From Single to Formation Flying CubeSats: An Update from the Delft Programme

In the recent two decades, very small satellites, such as Cubesats, are attracting more and more attentions from academia, industries and space agencies due to their low cost, short development cycle and promising capability. Currently, most Cubesat missions are used for technology demonstrations or education, which only explore the capability of an individual satellite. However, the capability of Cubesats can be extremely enhanced by flying a cluster of satellites. For example, several missions such as QB50 and OLFAR have been proposed for this purpose. This paper provides an update of the Delfi programme of the Delft University of Technology (TU Delft). Delfi-C3, the first CubeSat in the Delfi programme, was launched on Arpil 29, 2008 and is still operational after more than five years. Delfi-n3Xt, the second Delfi CubeSat, has been completed in January 2013 and is waiting for launch. The perspective of TU Delft on future small satellites motivated DelFFi, the third Delfi CubeSat mission, which is expected to be launched in 2015 within the QB50 framework and to demonstrate autonomous formation flying using two CubeSats named Delta and Phi. This paper consists of three primary parts. The first part provides an overview of results and lessons learned from the development and the mission implementations of the Delfi-C3 and Delfi-n3Xt satellites, with emphasis on subsystem development, satellite design, Assembly, Integration and Test (AIT) and project management. The second part of the paper presents the differences and improvements from Delfi-C3 and Delfi-n3Xt towards DelFFi. One of the important improvements is an advanced version of the Attitude Determination and Control Subsystem (ADCS) with sensors and actuators for 3-axis control. Delfi-C3 has no active attitude control, and on Delfi-n3Xt 3-axis attitude is only an experiment. While on the two DelFFi satellites, a more mature ADCS will be an integrated part of the platform. Another improvement is on the project management, where an innovative “spiral development” strategy is utilized. Different with the traditional phase-based project management process, in the DelFFi development several design cycles will be implemented and in each cycle improvements are applied to only part of the satellite. This strategy will provide every student with an opportunity of performing end-to-end systems engineering within their limited thesis work period, which is extremely preferred for training purpose. The third part of the paper focuses on the payloads of DelFFi that enable the autonomous formation flying. Here the technology developments are threefold: communicating, which concerns on inter-satellite communication and ranging using an innovative and very miniaturized device; processing, which utilizes multi-agent based artificial intelligence technology for cooperative control of the two CubeSats; and actuating, which performs formation control using a solid cool gas/micro-resistojet combined propulsion system with high volume efficiency and a specific impulse at 150s. Details of these technology developments are addressed