CubeSats provide a cost effective means to perform scientific and technological studies in space. Due to their affordability, CubeSat technologies have been diversely studied and developed by educational institutions, companies and space organizations all over the world. The CubeSat technology that is surveyed in this paper is the propulsion system. A propulsion system is the primary mobility device of a spacecraft and helps with orbit modifications and attitude control. This paper provides an overview of micro-propulsion technologies that have been developed or are currently being developed for CubeSats. Some of the micro-propulsion technologies listed have also flown as secondary propulsion systems on larger spacecraft. Operating principles and key design considerations for each class of propulsion system are outlined. Finally, the performance factors of micro-propulsion systems have been summarized in terms of: first, a comparison of thrust and specific impulse for all propulsion systems; second, a comparison of power and specific impulse, as also thrust-to-power ratio and specific impulse for electric propulsion systems.

In this paper, we study pure peer-to-peer (henceforth abbreviated as P2P) and mixed (combined single-spacecraft and P2P) satellite refueling in circular orbit constellations comprised of multiple satellites. We consider the optimization of two conflicting objectives in the refueling problem and show that the cost function we choose to determine the optimal refueling schedule reflects a reasonable compromise between these two conflicting objectives. In addition, we show that equal time distribution between the forward and return flights for each pair of P2P maneuvers does not necessarily lead to the optimum cost. Based on this idea, we propose a strategy for reducing the cost of P2P maneuvers. This strategy is applied to pure P2P refueling scenarios as well as to mixed refueling scenarios. Furthermore, for the case of a mixed scenario, we propose an asynchronous P2P strategy that also leads to more efficient refueling.

In this paper we revisit the problem of peer-to-peer refueling of a satellite constellation in orbit with propellant. In particular, we propose the egalitarian peer-to-peer refueling strategy that relaxes the restriction on the active satellites to return to their original orbital slots after all fuel exchanges have been completed. We formulate the problem as a minimum cost flow problem in the so-called constellation network, and minimize the total V subject to flow balance constraints, along with certain additional constraints introduced to avoid conflicts between active and passive satellites. Recognizing that the actual objective is to minimize the total fuel expenditure, instead of V, we also propose a method to improve the results by performing a local search around the minimum-V solution. We also provide explicit upper and lower bounds on the suboptimality of the obtained results. With the help of numerical examples, it is shown that the proposed egalitarian peer-to-peer refueling strategy leads to considerable reduction in terms of the total fuel expenditure over the baseline peer-to-peer strategy.

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