relative to each other, and carrying the vibration loads. These also incorporate ‘kick’ springs to aid in their  deployment, and to alleviate any possible “stiction” from the alignment blocks.  The boom is not retractable once deployed. Re-stowing is achieved by removal from the spacecraft, and  hand retraction of each set of pins followed by each tube segment being (de) telescoped; after which the  Stacer is compressed into its canister; and the harness and flyweight brake are rewound. Finally, the  SMAR is reset reinstalled, and preload is set.  Verification  The Impact Boom’s qualification activities were based on GEVS SE, as modified by JHU-APL for mission  specific needs. The test regime selected for the Stereo mission was Protoflight, meaning new (unflown or  non-heritage) hardware is tested with a combination of prototype (EM) levels (i.e., temperature or  vibration) with flight (FM) durations. This method is typically used to shorten development times by  eliminating the engineering / qualification model fabrication and test  period. However, the Impact Team did build up an engineering model,  and tested all 3 assemblies to the protoflight levels. The main changes  and additions pertinent to this paper: Level 300 cleanliness, UV + Visible  light inspection, no silicones used for fabrication, and testing for silicone  residuals. Vibration levels were taken from the Delta II user’s manual*  modified by APL analysis for the ‘stacked’ configuration. Stringent  EMVEMC levels were levied, due to the extremely sensitive radio  receiver and magnetometer on board. r  Testing procedures were standard NASA mission fare. The main tasks to  be performed for this application were: demonstration of sufficient force  (torque) margin for extension of telescoping sections throughout the  Boom’s stroke; thermal design validation at 25K (discussed previously);  and thermal vacuum cycling and deployment verification at hot and cold  operational temperatures.  As the team worked on the testing it became clear that Stacer thrust  force is not easy to measure accurately or repeatably. The deployment of  a Stacer is a ‘stick - slip’ affair: and once stopped, it sticks, then when  released slips, giving a wide range of force values due to the hysteresis  built into the inter coil friction. For consistency, the force value used at  any point was the force needed to start the Stacer being pushed back  into the canister, after overcoming the ‘stiction’. This does not accurately  convey the sliding force, but is as close as can be statically measured.  Attempts to measure Stacer force dynamically were fruitless. Another  difficulty lay in measuring the drag from rollers and harness. Each tube  has a 1.1 -m stroke, and pulling steadily for that distance vertically while  monitoring force is a challenge. The weight of the tube assembly was  subtracted from the pull out force, giving the drag value.  Finally it was seen that proving force margin analytically was not  conclusive as the uncertainties in each measurement, when combined,  exceeded the margin requirements. A different path was chosen: show  that the boom deploys while using 113 of the available Stacer force. By definition, there is sufficient  margin. This is how the boom was verified. Figure 15. Thermal  Vacuum Chamber  Delta II Payload Planner’s Guide, The Boeing Company, Huntington Beach, CA 92647 8  59