Given that each of the two QE class carriers was built to carry 40 F35s, but that we only have 3 a/c on order for delivery by 2022 (only half of the these for RN, rest RAF), one assumes there will, or the foreseeable future at least, be PLENTY of storage space left over on the hangar deck for capital spares such as engines which would be difficult to bring on board at sea.