Miniature Orbital Dexterous Servicing System Dr. David L. Akin*, Katherine M. McBryan*, Nicholas M. Limparis*, Christopher J. Carlsen*, Kevin P. Davis* *Space Systems Lab, University of Maryland, United States of America e-mail: [email protected], [email protected], [email protected] [email protected], [email protected] Abstract cuit board replacement using specialized tools for the crew in extravehicular activity (EVA). [2] Current concepts for robotic satellite servicing use robotic systems similar in size to candidate client space- Sullivan [3] divided satellite servicing tasks into five craft, requiring similar launch vehicles and roughly equal generalized categories: orbital correction, deployment as- costs. To achieve economic viability, proposed systems sistance, component repair, consumable resupply, and re- require multiple clients on each mission, restricting ap- moval/disposal. Studying all spacecraft failures from plications to geostationary orbit to ensure an adequate 1984 to 2003, he concluded that there are annually an number of potential clients and limiting each system to average of 4.4 component level failures, 0.3 deployment one repetitive common task such as refueling. This pa- failures, and 3.8 more complex failures. In addition, an per investigates the potential to create a new class of dex- average of one GEO satellite per year is deployed at an terous servicing vehicles based on small satellite tech- incorrect orbit and is declared a complete loss and another nologies, including recent developments in highly capa- thirteen satellites must use up station-keeping fuel in or- ble lightweight dexterous manipulators. A notional de- der to relocate from GEO to a superstationary retirement sign for a Miniature Orbital Dexterous Servicing System orbit. This reduces the lifetime, and profit, of these satel- (MODSS) vehicle was developed and applied to a can- lites. didate servicing opportunity from recent flight history. While HST, as the “existence proof” of on-orbit re- While much work remains to be done, all indications are pair and upgrade, was serviced by astronauts, robotic that a MODSS system offers a realistic potential for eco- systems have demonstrated in ground-based simulations nomically viable missions to a single client, allowing in- and on-orbit experiments that they are capable of a sim- dividualized logistics supply and launch-on-need to any ilar level of dexterity upon need. The Space Systems required orbit. Laboratory (SSL) performed the first robotics servicing demonstration on HST in neutral buoyancy in 1987, and 1 Introduction performed extensive servicing both robotically and via EVA/robotic collaboration in 1989. Ranger, a more capa- On-Orbit satellite servicing is rapidly becoming a ble robotic system developed by the SSL, used HST as the common phrase in the space industry. More and more ser- source for its standard canonical servicing tasks through- vicing experiments and missions are being planned and out the 1990’s. Developed under NASA funding as a low- executed, not merely as technology demonstrations, but cost flight demonstration of on-orbit dexterous robotics because satellite servicing has demonstrated that it has the for servicing applications, the four-manipulator Ranger potential to be financially profitable for both the client and system was the first U.S.-built dexterous robot to pass servicer. Nevertheless, it is a common belief that satellites NASA payload safety reviews for both flight and opera- must be specially designed in order to be serviced. Past tion on the Space Shuttle. [4] After the loss of Columbia, studies indicated that the cost of making a satellite ser- NASA Goddard Space Flight Center was directed to de- viceable would represent a 5-10% weight increase, in ad- velop the Hubble Robotic Servicing and Deorbit Mis- dition to increasing developmental and recurring costs. [1] sion (HRSDM). While mission manager conservatism led However, as experience with satellite servicing has grown, them to pass over the flight-qualifed Ranger system in fa- results indicate that satellites do not need special, expen- vor of the Canadian Special Purpose Dexterous Manipu- sive, hardware on board in order to be serviced. For ex- lator (SPDM) system, Ranger was modified by the Uni- ample, the Hubble Space Telescope (HST) was designed versity of Maryland to replicate the kinematics, configu- to be serviced only at the orbital replacement unit (ORU) ration, and end effector dexterity of SPDM to perform in- level; over the course of multiple servicing missions, re- dependent validation and verification in neutral buoyancy pair activities became more ambitious, even down to cir- simulations. Over the existence of the HRSDM mission, cially since insurance frequently covers