On a hot sunny day, if you decide to fuel up an SR-71 plane while it is on the ground, the fuel won’t stay inside the plane and will leak out on the field. To start the engine, you can not use a standard airport start cart. Instead, you have to use a pair of V8 muscle car engines. Now to get fuel ignited, you must have to inject a rocket propellent-like particular, toxic, high-temperature hypergolic chemical.
Even after all that, the plane cannot get in the air with a full tank. Neither can it fly for too long without one. Therefore, a tanker must be ready to take the plane in the air. The leaks mentioned earlier only get sealed up when the plane reaches its cruising speed and gets heated up. After getting re-fueled a few times, the plane becomes ready to get deep inside the enemy’s territory at its full speed. SR-71 can achieve speeds up to Mach 1.5. At this speed, if you touch the glass with your bare hands, your hands would get burned.
In the scenario of engine shutting down, you have only three tries to get it restarted because starting this plane is not like turning the key. You have to carry that hypergolic restart mix on board enough to not make the plane a bomb. Flying an SR-71 plane is very dangerous and very expensive at the same time. As soon as the military realized that they could get the job done without this plane, they grounded it. This a shame as well because the SR-71 was the closest thing to a space plane ever made. However, U-2 still flies still it doesn’t have the drawbacks which SR-71 was bringing with it.