Roland was an engineer so while things like making concrete was within his knowledge, making bombs wasn't.
So in their first attempt at making a bomb to be dropped from high altitude, they did a lot of test drop and use the witches' abilities to check if the fuse inside lit properly and what to do to adjust it to ensure that when the real bomb get drop, it'll actually detonate.
Heck even when trying to make steel Anna went through thousands of combinations of different component ratios to test which create the best one.