Rocket Heater dimensions question

Hi all,
I have a dimensions question concerning the Rocket Mass heater.

I've went through the book, and I'm concerned about MY dimensions , compared to what the book says.

the first dimension i'm concerned with is the burn tunnel, "C" in the book. it says that it should be 6x8 or 7" squared- but if i don't get it to that am I screwed? I have only 3 9"firebricks, and the rest are only 8" old building brick. I'm not sure what the length of it should be exactly; the book just gives dimensions in proportionality as far as I understand it. Don't let this this and this be less than "C." So...I guess to put in down, does the dimensions of burn tunnel have to be exact? if mine is 6 high by 7.5 inches wide, will it blow up and kill everyone within a 3 mile radius ( or just not work?)

My second questions is concerning the feed tube and heat riser. I had planned on building both out of brick ( although I'm looking for a metal tube for the heat riser.) If you put bricks together so that they interlock and form what reminds me of a swastika in it's own way, the tunnel is about 6"- less than the 7" called for in the book. Will this kill the draw?

these are the main problems I've had with building this thing so far. I don't want to put it all together, and then realize that i'm gonna have to smash it down because I didn't put the dimensions to the correct proportion/ and or size.

Any help will be rewarded with delicious cake.

What you are looking for is constant CSA (cross sectional area) through the system. Generally the squarer the better. If one part will be smaller than others, it should be the riser, but not by much. So size the riser first. 6inch seems to be the minimum size that works well. That is 6inch diameter for a round riser at 28sqin CSA. I do not know if 5.3 inchs square would be fine in a square configuration or if it should still be 6*6. Generally all of us have built a massless setup to test what we have outside before installing it inside. So figure out the CSA of your riser first then go through all of your runs and calculate them to make sure they are the same or slightly larger. Then look at each transition from one run to the next and make sure they are ok. From what I have seen, most people who have problems have had a bad transition (barrel to mass pipe mostly because it has to be hand formed of cob and is therefore hard to judge the CSA. In that case oversize is better. My core is steel and so was easier to make sure of.

Len Ovens wrote:What you are looking for is constant CSA (cross sectional area) through the system. Generally the squarer the better. If one part will be smaller than others, it should be the riser, but not by much. So size the riser first. 6inch seems to be the minimum size that works well. That is 6inch diameter for a round riser at 28sqin CSA. I do not know if 5.3 inchs square would be fine in a square configuration or if it should still be 6*6. Generally all of us have built a massless setup to test what we have outside before installing it inside. So figure out the CSA of your riser first then go through all of your runs and calculate them to make sure they are the same or slightly larger. Then look at each transition from one run to the next and make sure they are ok. From what I have seen, most people who have problems have had a bad transition (barrel to mass pipe mostly because it has to be hand formed of cob and is therefore hard to judge the CSA. In that case oversize is better. My core is steel and so was easier to make sure of.

That- THe csa-- was something I wasn't , well wasn't sure what the book meant by it. I know it said that CSA had to be greater than the burn tunnel, but I was confused because it didn't include the heat riser in that calculation.

Yea, I've been fooling with this for a few days in my off time, building a dry run of the combustion unit. I built it up once , put some hardly burning cardboard in the unit that i'm sure had no proper dimensions, and was dismayed by the fact that it didn't start producing near fusion quantities of energy.
I appreciate the help! If I actually get this thing going I'll post pics with myself standing over my creation triumphant.

The firebricks I just bought are about 9" x 2.5" x 4.5" and about $2 each from a Chimney/Fireplace supply company(though $3.50 each elsewhere). I think (like others said) the biggest important factor is everything being about the same size. If you have a bunch of them, you could make a circular riser by standing them end to end vertically and in a circle, and use an outter tube to keep them from falling outward. If they are mortared they wouldn't be able to fall in. Just an idea.