Which major U.S. government agency reinforces NASA's dependency by favoring SpaceX via its launch program?
The Department of Defense (DoD) through the National Security Space Launch (NSSL) program.
The reliance on SpaceX is further cemented by procurement decisions made by other significant government entities, most notably the Department of Defense (DoD). The DoD manages the National Security Space Launch (NSSL) program, which involves launching the highest-stakes and most complex national security satellites. After competitive selection rounds, the DoD heavily favored the Falcon 9 for these critical payloads. This external validation from the DoD regarding reliability and capability extends beyond civilian spaceflight and aggregates demand, which in turn feeds back into the operational experience benefiting NASA missions.

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How The U.S. Became So Dependent On SpaceX - YouTube