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With the ever-shrinking military budget constraints facing military and civilian contractors, the ability to extend the operational life of any system for minimal cost compared to a replacement is desirable. This fact has never been more true than in today's space industry. This thesis addresses the possibility of extending satellite life through the use of on-orbit refueling. Through compilation and analysis of satellite operational life span data, it is shown that maneuvering fuel depletion has a significant impact on satellite operations in geosynchronous orbit. If these satellites could be refueled economically this would prove not only cost effective but also improve satellite tactical employment for space support to the warfighter. Through the manipulation of satellite data, launch/design cost, on-orbit refueling vehicle design/construction costs and on-orbit operational requirements, it can be shown that on-orbit refueling can be done cost effectively. Single versus multiple satellite refueling operations were evaluated to determine the concept's viability.
Existing on-orbit refueling systems have steadily increased the mass of propellant they can refuel from Orbital Express' 22 kilograms in 2007 to Tianzhou 2's delivery of 2000 kilograms in 2021, yet refueling rates like the industry-leading Robotic and Refueling Mission's .155 kilograms per second demonstrated in 2012 shows that some technology areas could still hold back legitimate refueling operations. Refueling system designers should consider a few insights from this research to help define architectures that close existing gaps in refueling system performance.
Satellite constellations are an increasingly attractive option for many commercial and military applications. They provide a robust and distributed method of accomplishing the goals of expensive monolithic satellites. Among the many challenges that satellite constellations engender (challenges in control, coordination, disposal, and other areas), refueling is of particular interest because of the many methods one can use to refuel a constellation and the lifetime implications on the satellites. The present work presents a methodology for carrying out peer-to-peer refueling maneuvers within a constellation. Peer-to-peer (P2P) refueling can be of great value both in cases where a satellite unexpectedly consumes more fuel than it was alloted, and as part of a mixed refueling strategy that will include an outside tanker bringing fuel to the constellation. Without considering mixed-refueling, we formulate the peer-to-peer refueling problem as an assignment problem that seeks to guarantee that all satellites will have the fuel they need to be functional until the next refueling, while concurrently minimizing the cost in fuel that the refueling maneuvers entail. The assignment problem is then solved via auctions, which, by virtue of their distributed nature, can easily and effectively be implemented on a constellation without jeopardizing any robustness properties. Taking as a given that the P2P assignment problem has been solved, and that it has produced some matching among fuel deficient and fuel sufficient satellites, we then seek to sequence those prescribed maneuvers in the most effective manner. The idea is that while a constellation can be expected to have some redundancy, enough satellites leaving their assigned orbital slots will eventually make it impossible for the constellation to function. To tackle this problem, we define a wide class of operability conditions, and present three algorithms that intelligently schedule the maneuvers. We then briefly show how combining the matching and scheduling problems yields a complete methodology for organizing P2P satellite refueling operations.
The opportunity to perform on-orbit servicing and refueling of satellites using a refueling tanker has prospectively been previously considered. Although the docking capability for refueling satellites has been demonstrated by industry, effective optimization of a rendezvous sequence between the satellites in need of servicing remains under investigation. This research models the trajectory optimization for a satellite refueling tanker rendezvousing with a series of satellites in geosynchronous-Earth orbit as the traveling salesman problem, and investigates how ant colony optimization paired with the Hill-Clohessy-Wiltshire equations can be used to minimize the total velocity change required to complete orbital transfers between the satellites. The results of this research include a trajectory optimization algorithm for satellite refueling applied in MATLAB, and an analysis of the refueling tanker's optimized trajectory sequence. This research has found that the described methodology provides an effective means for estimating the total delta-v required to complete the rendezvous trajectory between satellites in need of refueling.
On-orbit servicing (OOS) of space systems provides immense benefits by extending their lifetime, by reducing overall cost of space operations, and by adding flexibility to space missions. Refueling is an important aspect of OOS operations. The problem of determining the optimal strategy of refueling multiple satellites in a constellation, by expending minimum fuel during the orbital transfers, is challenging, and requires the solution of a large-scale optimization problem. The conventional notion about a refueling mission is to have a service vehicle visit all fuel-deficient satellites one by one and deliver fuel to them. A recently emerged concept, known as the peer-to-peer (P2P) strategy, is a distributed method of replenishing satellites with fuel. P2P strategy is an integral part of a mixed refueling strategy, in which a service vehicle delivers fuel to part (perhaps half) of the satellites in the constellation, and these satellites, in turn, engage in P2P maneuvers with the remaining satellites. During a P2P maneuver between a fuel-sufficient and a fuel-deficient satellite, one of them performs an orbital transfer to rendezvous with the other, exchanges fuel, and then returns back to its original orbital position. In terms of fuel expended during the refueling process, the mixed strategy outperforms the single service vehicle strategy, particularly with increasing number of satellites in the constellation. This dissertation looks at the problem of P2P refueling problem and proposes new extensions like the Cooperative P2P and Egalitarian P2P strategies. It presents an overview of the methodologies developed to determine the optimal set of orbital transfers required for cooperative and non-cooperative P2P refueling strategies. Results demonstrate that the proposed strategies help in reducing fuel expenditure during the refueling process.
The ever-increasing number of space assets and interplanetary missions is creating a requirement for spacecraft service on-orbit. A large demand for this servicing in space is the resupply of consumables and propellants. The benefits of refueling, or refueling as it will be come to be called, are shown in this paper. A discussion of the issues involved with transporting fluids in zero-gravity is also included. Furthermore, the technology necessary to make spacecraft refueling feasible is demonstrated. The main focus of the paper is on the implementation of a servicing vehicle. The propulsion performance necessary for both a manned and automated servicer is investigated using an existing system design that would be based at the International Space Station. The advantages and disadvantages of each system are discussed. The importance of a high performance propulsion system is shown and recommendations are made on this subject. Overall, the servicing vehicle is shown to be most efficient if it were automated and able to provide many services.
The objective of this work was to develop new methods for high-level decentralized control of multiple space agents (i.e., satellites and spacecraft) with the objective of coordinated action and decision making. The blanket underlying assumption in this work was the sharing of a common resource (information, consumables, fuel, etc) so that all agents satisfy their own needs in a time-critical, cost-effective, optimal fashion. As a specific example of interest to the US Air Force we have addressed the problem of coordinated refueling between several satellites in a constellation. Satellite refueling has the potential to revolutionize future spacecraft operations. Apart from eliminating the need to replace (otherwise perfectly operating) satellites due to depletion of onboard fuel, a satellite constellation with refueling capabilities could easily change orbital planes or even have satellites move in non-Keplerian orbits. As a matter of fact, true formation flying (as opposed to orbiting) of spacecraft requires continuous thruster firing and the subsequent depletion of onboard fuel. Having the capability to continuously change the orbit of the satellites in a completely unpredictable manner will give unprecedented advantages to the US intelligence community.