How to transfer propane from a SMALL cylinder to a SMALL metal cylinder I made a metal cylinder that is the SAME size as my 400g propane cylinder (picture: here). I want to transfer ALL (or almost all) of the propane from the propane cylinder to my metal cylinder, preferably without using an air compressor or something like that. The reason why I'm doing this is because I'm building a rocket and I can't get the propane to flow out of the propane cylinder fast enough. 
 A: The only way I can think of that doesn't need specialist kit is to cool the metal cylinder and warm the propane cylinder. The boiling point of propane is about -42ºC, so if you cool the cylinder with dry ice you'll be able to condense the propane into it. Warm the original propane cylinder slightly, say to 50ºC, to get as much propane as possible into your metal cylinder.
Dry ice will get you down to around -70ºC, so it will easily be able to liquify the propane. Traditionally you use a mixture of dry ice and a solvent like acetone to improve thermal contact and increase the cooling rate.
You don't say what the pressure in the containers is, but the boiling point of propane rises quite rapidly with pressure. At around 5 atmospheres the boiling point rises to around 0ºC, so if the pressure is high enough you may be able to use a less exotic cooling system. There's a graph of the propane vapour pressure here. Propane liquifies if the ambient pressure is greater than the vapour pressure.
I've never had to buy dry ice myself (the lab always provided it) but I don't think it's especially hard to get.
