RMH build - weak rocket

Hello - have been building my first RMH and am getting a weak rocket. The stove appears to burn properly as long as the wood added is short but anything more than half the way up the feed tube will eventually have flames work there way up far enough to get smoke and limited flame into the room. So I have questions:

The heat riser is brick and clay mortor - and reaches to just under 2 inches of the top of the barrel. Could the fact that the clay is still some what damp hurt the draw?

The flu is about 35 feet - could this be too long (six inch flu pipe) or could the fact that I don't have mass around the flu pipe yet have an affect?

The burn chamber is 7 inches wide by 5 inches high and the heat riser is 6 inches by 6 inches - could there be an issue there? I narrowed the feed tube with little difference.

If I just drop short piece of wood in the feed tube the stove burns well - a tiny bit of smoke and or steam and some condensation is all that come out the end of the flu.

Sure would be interested in your comments - this is in my shop by the way - Thanks in advance - Jon

My guess is that your flue pipe is providing too much resistance to air flow. The addition of resistance when you fill up the feed tube tips the balance and the heat riser can't power through it.
Your burn chamber is 35 in^2, heat riser is 36 in^2, and flue pipe is 28.3 in^2. This is about 21% reduction in area going into the flue pipe which may be too high. I don't have enough experience with rmh to know if 35 ft is too long of a flue pipe.
The lack of thermal mass around the flue pipe would also have a strong effect. Without mass, the gases are allowed to cool much more quickly within the flue, which I think is a lack of energy to overcome the friction in the pipe.

The wet clay might be hurting it too, taking energy to evaporate water and lowering the combustion efficiency in the heat riser. But I think the flue is the main issue.

If you need the long duct length, you could try a booster fan at the end of the flue to get air moving. The exhaust should be cooled enough by then to not hurt the fan. This would also make it easier to get started.
http://www.homedepot.com/buy/inductor-8-in-in-line-duct-booster-fan-db208.html

Any pictures?

What kind of wood are you burning? Soft and/or pitchy wood will ignite too fast for a sideways fire-- the flames travel up the wood before the rest of the thermo-dynamic process has completed, carrying the burn out of the combustion chamber into the feed tube.

Can you show some pics of your burn chamber...empty and loaded?

Thanks for the input - I am shortening the flu 10 feet and will try it that way. Then I am going to make an metal heat riser (6 inch inner tube and 8 inch outer) and give it a try. Last thing will be to extend the mass and see what that changes. Hopefully I will learn a bit from the different approaches.

Also I have been burning both types of wood and you are right the apple wood burns more like I was expecting.

Also (but this may be a bit harder to change without a full rebuild), you say that your burn chamber CSA is 35 sq.in. (7x5), your feed tube CSA is 36 sq.in. (6x6) and (I'm assuming) your exhaust is 6" diameter, giving it a CSA of 28.26 sq.in. Your exhaust is your choke point, when it is your combustion throat that needs to be the smallest of the three. To take full advantage of Bernoulli's principle, and get that really "rockety" burn, you should have a burn channel with a CSA that is less than all other critical CSAs in the system (feed tube, heat riser, re-burn gap, radiant heat exchange, thermal pass, exhaust flue). All of the thermal exchange that happens in the mass and the barrel are only part of the equation--it is the constriction of the burn point that is the real 'engine' behind the design. By coupling the point of thermal expansion with a point of acceleration, you truly create a rocket.

So, aside from restricting your burn channel down to something below 28, the only other option would be to step up your exhaust to 7".

Thanks Chris - that has been my concern since the weak burn. I had read that point in Ianto Evan book but hoped I was close enough. Thinking now I will restrict the burn chamber by an inch as one more of my learning processes. Lots to learn in these little buggers.