This article summarizes why they refueled after takeoff: https://theaviationgeekclub.com/former-sr-71-driver-explains-why-the-blackbird-had-to-refuel-after-take

This article summarizes why they refueled after takeoff: https://theaviationgeekclub.com/former-sr-71-driver-explains-why-the-blackbird-had-to-refuel-after-takeoff/ https://theaviationgeekclub.com/former-sr-71-driver-explains... TLDR: They needed full tanks (more or less) or an inert layer of nitrogen over the gas in the tanks before hitting Mach 3 or else the fumes in the tank might ignite. At the end you'll see that they had a system for fueling the tanks on the ground in order to hit mach 3 right after takeoff but it was a maintenance nightmare. If I remember right, the SR71 could fly at Mach 3 for up to 90 minutes between refuelings. That's significantly longer than any jet ever made.