One of the most controversial design choices of the UK's new Queen Elizabeth class aircraft carriers is that, despite their size, they are designed around the VSTOL variant of the F-35 L:ightning II Joint Strike Fighter, the F-35 B, meaning the ships are fitted with ski-jumps for short take-offs and no arrester gear because the aircraft can land vertically. The USA with its nuclear powered Nimitz class ships operate the F-35C CATOBAR aircraft, but (and here's the important bit) can also operate a wider range of fixed wing aircraft using catapults and arrester gear. However, such systems are expensive to maintain, take up space in the ship, and require a source of steam to operate. There's no shortage of steam on a nuclear vessel of course, but the Queen Elizabeth Class are powered by diesel and gas turbine engines, not steam or nuclear. A steam boiler would need to be added to create steam to power a catapult.
However, a new technology means that it's now possible to power a catapult with electricity rather than steam: EMALS, or Electro-Magnetic Aircraft Launch System. This new form of catapult uses a linear induction motor, rather than a steam piston, to launch aircraft. The system first saw use on the latest US Aircraft Carrier, USS Gerald R Ford, and is now being introduced on China's newest aircraft carrier, the Fujiyan. Like any new system, EMALS has gone through significant development and testing by the US Navy, hitting many bumps along the road, but the design definitely shows promise, with many notable advantages over steam catapults, including smoother acceleration of aircraft, more precise adjustment for weight of aircraft, and taking up less space below decks. But one of the most obvious advantages is that it doesn't need a source of steam (although Gerald Ford still has that, being nuclear powered). There are conflicting reports on the Fujiyan's power plant, with Wikipedia (not the most reliable source) suggesting it may use steam turbines, like earlier carrier designs, or Integrated Electric Propulsion, like the Queen Elizabeth class. More up-to-date videos on youtube imply the latter.
Which brings us back to the Queen Elizabeth Class. Could the British carriers be retrofitted with EMALS and arrester gear? Certainly in the initial design of the carriers consideration was given to using a CATOBAR design, but this was dropped due to the cost of such a system. EMALS is still a new, expensive, and untested technology, but it should be technically possible to retrofit the ships. The question then becomes one of funding, as well as purchasing additional CATOBAR aircraft to take full advantage of the facility, plus additional pilot training. One thing never mentioned in these debates is that CATOBAR is less safe than VSTOL, with aircraft landing at speed and having to catch an arrester wire to stop, which is a difficult method of landing requiring precision flying from the pilots. With a vertical landing, the plane comes to a hover before landing, eliminating the old risk of missing the wires and having to go around again.
In summary, VSTOL is safer and cheaper than CATOBAR, but CATOBAR offers the possibility of longer-range fixed wing aircraft with a wider choice of fixed-wing aircraft. While it's fun to speculate about the possibilities, the current finances of the UK will probably preclude the adoption of EMALS for the foreseeable future.
