Some of the perfectionists are really not going to like this post on using electric fans to help start your rocket heaters, but there are some days when you just don't care what other peoplethink. This is also probably the best place in the world to argue about it so bring it on.
Seriously though. I love you all. Especially the ones I like to argue with.
A chimney draws or drafts due to warm air being lighter than cold air, so it tends to rise, which lets your exhaust gasses escape AND draws more air in to the heater to feed the fire. A Rocket Mass Heater breathes a lot of air in order to completely burn the fuel. The hotter the fire is the more your temperature difference is and the more powerful your draft will be, and the hotter and cleaner and more efficient your fire will burn. In the deep of winter the chimney at room temperature will already be "warm" compared to the outside air so a chimney will draw well enough right from the start for the fire to get the air it needs.
As an aside note, your chimney will always be trying to pull the warmer air out of your house so it is important to be able to completely close off your heater air intake when you are not burning it so the chimney isn't pulling warm air out of your house unnecessarily.
As far as starting the fire, there will be times of the year usually in the fall and spring when there is not enough temperature difference between the air outside and inside for your chimney to draw properly and you can fill your house with smoke trying to start the fire and get the chimeny warm enough to fire. I have seen some lovely days where the sun came out and it was actually warmer outside than it was inside, so getting the fire to draw was even more challenging.
This video discusses some of my favorite options for draft assist, or as my friend likes to say, "suck starting" the fire.