Any type of unforeseen event encountered in space operation can be critical. The objectives for this project will be to eliminate and/or reduce these risks by offering an automated method for detection of both oxygen outgassing and thermal shielding damage/defects on the exterior of a given spacecraft. In order to accomplish these tasks, a free flying robot that has the prerequisite specifications will be proposed. The probe will be required to locate itself and control its position around the spacecraft – while performing collision avoidance with objects in and around its flight path. In addition, the flyer will need the capability of detecting, locating, and analyzing possible hull damage and sending the processed information back to the target craft. The level of autonomy should be determined so that it will not interfere with other near-space operations, including the operation of the spacecraft itself. One of the important problems for the International Space Station and other space vehicles is locating the source of leakages of air and other such precious resources. Determining the mere existence of an air or other such leakage is not the primary objective of the flyer – rather the autonomous detection and location of such a leak should be considered. In this project, several possibilities for approaching the leak location problem will be discussed, as well as methods for the repair of such damage. Locating a leakage or other such physical abnormality is done through the use of sensing instrumentation to electronically recognize and quantify the physical process under consideration. Two possible free flyer configurations are offered to meet these sensing requirements. A one-probe sensor will be considered as a possible design modification to the Mini AERCam robot being developed by National Aeronautics and Space Administration Johnson Space Center. This design would serve as a modular upgrade to the AERCam’s current systems, utilizing existing technology to provide a source of external mobility to the sensor array. All leak detection functionality would be left entirely to the upgraded sensing module, allowing for as little modification as possible to the current AERCam’s specifications. The second configuration is that of a sender – receiver pair of survey robots operating in tandem formation about the exterior of the spacecraft. By using two probes instead of one, the number of different applicable sensor arrangements can be extended to allow for additional, more specialized tasks. Although there are several technical issues that must be addressed in the design of autonomous, formation flying craft, it is hoped that this arrangement will allow for a more time efficient assessment of the exterior of the target spacecraft. Either of these two possible probe configurations will attempt to employ and integrate existing technology in their design in order to minimize development costs, as well as production lead times.
The other relevant design point of this project is the detection of a surface defect and/or crack on the exterior hull of a given spacecraft. The capacity for performing such a task will necessarily require a different set of sensing and analysis algorithms than were employed for outgas detection. In order to minimize astronaut EVA activity, the probe will be required to perform hull inspections with little or no assistance from a human operator. Several possibilities currently exist as to the actual sensing method utilized by the flyer to detect regions of probable surface damage. Visible light sensors provide the most promising possibility. By cross checking current structural features with an internal model of acceptable variations in hull composition, the probe will be able to identify and isolate any hull regions were damage currently exists and/or exhibits symptoms of developing undesired structural irregularities. Several key issues must be addressed with regard to virtual standard the probe employs in the surface scans. The environment local to the ship’s hull is expected not only to be a function of time as the mission progresses, but also a function of position at any given survey time. As a result, a dynamic model or standard must be developed as an internal source of sensor calibration. The sensor should also operate in such a fashion that possible damage locations are not overlooked and that false/anomalous identifications of structural defects should be minimized. The level of autonomy for the flyer should be such that it can accomplish its mission objectives with as little human intervention as possible. The required probe motion will be defined by the limitations imposed by the specific sensors and ancillary instrumentation chosen for this particular application. Each of these key issues will be addressed in the design process.
One of the primary goals of this project is to design a robot that carries out the required tasks with complete autonomy, but achieving this level of functionality is an ambitious challenge, given the current state of robotic technology. Thus, in this project, a simplified robot model will be considered – and in some cases human intervention will be assumed to assist the flyer in the gathering, analysis, and transmission of desires information. Clearly, for the robot to operate, a rudimentary operating system which connects these processes smoothly and quickly must be considered. Regardless of the configuration of the flyer itself (i.e., single, double, or multiple drones), the possibility of accidental collision with the target spacecraft exists at all times. This situation must be avoided at all costs, and will remain a major focus throughout the design phase of this project. Several current, well-tested technologies exist that may prove to be amicable to modification for use in space – for instance the Traffic Alert Collision Avoidance System (TCAS) deployed for civil and military aviation applications. In addition to collision avoidance during nominal maneuvering, instances of component malfunction should also be taken into account. Failure of the propulsion system, sensor arrays, or the flyer’s internal programming could render the probe a potential hazard to all surrounding spacecraft. Each of these potential scenarios should be considered, and an appropriate backup system designed accordingly. Complete flyer failure is also a relevant consideration, and will be addressed during the design phase.
The addition of autonomous path planning and collision avoidance capability to an existing robot configuration will also be considered. For example, NASA’s AERCam robot is designed to fly around the space shuttle, while performing automatic attitude and motion control. Augmenting this system with the capability of identifying, planning, executing, and maintaining a desired trajectory will significantly increase its present utility. This type of autonomous maneuvering will require the system to estimate the location of the robot with respect to some pre-determined reference point and then perform the appropriate attitude and thrusting maneuvering necessary for it to hold the required course to the objective. Based upon the previous discussion, the preliminary design objectives have been narrowed to the design of a partially autonomous robotic flyer, with the explicit task of orbiting a given spacecraft in order to locate regions of structural damage and/or outgassing of stored oxygen. In order to achieve the minimum development time and cost, primary consideration will be given to the modification of existing robotic technologies.
The design objective will be broken down to several, smaller blocks to better facilitate the analysis of the complete system. The major sub-modules have been identified as:
Whenever possible, existing technologies will be employed in the design of all system components – with the intent of developing a modular final system design. The team will pick up some of the existing technologies that seem suitable for these four applications and analyze them. If necessary, the development of new technologies to meet the required design objectives will be considered. The final robot design is to be subjected to conceptual testing in order to determine if the desired design objectives were met. If possible, selected components of the flyer system will be constructed and/or modeled in order to test the functionality of the design.