Europe’s rearmament is usually measured in finished systems: tanks ordered, air-defence fire units delivered, production lines opened. Beneath those systems sit specialised functions that are hard to separate from the configurations for which they were developed. Rotating electrical interfaces are one of them. They carry power, signals, data and, in some designs, optical, radio-frequency or fluid connections across the boundary between a stationary structure and a rotating one, in turrets, sights, periscopes and radar antennas. A tank turret that cannot pass power and data to the hull, or a radar that cannot pass signals from its rotating antenna, does not function. Such interfaces are rarely bought from a catalogue: they are engineered to an envelope, a load profile and a set of acceptance tests, and then produced and repaired under controlled conditions over a service life that can outlast several upgrades of the host system. The question this raises for European supply security is not whether Europe can make slip rings. It is whether the companies, engineering knowledge, rights, test arrangements and customer approvals needed to keep a particular accepted configuration in production, to repair it and, if necessary, to make it elsewhere are actually within European control, or merely located in Europe.