From these calculations: the ramjet is to burn 76 kg hydrogen and 608 kg oxygen per kilometer of (2m-radius) tunnel. Andrzej Nowicki's Ice Gun Page has this: If the ramjet flies in pure hydrogen, about 80% of [jet] mass is oxygen. If the ramjet flies in the mixture of hydrogen and oxygen, less than 60% of its mass is oxygen.
If in unmixed H2, NASA's 100 "minimum" tonnes of ramjet has 20 tonnes of cargo-chassis Dry Mass and 80 tonnes of oxygen; or we can have 100 tonnes+400 O2. NASA offered the 1000 km tunnel which takes 250 seconds, and the 100 km 10 G skullcrusher which takes 45.45 s. Note the former requires 608 total tonnes of oxygen; the latter, 60.8. So either the 1000 km is for an order of magnitude more cargo, or else it must include oxygen in the 2 bar hydrogen barrel.
Chemically the ambience can be 30.4 H2 : 6.08 O2 by mass. "Oxyhydrogen" at this 2 bar pressure might burn; but, as I look into flame-speeds, this burn runs 3 m/s, which our craft is already outrunning the moment it becomes a ramjet, at 1 km/s. So why are we still talking about lugging 50-something percent of oxygen and its tank up the barrel? It's not the barrel that we're risking, it's the cargo!
The real reason we don't dilute our H2 is, I think, that it increases the mass of the (forward) gas to lower our Mach. Ramjets, it seems, like Mach Four and Stagnation Temperature gets bad at Eight. If the barrel have too much non-hydrogen its ramjet doesn't ever hit 8 km/s but runs more like a ramjet in 30 Ar+N2 : 7.5 O2; that is, like a ramjet over standard atmo.
This makes me wonder if the 100 km option going to 4.4 km/s could, in fact, push their barrel gas-ratio. Mind, this particular ramjet is likely carrying the same oxygen anyway, plus something with hydrogen, to get into orbit; since 4.4 km/s ain't doing that on its own.
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