NASA is in the process of submitting to Congress its path forward to comply with the mandate for a human-rated, heavy-lift Space Launch System (SLS). Under its Heavy Lift and Propulsion Technology Systems Analysis and Trade Study program, NASA tasked 13 companies, including Boeing, to perform various vehicle architecture trade studies, which were provided to the Marshall Space Flight Center in May. Both NASA and industry have considered numerous options from an architecture perspective, and all agree that the ability to accelerate 50 metric tons to escape velocity is required to support deep space exploration. The primary decisions NASA must make are which rocket variant will best meet the requirements, what evolutionary path it will take and when it can be made available.