Brazing Tungsten Carbide (WC) to Titanium  To help reduce the overall system mass for the TRIDENT ETU, a titanium drill string was used instead of a  stainless-steel drill string (as was used on RP15). This change introduced a new challenge of bonding the  tungsten carbide (WC) cutter to the titanium drill string. Given the properties of titanium, a vacuum brazing  approach is required. Also non-traditional brazing materials must be utilized. Because this is not a common  process, there are not many vendors that have the capabilities or knowledge to perform this properly. In  this case, a third-party vendor (Titanium-Brazing) was used to perform this work.  A standard titanium-based filler metal TiBraze200  (AWS BTi-5) reinforced by 20% of niobium particles  was used for brazing. Melting temperature of the filler  metal TiBraze200 is 863°C, melting temperature of  niobium is 2477°C. Therefore, niobium powder is not  melted together with the matrix TiBraze200 but is  reinforcing it to improve strength of brazed joints. Also,  the un-melted niobium powder increases viscosity of  the melt to fill out uncontrolled, non-capillary gaps that  may appear after assembling carbide tips with  titanium drill bodies.  Brazing paste was deposited inside the slot in the drill  body, then the carbide tip was inserted into the slot  and compressed. A simple graphite fixture was  manufactured to support assembled parts and fix  them in the vertical position during heating and cooling  in the vacuum furnace. Assembled drill bits with the  deposited brazing paste are shown in Figure 15. The  graphite support with parts to be brazed were  positioned in the vacuum furnace. An alumina ceramic  plate loaded by the "dead weight" ~250 g (0.5 lb) was  placed on top of carbide tips in order to compress  them after melting the brazing filler metal.   Overall, the brazing process was successful at bonding the WC cutters to the Titanium auger. No voids  were found in the brazed joints, but there were rolls of excessive filler metal on the surface of the titanium  bodies. The rolls were formed because brazing paste had to be deposited outside of the gap to provide  enough filler metal. Also, a few of the WC bits were bonded noticeably off-center as they shifted during the  brazing process. Recommendations to improve this process would be to add holes or grooves to the WC  insert to provide more volume for the filler metal to reside between the insert and the Titanium auger. Also,  an improved fixture design is necessary to help keep the insert on center with the auger axis during the  brazing process.  Flexible Printed Circuit Cable Harness  TRIDENT ETU’s cable harnessing system consists of three unique flexible printed circuits (FPCs). The first  circuit carries motor power signals, the second circuit carries motor feedback signals, and the third carries  heater and RTD signals. Each circuit is around 65 in (1.65 m) in length, 2 in (5 cm) in width and less than  0.015 in (1.3 mm) in thickness. The three circuits are stacked and joined together via twelve thru-hole right  angled MIL-DT-83513 Micro-D connectors, which in turn translates to 148 discrete wires with a current  carrying capacity of 3 ampere on each pin.      The FPC assembly approach offers numerous advantages over a discrete wire harness assembly. The  motor power and motor feedback FPCs each have its own integrated crossed-hatched copper shields to Figure 15. Assembled drill bits with deposited  brazing paste in graphite support.  NASA/CP—2018-219887 25