276 ensure smoothness and robustness are paramount when utilizing this method. One consequence of this  design is added parasitic drag however the graphite to hardened steel interface is inherently low -friction . It  has also been found that this system is robust and that the  component receiving the most wear  is the  harde ned steel in which minor <0.001- inch (25- µm) grooves are present after extended use.  

Figure 8.  Edge Roller Design: Pulley Wheel (left), Hard Metallic Stop ( right)  

As a few final no tes regarding the boundary condition restraints , the inclusion of adjustability to add preload  to the edges and select surface rollers is a helpful feature. The variability in the composite fabrication  process during a system development and the need for t uning can cause tedious tuning where ease of   adjustability will help quicken this effort .    In addition, in some cases  one can artificially decrease the need for the x4 or greater envelope factor  (discussed above) by pinching the boom at the forward roller  plane to more closely resemble the fully  formed geometric cross section. This can be done within reason however does add additional strain to the  composite, handling challenges during integratio n and increased parasitic drag. Roccor has successfully  imple mented this practice on the ‘ NASA CubeSat Magnetometer  Boom ’ deployer.  

Drivetrain   Within the Roccor deployer family, the drivetrain  design widely varies due to the requirements imparted for   each application . The inclusion of internal motors vs external drives , the need for slip clutches to enable  boom  synchronization and the ability  for mechanical  pass -through to adjacent boxes  are some examples  the Roccor team has incorporated when considering drivetrain designs . A detaile d discussion of this area  is beyond the scope of this paper however  the topic of the drive wheel design and placement is an important  area to review . Within the Roccor family of boom depl oyers, it was found that  the best method of driving  the boom  for deployment and retraction is via a friction wheel interface placed outside of the spool . Many  existing systems for metallic slit -tubes incorporate a motorized spool  inside the rolled boom  as the primary  drive mechanism . In conjunction with this, numerous rollers and pins  need to be placed around the spool  and boom exit point  to 1) control the strain energy of the coil prior to applying drive forces,  2) counteract   very high forces associated with radial expansion of the rolled boom imparted f rom the motorized spool  and  3) ensure that the flattened boom  does not buckle due to the spool drive forces as it exits the spool . When  upgrading the system to include HSC booms, the need to constrain the spool is no longer  present and  hence placing the drive wheel outside of the spool reduces significant complexity and cost . For all four  deployers outlined in this paper, a simplistic drive wheel with high durometer elastic material has been used  with success as shown in Figure 9. For the case of the ‘NASA ARM  Boom ’ deployer, an axial preload of  >20 lbf (89 N) has been successfully demonstrated with this system .