Onceapilot:
Like the Stirling engine, the efficiencies claimed for this regenerative heat cycle and cryogenic cooled, air-compressing, condensing and breathing/pure LOX/LH2 variable cycle rocket engine do not stack-up. The theoretical efficiency factors do not translate into the weight-limited reality that would be needed to make it competitive in orbital operation. The plant required to achieve the theoretical efficiencies is worthy of a ground based Nuclear powerstation.
ESA think it might. The US AFRL think it might. The people involved are actual rocket scientists one of whom also worked at JET on nuclear fusion. Weight limiting is the whole point because LOX is very dense and they aim to have to carry less.
The whole point of their cycle is the way it uses energy that the LACE (Liquid Air Cycle Engine) concept wasted. The innovation is the thermodynamic cycle and the very lightweight heat exchangers that enable it.
Somehow, it is proposed that the efficiency of wing-borne lift within the atmosphere, burning Hydrogen fuel with a small amount of ingested air, cooled and compressed by energy from carrying extra liquid Hydrogen fuel that also provides cryogenic cooling of the air through bulky heat exchangers, will provide some earth shattering increase in efficiency. Of course, this also ignores the extra weight of the wing-borne structure and it's aerodynamic controls, the extra weight of the complex engines that have to be lifted all through the flight and reentry, the extra Hydrogen fuel/coolant to operate the air-breathing rocket, the take-off and landing apparatus, the extra propellant burn during the slow ascent and, the deadweight of the additional propellants that are needed to accelerate the relatively slow spaceplane to orbit after the interface between wing-borne/air breathing and the pure rocket/ballistic operation above the atmosphere, compared to a non wing-borne multi stage pure rocket.
You're implying that these people can't do basic sums about weight which if you think about it would be quite unlikely. In fact they are an engine company and their reason for designing a notional spaceplace (Skylon) was to demonstrate that the sums in fact did work out and that the weight of the vehicle was amply compensated for by the reduction in the amount of LOX that it had to carry. On top of that the US AFRL has done work on other concepts with smaller versions of the same engine and a TSTO orbit configuration and considered it feasible.
There is also the question about the construction of a large spaceplane that is intended to be re-used, the weight of the structure to survive reentry, heating, flight loads and landing. Oh yes, landing. On a runway, like the takeoff?
However, the benefit of wing-borne lifting a ballistic rocket to some altitude and speed for launch to orbit (like the Pegasus launch system) does have some performance advantages but, of course, there are the practicalities of size.
Now, there are certainly interesting possibilities for high altitude atmospheric flight but, I reckon that the the multi stage rocket has got orbital lifting well and truly wrapped up for generations.
That depends on many factors including whether they can recover upper stages. It also means that you indeed need a landing pad in a place where you can accept the risk of a rocket falling somewhere perhaps not exactly where it should. Rockets are very unreliable in comparison to aircraft. If totally reused they might get to a much higher level but they're not thus far etc.
Skylon's cost has always presented problems because it was a big upfront investment. With cost of that investment entirely laid at the door of one vehicle it would compete with a fully reusable rocket and the issue is why would anyone go to that effort to be "as good". On the other hand if you start using the SABRE engine in military aircraft and perhaps start with smaller TSTO spaceplanes and if there are numerous spinoffs for the heat exchanger technology in conventional jet engines and .... nuclear submarines or whatever... then the economics could see quite a big change.
So the real problem is whether the market will be big enough to support all these companies. It might be growing but I don't think that's easy to predict. The proposed internet satellite constellations might be the way to have enough launches to justify the many ongoing developments by various companies and I think the SpaceX one is important to their plans for making launches cheaper but I wonder if there are slightly too many of those constellations in planning.