The bottle rocket is a staple of science demonstrations: A plastic bottle, inverted, part-filled with water, stopped with a cork and pressurised via hand- or foot-pump. When pressure grows high enough the cork is forced from the bottle, rapidly followed by the water at significent velocity. By conservation of momentum, the rocket is launched.
Too conventional. So how can this idea be taken up to a higher energy level? Explosives are a natural choice, but hard to get in this time of counter-terrorism. Liquid-fuel rockets are far beyond our skill and budget. Steam, though, could work. Or flash-boiled steam.
And we have the Explodium. Four kilovolts. Amperage too great for us to measure. That'll do it.
This was the third test. The force of the 'launch' is clear - but the bottle couldn't take it.