Ullage motors as they were on Saturn V (and in most other cases) are solid fueled. Making them incompatible with reuse. SpaceX emphasize reusable systems even w

Ullage motors as they were on Saturn V (and in most other cases) are solid fueled. Making them incompatible with reuse. SpaceX emphasize reusable systems even when not recoverable because that allows for testing of the exact flight hardware (eg mechanical separation systems over explosive bolts). A reusable approach would involve some form of gas thruster, so might as well just try hot staging.