Deflection Analysis of the Space Shuttle External Tank Door Drive Mechanism 

Michael A. Tosto*, Bo C. Trieu**, Brent A. Evernden+, Drew J. Hope,   Kenneth A. Wong and Robert E. Lindberg 

Abstract  Upon observing an abnormal closure of the Space Shuttle’s External Tank Doors (ETD), a dynamic  model was created in MSC/ADAMS to conduct deflection analyses of the Door Drive Mechanism (DDM).  For a similar analysis, the traditional appro ach would be to construct a full finite element model of the  mechanism. The purpose of this paper is to describe an alternative approach that models the flexibility of  the DDM using a lumped parameter approximation to capture the compliance of individual parts within the  drive linkage. This approach all ows for rapid construction of a dynamic model in a time-critical setting,  while still retaining the appropriate equivalent stiffness of each linkage component. As a validation of  these equivalent stiffnesses, finite element analysis (FEA) was used to iteratively update the model  towards convergence. Following this analysis, deflections recovered from the dynamic model can be used to calculate stress and classify each component’s deformation as either elastic or plastic. Based on the  modeling assumptions used in this analysis and the maximum input forcing condition, two components in  the DDM show a factor of safety less than or equal to 0.5. However, to accurately evaluate the induced stresses, additional mecha nism rigging information would be necessary to characterize the input forcing  conditions. This information would also allow for the classification of stresses as either elastic or plastic.     External Tank Door Background  The ETD cover openings in the Orbiter’s underside, which are access regions for the umbilical and  structural connections between the External  Tank (ET) and Orbiter. The ETD sits at the aft underside of  the Orbiter, and is prominently visible in its open configuration in Figure 1.   After jettisoning the ET during ascent, these doors are clo sed while on orbit and remain closed throughout  the duration of flight and descent, until they are opened after landing for inspection. Proper door closure is critical to avoid excessive aero-heating during descent through the Earth’s atmosphe re. Thermal analysis  has shown that if the doors are not fully closed and aligned with surrounding TPS tiles within 3.8 mm  (0.15 in), a safe descent would be questionable [1]. 

The three main ETD subsystems are the Centerline Latches (CL), Door Drive Mechanism (DDM), and Uplatch Mechanism (UM). The CL locks the doors in their open configuration while the ET is connected to  the Orbiter and through out the ascent stage. With the ET jettisoned, and while in orbit, the DDM is  commanded to move both doors from their open to closed configuration. Finally the UM, which has three hooks, is activated to latch the doors in their closed configuration and compresses the thermal and  pressure seals for proper closure. Figure 2 shows a close-up of the starboard-side door with the DDM  and the three hooks of the UM highlighted.                                                        

* National Institute of Aerospace/University of Virginia, Hampton, VA  ** NASA Langley Research Center, Hampton, VA  + NASA Johnson Space Center, Houston, TX 

Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  249