> an a air-hydrogen mix breathing-rocket hybrid design
Tough to use a breathing rocket anywhere but Earth. And SpaceX's goal is to use this to land on the Moon, Mars, everywhere.
Adding another engine means, one more production line each with less production volume (i.e. less efficiency of scale).
And the estimate cost of that vehicle were very optimistic with lots and lots of problems to overcome. A proposal like that from people who have never really done all that much is highly speculative.
The projected cost they could reach with that vehicle, is already beat by Falcon 9.
Elon Musk has even told the Royal Aeronautical Society that he thinks air-breathing engines are a bad idea. Massive complexity for not so much gain. Invest that money into better first stage engine, push threw the atmo quickly get out of it, and make optimal use of your second stage engine.
Its pretty save to say SpaceX will not invest in what is mostly a paper rocket with an engine that has only ever tested the cooling system.
That much cryogenic hydrogen and massive temperature differences is as the limit of engineering possibilities.
I think they would be very happy to get a factor of 2 in one step. It would only 7 such steps to get that factor of 100.
Having said that, they aren’t aiming for the most modern rocket technology as that implies “less well tested”, and will be less reliable. They’re building T-34s, not Tiger tanks.
Single stage to orbit is sexy, but multiple stage to orbit is inherently more efficient and I see no reason why it Would be more expensive if you reuse all stages.
Then again, if that stage were to be limited to a maximum height of ~20km it might as well be equipped with a stack of upgraded J58 engines (as uses in the SR-71), saving the weight of the cryogenic coolers used to produce LOX for those rocket engines. I guess SABRE makes more sense for a space plane than it does for a vertically launched rocket.
There's the criminally underfunded SABRE engine, an a air-hydrogen mix breathing-rocket hybrid design from Reaction Engines that rated to reach speeds of Mach 25 (roughly 35,000 miles per hour).
As modelled, it's more efficient than any other current or proposed chemical propulsion technology, and the proposed single-stage runway launchable system would be capable of delivering around 11 tons of cargo to the ISS. The costs to go to orbit and beyond using this engine could be less than half that of current best in class technology; even less after development cost amortization. Not sure why SpaceX haven't invested in it yet. Perhaps it doesn't fit in with their current vertical integration philosophy, but it seems like a game changing technology.