the lost revenue due to early system failures. To make a viable business case, many satellite servicing architectures are predicated on a requirement of servicing multiple clients, to allow the cost per client to be reduced by spreading the large development and launch costs between multiple planned missions. This in turn leads to business plans aimed at “low-hanging fruit”, such as specializing in orbital modi- fication or refueling. In order to service multiple satellites, the clients must have similar orbits and orbital planes to allow for ma- neuvering between orbits without exorbitant costs. For this reason, most servicing plans default to specializing in satellites in geostationary orbit (GEO). [3] Even large Figure 1. Ranger in the same configura- constellations of low Earth orbital satellites such as Irid- tion as SPDM/Dextre servicing a HST ium have assets in multiple orbital planes, with no feasible mockup ability to transfer a servicer between the planes. Satellites in GEO are by definition in a single orbital plane at 0o in- clination, allowing transfers between widely separated or- bital latitudes as long as time is not a critical issue. Other Ranger performed almost all HST servicing tasks (Fig- than single-digits of satellites in a single orbital plane of ure 1) eventually performed on-orbit by astronauts after a constellation, GEO is the only orbital destination with a a new NASA administrator canceled the robotic program large number of satellites at a single orbit, differing only in favor of another EVA servicing mission on the shuttle. in longitudinal positions. Also, the vast majority of com- Since that time, the DARPA Front-end Robotics Enabling mercial revenue comes from GEO satellites, making them Near-term Demonstrator (FREND) project has performed better targets for commercial servicing. Current commer- ground demonstration of autonomous grappling of a Mar- cial servicing entities, therefore, are focused on dedicated man band and bolt holes. [2] Common interfaces, such single-task missions (e.g., refueling) along the geostation- as Marman bands, allow servicers to grapple to provide ary arc. assistance without requiring the client to have specially With an eye to relieving the inherent limitations designed system. The NASA Goddard Robotic Refueling in “large” servicing systems, in recent years the SSL Mission (RRM) experiments on the International Space has been focusing on robotic and spacecraft technology Station have demonstrated the ability to perform com- miniaturization, with the goal of validating a feasible ap- plex servicing tasks such as refueling of satellites through proach to on-orbit robotic servicing based on a small satel- ground-service ports, even with the limited dexterity of lite bus, and taking advantage of the rapid development the SPDM. of cubesat technologies. The Miniature Orbital Dexter- When astronauts are not available, dexterous manip- ous Servicing System (MODSS) is a “smallsat”-based ulators are necessary to repair the majority of spacecraft robotic system concept capable of simple modular re- failures. [5] Typical servicing configurations utilize three configuration, creating wide applicability to human and manipulators, to allow one to grasp the client while still robotic space missions. MODSS is significantly smaller having two manipulators to perform servicing tasks, ei- and lighter than FREND and other large servicing sys- ther as a two-handed coordinated task or alternating be- tems currently under development. As a result, both de- tween different end effectors for specific tasks. Generally, velopment and launch costs are significantly reduced, to this approach assumes the “grappling” manipulator must enable economic viability for a single dedicated servicing be large enough to reach all areas of the client’s surface target. This will enable a number of other supporting ca- which represent potential workspaces while remaining at- pabilities, such as launch-on-need and the ability to tailor tached to a singular grapple. Dexterous manipulators tend robotics configuration and logistics of replacement parts to be be sized appropriately as well, particularly needing to each individual servicing client. to grow longer as the servicer bus increases to avoid po- tential contact issues. As the size of the servicing vehicle approaches the size of the client satellites, they require the 2 MODSS Overview same launch vehicles, at the same cost, as a replacement satellite. Given the general advance in satellite capabili- MODSS consists of a base spacecraft, which pro- ties with the advancement in technology, the owner gen- vides command and data handling, electrical power, atti- erally opts for a new satellite rather than a repair, espe- tude determination, propulsion,