KSat Stuttgart e.V.

SSETI Express is a microsatellite launched on 27. October 2005 from Plessezk (Russia). It was the first student designed satellite and the first completed mission of the SSETI project. The project was driven by the desire to enable the student teams of SSETI an active spacecraft. Its main objectives were to demonstrate the students’ capabilities and the maturity of their developed hardware.

SSETI Express

SSETI Express

ncube-2, Norway

UWE-1, University Würzburg

XI-V, University Tokyo

The three CubeSats aboard Express

In 2003, several students participating in SSETI were nearing graduation and risked to give up their projects prematurely. As a consequence, all hardware developed within the scope of SSETI was reviewed in fall 2003. A kick-off-meeting was held in December 2003 to consult what a simple, inexpensive and interesting mission relying on this hardware could look like.

In Phase A (feasibility study) four objectives were defined:

  • Support the students’ education
  • Release of three CubeSats
  • Validation of the subsystems
  • Photographing earth

 Launch

A challenge was to overcome obvious high-tech-problems with the existing low-tech-solutions. , so after defining the mission’s experimental objective, refining the hardware and integrating it to form a functioning satellite, SSETI Express was launched on 27. October 2005 at 08:52 CEST aboard a KOSMOS 3M – less than two years after the initial kick-off-meeting. The launch proceeded uneventfully and at 10:30 CEST the satellite’s first messages were received by the SSETI team. Likewise, the SSETI team established communication with the satellite. Of interest was especially the housekeeping data, which allowed a detailed reconstruction of the satellite’s situation at all times until communication was lost again.
Launch

A reason for the problems encountered was discovered afterwards. To ensure the CubeSats’ safety, Express remained inactive for ca. 65 minutes. During that time, the solar panels were generating power, yet lacked a load. The onboard energy dissipation system failed and overheated, preventing the charging of the batteries after activation. Thus, the batteries slowly discharged and finally left the satellite without power 6.5 hours after launch. Reactivation attempts proved unsuccessful.

The main objective, educating and inspiring students, was accomplished. Two of the three CubeSats were released and activated. Approximately 70 % of the satellites subsystems could be tested. The third CubeSat was possibly released, but did not activate itself. Unfortunately, the malfunction of the power supply unit aborted the mission prematurely and prevented the use of the attitude and reaction control systems developed by the Stuttgart team. SSETI Express is considered a success nevertheless. It was especially applauded for its compact timeframe and the pan-european cooperation.

Our contribution - RCS & ACS

System Overview

The Stuttgart team was responsible for the attitude and reaction control systems (ACS & RCS). This was implemented through eight cold gas thrusters, four for ACS and RCS each. Respectively two of these were positioned on opposing sides of the satellite, enabling precise three axis stabilization. The ACS gas at a pressure of 1.5 bar to create a thrust of 0.13 N each, while the RCS received gas at a pressure of 4.5 bar for a thrust of 0.39 N each. The ACS would be used to attain a certain attitude, while the RCS should counter thrust vector misalignment of an optional main engine. The thrusters have a length of 2 cm and are made of aluminium. They are attached to Express via custom developed “Mounting Device” and “Valve Clamps”. Only the controlling valves have been sourced from medical technology.

Fuel System

As fuel, nitrogen was chosen. It is contained in standard compressed-gas cylinders made from CFRP. This particular model is also in use as firefighter’s equipment and was made by Mannesmann Cylinder Systems (MCS). The extreme conditions of firefighting and the low weight (2.5 kg) made it predetermined for the use in space. Each container contains 6 l of nitrogen, equivalent to 2 kg at 300 bar. Special fasteners for the cylinders were developed to ensure a safe transport. Prior to launch, the containers’ space qualification was validated through vibration tests at the Institut für Statik und Dynamik in Stuttgart and at IABG in Munich. The single container aboard SSETI Express survived the launch undamaged.

Fuel Tank

3D-Model

Fasteners Testing

Tubing

Connecting the gas cylinders with the thrusters was the final task. Steel tubes with .25” and .375” diameter were preferred over titanium tubes for cost reasons. The weight impact on the overall satellite is negligible. The tubes were connected with each other by welding or with adapters (types AN and Swagelok). Welding was performed at the Insitut für Kernenergetik in Stuttgart using electron beam welding. The tubing was fixed with custom fasteners, which are glued onto the satellite structure with Araldite AV938. It is important to supply each thruster with gas of the same pressure. To accomplish this, for all thrusters are similar. For future projects, it is advised to use welding for more joints, as welding leads to 100 % airtight tubes. Alternatively, more extensive use of AN adapters may also result in improvements.

Tubing system overview and details

A Propellant Management System (PMS) was implemented to provide each system with the appropriate pressure, to shut down the system during launch , and to ease (re)fueling. A pyrotechnical valve closes the fuel tank during launch and is activated in orbit. Sensors for pressure and temperature supervise the operation and help regulate the system, to the point of shutting it down in case of abnormal readings. The so-called ACS-Branchvalve closes off the fuel tank when necessary, e. g. during prolonged inactivity. Connections within the PMS and to the tubing system were mostly done with Swagelok adapters, only the low-pressure controller and the ACS-Branchvalve were welded.

Testing the PMS
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