The STR system's task is to offer a strong structure and mechanical systems for the satellite. All other systems and the payload are attached to the structure. The weight of a spacecraft structure should be as small as possible, but the structure should be still strong enough against forces to the satellite in the launch phase and in the zero gravity environment. In the building of ESTCube-1 subsystem the Cubesat Design Specification is followed. The standard gives dimensions of the satellite and its weight. Material selection is based also on the Cubesat Design Specification and the materials used should be meant for space use. The main task will be to keep the mechanical systems as simple as possible, so that they would later work in space and not fail.

Requirements
The structure of the Cubesat must be strong enough to survive maximum loading need in the testing requirements and cumulative loading of all required tests and the launch. The Cubesat structure must be compatible with the P-POD. Here are listed the requirements for STR from the Cubesat Design Specification and the ESTCube-1 mission:

• Structure has to be as light as possible
• Structure has to be strong against launch loads
• Provide a radiation shielding for inner parts
• Provide interface for other subsystems and payload hardware
• Provide housing for the payload with tether
• Fulfill deployer and launcher requirements

Launch loads

The highest loads to the satellite structure will appear during the launch phase, when it is in the P-Pod. Important for the structure is that the natural frequency of satellite shall be higher than the natural frequency of the launch vehicle. Another danger is random vibration, which comes from engines and other sources like noise. It can damage thin parts like the side- and solar panels. The launch loads are given by the lauch providers. For example, the European Space Agency’s VEGA launcher specifies:

• Maximum axial Quasi Static g-Load: 5,0
• Integral level of sound pressure: 138,5 dB

According to the VEGA user's manual the fundamental frequencies must be

• In the lateral axis 15 Hz for spacecraft mass up to 2500 kg
• In longitudinal axis: 20 Hz - 45 Hz for spacecraft mass up to 2500 kg

Material

In the Cubesat Design Specification there are two types of aluminum suggested for the main structure. The types are AW 6061-T6 and AW 7075. If other materials are used, the thermal expansion must be similar to that of Aluminum 7075-T73 (P-POD material) and approved by Cal Poly launch personnel. For ESTCube-1 satellite, one of the suggested aluminums will be used because they are already space qualified.

Selection criteria for the satellite material are:

• Specific strength
• Specific stiffness
• Stress corrosion resistance
• Fracture and fatigue resistance
• Thermal expansion coefficient and conductivity
• Ease of manufacture

Layout proposal

The satellite body consists of the main frame and side panels that are attached to it. For the structure of the main frame there are two possible solutions. One solution is that the frame contains six parts (rails and beams, see explanation below), the other solution would be to manufacture the whole frame out of a solid block of aluminum. The first option would be cheaper, but the one piece frame would be stronger and a little bit lighter, because there would not be any connections. Another option for ESTCube-1 is a monoblock frame. The structure is lighter and stronger against vibration and forces from outside.