Why would the blackbird fly so high? I looked it up and it says they can fly up to 90,000ft.
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ELI5 – the air up there is less dense, it’s easier to fly faster at 70,80 or 90K feet above the ground. [Flying high and fast put it out of the reach of Soviet Surface to Air Missiles (SAMs) for decades.](https://theaviationgeekclub.com/even-if-it-was-faster-than-the-blackbird-the-sa-2-sam-was-never-able-to-hit-the-sr-71-heres-why/) If when they could climb that high, systems weren’t that accurate or the missiles were never fast enough to catch the SR-71.
When missiles were introduced that could pose a threat ([starting with the SA-5](https://en.wikipedia.org/wiki/S-200_(missile))) the Blackbird wasn’t allowed to fly over the USSR and some of it’s client states that got those missiles.
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