530 The reflector assembly i s mounted on a single axis rotary actuator with a range of motion of ±45 degree.  This system required two RF channels  for receive and transmit across this rotary joint , and accordingly the  HGA assembly was designed to accommodate two FWG s connect ing the feed assembly to the RF module  inside the spacecraft through the bulkhead waveguide.  The use of the FWG  was explored and validated by  a development program which consisted of detailed analysis  and extensive testing . The structural, thermal  and life cy cle characteristics  of the waveguide were evaluated with specially developed tests and  techniques . This program also experienced a process issue during the flight batch qualification. In  coordination with the  manufacturer, Custom Microwave, the root cause was quickly identified and an  improved process was employed,  resulting in a robust component.  

Trade Study  

The PSP HGA actuator  had a relatively smal l range of motion and a low life cycle requirement of 110 cycles  but was required to fit within a tight mass budget and envelope. A trade study was performed to identify the  most suitable RF transmission technique. The RF requirement precluded the use of a flexible coaxial cable.  The options considered for this trade were a n RF Rotary Joint and an FWG. The waveguide c onsidered for  this application wa s electroformed Ni -Co waveguide. This ty pe of flexible waveguide is highly elastic and is  capable of repeat ed cycling in bending without permanent deformation. The PSP RF system required a  WR-34 waveguide and the flexible length was chosen in consul tation with the vendor to minimize bending  stresses  while limiting launch loads  due to vibration environment. The RF rotary joint is an  electromechanical device consisting of a rotor assembly mounted to the stator through duplex bearing pairs.  The part con sidered for this trade was a standard WR -34 joint with waveguide interfaces  at input and output.  These devices have good RF performance and spaceflight heritage.  

Both options were evaluated for mass, volume, alignment requirement and heritage for the prop osed usage.  

− The mass of each w aveguide was 40  grams . The mass of each RF rotary joint was 300 grams and  the two waveguides were mounted to a bracket of mass 250 grams . The tot al mass of the RF joint  system wa s 850 grams compared to 80 grams for the FWG s.   − The FWG option required a much smaller envelope than the rotary joint assemblies .  Accommodation of two of the rotary joints along with the routing of the waveguides required a larger  envelope. This envelope was  particularly significant for the PSP  mission since the volume available  for HGA accommodation was very limited.  One of the unique features  of the PSP spacecraft is a  Thermal Protection Shield (TPS), which was a sandwich panel made up a ca rbon- carbon  composite facesheets  and a carbon foam core. During close flybys of the Sun, the spacecraft is  oriented with the TPS facing the Sun so as to protect  all spacecraft components within the umbra  of the TPS. This  need to fit within the umbra of the TPS imposed severe restrictions on t he  envelope.   − RF rotary joint s consist of preloaded duplex bearing pairs and require very tight alignment with the  actuator  axis. Any misalignment due to installation or on- orbit thermal deformations would impact  the RF performance of the rotary joint as well as the actuator  mechanical performance. The FWGs   can tolerate higher misalignments relative to the rotary axis.   − The cold operational temper ature of the RF joint is driven  by the bearing lubrication which is  typically about - 40°C  for wet lubricated sys tems . The flexible waveguide can be operated at much  lower temperatures.   − RF rotary joints have been used successfully for thousands of cycles on multiple space missions.  The FWG  considered for this application has extensive flight heritage but  its primary function was  to accommodate misalignments. There are a few instances  of FWGs  used on one-time deployment   mechanisms . However,  there is very li mited information on the use of flexible waveguides for cyclic  operations  in spac e over the life of the mission.