Combined cycle engine.

[Cloakedwand72]

Can the combined cycle SSTO engine work on a large scale like the skylon?

[jimmymcgoochie]

Theoretically, yes. From what I've seen/heard, the precooler system required for SABRE to function has been developed and tested (although perhaps not at full scale?), while the engine itself will use expansion deflection to improve its ISP both in atmosphere and in vacuum, a technology that is relatively unproven but still very much possible. I've seen concepts for a fully reusable launch up to geostationary transfer orbit, where the Skylon itself waits in LEO while an upper stage boosts the payload up, then returns to Skylon and docks to it again before returning to the surface.

With modern composite materials to take the heat of re-entry, there's no reason why Skylon couldn't work, except for one pretty big one: cost.

Getting Skylon operational will be expensive. If (and hopefully when) it does, it'll be entering a pretty saturated market- even without the much-hyped Starship-Superheavy lobbing a hundred tons to LEO per flight in a fully reusable manner, carving out a niche against both current and future conventional rockets will be difficult- Falcon 9, Soyuz, Ariane 5 and GSLV/PSLV are all in the ballpark of Skylon's payload to orbit, with plenty more rockets- Vega C, Angara, Neutron, Ariane 6 to name but a few- all under development or nearing operation. To compete with all of those, Skylon will have to be very reliable and have a very rapid turnaround time between flights, something that won't be easy when the whole thing goes to orbit and back every time. There's also the not so trivial issue of producing all the liquid hydrogen needed for such a rapid launch cadence. Once up and running, Skylon will be very cost-effective, but until that point money is the main problem to be overcome.