The Lockheed Skunk Works A-12 was the immediate predecessor of the SR-71, a single seat recon vehicle in some ways a bit superior to the SR-71, in others not as good. One of the odder ideas put forward was to use the A-12 to carry a modified Polaris missile with a recon satellite on a once-around mission… a way to turn the Mach 3+ A-12 into a global range hypersonic, nearly orbit-capable recon platform. It was bonkers, but the math checked out; nevertheless it was not built. It is described in greater detail in:

Lockheed SR-71 Blackbird – Origins & Evolution

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