Pogo is a nick-name for a fluid-structure interaction phenomenon in which the vehicle structural modes can interact with the propulsion fluid system modes. The most common approach to preventing pogo stability problems is to use an accumulator to decrease the frequency of the fluid system mode to well below the vehicle structural mode frequency. This paper describes some of the analysis perfomed on a bellows type pogo accumulator designed for the Antares vehicle. One of the challenges of the bellows type pogo accumulator is managing the compliance of the accumulator without allowing the bellows device to ever reach either the compression or extension hardstops. It is desirable to demonstrate that, for all possible combinations of variables, there is no chance of the bellows reaching the hardstops. At one point during this development program, the worst possible combinations of variables demonstrated some risk of reaching the hardstops during the main engine cut off event. To evaluate the likelihood of this, a probabilistic analysis was completed, focused on bellows motion during the engine cut off event. Using a probabilistic approach, it was possible to demonstrate that hardstop contact was an extremely low probability event. However, before the design was complete, new updates to the analysis resulted in more margin using worst case combinations, thus negating the need for the probabilistic approach. The approach documented in this paper remains a useful example of the potential necessity of probabilistic methods, despite not being finally implemented in this design.
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