Perpetual motion machines are physically impossible by the laws of thermodynamics, requiring no specialized citation to confirm.
The claim states that perpetual motion rockets driven by the cosmic microwave background are impossible. This is a direct consequence of the laws of thermodynamics (specifically the first and second laws), making the statement trivially true by definition and everyday observation of physical law. Therefore, it falls under the COMMON_KNOWLEDGE category where no citation is needed, and none of the retrieved papers directly address perpetual motion rockets (they discuss CMB surveys, dark sectors, and thermodynamic limits of energy harvesting).