Rocket (mass?) oven?

Hi there experts,

At my previous house, we had one of the traditional style outdoor ovens (built by the previous owner) where one builds a fire, lets it get to the coals stage, then shovels them to the sides and start cooking.

We only ever used it to make pizzas, so you waited a while before starting, because the first one or two would have incinerated cheese from the radiant heat, then you had a "sweet spot" where you'd get maybe four beautifully cooked ones, and then after that the pizzas would dry out before they got "crusty" because the temperature was getting too low. All in all, this sucked.

So I am now thinking about whether it is possible to have a wood fired oven that works on the principal of continuous convective heating from a rocket type combustion chamber. The idea would be to maintain an appropriate sized heat source for the duration of the cooking. I am primarily interested in using this for roasting meats, etc., with a possible secondary use as a smoker at other times.

Please see my highly out of scale schematic (and I use that word very loosely). I sense that you would need to have some sort of diffusion of the heat as it exits the heat riser, and some sort of "front wall" before it exhausts.

Doing a little more research, and I can't find anything like this on the internet. This means I am either brilliant, or crazy.

Another question pertaining to this, but possibly relevant to RMH in general: if you have an 8" system, can you maintain a smaller fire in it (just less fuel and more air in the feed chute) and keep all the rockety burn goodness happening?

EDIT: Aha! I sort of found something similar

Phil, here is a photo tutorial of a nice example of what you are describing....

https://picasaweb.google.com/Jonnygms/RocketOven

I personally prefer a little bit finer control over the temperature, so I build them on top of a fairly traditional Rocket Mass Heater barrel. I cut a hole in the top of the barrel and put the oven there. That way, with an adjustable damper over the hole, you can carefully regulate the temp in the oven while the fire burns and as well you can harness the excess heat into a bench for heated seating if desired. I use this same configuration for many different types of cooking. In my opinion it's a big improvement over the traditional "hot or freaky hot" options of a direct vented rocket stove for cooking.

Here's a pic of a small oven I built using this system, it was a temporary setup, only there for a weekend, so it's not very finished. Should give you the idea though.

Thanks Matt. "Validation!" as Paul might say.

Both the photo set you sent, and also the video I found show the J tube venting unimpeded into the oven. I sense the some sort of heat diffuser would be useful here, possibly venting around the outside walls so that the heating effect would be similar to the coals around the edges of a 'conventional' wood oven. I am planning on using the RMH core from Weisner Corp's 6" heater plans (which I still intend to build, but the building to house it is still unfinished ) so presumably 12 3x1" vents around the outside would maintain 36sq.in. cross sectional area.

I am thinking I'd like this oven to be pretty big (8-12 cu. ft), but I think the 6" RMH system should be able to keep that sort of volume pretty hot. Looking at the photo essay you sent, those sort of numbers seem to bear out.

Glad it was helpful Phil. If you look closely at the photo of my ugly weekend oven there you can see I do use a diffuser inside the oven. In that case it was just a flat rock, but I've successfully used pizza stones and kiln shelves there. The rock won't hold up long, but it baked some good bread over the weekend. 6" will provide a lot of heat, and if you build it on top of a barrel that has a bottom exit like a typical RMH you won't need to worry about system CSA at the top. In my photo you can see a wooden handle sticking out of the right side just below the oven. That's coupled to a damper that adjusts the opening on top of the barrel and controls the heat to the oven. In a 6" system I have just a sliver of opening there to try to hold it in the 500*F range. There's a LOT of potential heat right there, which is why it's nice to be able to shunt it into a bench. You just don't need much of the stove's output for cooking. Let's see if I can find a picture, hold on....

Like so.....



It pivots from right at the edge of the barrel there, and like I said, it's usually mostly closed. That's a 4" opening on the barrel top, which is plenty. It's not uncommon to have a flame column shooting 2' out of that hole when the stove is really ripping.

Cool. Just to make it clear, I am seeing that the oven would be essentially en-route to the exhaust - whereas if I understand your 'temporary' design, it is a conventional RMH with a selective bypass allowing some of the exhaust to route into the oven?

Yep, that's it exactly. By doing the "selective" routing you get pretty accurate temperature control, and the ability to use the heat downstream, not to mention you can send most of the exhaust away from the cooking area.

Hmm, interesting. I had planned to build a couple of additional "J tube" combustion chambers off to the side to provide wok-style burners for cooking using pots and pans - I suppose with a valve not unlike what Ernie and Erica show in the underfloor heating video, one could selectively redirect some of the combustion out of a different "chimney".

As a different approach, what if you had a valve allowing additional cool air to be sucked in, entering part way up the combustion chamber?

You definitely don't want to do the second. You need all the draw pulling through the fuel and any secondary air* will screw up the stove's performance. The first idea, I dunno. Might work fine, maybe not.

*with the exception of the Peter Channel as discussed on Donkey's board in the "small scale development" thread.

Why are these types of ovens generally domed (or at least circular in profile). Is it to do with heat distribution?

I believe they're parabolic in shape as that's self supporting whereas a flat or hemispherical dome is not self supporting.

https://www.theguardian.com/lifeandstyle/gallery/2011/may/04/building-cob-pizza-oven-pictures

In image 11 the student stands on the dome to show how strong it is.

Graham Chiu wrote:I believe they're parabolic in shape as that's self supporting whereas a flat or hemispherical dome is not self supporting.

https://www.theguardian.com/lifeandstyle/gallery/2011/may/04/building-cob-pizza-oven-pictures

In image 11 the student stands on the dome to show how strong it is.

Hi Graham,

Many arc shapes are self supporting even though they may not be parabolic.  If my memory of algebra is correct, the advantage of the parabola is that the rising heat which is vertical will bounce off of the roof and be directed towards the focal point of the parabola.  This will be towards the center area of the cooking surface.  It thus concentrates and helps retain the heat while simultaneously meeting the principle cooking area hot.

Sincerely,

Ralph