486 SpaceX Development Approach  

The SpaceX team has taken a very integrated, systems -based approach to the development of the SpaceX  docking system . This integrated approach has been a cornerstone through early concept development,  qualification, and flight integration, build  and test .  

Starting with the early concept development, close interaction between GNC and the design engineering  teams ensure d accurate system requirements for misalignments and speeds  and informed design  selection. During the prototype phase, the mechanisms, software and avionics teams worked tightly  together to identify  the optimum systems design.  

A system -level qualif ication campaign was undert aken as early as possible to inform docking performance   parameters . The SpaceX docking system has undergone extensive qualification testing at both the system  and subsystem level, utilizing SMC -S-016 and NASA -5017 mechanisms requ irements . The system as a  whole completed shock testing, vibration testing, thermal -vacuum and static structural testing. These  system level tests included full system performance tests at thermal extremes.  This system level  qualification testing highlight ed interactions that may have been missed with solely subassembly  qualifications, such as dynamic harnessing effects . This testing campaign allowed the validation and  refinement of flight software throughout the testing process .  

In addition, the soft capture system was subjected to extensive performance testing on the Six-Degree- ofFreedom Dynamic Test System (SDTS) at the NASA Johnson Space Center (JSC). The SDTS was used   to simulate the relative dynamics of Crew Dragon docking t o ISS and was critical to understanding system  level performance and interactions. Through two testing campaigns, the SpaceX docking system completed  more than 450 soft capture events . Significantly, the system and subassembly qualification testing allowed   validation of the docking dynamics simulation.  

Throughout the first and now subsequent flight builds, the close interaction between the integrated teams  at SpaceX, as well as dynamics modeling  team, has been maintained to ens ure each docking system is  built and validated to system requirements .  

Sub-System Modeling, Correlation, and Lessons Learned  

The SpaceX Docking System is modeled in NASA /JSC ’s Trick environment . This docking dynamics model  was developed by Dynamic Concepts Inc. as a contractor of SpaceX, and highly tuned via subassembly  and system level testing. Simulations were created for docking events as well as sub- scale testbeds, which  allows for highly tuned as -built data. This simulation was then able to correlate to Demo 1 docking.  

Sub-System Lessons Learned – Attenuator Arm Acceptance Testing   In general, while the attenuator arm is conceptually simple with two springs and a damper, the complexity  of mo deling comes from non- linear effects . For the modeling of initial response and peak loads and  dynamics, most components can be modeled linearly (all mechanisms have some degree of non- linearity) .  However, effects like dead bands, frictions, gaps, local sti ffnesses, all have a major role in accurately  simulating the behavior of the attenuator arms.  

In particular, t he correlation of the attenuator arm simulation resulted in several  findings  regarding the  carriage  and the damper. Initial modeling of the deploy structure, or carriage,  assumed it was static device,  but through correlation with ATP test data and additional testing specific to the carriage, it was found that  the carriage not only has significant dead  band but also  has important  back structure stif fness . These effects  are critical to model properly due to the high stiffness of the rebound spring.  The nature of the eddy current  damper, with its high gearing reduction results i s a slight dead band when velocity reverses . This is a minor  effect  but bec omes more significant at the end of attenuation when the relative velocities between the  Dragon and the ISS are small . An example of the correlation in the forces in the strut connecting linkage  between the acceptance test and simulation is shown in Figure 5.