I have long been thinking (though I'm no chemist) that solid propellants are just one of the ways to get a bullet going.

Has anyone ever considered using a liquid propellant?
Obviously, something like this would need to be tested under laboratory conditions and extreme safety measures would have to be taken while testing.
I was thinking that internal combustion engines are similar to firearms in many ways. Gasoline and oxygen are powerful enough to propell vehicles weighing thousands of pounds down the highway. Could they not be used to propell a bullet? Imagine a case lined with some sort of absorbent material soaked in a liquid fuel. (Cloud be gasoline, diesels, model airplane fuel, whatever.) Or alternately, combine the fuel with some kind of binding agent to make it into a jelly. If the case could hold enough oxygen to facilitate a clean, complete combustion of the fuel, it might have enough power to propell a bullet at workable velocities. If not, you might have to find a way to introduce oxygen chemically.
I'm not sure if there would be many advantages to this system, and the seal between the bullet, primer, and case would have to be airtight. But, it occurred to me that if it could be proportioned to burn clean, it might significantly increase the amount of shots you could fire before you have to break out your cleaning kit. (Though you would still have residue from the primer, I imagine.)

I know this really has nothing to do with traditional handloading, but I thought this topic would be of more interest to people that frequent this area of the site.

I know the Army and Navy have both tested aerosol propellant artillery. Ignition is an electric arc as I recall. I'm sure other nations have done so with their "big guns." How transferable to handheld or shoulder-fired small arms such technology is, that I can't say. I'm inclined to think that on a personal level, liquid propellants could be very dangerous to accidental ignition. Long term stability could also be a significant problem.

Ah, yeah. You do have to put some kind of chemical in gasoline if you're going to store it for a while.
It is interesting that they tried using that for artillery. I guess on that scale it's a lot more practical.
If it could be adapted to small arms, it might save a lot of dough. When you imagine that a model airplane engine has a cylender volume that approximates that of handgun ammunition, and can run at 20,000 RPM for several minutes on a tiny fuel cell, it seems like an efficient energy source..

Potato cannon. WD-40, hairspray, etc..... lotsa fun!

Hehe, you should try it the complicated way! I found some plans online and built one using a lawn sprinkler solenoid valve. You make a pressure chamber from pvc and install a tire valve in the endcap. You can fill it to the solenoid valve's rated pressure (providing it doesn't exceed that of the PVC pipe) and fire it with a button. The one I built would fire potatos about 120 yards and made a loud honking noise as air passed the diaphraghm in the valve. Pretty neat. :)