For the purposes of torque (force) margin analysis, the initial push is 45 N, final thrust 18 N (the lowest  value obtained). This lower value was used to bound the design force available for deployment. The  Torque (force) Ratio (tR) requirement from GEVS SE (Sec. 2.4.5.3) is:  t~ = tavaill trequird 2 3-0  and the Torque (force) Margin (tM) requirement from GEVS is: (Equation 1)  tM = tavail trequired - 1 2.0 (Equation 2)  for systems dominated by resistive torques due to friction. This assumes worst case for the boom, taking  the lowest force for the Stacer and applying it to the entire stroke. There is additional margin as there is a  significant mass at the end of the boom SWEA / STE-D which contributes momentum towards full  deployment stroke. Using the given values it can be shown that the force available, the minimum Stacer  force of 18 N, divided by the force required, the tube drag of 3.1 N yields a torque ratio of 5.8, and a  torque margin of 4.8. The Stacer satisfies the force requirements by analysis. Still, the device must show  functionality to prove that manufacturing has been in accordance with design.  The graph shows the need for a deployment initiator. The stowed Stacer is in a ‘meta-stable’ condition. If  left by itself, it would partially deploy in either direction, therefore a back plate on the canister is required.  To ensure that it deploys a deployment assist device (DAD) is incorporated. The final upturn in the force  curve is an artifact of how the force was measured. The Stacer in this case is 5-m long, and when it is  fully deployed, the coils have tightened onto themselves. The force value was taken at the moment the  Stacer began to slip back into the canister. For this case, the coil needs to be expanded significantly, and  requires greater effort.  There is an additional use for the Stacer after deployment as the secondary  EMVEMC shield. While each of the conductors in the harness is shielded, the  mission’s low noise requirement demanded a second, ‘over-shield’ for all  conductors. Since the harness runs down the center of the Stacer, the Stacer  was tied to ground, and serves this purpose.  Deployment Sequence  Deployment is initiated when a TiNi Aerospace shape memory alloy release  device (SMAR, Model P50-810-1 RS) is triggered causing the restraint pin to  pull out of the tail of the Stacer tip piece. To give the stacer and tube  deployment an initial ‘kick’, a deployment assist device (DAD) is incorporated  between the SMAR mount plate and the 50-mm tube base. The DAD consists  of three long coil springs compressed when stowed, and when released  provide -90 N of push at the very beginning of the stroke. After the first  100 mm of travel, the initial coils of the Stacer are fully formed around the tip  piece, and the flyweight brake has been spun up to speed. At this time the  DAD has completed its stroke. The Stacer is attached to the base of the 50-  mm tube via a swivel, allowing the Stacer to wind down while extending,  recapturing the strain energy stored when the Stacer was wound ‘out’ against  the canister. At the end of the 50-mm tube travel, the six lock pins pop into  their sockets, and transfer the Stacer push force, as well as momentum, to the  90-mm tube, pulling it along until it latches; the process continues with the  130-mm tube and the 170-mm tube, and finally the entire four tube rigid  assembly locks onto the 210-mm tube, which is fixed to the spacecraft. While  the actual sequence follows this description fairly closely, occasionally the  tube drag would cause one or another tube to partially deploy. There is no  provision or requirement for any tube to deploy in any set sequence. To  control the velocity of the tubes during deployment, the flyweight brake is  attached to the 50-mm tube via a lanyard, limiting the speed of deployment to  -0.5 m/s, giving a total deployment time of -10 sec. There are position  alignment blocks for stowed (launch) condition holding the tubes aligned 1. i -.  L-he-  Figure 14. Impact  Boom after a thermal  vacuum deployment  58