On a spacecraft, every subsystem fights every other for mass. One way to win that fight is to make one part do two jobs. US11352150B2, granted to Momentus Space LLC on June 7, 2022, does exactly that, claiming a "spacecraft structure configured to store frozen propellant."
The CPC sits at the structure/propulsion boundary: B64G 1/402 (propulsion plant arrangements), B64G 1/401 (propellant tanks), and notably B64G 1/546 (thermal insulation/control). The thermal code is the giveaway — keeping propellant frozen is a heat-management problem as much as a structural one.
The mechanism is integration. Instead of a separate propellant tank bolted into a separate structural frame, the claimed design uses the vehicle structure itself to hold propellant in frozen, solid form. Paired with Momentus's microwave water-plasma thruster (see US11527387B2), the frozen propellant is plausibly water — stored solid for density and handling, then melted and fed to the thruster. The thermal-control classification covers keeping it frozen until needed.
This is a genuine crossover patent: it belongs to neither the structures shelf nor the propulsion shelf cleanly, because its whole point is to merge them. That kind of multi-function integration is where smallsat and transfer-vehicle designers find the mass savings that close a mission.
The familiar discipline: this protects a specific integrated structure-plus-storage approach, not the broad idea of structural tanks (which has aerospace prior art) or water propulsion. And as with all of Momentus's portfolio, a granted claim is a design intent, not proof of a reliable flown system. The value of the record is in showing how aggressively the company tried to squeeze mass out of its architecture — and where structural and propulsion engineering blur together in a water-fueled tug.