The CR-7 R.A.P.I.E.R. used in game has about isp of 3200secs on Ascent and 310 at Vac, while the S.A.B.R.E. has about 3600s isp and 460s respectively. Add that to the ability of compressing some of the required oxidizer on the way up, which makes the projected Skylon to have less initial mass on take-off, makes S.A.B.R.E. much more adorable engine. With that said, required Delta-V of LEO is about 7200m/s for about 400km altitude, with 0.25 eccentricity, compared to 3600 for the similiar orbit on LKO. The S.A.B.R.E. propulsion is 1,5x efficient but LEO is about 2,5x harder to achieve than LKO, so in total yes, it is a great engine and Skylon is not unachievable with current tech.
They promised the first test flight on this very year; we shall see if everythings go scheduled.
Virgin Orbit architecture is TSTO, which is a bit better concept than Skylon, if you want to reach further than LEO, for example, to LMO or Lunar landing.
But, (BIG BUT) I have this spacecraft design that ejects only the tanks, but not carrying waste engines to second stage, and works with great efficiency, I made it to Saturn with enough propellant to return to Earth, and gonna have Titan lander as I develop this craft architecture a bit more. I don't know why other than Space Shuttle they didn't use this technique, and then again, even at the Space Shuttle, they had to carry excess engine. But isp of Rocketdyne's F-1 back then was 304s, while Raptor engines have 360s nowadays.
Bonus Attachment: The single-engine-stage but two-tank-stage podracer craft design: https://ibb.co/Vg7KhZy
note: reverse triangular landing gear design is because the heavy part of the craft is forwards, it makes controls much easier, since the weight is always close to frontiers, less elevators are required to keep it stable.
Bonus Attachment: SSTO with the same design, only using 2 elevon4's and having more control authority than most youtuber's 6 elevon winged designs. https://ibb.co/SDCqjnR
