Rocket cob oven

[shuboyje]

Another idea I had would be two rocket stoves fueling the oven. One under the floor as already proposed heating the hearth and a second protruding above the hearth heating the dome and providing open flame in the oven chamber for pizza.

I think you would have a better chance of getting up to pizza temperatures, but if retained heat baking is the goal the uninsulated floor is gonna be a major issue in any of these designs. I guess you could keep a fire going to keep the heat up but that may be a bit tricky and you would probably lose the fuel efficiency you gained with the rocket design by burning wood when a traditional black oven would be operating off of stored heat.

[Archena]

As long as you close the air intake and fuel opening I wouldn't think it would be that much of a heat sink. Granted, it won't retain as well, but it should do okay. Would depend on what you planned to do with the retained heat - if you're still gonna cook then the rocket design might be problematic. If you're gonna have the party and walk away, it should be fine.

Although I suspect you're right if we're talking bread oven. Any fuel advantage would be quickly lost by one of those.

[arthuritus]

Archene, I've already built my first cob oven so I've done some cob insulation there. It makes me want to use something more durable for given that it would be difficult to access for repair. Regarding chimneys, they function slightly differently in rocket stoves. In normal ovens they're a bit of a luxury to stop your eyebrows getting scorched and the front of your oven from getting sooty. Things like draft etc are an added bonus. Whereas in rocket ovens the draft is much more important. Its just coincidence that this is starting to look a bit like a white oven, that was not my focus at all.

All the rocket parts will need to be well insulated anyway - think molded perlcrete. Any extra convective heat transfer will probably be beneficial for cooking with retained heat because there'll be a bit hot thing underneath the hearth. Conductive transfer should be minimal because there will be an air gap directly under the hearth and then more perlcrete or similar. Radiant... hmm idk. Would that change?
But yeah, bung a cork in the feed and the chimney when baking and it should be sweeet.

To me, dual rockets would be slightly overkill given that they use rockets as a kiln for low temp ceramics (>1250 F). Better to make one slightly bigger one then figure out how to redistribute the heat from the bottom of the oven to the top a bit better. By the way, I've often heart 450 C quoted but is that the hearth temp or the dome or the air or what?

[Archena]

Hmm, it just occurred to me - won't the rocket need some kind of vent on the upper end? If the air under the hearth is stagnant, will it even draw? I don't recall seeing one - but I could have missed it.

Not sure about the temp - I presume the air but someone else can probably answer better than I.

[arthuritus]

The vent on the upper end is the chimney, connected to the main part of the rocket by the air gap around the hearth.

[Archena]

Ah, okay. Sorry, late night posting with irrational rambling.

:blush:

[shuboyje]

Originally Posted by arthuritus

All the rocket parts will need to be well insulated anyway - think molded perlcrete. Any extra convective heat transfer will probably be beneficial for cooking with retained heat because there'll be a bit hot thing underneath the hearth. Conductive transfer should be minimal because there will be an air gap directly under the hearth and then more perlcrete or similar. Radiant... hmm idk. Would that change?
But yeah, bung a cork in the feed and the chimney when baking and it should be sweeet.

I don't see where you are coming from here. Unless you keep the rocket running the air under the hearth will cool. The hearth will lose heat via convection and radiation on both the top and the bottom, as opposed to just the top in a standard black oven. I honestly think this will have a negative effect on retained heat cooking. As I already stated keeping the rocket going would be an easy solution, but your gonna lose a lot of the efficiency you gained using the rocket to begin with.

To me, dual rockets would be slightly overkill given that they use rockets as a kiln for low temp ceramics (>1250 F). Better to make one slightly bigger one then figure out how to redistribute the heat from the bottom of the oven to the top a bit better. By the way, I've often heart 450 C quoted but is that the hearth temp or the dome or the air or what?

Not sure I could disagree more. Nothing worse then building an oven only to have it unable to get up to temperature. I've seen a few rocket oven builds and nobody has come close to getting the oven to pizza temperature with a single rocket.

Kilns and pizza ovens are two very different beasts. Kilns take a huge amount of time to get up to temperature, I think 100 degrees per hour is generally used. Kilns do not have a huge gaping door leaking heat out the front. Kilns use a down draft air pattern that places the flue entrance at floor level allowing as much efficiency as possible(this requires a tall flue to generate the draft needed to pull the flame downward). Pizza on the other hand requires a hot surface to conduct here to the crust and a big open flame to cook the top. There are more heat sources then that in a standard black oven, but these two are the big ones. A single rocket will not produce a big enough flame. Say you build a 36 inch oven with a 16 inch dome. You need a flame about 7 feet long coming out of the rocket to enter the bottom front of the oven, go all the way to the back, up to the top and then all the way to the front. Not gonna happen.

If you need one more note of caution, look into french Guillard style bread ovens. These ovens have an external firebox under a fully insulated floor. The firebox produces open flame that enters the oven through a "throat" in the front. From my reading these ovens do not get to pizza temperature and they use a fire box so large you would need literally the worlds biggest rocket stove to equal it, even with the rocket stoves greater efficiency.

So to sum it all up, if you want to cook pizza with rockets providing all the heat I would highly suggest you go with two as I mentioned before. On the other hand if you are looking for an oven that is simply supplemented with the rocket stove AND has good retained heat functionality I would build a standard black oven with a rocket stove that protrudes through a fully insulated floor.

[arthuritus]

Thanks shuboyje but if you read previous posts I've tried to make it clear that I only want the rocket to supplement the 'standard black oven'. Although as you pointed out it could be a net drag on efficiency if I want to do retained-heat baking; if I get around to making it I'll be sure to insulate as much as possible.

And yes, they have used rockets for low temp kilns. One was a rocket oven that they'd crank up a bit more to use as a kiln. >1250 F, whatever that is. See one of the links i posted previously.

[shuboyje]

Sorry if my post wasn't clear. I agree there are functional kilns based on rocket stoves, I was simply pointing out the differences in kilns and pizza ovens whether rocked stove based or not.