SABRE might be an option for a first stage on vertically launched vehicle, to be used in that part of the atmosphere where the engine can get most - or even all

SABRE might be an option for a first stage on vertically launched vehicle, to be used in that part of the atmosphere where the engine can get most - or even all - of its oxygen from the air. Then again, if that stage were to be limited to a maximum height of ~20km it might as well be equipped with a stack of upgraded J58 engines (as uses in the SR-71), saving the weight of the cryogenic coolers used to produce LOX for those rocket engines. I guess SABRE makes more sense for a space plane than it does for a vertically launched rocket.