The video below is from a commercial site, but there are how-to guides on You Tube to make your own for free.
The key factor is this. Its made to use very small amounts of wood. Twigs and sticks.
If the SHTF your propane stove will run out of fuel. With a rocket stove, there are ALWAYS twigs, sticks and scrap wood lying around. You will never run out of fuel with this little gem!
First the commercial stove, its good to show how little fuel you need to cook a big pot.
[youtube=425,350]http://www.youtube.com/watch?feature=player_embedded&v=rSL1a_qk7Zg[/youtube]
How to make a rocket stove out of bricks...
[youtube=425,350]http://www.youtube.com/watch?v=8qoyBVKC0nI&feature=related[/youtube]