Looks like way too much air gap above the fuel. The air is likely passing over the top and up the chimney, rather than through the fuel.
Also, I think your fuel looks too thick. That piece of wood would burn better split in half, and with plenty more fuel in there as well. Rockets basically are best operated at full heat regulating it down to cook on can be tricky.
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Hi Holly; Welcome to Permies! And welcome to your first attempt at rocket science!
I agree , Michael was spot on about your rocket.
Maintaining a set size is critical. Enlarge a dimension and the whole thing stops behaving like a rocket and just becomes a stick fire..that dwindles out.
I think if you can find or buy red clay brick you will be much happier with the results. Home depot sells clay brick for .50 each,or less. Craigs list could have them for free.
Are you familiar with how a rocket is built? The sizes that it needs to be?
The cinder block set up you have is really called an L tube, they do work but require more fussing with the wood rather than a J tube rocket that self feeds by gravity.
Also you need to seal any air leaks with mud /clay.
You want all the air rushing thru your wood, not sucking air in cracks between the bricks.
If you think you can acquire clay bricks and mud , and want more info on how to proceed.
We will be happy to guide you.
First lay out a solid base. Dimensions from there depend on size.
A 6" J tube would be apx. 5.5 " square. Keep that dimension all the way thru , feed tube burn tunnel and the riser.
Your feed tube should be apx 16" tall, from the feed tube edge (riser side) your burn tunnel roof should be 10"-12" long and your riser should be 5.5" square and as tall as you would like.
An 8" J tube would be 7.5" throughout. with the same burn tunnel roof length.
Bricks covering the burn tunnel should be stood on edge, so with 2.5" bricks, 4 wide would be plenty for your application.
For an outdoor rocket stove real firebricks are overkill. Plain clay bricks would work as well.
Home Depot sells new clay brick for .50 each ...
EDIT) I've included a photo of my cast core as an example. You are building the same thing with brick. Add a riser and your canning.
Firebricks for your burn tunnel roof and riser might be a good idea.
At least if you plan on keeping this built.
Also note) Do not just dry stack your brick. They must be mudded to eliminate air leaks. All incoming air must pass into the top of the feed tube.
Hi Holly; The burn tunnel bricks are bridging across the burn tunnel.
A standard fire brick is 9 x 4.5 x 2.5 thick
A standard clay brick is 8 x 4 x 2 3/8" thick.
So four firebricks stood across your burn tunnel would be 4.5" tall and 10" thick (2.5 x 4) for a 10" burn tunnel roof.
Four clay bricks would be 4" tall and 9.5" thick so with clay bricks you would use five bricks and be 11 3/4" roof on your burn tunnel.
Have I confused you more or explained it ?
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