(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.)
-- neutrinos_shadow, May 03 2022

water go boom (-:
-- Voice, May 04 2022


I think the principle is fine, its the niggling details of making the breech walls and the containment clamps and the seal between breech and projectile strong enough that might cause some* issues.

*i.e. complete failure to do anything
-- pocmloc, May 04 2022


That's not fair, [poc]. If the water is heated to supercritical you're GUARANTEED to have a boom. It may or may not be a disassemblish boom, but boom it shall.
-- Voice, May 04 2022


//If//. A leaky seal will stop it going supercritical.
-- pocmloc, May 04 2022


At what pressure does Hydrogen and oxygen ignite on it's own? I imagine the springy heat energy of all the supercritical water molecules will be more than the energy released of a lesser amount of hydrogens and oxygens moleculing up.

How many goes you would get from the precision sealing would be an cost issue.
-- wjt, May 07 2022


[wjt]; good point. A (very) quick search said nothing about pressure, but auto-ignition occurs at 570°C at 1 Bar (which would actually help; it wouldn't need an independent ignition source). I would assume higher pressure = lower temperature.
-- neutrinos_shadow, May 08 2022



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