273 stable  laminate slit-tube architecture passively deploy s into an extended beam  when unrestrained to the  other extreme where the boom  passively retract s into a compact  coil. With proper tuning, a hybrid laminate  design can be implemented that is neutrally stable – possessing equal strain energies in the stowed and  deployed configurations . This attribute allow s for the slit-tube to be manipulated between its stowed and  deployed configurations without managing a significant change in strain energy and thus enabling a  simplistic deployer system . The concept was first analyzed by Murphey and Pellegrino [10] and later by  Shultz, Hulse, and Keller [ 11]. Figure 4 shows an example of a fiberglass composite slit -tube that is neutrally  stable  and remains partially coiled without external constraints.  

Although it may seem ideal to tune the laminate design to be neutrally stable , often this can be challenging  to achieve practically while balancing with other competing laminate requirements such as thermal stability  or structural performance. For the case of the ‘DARPA Phoenix  Camera Boom ’ and ‘NASA ARM  Boom ’  deployers, these two designs had the flexibility to enable a neutrally stable boom . This resulted in deployer  systems that contained fewer mechanisms and an overall lower unit cost . In the ‘DARPA Phoenix HIMast   Boom ’ and ‘NASA Cube Sat Magnetometer  Boom ’ cases, this was not possible whereas the laminate design  required added features  to ensure tight packaging on the spool . Without this assistance, the coil diameter  naturally grew to be ~50% larger than what was ultimately desired. For both of these cases, this was  relatively easy to overcome by either adding a small retraction spring inside the spool or in the case of the  ‘DARPA Phoenix  HIMast  Boom ’ system, a chain and slip- clutch coupling to apply a desired and constant  spool preload during retraction.      

Figure 4.  Example of a Neutrally Stable Boom (Glass Fibers)  

Boom Formation Region   As the HSC material is rolled off of the spool, there is a ‘formation’ region where the boom transitions from  a flattened geometry on the spool, to a fully formed cross section that is capable of supporting loads . It is  the responsibility of the deployer to guide the boom through this transition and when enough depth has been established, create a stout boundary support to enable a rigid deployed structure. In practice, it has  been found that the length of this formation region for a neutrally stable boom is approximately 9x the  diameter of the coiled boom or the envelope form factor . Note that this length is substantially s horter than  the factor 8 π analytically predicted 50 years ago by Rimrott for isotropic slit -tube booms, which illustrates a  further advantage of neutrally stable  laminate designs [ 12]. 

In ideal systems, the deployer envelope would fully encompass  this envelope form factor  to ensure that the  boom exits the box with its fully formed cross section  established and to provide adequate structural support  to the base of the boom once deployed. However it has been Roccor’s experience that most practical  spacecraft envelope constraints do not allow for this “luxury .”  For the Roccor deployer family, the portion  of the boom formation region covered by the deployer  varies widely from the ‘ DARPA Phoenix  HIMast   Boom’  system of 2 x to the ‘DARPA Phoenix  Camera  Boom ’ system of 6 x. The optimized deployer length  depends upon the design loads . High axial loads can be sustained with a relatively short deployer length  while bending loads necessitate a relatively long deployer length. In the case of  ‘DARPA Phoenix  HIMast   Boom’  the system design and requirements were able to close with the envelope form factor  of x2 due to  the mission architecture where multiple units would be used in tandem to form a hierarchal structure. For  the cases where the boom deployers are less than x4 in length, additional considerations such as the distal  end attachment  method (see the Distal End  section below) and the boom structural knockdowns must be  considered.