Ramjet in ice

Second installment, on ice gun physics. Looking here for how to get this thing from zero to 8 km/s.

In fact it does not go from zero. Here is the claim: At launch the ramjet is placed behind the breech of the gun. When the breech valve is opened, hydrogen flows through the ramjet at about 1 km/s; fast enough to generate thrust. At face value, something is blowing a 1 km/s gale through the pipe. Of mostly hydrogen. In mph: 2237.

And what's Mach-One in 200K 2bar H₂? NASA were mixing it with oxygen, such as to put it less in the km/s range and more in the hectometer/second range. There is already going to be a sonic-boom on this thing.

No, man. The windtunnel's not happenin'.

What's happenin' - possibly - is the ramjet running into the hydrogen at (supersonic) 1 km/s. Maybe there's a jetpack behind it. Maybe (my preference) it's accelerating into the gas by some other means. Partly by gravity perhaps; magnetic rail seems more-helpful. Since I think we aren't letting the ramjet touch the sides.

Another factor is the pressure of the gas. Best I can tell, they want pressure as to spark the ramjet process and to kick it from 1 km/s. Then, 4.4 km/s (if on the 100 km plan). Then maybe to 8 m/s and LIFTOFF. NASA thought they could pop the cork at the "muzzle" end to have the H₂ pressure at the last stretch be lower.

So: railgun to 1 km/s. Put up a brittle barrier (or a door with hinges) behind which is 2 bar of hydrogen - smash that barrier. Spark the ramjet. As it gets to 4.4 km/s, let the hydrogen attenuate (as it will naturally do, as the hydrogen leaves out the back).

I don't even know that 8 km/s is the limit in this scheme; but, if we only have 3.4 G and 1000 km to play with, that's probably about as good as we'll get on Earth.