Yesterday i have an idea. I know thath the rocket fuel make gas and this gas propels the rocket, but this fuel is very dangerous. This is my idea: if we put together bicarbonate of sodium and vinegar (nautralization reaction) we obtain gas and a liquid, this gas produce pressure when we contain this reaction. This reaction can whit the apropiates conditions propels a very litle probe in an sub-orbital flight. all this thing is only an idea if you have more opoinions or comments about this topic, im very interested to see it Thanks ppcastroe:) p.d: Sorry about my english