RASCaL

(Another one that's been evolving in my head...) Ever since seeing the Mythbusters explode a domestic water heater, I've wanted to use that "steam explosion" in a useful way.
There have been steam cannon concepts in the past (way back to Archimedes), but they are all quite pitiful (IMHO...). They all simply heat up an empty vessel behind the breech (with the projectile in place), then spray in a little water, which flashes to a little steam & bang! Projectile goes pfft.
Rather, have the projectile held in place in the breech. Behind it, the heated vessel is completely full of water, which gets heated (electrically) to 400°C or so. Because it is contained, the water can't turn into steam & goes super-critical, raising the pressure. A lot.
Once everything ready, the clamps holding the projectile let go. Suddenly the supercritical water finds that it is no longer under pressure, & it flashes to steam, increasing it's volume by ~800x. This makes the projectile leave in a hurry. If my calculations are correct (questionable...) 2 litres of water heated 400°C & a 1kg projectile gives (theoretically) 2,592m/s, about Mach 7.6; there will obviously be losses.
Then I discovered ram accelerated projectiles.
Like a ramjet, RAP cannons require the projectile to already be going fast; then it goes faster. So it becomes "Stage 2" to the steam cannon.
In keeping with the "electricity + water" theme, I figured hydrogen & oxygen (split water) would be a convenient fuel for the RAP stage (whether it's the BEST option, TBC). A modification I haven't seen in digging around RAP research is to have a non-constant radius to the barrel; as the projectile accelerates, the barrel (radial gap) decreases to provide more compression. Multiple stages allow changing initial pressure of the H/O2 mix; a simple stretched plastic film between will probably be enough, except right at the muzzle, where a fast- acting mechanical door will have to be used.
Since it will be big & very slow, the primary use I see would be micro-satellite launching. Maybe hang it from a balloon, or mount it along the whole length of an aircraft.
So, a Ram Accelerated Steam Cannon Launcher. RASCaL.
(I'll link models & drawings when I sort out my Photobucket.)