The International Space Station (ISS) is a research facility now operating in low earth orbit as an orbital scientific platform. Construction on the Station began in 1998 and was completed in 2011. The power required to support the scientific and operation functions of the ISS is provided by arrays of solar panels. Designing a system to transfer signals and power across several rotating joints at high voltage and current in a hard vacuum environment presented complex issues for the ISS designers.
The components of the ISS present difficult design and engineering challenges. Wear debris created by slip rings can cause short circuits and can result in the formation of destructive plasmas and failure of the transfer path. The drag torque of slip ring transfer mechanisms can be undesirably high when large amounts of power are transferred because of the many brushes required. The vacuum environment also requires special lubrication consideration. This lubrication system must also be compatible with the atmosphere of the earth environment.
After developing a high power rotary transfer capability based upon the Roll-Ring® technology in which the listed problems were eliminated, Pete Jacobson led an effort to accommodate the ISS power transfer requirements and to maximize the power transfer efficiency. This configuration accommodated the vacuum environment since the rolling interfaces had minimum debris generation, did not require any lubrication and had inherent torque which was reduced by several orders of magnitude as compared to that of slip rings. An additional benefit toward reliability is the fact that the rolling interconnections in Roll-Ring® configurations do not require any adjustments but instead establish proper preloads and alignments at assembly. This eliminates a source of potential performance inconsistency.
These features and proven performance resulted in a NASA selection of Roll-Rings® for the electrical transfer in all rotational joints between the ISS solar arrays and the central Command Module. The first eight circuit evaluation prototype unit to validate the technology was life tested at the NASA Lewis Research Center(LeRC) in the late 1980’s. The success of these tests and the demonstration of a power transfer efficiency of 99.966 % resulted in a NASA decision to use Roll-Rings on all rotational axes on the Station. This included use in mechanisms which are used for both oscillatory motion and full rotation. The validated electrical transfer efficiency is significantly greater than that achievable with slip rings or a rotary transformer. The exceptionally high transfer efficiency is especially important because of the difficulty transferring loss heat from the station to space.
The power transfer design consisted of the conventional Roll-Ring® configuration but with the integration of an additional design feature which provides for multiple flexures within each circuit in electrical parallel. The basic design concept provides for multiple paths and resultant current density reduction for each circuit. This in turn results in the effective increased circuit transfer efficiency and minimized temperature rise while transferring power within the ISS. The prototype eight circuit test unit completed 800,000 revolutions in hard vacuum with minimal wear evidence in post-test tear down. These tests included DC currents of 100 amps and 20 KHz AC power levels at 440 volts.
Roll-Rings® have been integrated into three power and signal transfer mechanisms on the ISS under several NASA contracts; two Utility Transfer Assemblies (UTA) one in each SARJ, eight Bearing Motor Roll Ring Modules (BMRRM) in the BGAs for the solar arrays and six Power and Data Transfer Assembly (PDTA) for the thermal radiators. A total of sixteen Roll-Ring® devices serve the International Space Station.
Components of the three Roll-Ring® mechanisms went to space over intervals between November, 2000 and March, 2009 and have been operational starting January, 2001.
The primary unit is the Utility Transfer Assembly (UTA) used on the station Alpha axis, one for each SARJ, which provided twenty-four high current paths and thirty-six HF data and video circuits. The power transfer section was a flight quality version of the prototype design tested at the LeRC. The signal circuits were provided by four nine-circuit Roll Ring modules. Two UTA modules are required for the ISS. The first UTA was launched in Sep, 2006 and the second on June, 2007.
The ISS Beta Gimbal Assembly (BGA) consists of a mast canister which houses the Solar Array Wings (SAW) and the Bearing, Motor and Roll Ring Module (BMRRM) which is used to rotate the solar arrays and distribute the DC power. The qualified power level is 35KW per BGA. There are eight BGA’s and eight SAW’s. The BMRRM is used to transfer power from the solar arrays to the Alpha Axis.
The Beta Gimbal Roll Ring Sub Assembly consists of five high current / high voltage circuits and twelve data and video circuits. The data circuits were configured with two modules similar to those used in the UTA. Eight Beta joints have been launched to space for the ISS. The first two modules went to space Nov, 2000 and have been operational since Nov, 2001. The next two were launched in Sep, 2006, the next two in June, 2007 and the last two in March, 2009.
A third module design used in the ISS is the Power and Data Transfer Assembly (PDTA) used to dissipate heat to deep space. This unit uses two Roll Ring modules which provide for sixteen circuits of low level power and for data and video transfer.