ISSUES TO BE SOLVED DESIGNING A CUBESAT
In this article we present an extract of the thesis work of our colleague just graduated Emanuele Sartorini, who deals with any problems that may occur in the design phase of a Cubesat.

Cubesat Standard
Together with the design and implementation of the first micro and nano-satellites, the first design standards were born in terms of weight and size. The most famous and used is the Cubesat 1U standard. It defines the physical dimensions in terms of weight and volume of a nanosatellite.

Fig. 1: Various CubeSat Layouts

A cubesat unit must have a 10 cm cubic-shaped factor, thus corresponding to a volume of 1 dm3, and a mass not exceeding 1.33 kg.
Combining several single units, as in Figure 1, you can increase the size, space for internal systems and maximum weight to get Cubesat 3U, 6U or 12U just like for Lego bricks. Each unit will be filled with different subsystems that we will call payloads whose characteristics depend a lot on the type of mission for which the Cubesat has been designed.

In general, however, you can find subsystems common to all nano-satellites: solar panels, antennas, on-board computer (OBC), trim control (ADCS), power management unit (EPS), radio module (TRX) and one or more secondary payloads. The various payloads and communication buses will be resumed and deepened later

Mechanical Design
During the structural design of the cubesat the choice of materials to be used is critical as it must consider the phenomena related to the extreme environment (Space) in which the same will operate. The environmental variables that characterize the space are mainly: the absence of atmosphere and the presence of extreme temperature ranges with temperatures that in low orbit (LEO, Low Earth Orbit) can go from -170 C to 123 C. The latter creates the problem of thermal expansion of the material and hinders or excessively favors of the heat dissipation generated by the internal systems of the cubesat.

Fig. 2: 3D CAD model of a main structure of a Cubesat 3U

Thermal expansion
Thermal expansion is a physical phenomenon that occurs when a body increases in volume as the temperature increases. This increase is caused by the variation of the oscillation of the atoms around the equilibrium point, which is normally identified with the bond length.

During the design of structures, electronic boards and antennas which are generally of heterogeneous multilayer type, you will have to consider the coefficient of the thermal expansion of the various materials in contact with each other. In particular with regard to the antennas, in order to avoid separation between the dielectric and the metallic layer, the two must have a very similar thermal expansion coefficient.

Outgassing and Whiskering
Among the problems generated by the almost total absence of air particles and therefore of pressure we find the phenomena of outgassing and whiskering. The first is intrinsic to any material while the second is of some metals. Let’s start with the phenomenon of outgassing which is defined as the ability of a material to release gas previously trapped.
It is a phenomenon not to be underestimated in the design because the atmosphere, which is a gas, trapped within the satellite subsystems, will try to exit due to the difference in pressure between outside (vacuum) and inside (atmospheric pressure) causing damage except in cases where the process is not easy. In Pcbs (Printed Circuit Board), outgassing can lead to damage to soldermask, a thin layer of polymeric material that protects copper tracks.

Wiskering, on the other hand, is a phenomenon for which, in a vacuum condition, thin metal crystals are created that can give rise to short circuits between two close electrical contacts. It is just some of the materials among which there are: zinc, brass and cadmium.

Materials used for the structure
Generally the materials used for the primary structure (Figure 2) are both metallic and non-metallic. Among the metals we often find aluminum that gives strength to the structure without weighing it down too much. For composite materials, however, the topic is much wider, in fact, we are studying always better and innovative materials oriented to use in the additive industry (3D printing). One of the latest findings in the low-orbit space field suitable for 3D printing technology, is a polyamide-based composite material loaded with carbon fibers called Windform® XT 2.0.

Emanuele Sartorini