Thanks skinkinthegrass!
There was an article about this in PopSci a few, hmm, maybe five years back. It’s an interesting idea — supercooling the oxygen is a necessity in order to get any thrust out of it. Ordinarily, superheated oxygen merely oxidizes the fuel, bring the thrust down to near-zero like right about now, boom. So various bright people thought, wait, what if we cool it?
The savings of not hauling oxygen — and cryo oxygen is very compact in comparison with the hydrogen this craft would also be hauling — add a little to the mass budget. I should point out that single stage to orbit (SSTO) vehicles are and have been feasible, technically, for some time now. But they are rather like that joke about isometric exercise — isometrics are great for people who want to do more isometrics.
The point of the SSTO is said to be one thing, but basically it would result in a very small payload into orbit. SSTOs are a beguiling problem, but they are an end unto themselves, rather than a solution to other problems. Perhaps the only eventual use for any SSTO is to deliver unmanned smallish payloads to orbit, perhaps somewhat cheaper (though probably not), and tumble back through the atmosphere for recovery and recycling for another flight, not unlike the SRBs from the Space Shuttle.
The streaming vids of those separating and tumbling back to Earth are awesome, btw.