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In January 2004, the National Aeronautics and Space Administration (NASA) received new strategic guidance for Space Exploration. With this new guidance, the manned spaceflight community was given an exciting opportunity to develop new human qualified space vehicles based on the latest technology and methodology. The scope of NASA's Constellation program encompasses all elements that must work together to successfully complete the mission of returning humans to the moon. These elements include a launch system, crewed vehicle, and landing module, to name a few. Each element within the Constellation Program is considered a separate development project and this has led to the selection of different avionics architectural approaches on different vehicles. Additionally, legacy systems such as the International Space Station must also interface with the Constellation system. Interfaces between these elements (new and legacy), and especially between their avionics systems, must be carefully integrated to ensure mission success. This paper discusses considerations for the interoperability of varying avionics architectures within a complex system of systems such as Constellation. Currently, two of the major elements within the Constellation system are advancing into the design phase: the Orion Crew Exploration Vehicle (CEV) and the Ares I Launch Vehicle. These vehicles have chosen different avionics architecture approaches. The Orion vehicle is implementing an Integrated Modular Avionics (IMA) architecture with high integrity self-checking pair processors for fault management, while the Ares I element is implementing a Federated Avionics architecture with some integrated characteristics and a voting scheme for fault management. This paper discusses implications on the design of each vehicle due to the interface requirement between these two different avionics approaches.