used to assets,hereafter referred to as targets,in situ.The calculate the performance of the designs and their targets may be cooperative (designed for servicing) utility to several types of users.Designs of interest are partially cooperative(e.g.maneuverable in ways further explored using Integrated Concurrent helpful to the tug),uncooperative (inert),or even Engineering techniques,resulting in complete hostile.The later case covers spinning or tumbling conceptual designs.The results give an understanding vehicles that would be hazardous to approach.A tug of the trade-space for such vehicles,including changes the orbits of these targets for operational sensitivities to both design variables and assumed user reasons(e.g.life extension),to retrieve the targets needs.This clarifies some of the challenges involved bringing them out of orbit or to other assets(e.g.Shuttle such as physical constraints and sensitivities to or ISS),or to eliminate debris.Similar vehicles may uncertain user preferences.Several potentially viable interact or service targets in a variety of other ways. designs are identified including an electric-propulsion The ability to interact with objects in space is a high delta-V vehicle dubbed the Electric Cruiser,and a desirable capability,but clearly the range of possible class of lower delta-V vehicles dubbed Tenders which approaches is large,and it has proven difficult to design are studied in a companion paper viable tug systems. NOMENCLATURE The concept of tug vehicles goes back to the early years of the space program.A literature review is included in C Cost ($ a companion paper.'Here,we will only note that Cd Dry mass cost coefficient($/kg) hypothetical tugs designed for a single mission rarely Cw Wet mass cost coefficient(S/kg) show an economic pay-off,although there is some Isp Specific impulse (sec) evidence that if an infrastructure for on-orbit service Acceleration due to gravity (9.8 m/sec could be created it would have positive value.The Bus mass(kg) concept in practice is made difficult by unfriendly mbf Bus mass fraction coefficient orbital dynamics (many desired maneuvers are M Mass of observation/manipulator system (kg) extremely energy-intensive),environments (the vehicle M Dry mass (kg) must be radiation hard,and/or hard against some level M Fuel mass (kg) of debris damage,to last useful lifetimes in many Mp Mass of propulsion system(kg) orbits).and economics (markets are uncertain,and mipo Propulsion system base mass(kg) payoff is difficult to prove).Some missions require Propulsion system mass fraction coefficient nuclear or other advanced propulsion systems,and most M Wet mass (kg) require advances in control systems and docking or Total utility grappling hardware. Single attribute utility for capability Single attribute utility for response time In this work,new space system architecture and Single attribute utility for delta-V conceptual design techniques have been applied to the Utility weighting for capability tug problem.A capability referred to as Multi-Attribute Utility weighting for response time Tradespace Exploration (MATE)with Concurrent W Utility weighting for delta-V Engineering(MATE-CON)was used.MATE is a Change in velocity (m/sec) method for examining many design concepts to Senior Special Projects Engineer,Associate Fellow AlAA Graduate Research Assistant.Department of Aeronautics and Astronautics Copyright 2003 by Hugh L McManus.Published by the American Institute of Aeronautics and Astronautics,Inc.with permission. American Institute of Aeronautics and AstronauticsAIAA Space 2003 Conference and Exposition Long Beach, CA, Sept. 23-25, 2003. AIAA 2003-6370 1 American Institute of Aeronautics and Astronautics UNDERSTANDING THE ORBITAL TRANSFER VEHICLE TRADE SPACE Hugh L. McManus* Metis Design, 46 Second St., Cambridge, MA 02140 and Todd E. Schuman† Massachusetts Institute of Technology, Cambridge MA 02139 * Senior Special Projects Engineer, Associate Fellow AIAA † Graduate Research Assistant, Department of Aeronautics and Astronautics Copyright © 2003 by Hugh L. McManus. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. ABSTRACT This study uses new methods to explore the theoretical performance of over a hundred possible orbital transfer vehicle designs. The designs have varying propulsion types, fuel mass fractions, and grappling/observation equipment capabilities. Simple sizing rules are used to calculate the performance of the designs and their utility to several types of users. Designs of interest are further explored using Integrated Concurrent Engineering techniques, resulting in complete conceptual designs. The results give an understanding of the trade-space for such vehicles, including sensitivities to both design variables and assumed user needs. This clarifies some of the challenges involved such as physical constraints and sensitivities to uncertain user preferences. Several potentially viable designs are identified including an electric-propulsion high delta-V vehicle dubbed the Electric Cruiser, and a class of lower delta-V vehicles dubbed Tenders which are studied in a companion paper. NOMENCLATURE C Cost ($) cd Dry mass cost coefficient ($/kg) cw Wet mass cost coefficient ($/kg) Isp Specific impulse (sec) g Acceleration due to gravity (9.8 m/sec2 ) Mb Bus mass (kg) mbf Bus mass fraction coefficient Mc Mass of observation/manipulator system (kg) Md Dry mass (kg) Mf Fuel mass (kg) Mp Mass of propulsion system (kg) mp0 Propulsion system base mass (kg) mpf Propulsion system mass fraction coefficient Mw Wet mass (kg) Utot Total utility Vc Single attribute utility for capability Vt Single attribute utility for response time Vv Single attribute utility for delta-V Wc Utility weighting for capability Wt Utility weighting for response time Wv Utility weighting for delta-V Dv Change in velocity (m/sec) INTRODUCTION An orbital transfer vehicle, or “space tug,” is one instance of a broad class of vehicles that can perform a variety of on-orbit servicing functions. The simplest function of such a vehicle would be to observe space assets, hereafter referred to as targets, in situ. The targets may be cooperative (designed for servicing), partially cooperative (e.g. maneuverable in ways helpful to the tug), uncooperative (inert), or even hostile. The later case covers spinning or tumbling vehicles that would be hazardous to approach. A tug changes the orbits of these targets for operational reasons (e.g. life extension), to retrieve the targets, bringing them out of orbit or to other assets (e.g. Shuttle or ISS), or to eliminate debris. Similar vehicles may interact or service targets in a variety of other ways. The ability to interact with objects in space is a desirable capability, but clearly the range of possible approaches is large, and it has proven difficult to design viable tug systems. The concept of tug vehicles goes back to the early years of the space program. A literature review is included in a companion paper.1 Here, we will only note that hypothetical tugs designed for a single mission rarely show an economic pay-off, although there is some evidence that if an infrastructure for on-orbit service could be created it would have positive value.2 The concept in practice is made difficult by unfriendly orbital dynamics (many desired maneuvers are extremely energy-intensive), environments (the vehicle must be radiation hard, and/or hard against some level of debris damage, to last useful lifetimes in many orbits), and economics (markets are uncertain, and payoff is difficult to prove). Some missions require nuclear or other advanced propulsion systems, and most require advances in control systems and docking or grappling hardware. In this work, new space system architecture and conceptual design techniques have been applied to the tug problem. A capability referred to as Multi-Attribute Tradespace Exploration (MATE) with Concurrent Engineering (MATE-CON) was used. MATE is a method for examining many design concepts to