Small rocket stove failed?

So to test the concept of a rocket stove that burns sideways, I made a small scale one out of cob, and burned popsicle sticks in it.

Dimensions:
-4 inch vertical feed chamber
-12 inch horizontal burn chamber
-18 vertical smoke stack
- roughly 2 inch diameter for eveything

It just doesn't seem to work. When I start a fire in the feed chamber, it burns straight up instead of back through the burn chamber. It also seems to be choking itself out like it can't decide where to get air from. Can anyone tell me what I'm doing wrong, or show me an example of one that does?

In the case of j-tube burn tunnels, smaller is harder to do. Peter van den Berg designed a 4" burn tunnel which does work and has the same efficiency numbers as his 6" and his 8".

Did you let it dry first? Just ruling out obvious problems before moving to complex ones.

Mike Cantrell wrote:Did you let it dry first? Just ruling out obvious problems before moving to complex ones.

Well, to be honest I let it dry for only a day and it was a little damp, so I suppose that could be part of it. But I think it was dry enough for the concept to work. You're right though, I should wait for it to completely dry but i'm impatient like that.

Also, what do you guys think of using a 4x4 as a mold for the stove?

John, first test, you push newspaper in the burn tunnel, and light it. If your rocket can work, it will! Don't pack it too tightly!

John Smithington wrote:Well, to be honest I let it dry for only a day and it was a little damp, so I suppose that could be part of it. But I think it was dry enough for the concept to work. You're right though, I should wait for it to completely dry but i'm impatient like that. Also, what do you guys think of using a 4x4 as a mold for the stove?

As I understand it, you've built the thing out of cob. That's not insulative enough, a pile of wet mud won't do. Second, a 2" round stove is extremely hard to get going because of the unfavorable wall area/streaming core ratio. Even a 4" square isn't easy to get going without special measures. Maybe the little 2" core could run, but it should have virtually no mass at all, and being extremely insulative at the same time.

My advice: try the 4" round or square, the round one's cross section area is already 4 times as large as the 2". The 4" square is 5 times as large.

Ya, I plan on using a 4x4 for the next stove. I'm not convinced cob is a bad insulator though, earth from what I've seen is quite effective at keeping in heat, but of course it needs to be dry.

Cob is nice to store heat and release it slowly over time, but it isn't an insulating material per se. In order to make it more insulative you could mix it with vermiculite or perlite, the agricultural kind. This is lightweight, heat resistant and pretty insulative.

Insulating materials are lightweight, they contain empty spaces which slow heat conduction through it.

Peter Berg wrote:Cob is nice to store heat and release it slowly over time, but it isn't an insulating material per se. In order to make it more insulative you could mix it with vermiculite or perlite, the agricultural kind. This is lightweight, heat resistant and pretty insulative.

Insulating materials are lightweight, they contain empty spaces which slow heat conduction through it.

There is straw in cob, varrying amounts depending on what ratios you're using, so I'd imagine that acts as an emptry space somewhat. But ya, I hear vermiculite is ideal for insulation, but I don't think a lack of insulation is the number one issue with my test stove. From what I've read on hear from you guys, I've concluded that the primary problem is that the tubes are too small for sufficient airflow.

I guess Cob being good at storing heat and releasing over time is why they use it in those rocket mass heater thingies. Seems like a good use of a rocket stove.

my first exsperiment was 2 .25" exhaust pipe in a bucket of cob. it worked but you had to feed it constantly. the second i made from sign board and was 4" square when burned out. that one was much better i could let it burn almost an hour unattended without reloading. the second one i cast out of cob with perilite added. it got hot enough to turn the entire thing red. im assuming it fired the cob like a brick, so it was definitely hot enough. these were just cores and risers outside to play with. if i build one for the garage it will be at least 6".

one thing i did notice about my cob. when i mixed the batch for the second core, i mixed the clay and water in buckets with my drywall mixer. that made it very smooth and elastic like. i then mixed in the sand straw and perilite. i believe it made for better cob than the first mix, with just feet, because it didnt crack like the first one did. i have played with it all summer without it coming apart. it sets outside to with no cover. cob is much much more resilient than i thought it was! lol

Its good to know 4" will work better than my 2" one.

Sounds like insulation really makes a difference. I'll have to make a stove with some vermiculite in between the layers or something to test it.

With my experience with cob, i prefer mixing powdered clay and sand together in a 1:1 ratio, then add half as much water, then varrying amounts of straw. I've never had cracking problems with this method, but it is a pain to powder the clay before mixing it in.