While there are good reasons for a drop tower over an airplane, namely experiments requiring more precise control, I’m not remotely convinced that the Bremen tower was cheaper to build and operate than a Vomit Comet. For one, a 146m tall tower for which ZARM broke ground in 1988 and inaugurated the catapult in 2004; that sounds incredibly expensive. NASA uses old 727s, long out of production so a cost is difficult to assign but they were $20mm new, NASA’s planes were probably enroute to the boneyard when they acquired them.