Development Program Goals and Objectives  Since gas struts have not been used for separation on a manned vehicle, development testing is needed  to mitigate risk. The risk falls into three categories; performance related risk, reliability related risk, and programmatic risk. Programmatic risk is in some ways a sub-set of the stated technical risk because technical issues that arise in the strut development program could threaten the schedule for the launch of Ares I flight tests. This concern is one of the chief objections to this technology. A realistic approach to address this programmatic risk is to carry both BDMs and struts in the program until struts have demonstrated their capability. The struts are a bolt-on technology, using the existing hole patterns on the upper part of the Ares I interstage attach ring and a direct bolt through on the upper stage aft skirt, so they can be installed with little impact on other systems. The recurring cost of the struts will not likely increase because of the development program. Because of development testing, the qualification program cost for a strut separation system will be substantially reduced. Programmatic-risks are addressed in this paper by eliminating technical risk throug h a robust development test program.  

Resolving Performance Related Risk   The metering function of the MAG Strut system is determined by the size and pattern of holes along the metering rod. Development testing is required to characterize the strut performance with different  metering rods under different co nditions that simulate nominal operations and potential failures.  Mathematical models provide solid indications of the flow rates for struts with various metering rods; however, their accuracy is not good enough to use for qualification by analysis. The development testing would provide data that would validate the analytical flow models. The best way to establish the force vs. distance performance characteristics of the struts is to test them with several different metering rods moving different masses. A range of pressures could also be investigated to establish the performance characteristics of the struts under nominal and degraded performance scenarios. A relatively simple test set-up as shown in Figure 16 is required to perform the development testing. In this performance test, a mass of approximately 22,680 kg is released to be pushed by the strut. It will accelerate to approximately 6.17 meters per second and then disengage from the fitting mounted on the mass. After disengagement, the moving mass must be stopped by a snubber. Side forces acting against the fitting will be simulated by attaching a spring to the mass that ap plies a side force as it rolls down the track on its metal wheels.  High-speed video recording will measure any twang or motion oscillations.                           

Figure 16 – MAG Strut Performance Development Test Set-up    The development program would seek to characterize the performance of the struts for several separate  side force profiles that would represent a range of operational possibilities and off nominal load cases. The strut has Teflon slides on the piston and in the rod housing. If sufficient side force was present, a strut that was pressurized to less than 10% of the design pressure may bind at some point during the stroke of the strut. The mating conical interface of the rod fitting and the spike fitting on the upper stage is intended to gradually relieve side force as the struts disengage. If binding occurred on a partially charged  strut, this side load relief action is intended to preclude disengagement of the strut from the fitting while pressurized. Figure 17 shows the strut rod fitting and the spike fitting that is mounted to the upper stage. Because no failure scenario has been identified that indicates that binding is a problem, development  341